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Cell 201 > Midterm 2 > Flashcards

Midterm 2 Flashcards

1
Q

Cells producing steroid hormones tend to have extensive networks of ______ ER

A

Smooth

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2
Q

Cells involved in synthesis of secretory proteins have prominent _____ ER networks

A

Rough

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3
Q

Where is the tER, transitional Endoplasmic Reticulum, located?

A

At the edge of rough ER

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4
Q

What is the function of the tER, transitional Endoplasmic Reticulum?

A

The formation of vesicles that shuttle lipids and proteins from the ER to the Golgi: Assembling and budding vesicles for transport to other compartments or secretion (Sec12, Sar1, COPII proteins)

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5
Q

What does the smooth Endoplasmic Reticulum form?

A

Tubular structures

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6
Q

What does the rough Endoplasmic Reticulum form?

A

Large, flattened sacs

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7
Q

What is special about the sER?

A

It has no ribosomes

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8
Q

What is the function of the sER?

A
  • Membrane, Steroid and Lipid biosynthesis
  • Drug detoxification
  • Carbohydrate metabolism
  • Calcium Storage
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9
Q

What is the function of the rER?

A

It is the place where proteins are inserted into the ER
- Co-translational protein import
- Folding of secreted and membrane proteins
- Addition of carbohydrates to glycoproteins
- Recognition and removal of misfolded proteins
- Assembly and budding of vesicles for transport to other compartments or secretion

F A I R R A B

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10
Q

What is a special quality of the rER?

A

Sec61 translocon complexes with ribosomes on the cytosolic membrane side

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11
Q

What is vesicular transport?

A

The process of delivering components (proteins and lipids) to other compartments for formation of organelles and cell function

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12
Q

Anterograde transport is from ____ to the ______

A

From the ER to the PM

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13
Q

Retrograde transport is from ____ to the ____

A

From the PM to the ER

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14
Q

Why is vesicular transport so important?

A

It is critical for the balance and flow of lipids and membrane proteins

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15
Q

What is the Endoplasmic Reticulum?

A

It is a continuous network of flattened sacs, tubules and vesicles through a cell’s cytoplasm

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16
Q

What is the space inside the ER?

A

The ER lumen

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17
Q

What are the membrane-bound sacs inside the ER called?

A

Cisternae

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18
Q

What is the Endomembrane system?

A

A set of membranes that form a single functional and developmental unit

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19
Q

What does the Endomembrane system consist of?

A

The nuclear membrane, ER, Golgi, lysosomes, vesicles, endosomes, and the plasma membrane

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20
Q

How is the Endomembrane system connected?

A

Directly or indirectly through vesicular transport

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21
Q

Explain sER drug detoxification

A

Hydroxylation: adding hydroxyl groups to a drug in order to increase solubility and make it easier to excrete them from the body
Enzymes involved in the drug detoxification process can be upregulated and sER can proliferate due to drug exposure

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22
Q

Explain sER Carbohydrate metabolism

A

Breaking down stored glycogen using glucose-6-phosphatase, which is an enzyme unique to the sER. This enzyme hydrolyzes the phosphate from glucose-6-phosphate to form free glucose

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23
Q

Explain sER Calcium Storage

A

Calcium ions are actively pumped from the cytoplasm into the ER for storage, and calcium pumps and channels are enriched in smooth ER

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24
Q

Explain sER Steroid Biosynthesis

A

Enzymes responsible for steroid biosynthesis are present at the sER

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24
Q

What is the Golgi complex?

A

A series of flattened membrane-bound cisternae, and is functionally and physically linked to the ER

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25
Q

What is the function of the Golgi Complex?

A

It has a central role in membrane and protein trafficking in eukaryotic cells, where glycoproteins and membrane lipids from the ER undergo further processing and are sorted and packaged for transport

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26
Q

What is a series of cisternae called?

A

A Golgi stack

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27
Q

Secretory cells have how many Golgi stacks?

A

Hundreds or thousands

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28
Q

What are the two faces of the Golgi stack?

A

The CGN, Cis Golgi Network
The TGN, Trans Golgi Network

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29
Q

Describe the CGN

A

The cis face is oriented towards the ER, therefore the Golgi compartment on this side is called the CGN

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30
Q

Describe the TGN

A

The trans face is oriented away from the ER, therefore the Golgi compartment on this side is called the TGN

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31
Q

Proteins and lipids are delivered from the ___ to the ER

A

CGN

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32
Q

Proteins and lipids leave the Golgi in transport vesicles that continuously bud from the ___ destined for the cell surface or other organelles

A

TGN

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33
Q

What is between the TGN and CGN?

A

Medial cisternae, where much of the processing of proteins occurs

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34
Q

Each compartment shows _____ _______, containing specific proteins unique to each portion of the network

A

Biochemical polarity

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35
Q

Describe N-linked glycosylation

A

It involves the addition of an oligosaccharide to the nitrogen atom of certain asparagine residues

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36
Q

Describe O-linked glycosylation

A

It involves the addition of an oligosaccharide to the oxygen atom on the hydroxyl group of certain serine or threonine residues

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37
Q

What is glycosylation?

A

The addition of carbohydrate side chains to proteins, which is a big part of protein processing in the ER and Golgi

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38
Q

Where do the initial steps of N-glycosylation take place?

A

The ER

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39
Q

Glycosylation can occur co-translationally to promote…

A

Proper protein folding

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40
Q

All added chains in glycosylation initially have a common core oligosaccharide consisting of…

A

Two units of N-acetylglucosamine
Nine mannose units
Three glucose units

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41
Q

What do Calnexin (CNX) and Calreticulin (CRT) do?

A

They bind to monoglycosylated proteins and promote disulfide bond formation

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42
Q

Glucosidase I and II…

A

Remove 2 glucose residues

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43
Q

CNX and CRT are chaperones in that they…

A

Attempt to aid folding

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44
Q

The third glucose is removed by ________ after release from CNX and CRT

A

Glucosidase II

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45
Q

If protein folding is incorrect, a _________ binds to improperly folded proteins and adds back a ________, making the protein a substrate for CNX/CRT binding

A

Glucosyl transferase (UGGT)
Single glucose unit

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46
Q

The competition between UGGT and ER mannosidase I ultimately ______________

A

Determines the fate of the protein

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47
Q

What two enzymes compete to correct protein folding

A

UGGT and ER mannosidase I

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48
Q

ER mannosidase I can bind to remove a mannose residue which triggers _____

A

READ

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49
Q

Where does further processing of N-glycosylated proteins occur?

A

In the Golgi complex as the glycoproteins move from the CGN to the TGN

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50
Q

Terminal N- and O- linked glycosylations are…

A

Variable and create great diversity

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51
Q

The ER and Golgi contain hundreds of different glycosyl transferases?

A

True

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52
Q

What happens after budding from the TGN

A

Some vesicles move directly to the cell surface and immediately fuse with the PM

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53
Q

Constitutive Secretion is an ________ process, ______ and _______ of external signals

A

Unregulated
Continuous
Independent

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54
Q

What is an example of Constitutive secretion

A

Mucus secretion by the intestinal lining

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55
Q

What is Regulated Secretion?

