Cytoskeleton (ch 16)

Question 1

What are the 3 types of cytoskeletal filaments?

Answer 1

1. Intermediate filaments
2. Microtubules
3. Actin filaments

Question 2

What is the main purpose of intermediate filaments?

Answer 2

Provide mechanical strength to the cell.

Question 3

What are the 2 main purposes of microtubules?

Answer 3

1. Determine organelle positioning

2. Direct intracellular transport

Question 4

What are the 2 main purposes of actin filaments?

Answer 4

1. Determine the shape of the cell surface

2. Required for whole cell locomotion

Question 5

What is the most conserved protein in animal cells?

Answer 5

Actin.

Question 6

What animal species lack intermediate filaments?

Answer 6

Species which have exoskeletons.

Question 7

What are the relative widths of intermediate filaments, actin filaments, and microtubules?

Answer 7

Intermediate filaments: 10nm
Actin filaments: 5-9nm
Microtubules: 25nm

Question 8

Give an example of a way in which the cytoskeleton is dynamic.

Answer 8

Example: rapid cytoskeletal rearrangement in neutrophils allows them to chase down bacteria.
Example actin forms a contractile ring which pinches off the cells during cytokinesis.

Question 9

How are cytoskeletal filaments responsible for polarity within the cell?

Answer 9

They have +ve and -ve ends which direct cell components to one side of the cell or the other.

Question 10

Why does the cell disassemble and reassemble filaments instead of moving assembled filaments around?

Answer 10

Faster, less awkward to move them through the crowded cytosol.

Question 11

What energy source drives microtubule formation? What about actin filament formation?

Answer 11

Microtubules: GTP
Actin: ATP

Question 12

Why have a microtubule made from many protofilaments?

Answer 12

Protofilaments alone are thermally unstable, but a microtubule composed of protofilaments is much harder to break.

Question 13

How many protofilaments make up a microtubule in mammalian cells?

Answer 13

13.

Question 14

What is characteristic of actin in its T-form? What about its D-form?

Answer 14

T-form: bound to ATP

D-form: bound to ADP

Question 15

At what end of the microtubule or actin microfilament are new monomers most readily added?

Answer 15

+ve end.

Question 16

Which form of actin (T or D) is more likely to be depolymerized?

Answer 16

The ADP bound D-form.

Question 17

Is depolymerization of actin dependent on concentration? Please elaborate.

Answer 17

No. Subunits leave polymer end at constant rate regardless of concentration.

Question 18

Is polymerization of actin dependent on concentration? Please elaborate.

Answer 18

Yes. The # of monomers that add to the polymer per unit time is proportional to the [free subunit].

Question 19

What is the “persistence length” in relation to actin?

Answer 19

The maximum length at which two points along an actin filament will be oriented in the same direction. Beyond this length the filament is too bendy and the directions are uncorrelated.

Question 20

What is “treadmilling” in actin filaments?

Answer 20

When the actin filament is gaining and losing monomers at the same rate so the length of the filament remains constant.

Question 21

How can actin treadmilling propel cellular movement?

Answer 21

Net assembly at the leading edge pushes the cell forward while net disassembly at the lagging edge pulls up the rear of the cell.

Question 22

In actin polymerization, what effect will profilin have when it binds to free actin monomers?

Answer 22

It will promote rapid +ve end growth.

Question 23

In actin polymerization, what effect will thymosin have when it binds to free actin monomers?

Answer 23

It will totally inhibit growth at the +ve end of an actin filament.

Question 24

Where in the cell does actin filament nucleation most frequently occur?

Answer 24

At or near the plasma membrane.

Question 25

What 2 proteins are most closely associated with actin filament nucleation?

Answer 25

1. Arp complex | 2. Formins

Question 26

What is an Arp in relation to actin microfilaments? What is its function?

Answer 26

Actin-related protein. Arp2 and Arp3 form a complex on which actin monomers can start building a new actin filament.

Question 27

When Arp2 and Arp3 form a complex to initiate actin nucleation, what end of the new actin filament binds to the complex?

Answer 27

The -ve end (duh).

Question 28

In what orientation do actin protofilaments twist around each other?

Answer 28

In a right-handed orientation.

Question 29

How do Arp complexes allow branching nucleation of actin? At what angle from previous actin filaments are new filaments established?

Answer 29

Arp complex binds to existing actin filament and promotes growth of a new filament at a 70 degree angle from the original.

