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The Endomembrane System Flashcards

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1
Q

What is the secretory pathway?

A

A vesicular transport pathway that moves proteins and lipids from ER through the Golgi to the plasma membrane/extracellular space

The secretory pathway involves the transport of materials outward from the ER to various cellular locations.

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2
Q

How do proteins and lipids reach the Golgi apparatus?

A

They are made at the ER and transported to the Golgi apparatus in transport vesicles

This process is essential for the proper functioning of cellular components.

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3
Q

What initiates membrane lipid synthesis?

A

Begins at the ER

Enzymes on the cytosolic face of the ER membrane synthesize new phospholipids.

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4
Q

Which transporters transfer phospholipids non-selectively between leaflets of the membrane?

A

Scramblase transporters

They can transport any phospholipid and flip it inwards

This results in symmetric growth of both halves of the bilayer

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5
Q

What is the function of flippase transporters?

A

Located in the Golgi

Flip specific phospholipids from outer to inner leaflet

This process contributes to the asymmetry of cellular membranes e.g. plasma membrane

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6
Q

Is membrane orientation preserved during vesicular transport?

A

Yes

The original cytosolic surface always remains facing the cytosol.

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7
Q

What drives vesicle budding?

A

Assembly of a protein coat which forms on the cytosolic side

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8
Q

What role does dynamin play in vesicle budding?

A

Helps the bud pinch off from the donor compartment

Dynamin is a GTPase that changes conformation when it hydrolyzes GTP.

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9
Q

What are clathrin-coated vesicles primarily involved in?

A

Functions in plasma membrane to bring material into the cell
And in the Golgi to help select cargo proteins

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10
Q

What is the function of adaptins in vesicular transport?

A
  • Help clathrin attach to the membrane, forming clathrin-coated pits on the cytosolic face of the membrane
  • Bind cargo receptors which recognise specific sorting signals on cargo proteins, recruiting them into the vesicle
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11
Q

What is the role of Rab proteins in vesicle docking and fusion?

A
  • Rab proteins on vesicles bind to tethering proteins on target membranes
  • Specific pairs of Rabs (identity marker) and tethers (marker)
  • Initial recognition between vesicle and target membrane
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12
Q

What do SNARE proteins do?

A

Catalyze fusion of vesicle and target membranes

They are essential for the final fusion event that allows vesicle contents to be delivered.

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13
Q

What type of sorting signal is the KDEL sequence?

A

ER retention/retrieval signal

It ensures that soluble proteins are returned to the ER.

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14
Q

What determines the final destination of proteins in the secretory pathway?

A

Sorting or trafficking signals

These signals guide proteins to their appropriate cellular locations.

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15
Q

What is the Golgi apparatus typically composed of?

A

A stack of membrane-enclosed sacs called cisternae

The Golgi is crucial for modifying and sorting proteins.

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16
Q

What are the two models explaining protein movement through the Golgi?

A

Vesicular transport model and cisternal maturation model

Both models help explain how proteins are processed and transported within the Golgi.

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17
Q

What modifications occur to proteins at the Golgi?

A
  1. O-linked oligosaccharides added to some OH side chains of serine and threonine in proteins as they transit the Golgi
  2. N-linked oligosaccharides added to proteins in the ER can be trimmed and rebuilt at the Golgi
  3. N-linked oligosaccharides on lysosomal proteins modified with mannose-6-phosphate
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18
Q

What happens at the trans Golgi network?

A

Cargo proteins are sorted into transport vesicles bound for different destinations

This sorting is critical for the distribution of proteins throughout the cell.

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19
Q

True or False: The secretory pathway only moves materials inward into the cell.

A

False

The secretory pathway is responsible for transporting materials outward from the ER.

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20
Q

What are the two types of exocytosis?

A

Unregulated (constitutive) = happens all the time
Regulated exocytosis = only occurs in some cells

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21
Q

What characterizes unregulated exocytosis?

A
  • Constant stream of transport vesicles bud from the trans-Golgi network and fuse with the plasma membrane
  • Supplies membrane proteins and lipids for plasma membrane growth (to replace damaged ones)
  • Allows protein secretion = cell surface, extracellular matrix or fluid
  • No signal required
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22
Q

What characterises regulated exocytosis?

A
  • Proteins are sorted into secretory vesicles and stored until a specific signal is received (that triggers fusion)
  • Only occurs in specialised secretory cells that make large quantities of proteins
  • Cargo such as hormones, mucus, digestive enzymes and neurotransmitters are secreted in response to specific signals
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23
Q

What types of cargos are commonly involved in regulated exocytosis?

A
  • Hormones
  • Mucus
  • Digestive enzymes
  • Neurotransmitters
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24
Q

What triggers the secretion of insulin from pancreatic β-cells?