A

When secretory vesicles accumulate in the cell and only fuse with the PM in response to specific signals

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56
Q

What is an example of Regulation secretion?

A

Neurotransmitter release where secretory vesicles carrying neurotransmitters move close to the site of secretion and remain there until receiving a signal

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57
Q

What is the role of depolarization in regulated secretion?

A

It is the signal that triggers vesicles to release their content by fusion with the PM

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58
Q

What is protein trafficking?

A

Proteins synthesized in the cell must be directed to a variety of locations. Once a protein reaches its destination, it must be prevented from leaving, and so proteins often contain a specific tag which targets it to a transport vesicle that will take it to the correct location

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59
Q

What is a protein targeting tag?

A

An amino acid sequence, hydrophobic domain, or oligosaccharide side chain which is used to target it to a transport vesicle, or to exclude material from certain vesicles

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60
Q

How do we maintain ER identity?

A

By preventing some proteins from escaping the ER and/or by retrieving others from the Golgi

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61
Q

What are some retention/retrieval tags that keep proteins in the ER?

A

RXR (Arg-X-Arg)
Dibasic (KK/disyline on the C-terminus)
RR/Diarginine
KDEL (mammals)
HDEL (yeast)

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62
Q

What happens when a protein with a retention tag binds a receptor in the Golgi?

A

The receptor-cargo complex is packaged into a transport vesicle for return to the ER

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63
Q

What is a Golgi-specific protein?

A

An integral membrane protein with one or more hydrophobic membrane-spanning domains

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64
Q

What does the length of the hydrophobic domain determine?

A

Into which cisternae each protein is incorporated

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65
Q

What is exocytosis?

A

The process by which secretory vesicles release their contents outside the cell. Proteins in a vesicle are released to the exterior of the cell as the vesicle fuses with the PM

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66
Q

What is endocytosis?

A

The process by which cells internalize external materials

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67
Q

What are some examples of exocytosis in animals?

A

Hormones, mucus, milk proteins, digestive enzymes

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68
Q

What are some examples of exocytosis in plants?

A

Enzyme and structural proteins for the cell wall

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69
Q

When a vesicle fuses with the PM in exocytosis, the luminal membrane of the vesicle becomes part of….

A

The outer surface of the PM

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70
Q

Glycolipids and glycoproteins that were formed in the ER and Golgi lumen will face the….

A

Extracellular space

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71
Q

What is Polarized secretion?

A

When exocytosis of specific proteins is limited to a specific surface of the cell

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72
Q

What is an example of polarized secretion?

A

Intestinal cells that secrete digestive enzymes only on the side of the cell that faces into the intestine

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73
Q

Why is membrane flow important?

A

To maintain a steady-state composition of the PM, which is defined by the balance between endocytosis and exocytosis

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74
Q

How do endocytosis and exocytosis influence membrane flow?

A

Endocytosis removes lipids and proteins from the PM, whereas Exocytosis adds lipids and proteins to the PM

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75
Q

What is phagocytosis?

A

The ingestion of large particles up to and including whole cells or organisms. The way unicellular organisms acquire food.

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76
Q

Explain phagocytosis in humans

A

Neutrophils and macrophages use phagocytosis as a means of defence, where they engulf and digest foreign materials or invasive microorganisms found in the bloodstream or injured tissues

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77
Q

What are the 4 steps in protein coating?

A
  1. Cargo sorting/Selection/Concentration
  2. Coat assembly/Membrane deformation
  3. Vesicle budding
  4. Vesicle uncoating
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78
Q

Why are protein and lipid vesicles called coated vesicles?

A

Because of the layers of proteins coating their cytosolic surfaces

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79
Q

What are some examples of coat protein systems?

A

Clathrin, COPI, COPII, Caveolin

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80
Q

What is the role of coat proteins?

A

They induce membrane curvature needed for the formation of the vesicles and participate in the collection of specific cargo molecules

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81
Q

What are Clathrin-coated vesicles?

A

Vesicles surrounded by coats make of two multimeric proteins: clathrin and adaptor protein (AP)

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82
Q

What does the shape of a clathrin protein and the way it’s assembled provide?

A

The driving force to form a spherical vesicle

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83
Q

What is the basic unit of a clathrin lattice?

A

A triskelion

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84
Q

Explain the structure of a triskelion

A

A multimeric protein composed of 3 heavy chains and 3 light chains which radiate from a central vertex, with the light chains associated with the inner half of each “leg”. Triskelions assemble into hexagons and pentagons of the lattice clathrin-coated pits and vesicles

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85
Q

All cargo must interact with the vesicle coat?

A

True, either directly or indirectly

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86
Q

Where do coats in the cytoplasm bind?

A

They bind to specific motifs contained in the cytoplsmic domain of transmembrane proteins

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87
Q

Why are many cargo proteins soluble?

A

They must be able to bind to receptors/adaptors which are transmembrane proteins in order to be collected into a vesicle

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88
Q

What are some examples of motifs for protein coat formation?

A
  • Dileucine (AP/clathrin)
  • YXX (Tyrosine - anything - anything)
  • Dilysin (COPI)
  • Diacidic (COPII)
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89
Q

What is required for constricting and closing a Clathrin-coated vesicle?

A

Dynamin

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90
Q

What is dynamin?

A

It is a cytosolic GTPase; as GTPase is hydrolyzed, dynamin rings tighten and separate the vesicle from the PM, which allows clathrin to dissociate once the vesicle has formed and budded

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91
Q

How is COPI and COPII vesicle coat assembly initiated?

A

By members of the Ras superfamily of small GTPases

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92
Q

What are the On and Off states of GTPase?

A

GTP = membrane-bound (on)
GDP = soluble/cytoplasm (off)

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93
Q

How is GTPase activated?

A

Guanine nucleotide exchange factor

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94
Q

What does NSF mean?

A

N-ethylmaleimide sensitive factor

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95
Q

What does SNAP mean?

A

Small NSF attachment protein

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96
Q

What does SNARE mean?

A

SNAP receptor

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97
Q

Where are v-SNAREs found?

A

On vesicles

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98
Q

Where are t-SNAREs found?

A

On target membranes

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99
Q

V- and T-SNAREs are complementary molecules that…

A

Allow recognition between vesicles and their targets

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100
Q

What is the role of COPI-coated vesicles?

A

Retrograde transport from the Golgi back to the ER

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101
Q

What are COPI-coated vesicles coated with?

A

COPI, ARF (ADP ribosylation factor), and a small GTP-binding protein

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102
Q

What factor mediates the assembly of a coat?

A

ARF

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103
Q

What is the COPI-coated vesicle cycle?

