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MCB2050 > Unit 5: Lysosome and Endosome > Flashcards

Unit 5: Lysosome and Endosome Flashcards

1
Q

What traffics soluble proteins to the lysosome?

A

M6P targeting signal and receptor

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2
Q

What is the function and key role of the lysosome?

A
  • digestive organelle that degrades all types of macromolecules
  • key role in autophagy = degredation of organelles/componenets
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3
Q

What types of enzymes does the lysosome contain for degradation? What conditions are they active?

A
  • soluble acid hydrolyase enzymes

- only active at low pH (4.6) of lysosome interior lumen

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4
Q

How are resident lysosomal membrane proteins protected?

A
  • shielded by attached lumen facing carbohydrate groups

- glycosylation in ER and Golgi

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5
Q

Where are products of degredation sent?

A
  • cytosol

- reused by other biosynthetic pathways

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6
Q

How is the low pH in the lysosome lumen maintained?

A

ATPase proton pump (pump H+ into lumen from cytosol)

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7
Q

What pathway traffics soluble proteins to the lysosome?

A
  • biosynthetic pathway

- lysosomal proteins synthesized and initially N- glycosylated in the RER and modified in the cis golgi cisternae

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8
Q

Why is the modfication in the cis golgi cisternae important for lysosomal proteins?

A
  • mannose 6 phosphate in protein core cuz 2 mannose residues phosphorylated
  • M6P is the lysosomal targeting signal
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9
Q

What recognizes the M6P bearing lysosomal protein and where?

A
  • M6P receptors
  • in the TGN
  • m6p receptor is an integral transmembrane protein
  • lumenal facing domain of the m6p receptor binds the m6p in the TGN
  • m6p receptor concentrates lysosomal proteins into nascent clathrin coated vesicles
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10
Q

The M6P receptor (on TGN): lumenal facing binds ___

cytosolic facing binds ____

A
  • lumenal: M6P of soluble lysosomal protein in the TGN lumen

- cytosolic: binds GGA adaptor coat proteins (multiple binding domains)

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11
Q

What do GGA proteins serve as?

A
  • linkers during clathrin coated vesicle assembly
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12
Q

What mediates the recruitment of GGA adaptor proteins from cytosol to TGN surface? What is it?

A
  • Arf1
  • like Sar1
  • GDP/GTP binding regulatory protein
    binds to TGN membrane
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13
Q

What promotes the initial bending/curvature of the TGN membrane to form the clathrin coated vesicle?

A
  • Arf-1- GTP (Arf1-GDP is cytosolic/inactive)

- binds to TGN membrane

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14
Q

Where are clathrin coated vesicles assembled?

A

TGN

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15
Q

What protein mediates the release of clathrin coated vesicle from the TGN (pinching)? How does this process work?

A
  • dynamin - large soluble GTPbinding protein
  • recruited from cytosol to connection stalk
  • polymerizes to form dynamin ring around the stalk
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16
Q

What is the step that causes the release of the vesicle?

A

GTP hydrolysis

  • conformational change in dynamin ring
  • if GTP hydrolysis does not occur (gammaGTP) extended budding of stalk but no cleavage
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17
Q

Once pinched off, how is the clathrin coat disassembled?

A
  • Arf1-GTP converted to Arf-GDP and released
  • GGA and clathrin triskelions released too
  • used for another round of clathrin coat assembly at TGN
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18
Q

After disassembly of clathrin coat, the vesicle contains the lysosomal cargo proteins. What happens next?

A
  • late endosome fuses

- this is mediated by vesicle/organelle specific Rabs and v-/t-SNARES

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19
Q

The late endosome has a ____ interior. Therefore, this causes ____

A
  • acidic interior pH 5.5
  • causes dissociation of M6P receptors from soluble lysosomal cargo proteins
  • TGN / TGN vesicles pH 6.4
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20
Q

Where do M6P receptors go?

A

recycled back from the late endosome TO the TGN

another round of trafficking

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21
Q

What is the direction of the endocytic pathway? Definition.

A
  • opposite direction of the secretory pathway
  • brings things IN
  • materials move INTO the cell
  • via vesiculation of PM
  • recycled back to the PM or transport to the lysosome for degredation
22
Q

What are two main processes for internalization?

A
  • endocytosis

- phagocytosis

23
Q

Define endocytosis.

A
  • SELECTIVE internalization of PM components

i. e. PM receptors and bound extracellular ligands

24
Q

Define phagocytosis.

A
  • uptake large, particulate materials from extracellular space of specialized cells
    i. e. micro-organism (cellular eating) ingestion by amoebasor macrophages (WBC)
25
Q

Name 2 forms of endocytosis (different mechanisms).

