Unit 5: Lysosome and Endosome Flashcards
What traffics soluble proteins to the lysosome?
M6P targeting signal and receptor
What is the function and key role of the lysosome?
- digestive organelle that degrades all types of macromolecules
- key role in autophagy = degredation of organelles/componenets
What types of enzymes does the lysosome contain for degradation? What conditions are they active?
- soluble acid hydrolyase enzymes
- only active at low pH (4.6) of lysosome interior lumen
How are resident lysosomal membrane proteins protected?
- shielded by attached lumen facing carbohydrate groups
- glycosylation in ER and Golgi
Where are products of degredation sent?
- cytosol
- reused by other biosynthetic pathways
How is the low pH in the lysosome lumen maintained?
ATPase proton pump (pump H+ into lumen from cytosol)
What pathway traffics soluble proteins to the lysosome?
- biosynthetic pathway
- lysosomal proteins synthesized and initially N- glycosylated in the RER and modified in the cis golgi cisternae
Why is the modfication in the cis golgi cisternae important for lysosomal proteins?
- mannose 6 phosphate in protein core cuz 2 mannose residues phosphorylated
- M6P is the lysosomal targeting signal
What recognizes the M6P bearing lysosomal protein and where?
- 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
The M6P receptor (on TGN): lumenal facing binds ___
cytosolic facing binds ____
- lumenal: M6P of soluble lysosomal protein in the TGN lumen
- cytosolic: binds GGA adaptor coat proteins (multiple binding domains)
What do GGA proteins serve as?
- linkers during clathrin coated vesicle assembly
What mediates the recruitment of GGA adaptor proteins from cytosol to TGN surface? What is it?
- Arf1
- like Sar1
- GDP/GTP binding regulatory protein
binds to TGN membrane
What promotes the initial bending/curvature of the TGN membrane to form the clathrin coated vesicle?
- Arf-1- GTP (Arf1-GDP is cytosolic/inactive)
- binds to TGN membrane
Where are clathrin coated vesicles assembled?
TGN
What protein mediates the release of clathrin coated vesicle from the TGN (pinching)? How does this process work?
- dynamin - large soluble GTPbinding protein
- recruited from cytosol to connection stalk
- polymerizes to form dynamin ring around the stalk
What is the step that causes the release of the vesicle?
GTP hydrolysis
- conformational change in dynamin ring
- if GTP hydrolysis does not occur (gammaGTP) extended budding of stalk but no cleavage
Once pinched off, how is the clathrin coat disassembled?
- Arf1-GTP converted to Arf-GDP and released
- GGA and clathrin triskelions released too
- used for another round of clathrin coat assembly at TGN
After disassembly of clathrin coat, the vesicle contains the lysosomal cargo proteins. What happens next?
- late endosome fuses
- this is mediated by vesicle/organelle specific Rabs and v-/t-SNARES
The late endosome has a ____ interior. Therefore, this causes ____
- acidic interior pH 5.5
- causes dissociation of M6P receptors from soluble lysosomal cargo proteins
- TGN / TGN vesicles pH 6.4
Where do M6P receptors go?
recycled back from the late endosome TO the TGN
another round of trafficking
What is the direction of the endocytic pathway? Definition.
- 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
What are two main processes for internalization?
- endocytosis
- phagocytosis
Define endocytosis.
- SELECTIVE internalization of PM components
i. e. PM receptors and bound extracellular ligands
Define phagocytosis.
- uptake large, particulate materials from extracellular space of specialized cells
i. e. micro-organism (cellular eating) ingestion by amoebasor macrophages (WBC)
Name 2 forms of endocytosis (different mechanisms).
- bulk phase endocytosis (pinocytosis - cellular drinking)
2. receptor mediated enocytosis
Bulk phase endocytosis
- pinocytosis
- responsible for non-specific uptake of extracellular fluids
- and PM protein turnover; replaces the entire membrane
Receptor mediated endocytosis: function
- 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
Explain the steps of receptor mediated endocytosis:
- 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
What is PI(4,5)P2
phosphatidylinositol
- unique membrane lipid
- SIGNAL recruiting cytosolic (soluble) adaptor protein AP2 to the coated pits
What is AP2?
soluble adaptor “linker” protein with multiple binding domain
- domains for binding include: PI(4,5)P2,
cytosolic domains for transmembrane receptors,
clathrin
What do AP2 adaptor proteins do at the cyotosolic domains?
- form the inner layer of the coat
What do AP2 adaptors and clathrin do?
- recruit clathrin triskelions which self assemble to form outer scaffold of the coat on growing vesicle
What is the purpose of the clathrin triskelions on the outer scaffold of the forming vesicle?
- lattice formation
- membrane curvature
For receptor mediated endocytosis, what pinches the clathrin coated vesicle from the PM?
dynamin
What processes is dynamin involved in?
- Receptor mediated endocytosis - release from PM
- release of vesicle from TGN
What happens to components after budding of vesicle in receptor mediated endocytosis?
- clathrin coat disassembles
- clathrin and AP2 adaptors released in cytosol
What happens to the nascent endocytic vesicle after it has budded?
- the vesicle with transmembrane soluble ligand cargo targets and fuses with early endosome
- mediated by Rabs and t/vSNARES
What is the early endosome responsible for?
- sorting and recycling via ENDOcytosis
What does the acidic environment of the early endosome do to the endocytotic vesicle?
when fused, the acidic environment causes the dissociation of transmembrane receptor from soluble ligands
Of the two compartments of the early endosome, what does the recycling compartment do?
- recycling compartment detaches frm rest of organelle and traffics back to PM (recycled receptors)
Of the two compartments of the early endosome, what does the sorting compartment do?
- sorting compartment contains soluble endocytic cargo from extracellular space
- AND PM receptors destined for degradation in lysosome
What is the late endosome originated from? How is the correct pH acheived?
- sorting compartment of early endosome
- pH 6 to pH 5.5
- ATPase pump activated
The lower pH of 5.5 in the late endosome triggers ____
dissociation of incoming M6P receptorlysosomal cargo protein from TGN (came from biosynthetic)
The endocytic cargo proteins are _____ into a portion of the late endosome and therefore _____.
- concentrated
- detaches ‘fragments’ from rest of organelle
What is the fragmented part of the late endosome called?
multivesicular body (MVB)
The late endosome contains the acid hydrolyases. What are the acid hydrolyase enzymes?
- soluble cargo proteins from TGN via biosynthetic pathway!
What does the fragment of the late endosome contain?
- the MVB contains:
- soluble cargo proteins (acid hydrolyases)
- soluble endocytic cargo (extracellular ligands)
- intralumenal vesicles (invagination of boundary membrane)
How does stuff get degraded?
all MVB lumenal materials are released to the lumen of the lysosome
Are the soluble cargo proteins activated in the MVB?
- yes! acid hydrolyase is activated at acidic pH 4.6 of lysosome
What is degraded?
- soluble endocytic cargo (extracellular ligands)
- intralumenal vesicles with PM derived receptors