A
  1. In the cytoplasm, ARF exists in complex with GDP
  2. Upon meeding a GEF (Guanine exchange factor) associated with the membrane, the GDP is exchanged for GTP
  3. The resulting conformational change in ARF attaches it to the membrane and leads to cargo and COPI association with ARF
  4. Assembly of the coat drives vesicle formation
  5. Once the vesicle is formed, a GAP (GTPase activating protein) in the donor membrane triggers hydrolysis of GTP to GDP, a conformational change in ARF and release of the coat
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104
Q

What is the role of COPII-coated vesicles?

A

Transport from the ER to the Golgi

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105
Q

In yeast, what is the COPII coat assembled from?

A

Sec13/31 and Sec23/24, and a small GTP-binding protein called SarI

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106
Q

What is SarI?

A

It has an amphipathic helix at the N-terminus, and is similar to ARF. Its process of coat formation is similar to COPI-coated vesicles

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107
Q

What is the SNARE hypothesis?

A

That sorting and targeting of vesicles involves two families of SNARE proteins… once vesicles are formed, additional proteins ensure delivery to the correct destination

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108
Q

What is the role of Rab GTPases?

A

They lock SNARE proteins together in order to facilitate membrane fusion

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109
Q

What are tethering proteins?

A

Tethering proteins act over long distances and attach vesicles to their targets before the SNAREs interact

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110
Q

What is Complexin?

A

A tethering protein that regulates neurotransmitter vesicle fusion, where is clamps SNARE proteins to prevent zippering/fusion (superprimed)

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111
Q

When do superprimed vesicles fuse?

A

When calcium enters the cell

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112
Q

Why is Receptor-mediated Endocytosis important?

A

Cells acquire some substances through this process, and other cells use receptors on the outer cell surface to internalize many macromolecules

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113
Q

How does Receptor-mediated Endocytosis regulate cell signalling?

A
  1. Receptors on the cell surface can be stimulated by molecules in the environment, leading to growth, cell division, motility, etc
  2. After receiving these signals, cells can internalize the receptors to become less responsive to the stimulus (Desensitization)
  3. After internalization of a receptor, the vesicle can fuse with other vesicles that are budding from the TGN to form early endosomes
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114
Q

What is Defective Desensitization?

A

Failure to internalize the receptor which can lead to overstimulation (excess cell division and tumour formation)

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115
Q

What is the role of early endosomes?

A

Sites for sorting and recycling of materials brought into the cell

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116
Q

How can we recycle plasma membrane receptors?

A

Receptors from the Golgi membrane can be recycled through fusion with other vesicles
Acidification of the early endosome can occur facilitated by an ATP-dependent proton pump
The lower pH promotes the removal of the ligand from a receptor to allow recycling back to the Golgi

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117
Q

What is a lysosome?

A

An organelle of the endomembrane system that contains digestive enzymes that are capable of degrading all the major classes of biological macromolecules. They maintain an acidic environment (pH 4.0-5.0) due to their ATP-dependent proton pumps

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118
Q

What are the enzymes inside the lysosomes called?

A

Acid hydrolases

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119
Q

How do lysosomes develop?

A

Lysosomal enzymes are co-translationally synthesized into the ER and trafficked through the secretory pathway to the TGN and then sent to endosomes in transport vesicles. Overtime, endosomes mature into late endosomes, with all the enzymes present, but not engaged in digestion. As the internal environment becomes more acidic, the acid hydrolases become activated through the pumping of protons or through fusion with an existing lysosome.

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120
Q

What are lysosomal storage diseases?

A

They are characterized by the accumulation of substances that cannot be broken down as needed, and most have no treatment

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121
Q

What is Autophagy?

A

When cellular structures that are damaged and are no longer needed can be broken down, and these damaged organelles can be wrapped in a double membrane derived from the ER, forming an autophagosome.

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122
Q

What is recycled in Autophagy?

A

Nucleotides, sugars, amino acids, etc

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123
Q

What can trigger Autophagy?

A

Starvation

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124
Q

How does Signal Transduction work?

A

Cell membranes can regulate the flow of ions between the interior and exterior of the cell. Nerve cells have special mechanisms for using electrical potentials to transmit information over long distances.
Cells can also communicate by sending and receiving regulatory chemical messengers. Receptors are located on receiving cells that can be quite distant from the secreting cell

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125
Q

What is Membrane potential?

A

A fundamental property of all cells, where cells at rest normally have an excess positive charge on the outside and a negative charge on the inside. The resulting electrical potential of the cell is called the resting membrane potential, which is -70mV

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126
Q

What is Electrical Excitability?

A

The unique feature of electrically excitable cells is their response to depolarization, where excitable cells respond with an action potential.

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127
Q

How to excitable cells respond to depolarization?

A

They respond with an action potential, where they have voltage-gated channels in their plasma membranes, and the coordinated opening and closing of the ion channel leads to an action potential

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128
Q

Sodium influx leads to a ______ charge inside the cell

A

Positive

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129
Q

How is resting potential established?

A

By the 3 Sodiums out/2 Potassiums in exchange

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130
Q

How is an action potential propagated?

A

A stimulus triggers the voltage-gated sodium channel and sodium flows into the cell with the gradient. The increase in sodium triggers the opening of potassium channels and they flow out with the gradient. The sodium/potassium exchanger then re-establishes the resting potential

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131
Q

What is an electrical synapse?

A

When a presynaptic neuron is connected to a post synaptic neuron via gap junctions, and the iosn move through the junctions between the cells with no delay in transmission

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132
Q

What is a chemical synapse?

A

When presynaptic and postsynaptic neurons are not connected by gap junctions, but instead a synaptic cleft. A signal at the terminus of the presynaptic neuron must be sent to the postsynaptic neuron chemically

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133
Q

What is spatial summation?

A

Action potentials that are received at a single synapse are usually not sufficient to induce an action potential, and so when many action potentials can neurotransmitter release simultaneously, it is more likely that an action potential with be induced.

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134
Q

Postsynaptic neurons can receive both inhibitory and excitatory signals?

A

True

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135
Q

Neurons can receive thousands of inputs from other neurons and physically sum the signals?

A

True

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136
Q

What is chemical messaging?

A

The second major means of intercellular communication: Cells produce signals by displaying molecules on their surface, or by releasing a chemical signal.

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137
Q

What are the different kinds of chemical signals?

A

Paracrine
Exocrine
Juxtacrine
Autocrine

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138
Q

Paracrine signals…

A

Are diffusible and act over a short range

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139
Q

Endocrine signals…

A

Are produced far from the target tissues, which they reach via the circulatory system

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140
Q

Juxtacrine signals…

A

Require physical contact between sending and receiving cells

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141
Q

Autocrine signals…

A

Act on the same cell that produces them

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142
Q

What is a ligand?

A

A chemical messenger that binds to a target receptor

143
Q

When a messenger reaches its target, it binds to a…

A

Receptor

144
Q

The ligand is a _____ messenger

A

Primary

145
Q

How do messengers bind to receptors in a highly specific way?

A

Through several noncovalent bonds, which is achieved by the binding site on the receptor fitting the messenger very closely with necessary amino acid side chains, positioned to form chemical bonds with the messenger

146
Q

What is a cognate receptor?