Question 30

What differentiates the function of an Arp complex from a formin?

Answer 30

Arp: produce branched actin Formin: produce straight, unbranched actin

Question 31

How does a formin homodimer interact with an actin filament?

Answer 31

It associates with the growing +ve end and promotes unbranched growth by capturing 2 actin monomers and acting as a template for connecting them to the existing filament.

Question 32

In vertebrates, what precentage of actin exists as a filament and what percentage is free-floating?

Answer 32

About 50-50%.

Question 33

Are Arps present in all eukaryotes? Please elaborate.

Answer 33

Yes. Likely arose from an early gene duplication event.

Question 34

Arps are very similar to actin, so how are they prevented from being co-assembled into a filament?

Answer 34

They have an identical +ve end but a very different -ve end.

Question 35

What proteins can cap the -ve end of actin?

Answer 35

Arp2, Arp3, tropomodulin.

Question 36

What proteins can cap the +ve end of actin?

Answer 36

CapZ (plus-end capping protein).

Question 37

What determines if there is growth or shrinkage of an actin microfilament?

Answer 37

The critical concentration of free actin monomers.

Question 38

What are the 2 kinds of bundled actin arrangements? What protein is associated with these?

Answer 38

Contractile bundles and tight parallel bundles are associated with formins.

Question 39

What are the 2 kinds of branched actin arrangements? What protein is associated with these?

Answer 39

Gel-like networks and dendritic networks are associated with Arp complexes.

Question 40

What protein dimer links actin filaments loosely in a contractile bundle? Why is this advantageous?

Answer 40

α-actinin links actin filaments and allows myosin to enter bundle for muscle contraction.

Question 41

What protein monomer links actin filaments tightly in a parallel bundle? Why is this advantageous?

Answer 41

Fimbrin links actin filaments and prevents myosin from entering the bundle.

Question 42

What protein dimer links branched actin filaments in a gel-like network?

Answer 42

Filamin dimers.

Question 43

What is the structure of a myosin?

Answer 43

They are a coiled-coil of two α-helices with light chains at the N-terminus (neck region) and nothing at the C-terminus.

Question 44

How many distinct families of myosin exist in humans? How many of these travel towards the negative end of an actin filament?

Answer 44

37 distinct families. Only 1 walks towards the negative end of the filament.

Question 45

What 5 steps outline the action of myosin 2 with actin?

Answer 45

1. Myosin starts bound to actin 2. ATP binds, myosin releases 3. ATP hydrolyzed to ADP, myosin moves 4. Myosin binds actin, releases Pi 5. Power stroke, loss of ADP

Question 46

What phase of myosin movement along actin is associated with rigor mortis?

Answer 46

The point when myosin is bound to actin just before or after a power stroke.

Question 47

Why is calcium required for muscle contraction?

Answer 47

Calcium binds to the troponin complex, moving tropomyosin off of binding sites and allowing myosin to bind to actin.

Question 48

How far does myosin II move along an actin filament during each contraction?

Answer 48

~8-9nm.

Question 49

What causes calcium to be released into the cell during muscle contraction? Where is calcium stored?

Answer 49

An action potential travels down the T-tubules, triggering the release of calcium from the sarcoplasmic reticulum.

Question 50

Starting at calcium release, outline the steps in the signalling cascade leading to muscle contraction.

Answer 50

1. Calcium released from SR 2. Calcium binds to calmodulin 3. Calmodulin activates myosin light chain kinase 4. Phosphorylation of myosin light chain 5. Myosins self-assemble into thick filaments for contraction

Question 51

What differentiates the function of myosin II and myosin V?

Answer 51

Myosin II: muscular contraction | Myosin V: cargo transport along actin filaments

Question 52

How far does myosin V move along actin during each swing of the lever arm?

Answer 52

~30-40nm.

Question 53

What are the base monomers of a microtubule?

Answer 53

α-tubulin and β-tubulin.

Question 54

What is the space in the middle of a microtubule called?

Answer 54

The lumen (same as most biological spaces).

Question 55

Where does nucleation of a microtubule begin? What protein enables this?

Answer 55

Begins at the microtubule-organizing center (MTOC/centrosome) and is enabled by γ-tubulin small complex (γ-TuSC).

Question 56

How does γ-tubulin small complex (γ-TuSC) enable microtubule nucleation?

Answer 56

By forming a "lock washer" shape spiral of 7 γ-TuSCs (dimers, 14 γ-tubulins) as a template for polymerization.