A

An increase in blood glucose.

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25
How does endocytosis contribute to cellular regulation?
* Nutrient uptake * Receptor downregulation * Response to extracellular signals * Recycling of cellular components
26
What are the three main forms of endocytosis?
* Phagocytosis * Pinocytosis * Receptor-mediated endocytosis
27
What is phagocytosis?
The uptake of large particles by specialized cells.
28
What role do macrophages play in phagocytosis?
They ingest microorganisms.
29
What is pinocytosis?
Uptake of fluid and macromolecules - non selective = occurs in all cells constantly - mediated by clathrin coated vesicles - small areas of plasma membrane & extracellular fluid internalised - rate varies between cells = can be very high - up to 25% cell volume or 200% plasma membrane internalised per hour in macrophages
30
What is receptor-mediated endocytosis?
A selective uptake mechanism of specific molecules Uses cell surface receptors to capture cargo = increases efficiency of uptake (1000 fold)
31
What happens to low-density lipoproteins (LDL) during receptor-mediated endocytosis?
LDL binds to LDL receptors on cell surface Is internalised by clathrin-coated vesicles = fuse with endosomes Dissociates in the acidic environment of endosomes.
32
What is the fate of endocytosed macromolecules?
They are sorted in endosomes and can be delivered to lysosomes for degradation.
33
What is the role of lysosomes in cellular metabolism?
- Contains approx. 40 hydrolytic enzymes to degrade macromolecules into useful products that are then pumped into the cytosol - optimal activity at pH 5 - inactive in the cytosol - acidic pH is actively maintained by an ATP dependent proton pump
34
What is the optimal pH for lysosomal enzymes?
pH 5.
35
How are enzymes sorted and delivered to lysosomes?
- Made at the ER - Follow secretory pathway to the Golgi - Modified with mannose-6-phosphate in the Golgi - Sorted by M-6-P receptors in trans Golgi networks = sorts & packages them into transport vesicles that deliver them to lysosomes
36
What is autophagy?
A process where damaged organelles or obsolete parts of the cell are removed Engulfed in a double membrane that forms in the cytosol Forms autophagosome
37
What forms when an autophagosome fuses with a lysosome?
The contents are delivered for degradation.
38
What is the role of endosomes in the endocytic pathway?
They act as the main sorting station.
39
True or False: Exocytosis only occurs in specialized cells.
False, it occurs in all eukaryotic cells.
40
Fill in the blank: The process of _______ allows cells to take in fluid, small and large molecules from their external environment.
endocytosis
41
What is the fate of receptors following endocytosis?
1. Recycled —> most receptors (e.g. LDL receptors) 2. Degradation —> EGF receptor 3. Transcytosis —> move to a different domain of the plasma membrane (transfer cargo from one extracellular space to another)
42
What type of environment do lysosomal enzymes require to be active?
An acidic environment.
43
Role of enzymes on the cytosolic face of the ER
Synthesise new phospholipids
44
What is the endocytic pathway?
Moves proteins & other molecules into the cell Mitochondria and chloroplasts are not connected to these vesicular transport pathways
45
How is membrane orientation preserved during vesicular transport?
- original cytosolic surface will always remain facing the cytosol - other side of the membrane faces the compartment lumen or outside the cell - regions of membrane proteins that were in the ER/golgi lumen end up on the cell surface = often glycosylated
46
Regulation of vesicular transport
- vesicles that bud must contain the appropriate cargo only - vesicles must fuse with the correct target compartment/organelle
47
What is involved in vesicle budding?
1. Coats help to deform the membrane into a bud (bends into vesicle shape) and capture cargo to be included in the vesicle 2. Dynamin helps the bud pinch off from the donor compartment 3. Uncoating = removal of coat proteins so it can fuse
48
Role of clathrin
- functions at Golgi & plasma membrane - forms a highly ordered basket like structure - shapes the membrane - captures cargo e.g. molecules to be taken into cells from the plasma membrane or outside the cell
49
Describe vesicle budding
- clathrin assembles into a cage causing the membrane to invaginate into a bud - Dynamin assembles as a ring around the neck of the bud to help it pinch off
50
What is Dynamin?
A GTPase Conformation of Dynamin changes when it hydrolyses GTP = this pushes the 2 lipid bilayers together Together with other proteins, this constricts the neck of the bud until it pinches off Budding vesicle pinches off
51
How does mutant Dynamin cause paralysis in flies?
- temperature sensitive mutant of Dynamin in drosophila - vesicles cannot pinch off from the plasma membrane - results in loss of neurotransmitter vesicles in the synapse - this causes paralysis
52
What is vesicle uncoating?