A

A receptor specific for a certain ligand

147
Q

When a receptor is bound to its ligand, it is said to be…

A

Occupied

148
Q

When a ligand binds to its receptor, it often…

A
  1. Induces a change in receptor conformation
  2. Causes receptors to cluster
    These both lead to activation
149
Q

What is a second messenger?

A

Small molecules or ions that relay the signal within a cell, due to ligand binding that triggers production of molecules

150
Q

What are some examples of second messengers?

A
  • cAMP
  • Ions
  • Gases
  • Membrane lipid derivatives
151
Q

What is signal amplification?

A

The multiplication of the effect of a signal

152
Q

What is signal integration?

A

Where cells integrate a multitude of signals in order to produce appropriate responses, as a single receptor can activate multiple pathways, or these pathways can converge into the same molecule

153
Q

What is signalling crosstalk?

A

Where activated components from one pathway affect components of another pathway

154
Q

What are the categories of receptors?

A
  1. Ligand-gated channels
  2. Plasma membrane receptors
  3. G-proteins
  4. Receptor-linked kinases
155
Q

What is the G-protein-coupled receptor?

A

Ligand-binding causes a change in receptor conformation that activates a particular G-protein. All G-protein-coupled receptors have a similar structure with seven transmembrane helices connected by alternating cytosolic or extracellular loops, and the extracellular portion of each receptor has a unique messenger-binding site

156
Q

How do G-proteins act?

A

Like molecular switches whose on and off states depends on whether they are bound to GTP or GDP

157
Q

Heterotrimeric G-proteins mediate signal transduction through…

A

G-protein-linked receptors and have G,G,G subunits

158
Q

How are G-proteins regulated?

A

The rate of hydrolysis… GTP hydrolysis is greatly enhanced by regulators of G-protein-signalling proteins (RGS)

159
Q

What is cyclic AMP?

A

cAMP is the second messenger of G-protein signaling formed from cytosolic AMP by adenylyl cyclase, an enzyme that is anchored in the plasma membrane. The enzyme is inactive until bound to activated G

160
Q

What is the function of cAMP?

A

Its main target is PKA, protein kinase A, which is regulates by separating the regulatory and catalytic subunits

161
Q

What is PKA?

A

Protein kinase A, which phosphorylates a variety of proteins on Ser or THR residues, using ATP as the energy source
Can phosphorylate amino acids on a receptor and inhibit them

162
Q

G proteins are active for a _____ time

A

Short
They can quickly respond to changing conditions

163
Q

cAMP is degraded by…

A

A phosphodiesterase

164
Q

Once a G protein becomes inactive, adenylyl cyclase stops making new cAMP

A

True

165
Q

What is a GRK?

A

G-protein-linked receptor kinase, which acts on activated receptors to inhibit their activity, and carry out the phosphorylation on amino acids in the cytosolic domain

165
Q

What is a GRK?

A

G-protein-linked receptor kinase, which acts on activated receptors to inhibit their activity, and carry out the phosphorylation on amino acids in the cytosolic domain

166
Q

What is IP3?

A

Inositol-1,4,5-triphosphate

167
Q

What is the role of IP3?

A

It functions as a second messenger. It’s generated from PIP2 when phospholipase C is activated, and it cleaves PIP@ into IP3 and diacylglycerol, both of which are second messengers in a variety of cellular events

168
Q

The role of calcium in cell signalling pathways?

A

Ca2+ is a part of the IP3 cell signalling pathways, and IP3 binding to receptors on the ER can trigger Ca2+ release. Ca2+ conc. are maintained at low levels through calcium ATPases in the PM and ER. Calcium can bind to effector proteins and alter their activity

169
Q

What is Calmodulin?

A

It is a protein that mediates many calcium-activated processes in the cell. It has a structure like an arm with a hand at each end, and each end binds two Ca2+

170
Q

Consequences of disruption in G-protein signalling?

A

Some bacteria cause their diseases through their effects on heterotrimeric G proteins, like Cholera.
When V. Cholera colonizes the gut, it secretes cholera toxin, which modifies G so that it cannot hydrolyze GTP, which alters the salts and fluids in the intestine and can cause death by dehydration.

171
Q

What is receptor affinity?

A

The relationship between the amount of ligand in solution and the number of receptors occupied

172
Q

What is Kd?

A

The dissociation constant, which is the amount of free ligand needed to produce a state in which half the receptors are occupied
Receptors with high ligand affinity have low Kd and vice versa

173
Q

Explain Kd = Koff/Kon

A

“On” is governed by the concentration
“Off” is governed by how tightly the ligand is bound

174
Q

What is Dynamic equilibrium?

A

The balance between two events (Koff and Kon), and at any given moment, there will be a certain number of unoccupied and occupied receptors

175
Q

What is an Agonist?

A

They activate the receptor to which they are bound (mimics)

176
Q

What is an Antogonist?

A

They bing receptors without triggering a change and preventing activation of the receptor (blocks)

177
Q

When is receptor down-regulation?

A

When receptors are occupied for prolonged periods, that the cell adapts to no longer respond to the ligand

178
Q

How do cells down-regulate receptors?

A
  1. Cells reduce the density of receptors on their cell surface via receptor-mediated endocytosis
  2. Cells can adapt to signals by desensitization, alterations to the receptor that lower it’s affinity for the ligand and prevent downstream intracellular events
179
Q

What is a common desensitization method?

A

Phosphorylation

180
Q

What are protein kinase-associated receptors?

A

Receptors that can also function as kinases, by which they are stimulated by ligand binding. Signalling of the receptor protein kinases is transmitted through a phosphorylation cascade

181
Q

What are growth factors?

A

Messengers in a serum that stimulate growth; Cultured cells in vitro will not grow unless blood serum is provided

182
Q

What is PDGF?

A

Platelet-derived growth factor; secreted by platelets as a part of the healing process to stimulate fibroblasts to form new connective tissue. Its receptor is a receptor tyrosine kinase

183
Q

What is the structure of a Receptor Tyrosine Kinase?

A

A single polypeptide chain with just one transmembrane segment
The extracellular part contains the ligand-binding domain
On the cytosolic side is the tyrosine kinase domain

184
Q

What is autophosphorylation?

A

When a receptor phosphorylates the same kind of receptor as themselves

185
Q

How is signal transduction initiated?

A

Upon ligand binding, where receptors form dimers upon ligand binding and phosphorylate each other
Phosphorylated tyrosine residues can be bound by proteins with Src-homology2 (SH2) domains, and can activate phospholipaseC, leading to the production of IP3 and DAG

186
Q

What is Src?

A

One of the first tyrosine kinases identified. It is regulated by phosphorylation at two sites.

187
Q

What is JAK?

A

Nonreceptor tyrosine kinase, which binds to the receptor and is activated in response to ligand binding

188
Q

Phosphorylated tyrosine residues can be recognised by…

A

SH2 domains

189
Q

Why are SH2 domains important?

A

There are important structural modules for signalling cascades, and proteins with SH2 domains are recruited to proteins that are phosphorylated by tyrosine kinases

190
Q

What is the Kinase cascade?