Question 57

How many γ-TuSCs exist on the centrosome matrix?

Answer 57

>50.

Question 58

What is a centriole?

Answer 58

A short cylinder of modified microtubules and accessory proteins which exists inside the centrosome matrix of the microtubule organizing center (/centrosome).

Question 59

Is a centriole required for formation of a centrosome? How can this be tested?

Answer 59

No. By severing a portion of the cell which contains microtubles we can see that they aggregate to form a new centrosome which lacks centrioles.

Question 60

Actin filaments undergo treadmilling to retain constant length while microtubules have _______ _________.

Answer 60

Dynamic instability.

Question 61

Describe dynamic instability in microtubles.

Answer 61

Rapid growth with GTP cap followed by loss of GTP cap (catastrophe) and rapid shrinkage. Regaining GTP cap (rescue) allows rapid growth again.

Question 62

What protein can associate with microtubules and increase the frequency of "catastrophes" or shrinkage events?

Answer 62

Catastrophe factor kinesin-13.

Question 63

What protein can associate with microtubules and decrease the frequency of "catastrophes" or shrinkage events?

Answer 63

XMAP215.

Question 64

What effect can MAP2 and tau have on microtubule arrangement when bound to microtubules?

Answer 64

Bind to the side of a microtubule and protrude, causing the microtubules to be more widely spaced.

Question 65

What are the 2 types of microtubule motor proteins? What direction do they travel along the microtubule?

Answer 65

1. Kinesins (travel to +ve end) | 2. Dyneins (travel to -ve end)

Question 66

How does Kinesin move along the microtubules?

Answer 66

It "walks" along. Each step is driven by ATP hydrolysis in the lagging head ("foot").

Question 67

What differentiates the structure and function of cytoplasmic dyneins from axonemal dyneins?

Answer 67

Cytoplasmic: homodimers, cargo transport Axonemal: heterodimers/trimers, cilia and flagella

Question 68

What drives dynein movement?

Answer 68

ATP hydrolysis.

Question 69

What is the general structure of a eukaryotic flagella? Do bacteria have the same flagella structure?

Answer 69

9 peripheral microtubule pairs with one central pair. Bacterial flagella have super different structures.

Question 70

How are dyneins able to drive flagella action and produce movement?

Answer 70

They try to move along the microtubules, but these microtubules are linked so that they only bend a certain amount. Resulting alternation in bending gives whip-like motion.

Question 71

What are 4 common characteristics of motor proteins?

Answer 71

1. Driven by ATP hydrolysis 2. Unidirectional movement 3. Motor in head 4. Tail determines cargo and function

Question 72

Describe the structure of an intermediate filament.

Answer 72

8 coiled-coil tetramers (32 monomers) are laterally associated and progressively elongated to form a filament.

Question 73

What are some examples of proteins which can make up intermediate filaments?

Answer 73

Desmin, keratin, neurofilaments, the list goes on...

Question 74

What part of the cell is most often targeted by toxins? What organism most often employs this strategy?

Answer 74

Plants often produce toxins for defense which target the cytoskeleton.

Question 75

Do bacteria have actin and microtubules? What about motor proteins?

Answer 75

They have homologs to actin and microtubules but none have motor proteins.

Question 76

What is the tubulin homolog in bacterial cells?

Answer 76

FtsZ protein.

Question 77

What are the actin homologs in bacterial cells?

Answer 77

ParM (plasmid segregation), MreB, MbI (cell shape).

Question 78

What are the 3 main protrusive structures of a cell? What cytoskeletal filament forms these?

Answer 78

All formed by actin: 1. Filopodia 2. Lamellipodia 3. Pseudopodia

Question 79

Describe a cell's lamellipodia?

Answer 79

A sheet of actin extending in front of the cell while the back of the cell retracts. A method of cell movement.

Question 80

Activation of ___ leads to stress fibers, while activation of ___ leads to lamellipodia, and activation of ___ leads to filopodia/microspikes.

Answer 80

Rho = stress fibers, Rac = lamellipodia, Cdc42 = filopodia/microspikes.

Question 81

How does a neutrophil track down bacteria it wants to engulf?

Answer 81

GPCRs on the membrane detect chemoattractant from the bacterium and trigger actin rearrangement to move the cell.

Question 82

Which protein associated with actin can bind to actin filaments and to other proteins and activate those upon binding?

Answer 82

Filamin.