A
  1. bulk phase endocytosis (pinocytosis - cellular drinking)

2. receptor mediated enocytosis

26
Q

Bulk phase endocytosis

A
  • pinocytosis
  • responsible for non-specific uptake of extracellular fluids
  • and PM protein turnover; replaces the entire membrane
27
Q

Receptor mediated endocytosis: function

A
  • concentrating and internalizing specific extracellular materials (ligands) bound to specific receptors on outer surface of the PM
  • involves clathrin coated vesicles
  • PM protein destined for degredation included
28
Q

Explain the steps of receptor mediated endocytosis:

A
  • PM transmembrane receptor ACTIVATED by binding to specific ligand
  • ligand receptor complex DIFFUSES LATERALLY in the PM and accumulates in coated pits (clathrin)
  • inner leaflet of PM at coated pits ENRICHED in unique membrane PHOSPHOLIPIDS (lipid microdomain) - PI(4,5)P2
29
Q

What is PI(4,5)P2

A

phosphatidylinositol

  • unique membrane lipid
  • SIGNAL recruiting cytosolic (soluble) adaptor protein AP2 to the coated pits
30
Q

What is AP2?

A

soluble adaptor “linker” protein with multiple binding domain
- domains for binding include: PI(4,5)P2,
cytosolic domains for transmembrane receptors,
clathrin

31
Q

What do AP2 adaptor proteins do at the cyotosolic domains?

A
  • form the inner layer of the coat
32
Q

What do AP2 adaptors and clathrin do?

A
  • recruit clathrin triskelions which self assemble to form outer scaffold of the coat on growing vesicle
33
Q

What is the purpose of the clathrin triskelions on the outer scaffold of the forming vesicle?

A
  • lattice formation

- membrane curvature

34
Q

For receptor mediated endocytosis, what pinches the clathrin coated vesicle from the PM?

A

dynamin

35
Q

What processes is dynamin involved in?

A
  • Receptor mediated endocytosis - release from PM

- release of vesicle from TGN

36
Q

What happens to components after budding of vesicle in receptor mediated endocytosis?

A
  • clathrin coat disassembles

- clathrin and AP2 adaptors released in cytosol

37
Q

What happens to the nascent endocytic vesicle after it has budded?

A
  • the vesicle with transmembrane soluble ligand cargo targets and fuses with early endosome
  • mediated by Rabs and t/vSNARES
38
Q

What is the early endosome responsible for?

A
  • sorting and recycling via ENDOcytosis
39
Q

What does the acidic environment of the early endosome do to the endocytotic vesicle?

A

when fused, the acidic environment causes the dissociation of transmembrane receptor from soluble ligands

40
Q

Of the two compartments of the early endosome, what does the recycling compartment do?

A
  • recycling compartment detaches frm rest of organelle and traffics back to PM (recycled receptors)
41
Q

Of the two compartments of the early endosome, what does the sorting compartment do?

A
  • sorting compartment contains soluble endocytic cargo from extracellular space
  • AND PM receptors destined for degradation in lysosome
42
Q

What is the late endosome originated from? How is the correct pH acheived?

A
  • sorting compartment of early endosome
  • pH 6 to pH 5.5
  • ATPase pump activated
43
Q

The lower pH of 5.5 in the late endosome triggers ____

A

dissociation of incoming M6P receptorlysosomal cargo protein from TGN (came from biosynthetic)

44
Q

The endocytic cargo proteins are _____ into a portion of the late endosome and therefore _____.

A
  • concentrated

- detaches ‘fragments’ from rest of organelle

45
Q

What is the fragmented part of the late endosome called?

A

multivesicular body (MVB)

46
Q

The late endosome contains the acid hydrolyases. What are the acid hydrolyase enzymes?

A
  • soluble cargo proteins from TGN via biosynthetic pathway!
47
Q

What does the fragment of the late endosome contain?

A
  • the MVB contains:
  • soluble cargo proteins (acid hydrolyases)
  • soluble endocytic cargo (extracellular ligands)
  • intralumenal vesicles (invagination of boundary membrane)
48
Q

How does stuff get degraded?

A

all MVB lumenal materials are released to the lumen of the lysosome

49
Q

Are the soluble cargo proteins activated in the MVB?

A
  • yes! acid hydrolyase is activated at acidic pH 4.6 of lysosome
50
Q

What is degraded?

A
  • soluble endocytic cargo (extracellular ligands)

- intralumenal vesicles with PM derived receptors

MCB2050 (10 decks)

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