A
  1. Autophosphorylation of the receptors recruits cytosolic proteins
  2. GRB2, which has an SH2 domain that recognizes the phosphotyrosine on EGFR, binds the receptor leading to the activation of Sos
  3. Sos stimulates Ras to release GDP and bind a GTP molecule, which activates Ras
  4. Activated Ras triggers a series of phosphorylations by Raf
  5. MAP kinases are activated
  6. MAPKs phosphorylate transcription factors that alter gene expression
  7. Once Ras is in its active state, it must be inactivated to avoid continual stimulation of the Ras pathway, which is accomplished by a GAP that facilitates GTP hydrolysis
    8/ GDP-bound Ras is now inactive
191
Q

Why is Ras important?

A

In regulating the growth of cells (it is a small monomeric G protein)
It can bind GDP or GTP, but is only active when bound to GTP
Requires assistance from a GEF (guanine-nucleotide exchange factor) called Sos to acquire a GTP molecule

192
Q

EPK/MAPK can inactivate Ras through a variety of mechanisms to turn off signalling?

A

True

193
Q

How do we make signalling more efficient?

A

Scaffolding proteins, where components are assembled into large multiprotein complexes

194
Q

What is the importance of Mating factor?

A

It signals between cells resulting in large-scale changes to polarized secretion, cytoskeleton, and gene expression?

195
Q

Type I diabetes is caused by…

A

Loss of insulin-producing cells

196
Q

Type II diabetes results from…

A

Insulin resistance

197
Q

What is a cytoskeleton?

A

A network of interconnected filaments and tubules extending through the cytosol
It plays roles in cell movement and division
It is dynamic and changeable

198
Q

What are the major structural elements of the cytoskeleton?

A

Microtubules
Tubulin
Microfilaments
Actin
Intermediate filaments
Various proteins

199
Q

How do we study receptor function?

A

Introducing mutations into the receptor: FGFs (Fibroblast growth factors) and their receptor tyrosine kinases… normal FGFRs undergo autophosphorylation in response to ligand binding

200
Q

What are dominant negative receptor mutations?

A

A mutant that overrides normal function;
Some mutant FGFRs can find ligands but cannot undergo autophosphorylation… these mutant receptors interfere with normal receptor function because they can dimerize with normal receptors

201
Q

What does a dominant mutation in FGFR-3 cause?

A

A form of dwarfism called achondroplasia

202
Q

What are constitutive mutations?

A

When mutations cause the receptor to stay switched “on” all the time

203
Q

What is a major class of serine-threonine kinases?

A

A family of proteins that bind TGFb (Transforming growth factor b) family members

204
Q

What is the TGFb family?

A

It regulates many cell functions including cell proliferation, programmed cell death, specialization, and key embryonic events

205
Q

Describe the TGFb signalling cascade?

A
  1. Growth factor finds the transmembrane receptor
  2. Upon ligand binding, the type II receptor phosphorylates the type I receptor, which then initiates a signal transduction cascade
  3. R-Smad is phosphorylated by a complex of anchoring proteins with the receptors
  4. Smad 4 forms a ultiprotein complex with phosphorylated R-Smadsl the whole complex can then enter the nucleus
  5. In the nucleus, the Smad complex can regulate gene expression
206
Q

What are endocrine hormones?

A

Hormones that travel from sending to receiving cells via the circulatory system
They are synthesized by endocrine tissues and are secreted directly into the blood stream, with a lifespan ranging from a few seconds to many hours
As they circulate, they encounter their receptors in target tissues

207
Q

What are hormones?

A

Secreted chemical signals that coordinate the function of cells and tissues over long distances

208
Q

What do steroid receptor proteins do?

A

Mediate the actions of steroid hormones such as progesterone, estrogen, testosterone, and glucocorticoids

209
Q

What are steroid hormones?

A

Lipid-based (cholesterol) signalling molecules
The hormone enters the target cell and binds its receptor, triggering a cascade of events that change gene expression

210
Q

How are Bacteria and Archaea structurally similar to Eukaryotes?

A

The actin-like MreB protein is involved in DNA segregation
The tubulin-like FtsZ protein is involved in regulating division
Crescentin is a regulator of cell shape

211
Q

What are microtubules?

A

The largest of the cytoskeletal components of a cell
Straight, hollow cylinders of varied length that consist of (usually 13) longitudinal arrays of polymers called protofilaments

212
Q

What are the two types of microtubules?

A
  1. Cytoplasmic microtubules
  2. Axonemal microtubules
213
Q

What are cytoplasmic microtubules?

A

They pervade the cytosol and are responsible for a variety of functions:
- Formation of mitotic and meiotic spindles
- Maintaining or altering cell shape
- Placement and movement of vesicles

214
Q

What are axonemal microtubules?

A

They include the organized and stable microtubules found in structures such as cilia and flagella

215
Q

What is the axoneme?

A

The central shaft of a cilium or flagellum, a highly ordereed bundle of microtubules

216
Q

What is a protofilament?

A

The basic subunit of a protofilament is a heterodimer of tubulin, one a-tubulin and one b-tubulin
They bind noncovalently to form an ab-heterodimer, which does not normally dissociate

217
Q

What is the structure of the a and b subunuts?

A

They have very similar 3D structure but only 40% amino acid identity
Each has an N-terminal GTP binding domain, a central domain to which colchicine can find, and a C-terminal domain that interacts with MAPs (microtubule-associated proteins)
All dimers in the microtubule are oriented the same way

218
Q

What is to polarity of microtubules?

A

Protofilaments have an inherent polarity
The two ends differ both chemically and structurally

219
Q

Cytoplasmic MTs are _____ MTs

A

Singlet, with 13 protofilaments

220
Q

Some axonemal MTs form ______ or _______ Mts

A

Doublet or triplet

221
Q

What are the characteristics of Doublet and Triplet MTs

A

They contain one 13-protofilament tubule (A tubule) and one or two additional incomplete rings (B and C tubules) or 10 or 11 protofilaments

222
Q

How do MTs form?

A

Nucleation: Reversible polymerization of tubulin dimers in the presence of GTP and MG2+
Dimers aggregate into oligomers, which serve as nuclei from which new MTs grow

223
Q

What is elongation?

A

The addition of more subunits at either end

224
Q

Howdo MTs assemble in vitro?

A

MT formation is slow at first, which is called the lag phase due to the slow process of nucleation
The elongation phase is much faster; when the mass of MTs reaches a point where the amount of free tubulin is diminished, the assembly is balanced by disassembly; the plateau phase

225
Q

What is the critical concentration?

A

The tubulin concentration at which MT assembly is exactly balanced by disassembly
MTs grow when the tubulin concentration exceeds the critical concentration and shrink when the concentration is below

226
Q

Do the two ends of a MT differ?

A

They differ chemically, and one can grow or shrink much faster than the other
This can be visualized by mixing basal bodies
The rapidly growing MT end is the plus end and the other is the minus end

227
Q

What is MT treadmilling?