After budding, coat proteins are removes = uncoating This requires molecular chaperons and ATP Produces ‘naked’ transport vesicles that can fuse with the target membrane
53
What are COPII-coated vesicles?
Originate in the ER and are destined for the Golgi
54
What are COPI-coated vesicles?
Originate in the Golgi cisternae and are destined for the ER
55
What is involved in the docking and fusion of transport vesicles?
After budding, the transport vesicle must deliver its contents to the correct destination or target compartment Often transported along Microtubules in order to reach its destination Involves Rab proteins and SNAREs = small GTPases
56
General role of Rab proteins & SNAREs
Ensure there is the right specificity so vesicles fuse with the right membrane
57
Role of SNAREs in vesicle docking & fusion
- Bring vesicles closer together to the target membrane - SNARE proteins in vesicles and target membrane - Complementary pairs v-SNAREs (present in vesicle) and t-SNAREs (present in target membrane) - These interact and dock the vesicle onto the target membrane
58
How do v-SNAREs and t-SNAREs catalyse fusion?
- t-SNAREs and v-SNAREs wrap tightly around each other - this pulls the membranes together to drive final fusion event - bilayers must be brought within 1.5 nm for lipids to fuse - water must be displaced = energetically unfavourable - fusion may require an additional signal (I.e. neurotransmitter release)
59
Describe the ER retention/retrieval signal
KDEL sequence (LysAspGluLeu) at the C-terminus of soluble proteins recognised by KDEL receptor in the Golgi Recruited into COPI vesicles that return the protein to the ER Prevents it from being secreted *Molecular chaperones & folding factors in the ER lumen including BiP and PDi contain KDEL sequences*
60
Name other sorting signals
- Addition of mannose-6-phosphate to N-linked glycans of some glycoproteins at the Golgi results in sorting to the lysosome - An unusually short transmembrane domain (18 amino acids long) retains proteins in the Golgi
61
Location of Golgi apparatus
Located near the nucleus (position maintained by Microtubules) and consists of stack of membrane enclosed sacs called cisternae Each Golgi stack consists of 3-20 cisternae
62
Structure & function of Golgi
**Cis Golgi network** - orientated towards nucleus - for entry - transport vesicles carrying proteins from the ER **Trans Golgi network** - orientated towards the plasma membrane - for exit - transport vesicles carrying proteins onwards
63
2 models of protein transport through the Golgi
Vesicular transport and cisternal maturation
64
Describe vesicular transport through the Golgi
- Golgi cisternae are viewed as static compartments (always exist) containing specific enzymes - Vesicles bud from one cisterna and fuse with the next = transporting cargo through each cisternae in turn - Cargo molecules present in small transport vesicles = 100 nm in diameter
65
Describe cisternal maturation in the Golgi
- Each Golgi cisterna matures as it migrates outward through the stack - Golgi resident enzymes carried forward with the cisternae are returned to an earlier compartment in vesicles - Explains transport of large cargo like procollagen (300 nm long) that are too big for a typical vesicle
66
Function of Golgi
Protein modification and sorting
67
Describe protein sorting at the cis Golgi network
ER resident proteins (contains KDEL) are sorted back to the ER Cargo proteins proceed onwards through the Golgi
68
Describe protein sorting at the trans Golgi network
Cargo proteins are sorted into transport vesicles bound for different destinations
69
How is secretion of insulin regulated?
- Proteins undergoing regulated secretion aggregate in the trans Golgi (high pH and Ca2+ conc.) - Aggregates do not enter the constitutive pathway - Packaged into secretory vesicles at a high concentration (x200) = can secrete lots of it
70
How fast is regulated secretion?
In neurons can be 5 ms after it was stimulated to fire Results in practically instant fusion
71
What balances exocytosis?
Endocytosis This maintains plasma membrane homeostasis
72
What is endocytosis?
Allows cells to take up fluid, small and large molecules from their external environment Material is enclosed by the plasma membrane and pinches off to form a endocytic vesicle = delivered to endosomes for sorting
73
Different types of phagocytosis in cells
- Protozoa take up food by phagocytosis - Macrophages & neutrophils take up microorganisms in animals - Phagocytic cels extends to pseudopods to engulf microbes - Forms phagosome which fuses with lysosome to destroy microbes Mycobacterium tuberculosis inhibits membrane fusion = multiplies in macrophages
74
Materials that enter cells via receptor-mediated endocytosis
1. Lipoproteins —> Cholesterol (LDL) 2. Essential metabolites —> Vitamin B12, iron 3. Signalling molecules —> epidermal growth factor (EGF) 4. Virus particles —> influenza, HIV, coronavirus

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