A

Addition of subunits at the plus end and removal from the minus end
If the concentration of tubulin subunits is above the critical concentration for the plus end, but below that of the minus end, treadmilling will occur

228
Q

How does colchicine affect the assembly of MT?

A

It binds to tubulin monomers, inhibiting their assembly into MTs and promoting MT disassembly

229
Q

How does Nocodazole affect the assembly of MT?

A

It inhibits MT assembly, and its effects are easily more reversed than those of colchicine

230
Q

What are Antimitotic drugs?

A

They interfere with the spindle assembly and this inhibit cell divison. It is useful for cancer treatment; commonly used for breast cancer

231
Q

How does Taxol affect MT?

A

It binds tightly to microtubules and stabilizes them, causing a depletion of free tubulin subunits causing dividing cells to arrest during mitosis

232
Q

What are Catastrophins?

A

They act at the ends of MTs and promote the peeling of subunits from the ends

233
Q

What do Katanins do?

A

They sever MTs

234
Q

What is the structure of Centrioles?

A

The walls are formed by 9 pairs of triplet microtubules, oriented at right angles to each other

235
Q

What role do Centrioles play?

A

They are involved in basal body formation for cilia and flagella
Cells without centrioles have poorly organized mitotic spindles

236
Q

Where do MTs originate from?

A

MTOC, microtubule-organizing center
Also called the centrosome
In animal cells, the centrosome is associated with two centrioles, surrounded by pericentriolar material

237
Q

How does GTP hydrolysis contribute to MT dynamic instability?

A

Each tubulin heterodimer binds two GTP molecules, a-tubulin binds one and b-tubulin binds a second
GTP is needed to promote heterodimer interactions and addition to MTs, but its hydrolysis is not required for MT assembly
The GTP bound to the b-subunit is hydrolyzed after the heterodimer is added to the MT

238
Q

MTs grow by ________ and _________ at plus ends

A

Polymerization and Depolymerization

239
Q

Growing MTs have ___ at plus ends

A

GTP

240
Q

Shrinking MTs have ___ at plus ends

A

GDP

241
Q

What stabilizes the MT?

A

The GTP cap at the plus end

242
Q

If the GTP cap disappears altogether…

A

The MT becomes unstable and loss of GDP-bound subunits is favored

243
Q

What is Microtubule catastrophe?

A

A switch from growth to loss of an MT

244
Q

What is Microtubule rescue?

A

A sudden switch back to the growth phase of an MT

245
Q

What is g-tubulin?

A

A large ring-shaped protein complex inside of a centrosome

246
Q

What are g-TuRCs?

A

g-Tubulin ring complexes, which nucleate the assembly of new MTs away from the centrosome

247
Q

Where do MTs grow outwards from?

A

The MTOC, which a fixed polarity with dynamic growth and shrinkage of MTs occuring at the plus ends

248
Q

What are MAPs?

A

Microtubule-associated proteins, which bind at regular intervals along a microtubule wall, allowing for interaction with other cellular structures and filaments
Tau causes MTs to form tight bundles in axons
MAP2 promotes the formation of looser bundles in dendrites

249
Q

What are +TIP proteins?

A

+-end tubulin interacting proteins, which stabilize the proteins that capture and protect the growing plus ends, while decreasing the likelihood that MTs will undergo catastrophic subunit loss

250
Q

What are microfilaments?

A

The smallest of the cytoskeletal filaments
Best known for their role in muscle contraction
Play critical roles in organelle structure, cell migration and endocytosis, development and maintenance of cell shape
It is the structural core of microvilli
Can assemble to form characteristic structures

251
Q

What is actin?

A

The building block of microfilaments
Folds into a globular-shaped molecule that can bind ATP or ADP

252
Q

What is G-actin?

A

Molecules that polymerize to form microfilaments, F-actin

253
Q

What are F-actin microfilaments?

A

Composed of two linear strands of polymerized G-actin, wound into a helix

254
Q

All actin monomers in the filament have the same orientation?

A

True (plus and minus end)

255
Q

G-actin polymerizes reversibly into filaments similar to tubulin assembly?

A

True

256
Q

What are Cytochalasins?

A

Fungal metabolites that prevent the addition of new monomers to existing MFs (bind the filament)

257
Q

What is Latrunculin A

A

A toxin that sequesters actin monomers and prevents their addition (bind to the monomers)

258
Q

What is Phalloidin?

A

Stabilizes MFs and prevent depolymerization (bind to the interface between subunits)

259
Q

What happens after G-actin monomers assemble onto the microfilament?

A

ATP bound to them is slowly hydrolyzed

260
Q

Growing plus ends have…

A

ATP-actin

261
Q

Most of the MF is composed of…

A

ADP-actin

262
Q

What affects the polarity of MFs?

A

The rapid addition of G-actin at the plus end

263
Q

What are filopodia?

A

Organized and polarized MF cables with the plus ends toward the tip of the protrusion
More organized than lamellipodia

264
Q

What are stress fibers?

A

Organized bundles characteristic of cells that adhere tightly to a surface, are needed for attachment

265
Q

What are lamellipodia and filopodia?

A

Characteristic of cells that crawl and generate force at the leading edge of the cell to allow them to move along a surface

266
Q

Cells regulate ____ and ___ MFs are assembled

A

Where, how

267
Q

What are actin-binding proteins?

A

Proteins that control where actin assembles and the organization of the resulting network
Control occurs at the level of nucleation, elongation, severing of MFs, and association of MFs into networks

268
Q

If the concentration of ATP-bound G-actin is high, MFs will…

A

assemble until the G-actin is limiting

269
Q

What is Thymosin 4?

A

Binds free G-actin

270
Q

How is the availability of G-actin controlled?

A

By regulating the amount of it bound to thymosin 4 by profilin

271
Q

What are capping proteins?

A

They bind the ends of a filament to prevent further loss or addition of subunits

272
Q

What does CapZ do?

A

Binds to plus ends to prevent loss/addition of subunits

273
Q

What does Tropomodulin do?

A

Bind to minus ends, preventing loss of subunits

274
Q

What does Selsolin do?

A

Break up MFs because it can bind to F-actin and sever the filament, cap the plus end to prevent addition or loss of subunits

275
Q

What can we do with the severed filaments?

A

Crosslink or bundle them to remodel the actin cytoskeleton

276
Q

Gelsolin activity is activated by ___ and inhibited by ___

A

Ca2+, PIP2

277
Q

What does filamen do?

A

Form actin networks from loose networks of crosslinked filaments.
Anchors two MFs together where they intersect at a fixed angle

278
Q

What does byfimbrin do?

A

Tightly bind MFs in a bundle

279
Q

How do proteins link actin to membranes?

A

Linking proteins, such as Band 4.1 or Ankyrin with Spectrin
They can connect to the PM and exert force on it during cell movement or cytokinesis

280
Q

What is the Arp2/3 Complex?

A

A complex of actin-related proteins, nucleates new branches on the sides of old filaments
Activated by a family of proteins that includes WASP and WAVE/Scar

281
Q

What is WASP?

A

Autoinhibited, activated by PIP2, and once activated will activate Arp2/3

282
Q

What are formins?

A

A protein regulating actin polymerization, move with the end of the growing filament to promote polymerization

283
Q

What are intermediate filaments?

A

The most stable and least soluble cytoskeletal component
Not polarized
Keratin
Support the entire cytoskeleton

284
Q

What is cell motility?

A

Movement of a cell or organism through the environment
Movement of the environment past/through the cell
Movement of components in the cell
Occurs at the tissue, cellular, and subcellular level

285
Q

What is cell contractility?

A

Describes the shortening of muscle cells, a specialized form of motility

286
Q

What are plakins?

A

Linker proteins that connect IFs, MFs, and MTs

287
Q

What is plectin?

A

A plakin found at sites where IFs connect to MFs and MTs

288
Q

What are intercalated discs?

A

Joins heart cells from end to end
Have a high concentration of gap junctions, so waves of depolarization spread easily from one cell to the next

289
Q

What is smooth muscle?

A

Responsible for involuntary contraction in various tissues
Contractions are relatively slow and of greater duration than in skeletal or cardiac muscle

290
Q

What is the structure of smooth muscle?

A

Long and thin with pointed ends, no striations
Dense bodies, plaque-like structures
Bundles of actin filaments are anchored to the dense bodies in a crisscross pattern, cross-bridges form in an irregular pattern

291
Q

What are integrins?

A

Transmembrane proteins:
Outside the cell: attached to the extracellular matrix proteins
Inside the cell: connected to actin filaments via linker proteins

292
Q

What are adhesions/focal contacts?

A

Integrin-dependent attachments

293
Q

What is Directional migration?

A

Occurs through the formation of protrusions predominantly on one side of a cell

294
Q

What is chemotaxis?

A

Whena cell moves in response to a chemical gradient

295
Q

What are Chemoattractants?

A

When cells move toward a higher concentration of the diffusible molecules

296
Q

What are Chemorepellants?

A

When cells move toward a lower concentration of the diffusible molecules

297
Q

Binding of the molecules to cell surface receptors leads to…

A

Corresponding cytoskeletal changes

298
Q

What does activation of the Rho pathway result in?

A

Formation of stress fibers

299
Q

What does Rac activation result in?

A

Extension of lamellipodia

300
Q

What does Cdc42 activation result in?

A

Formation of filopodia

301
Q

How does a smooth muscle contract?

A
  1. In response to a nerve or hormonal signal, extracellular calcium enters the muscle cell, activating Calmodulin
  2. The calcium-Calmodulin complex binds to MLCK, activating it and triggering myosin light-chain phosphorylation
  3. Phosphorylation leads to a conformational change in myosin, promoting its assembly into filaments and activates the cross-bridge cycle
  4. As the calcium levels in the muscle cell fall, MLCK is inactivated
  5. Myosin light-chain phosphatase removes the phosphate from the myosin light chain and the muscle cell relaxes
302
Q

What is cell crawling?

A

Movement of most cells in animals through MFs
Involves: extension of a protrusion, attachment to a substrate, and generation of tension which pulls the cell forward

303
Q

How do crawling cells generate force and movement?

A

Cells extend protrusions at their front/leading edge, and by actin retrograde flow, MFs move toward the rear of the protrusion as it extends

304
Q

What is retrograde flow?

A

Flow resulting from actin assembly at the growing protrusion and rearward translocation of filaments toward the base

305
Q

How do cells attach?

A

New sites of attachment are formed at the front of the cell, and contacts at the rear must be broken

306
Q

What are attachment sites?

A

Complex structures involving a number of proteins including integrins

307
Q

How do cells contract?

A

Under the control of Rho, nonmuscle myosin II at the rear of the cell is activated
The cell body is squeezed forward and releases attachments at the rear

308
Q

For new movement to occur, new attachments must be balanced by…

A

Loss of old ones

309
Q

What is phosphatidylinositol-4,5-bisphosphate (PIP2)?

A

Binds to profilin, WASP, CapZ, gelsolin and other proteins

310
Q

What are the Rho GTPases?

A

Rho, Rac, Cdc42
Monomeric G proteins

311
Q

How are Rho GTPases regulated?

A

They are stimulated by GEFs through the exchange of bound GDP for GTP
GAPs inactivate Rho GTPases bycausing them to hydrolyze their bound GTPs to GDP
GDIs sequester inactive Rho GTPases in the cytosol

312
Q

What is the Sarcoplasmic reticulum?

A

Has calcium ATPases to pump calcium into the SR

313
Q

How are calcium levels controlled?

A

By nerve impulses from motor neurons, calcium release into the cytosol of a muscle cell triggers contraction, for muscles to relax, calcium levels must decrease

314
Q

IF proteins are ___ rather than globular

A

Fibrous

315
Q

What is IF structure?

A

All have an homologous central rodlike domain conserved in size, secondary structure, and in sequence
Flanking the central helical domain are N- and C- terminal domains that differ greatly among IF proteins

316
Q

Why are IFs important structural determinants?

A

Because they are thought to have a tension-bearing role
Less dynamic than MFs and MTs but are not static structures

317
Q

Cell architecture depends on…

A

The unique properties of all the cytoskeletal elements working together

318
Q

MTs _____ binding when a cell is compressed

A

Resist

319
Q

MFs serve to ____ tension and force

A

Generate

320
Q

IFs are ___ and can withstand tensile forces

A

Elastic

321
Q

MTs and MFs provide a scaffold for motor proteins, or mechanoenzymes to…

A

Produce motion at the molecular level

322
Q

What is the structure of cilia and flagella?

A

Axonemes have a characteristic 9+2 pattern, with 9 outer doublets and 2 MTs in the center, the central pair
Each outer doublet of the axoneme consists of one complete MT (the A tubule) and one incomplete MT (the B tubule)
The A tubule has 13 protofilaments, whereas the B tubule has 10 or 11, the tubules of the central pair are both complete
Each A tubule has a set of sidearms that project from each of the outer doublets; these consist of axonemal Dynein

323
Q

What is the role of Axonemal dynein?

A
  • Axonemal dynein is involved in the sliding of MTs against each other, which bends the axoneme
  • The dynein arms occur in pairs, one inner and one outer arm
324
Q

What are primary cilia?

A

Primary cilia are present on almost all cells and serve as sensory structures
These have a “9+0” structure, i.e., lacking the central pair
Primary cilia are also important in development; defects in them can result in disorders such as deafness and left-right asymmetry reversals

325
Q

What are myosins?

A

Myosins are ATP-dependent motors that exert force on actin filaments
Currently there are 24 known classes of myosins
All have at least one polypeptide chain called the heavy chain, with a globular head group attached to a tail of varying length

326
Q

What are Myosins function?

A
  • Muscle contraction
  • Cell movement
  • Phagocytosis
  • Vesicle transport
    Type II myosins are the best understood
    They have two heavy chains (each with a globular head, a hinge region, a rodlike tail) and four light chains
    They use ATP hydrolysis to cause actin filaments to slide past myosin molecules, resulting in contraction of a cell or group of cells
    the functions of myosin?
327
Q

What is the structure of myosin?

A

The globular head binds actin and uses the energy of ATP hydrolysis to move along the filament
Most move toward the plus-end but myosin VI is an exception
The tail region varies among classes of myosin, which vary in the types of molecules or structures they can bind

328
Q

What is kinesin?

A

involved in ATP-dependent transport toward the plus ends of MTs, called anterograde transport

329
Q

What is dynein?

A

moves particles (cargo) toward the minus ends of MTs, called retrograde transport

330
Q

Proteins and neurotransmitters produced in the cell body must be transported….

A

to the nerve ending…diffusion is too slow.

331
Q

What is fast axonal transport?

A

involves movement of vesicles and organelles along MTs
Organelles can be observed moving along filaments at rates of about 2 m/sec (~80 minutes per cm).

332
Q

What is the structure of kinesin?

A

Kinesins consist of three parts
– A globular head region that attaches to MTs
– A coiled helical region
– A light-chain region involved in attaching the Kinesin to other proteins or organelles
Kinesins move along the MT in 8-nm steps; the movement is coupled to ATP hydrolysis
Kinesin movement looks like “walking” with the two globular head domains taking turns as the front foot
Each Kinesin molecule exhibits processivity
It can move long distances along a MT before detaching from it
Front foot is empty (apo) and bound to β tubulin
Back foot is bound to ADP and not bound to tubulin
ATP binding in the front foot forces the back, ADP-bound foot forward (force)
ATP hydrolysis in (the now) back foot promotes 1) Pi release and 2) the now ADP-bound foot to release tubulin
At the same time, binding of the ADP-bound front foot binds tubulin which forces ADP release

333
Q

What does Cytoplasmic Dynein do?

A

Move toward the minus ends of MTs, associated with Dynaction, which helps link it to cargo

334
Q

What does the sliding-microtubule model suggest?

A

The sliding filament model was proposed in 1954
According to the model, muscle contraction is due to thin filaments sliding past thick filaments, with no change in length of either. That sliding of MTs relative to each other is converted into localized bending because the doublets are connected to the central pair and to each other

335
Q

What is the role of calcium in contraction?

A

The regulatory proteins tropomyosin and troponin regulate myosin binding
When the calcium concentration is low, tropomyosin blocks the myosin binding sites on the actin filament
At higher concentrations, calcium binds TnC of the troponin complex, causing tropomyosin to shift, and allowing myosin to bind

336
Q

How are cross-bridges formed?

A

Cross-bridges are formed from links between the F-actin of thin filaments and myosin heads of thick filaments
Cross-bridges must dissociate repeatedly during contraction; each cycle of cross-bridge formation causes thin filaments to slide past thick filaments
The result is shortening of sarcomeres and muscle fiber contraction

337
Q

What does actinin do?

A

Keep actin filaments bundled into parallel arrays

338
Q

What does CapX do?

A

Maintain the attachment of the plus ends to the Z line and caps the actin in the filaments

339
Q

What does Tropomodulin do?

A

Binds the minus ends of the filaments to maintain stability

340
Q

What does Nebulin do?

A

Defines thin filament length and helps anchor it to the Z line

341
Q

What does Myomesin do?

A

It is present at the H zone and helps anchor it to the Z line

342
Q

What does Titin do?

A

Attaches the thick filaments to the Z lines and keeps think filaments in the correct position relative to thin filaments during contraction

343
Q

Explain Cross-bridge formation:

A

Structural states of myosin during the contractile cycle. Without bound nucleotide, myosin is strongly bound to actin (rigor state). ATP binding dissociates the complex actin-myosin. ATP is then hydrolyzed to ADP+Pi. There is a swing of the lever arm (green). Myosin can rebind to actin, release ADP+Pi and produce force by returning to the original state where myosin is again strongly bound to actin without nucleotide bound.

344
Q

What are cilia?

A

are about 2–10 m long and occur in large numbers on the surface of ciliated cells
They occur in both unicellular and multicellular eukaryotes
Cilia display an oarlike pattern of beating, generating a force parallel to the cell surface

345
Q

What are flagella?

A

move cells through a fluid environment
They are the same diameter as cilia, but usually much longer (up to 200 m)
They are limited to one or a few per cell and move with a propagated bending motion

346
Q

Describe the axoneme?

A

Cilia and flagella share a common structure, theaxoneme
It is connected to a basal body and surrounded by an extension of the cell membrane
Between the axoneme and basal body is a transition zone in which the MTs take on the pattern characteristic of the axoneme

347
Q

What are thin filaments?

A

Thin filaments interdigitate with the thick filaments
Thin filaments contain three proteins: F-actin, intertwined
with tropomyosin and troponin
One troponin complex associates with each tropomyosin
Together they constitute a calcium-sensitive switch that activates contraction in striated muscle

348
Q

Why are Z lines important?

A

The actin in thin filaments is oriented so that all the plus ends are anchored at Z lines
Myosin II moves toward the plus ends, so the thick filaments move toward the Z lines during contraction

349
Q

Each thick filament…

A

consists of hundreds of molecules of myosin, oriented in opposite directions in the two halves of the filament

350
Q

What is the difference between Kinesin and Myosin?

A

Myosin II is an efficient motor that “walks” along actin like Kinesin walks along microtubules
Both have two heads that walk along a protein filament, and both use ATP hydrolysis to change their shape
Kinesins operate alone or in small numbers to transport vesicles over large differences
* Myosin II molecules move short distances but operate in large arrays, in some cases billions of motors working together to mediate muscle contraction

351
Q

What is striated muscle?

A

The filaments in skeletal muscle are aligned,
giving myofibrils a pattern of alternating dark and light bands (e.g. striated muscle)
Dark bands are A bands and light bands are
I bands
The lighter region in the middle of each A band is called the H zone; the M line runs down the center
The M line contains myomesin, a protein that links myosin filaments together
In the middle of each I band is a dense Z line; the distance from one Z line to another defines a sarcomere

352
Q

What is involved in Muscle contraction?

A

Muscle contraction is the most familiar example of mechanical work mediated by intracellular filaments
Much of what is known about contractile processes is based on studies involving skeletal muscle
Each muscle fiber contains numerous myofibrils, each of which is divided along its length into repeating units calledsarcomeres
Each sarcomere contains bundles of thin filaments (containing actin, troponin and tropomyosin) and thick filaments (containing myosin)

353
Q

Why are MT motors important?

A

MT motors are important for dynamically shaping the complicated endomembrane system
For example, ER membrane extensions can be moved along MTs
The vesicles to and from the Golgi complex are carried by MT motors on microtubule tracks

354
Q

What is the importance of Cytoplasmic dynein?

A

ATP binding and hydrolysis causes the dynein to take a step forward
Pi release provides the force – remodeling of the complex pulls the cargo closer (towards the minus end)
Dynein hops and pulls

Cell 201 (1 decks)

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