LIBERTY UNIVERSITY MSCI 660
INTRACELLULAR MEMBRANE TRAFFIC AND
CELLULAR SORTING QUIZ QUESTIONS WITH
COMPLETE SOLUTIONS
Course
MSCI 660
1. Why are vesicles essential for intracellular protein transport?
Answer: Vesicles ferry proteins between membrane-bound organelles, enabling selective cargo
delivery while maintaining compartment identities.
2. Describe the roles of clathrin, COPI, and COPII in vesicle formation and directionality.
Solution:
Clathrin: mediates PM↔endosome/Golgi, with adaptin specificity.
COPII: leads ER → Golgi (anterograde).
COPI: drives Golgi → ER (retrograde).
3. A protein has a KDEL sequence and mistakenly reaches the Golgi. How is it retrieved?
Solution: KDEL sequence binds KDEL receptors in Golgi → packaged into COPI vesicles →
returned to ER.
4. Outline the process of vesicle docking and fusion.
Solution:
1. Rab GTPase binds vesicle and tethers to target.
2. SNARE pairing (v-SNARE to t-SNARE) pulls membranes close.
3. NSF/α-SNAP uncoil SNAREs post-fusion.
5. How does the Golgi apparatus process and sort proteins destined for lysosomes?
Solution:
ER enzymes add mannose-6-phosphate (M6P) in cis-Golgi → M6P receptors in trans-Golgi
bind → packaged into clathrin-coated vesicles → delivered to late endosome/lysosome.
,6. Explain the role of signal peptides in targeting proteins to the ER.
Solution: N-terminal signal peptides (hydrophobic) recruit the SRP, pause translation → dock at
ER → cleaved → protein enters ER lumen.
7. What energy source drives vesicle budding and uncoating?
Solution: GTPases like Sar1 and Arf hydrolyze GTP → coat recruitment and disassembly; ATP
via chaperones like Hsc70 uncoat clathrin vesicles.
8. Compare direct vs. indirect Golgi-to-lysosome trafficking pathways.
Solution:
Direct: Golgi → late endosome → lysosome
Indirect: Golgi → endosome → recycling endosome → plasma membrane → endocytic
pathway → lysosome
9. A mutation disables a vesicle’s v-SNARE. What’s the effect?
Answer: Vesicle fails to dock/fuse, so cargo accumulates in donor compartment, disrupting
downstream trafficking.
10. How does quality control in the ER ensure only properly folded proteins proceed?
Solution:
Chaperones (BiP, calnexin) assist folding.
Misfolded proteins are tagged with ubiquitin, extracted to cytosol by ERAD, and
degraded by proteasomes.
✅ Summary Table
Question Key Concepts
1. Vesicle necessity Compartmentalized transport
2. Coat protein roles Directionality in vesicle trafficking
, 3. KDEL retrieval ER retention mechanism
4. Docking/fusion Rab GTPases & SNAREs roles
5. M6P sorting Lysosomal targeting from Golgi
6. Signal peptides ER targeting & translocation
7. Energetics GTP/ATP in budding/uncoating
8. Trafficking pathways Alternate lysosomal routes
9. v-SNARE mutation Fusion blockade
effect
10. ER quality control Folding surveillance & degradation
11. What is the role of dynamin in vesicle formation?
Answer:
Dynamin is a GTPase that pinches off clathrin-coated vesicles from the plasma membrane or
Golgi by constricting the vesicle neck through GTP hydrolysis.
12. How do Rab GTPases ensure vesicle targeting specificity?
Solution:
Rabs bind specific effector proteins on target membranes → tether vesicles to correct
compartments → ensure spatial and temporal accuracy of vesicle fusion.
13. What happens if the Sar1 GTPase is non-functional in the ER?
Answer:
COPII vesicle formation fails → no anterograde transport from ER to Golgi → proteins
accumulate in the ER → possible activation of the unfolded protein response (UPR).
14. Why are lysosomal hydrolases tagged with mannose-6-phosphate (M6P)?
Solution:
M6P tags bind M6P receptors in the trans-Golgi → direct hydrolases to the lysosome → failure
to tag = secretion of hydrolases into extracellular space (I-cell disease).
INTRACELLULAR MEMBRANE TRAFFIC AND
CELLULAR SORTING QUIZ QUESTIONS WITH
COMPLETE SOLUTIONS
Course
MSCI 660
1. Why are vesicles essential for intracellular protein transport?
Answer: Vesicles ferry proteins between membrane-bound organelles, enabling selective cargo
delivery while maintaining compartment identities.
2. Describe the roles of clathrin, COPI, and COPII in vesicle formation and directionality.
Solution:
Clathrin: mediates PM↔endosome/Golgi, with adaptin specificity.
COPII: leads ER → Golgi (anterograde).
COPI: drives Golgi → ER (retrograde).
3. A protein has a KDEL sequence and mistakenly reaches the Golgi. How is it retrieved?
Solution: KDEL sequence binds KDEL receptors in Golgi → packaged into COPI vesicles →
returned to ER.
4. Outline the process of vesicle docking and fusion.
Solution:
1. Rab GTPase binds vesicle and tethers to target.
2. SNARE pairing (v-SNARE to t-SNARE) pulls membranes close.
3. NSF/α-SNAP uncoil SNAREs post-fusion.
5. How does the Golgi apparatus process and sort proteins destined for lysosomes?
Solution:
ER enzymes add mannose-6-phosphate (M6P) in cis-Golgi → M6P receptors in trans-Golgi
bind → packaged into clathrin-coated vesicles → delivered to late endosome/lysosome.
,6. Explain the role of signal peptides in targeting proteins to the ER.
Solution: N-terminal signal peptides (hydrophobic) recruit the SRP, pause translation → dock at
ER → cleaved → protein enters ER lumen.
7. What energy source drives vesicle budding and uncoating?
Solution: GTPases like Sar1 and Arf hydrolyze GTP → coat recruitment and disassembly; ATP
via chaperones like Hsc70 uncoat clathrin vesicles.
8. Compare direct vs. indirect Golgi-to-lysosome trafficking pathways.
Solution:
Direct: Golgi → late endosome → lysosome
Indirect: Golgi → endosome → recycling endosome → plasma membrane → endocytic
pathway → lysosome
9. A mutation disables a vesicle’s v-SNARE. What’s the effect?
Answer: Vesicle fails to dock/fuse, so cargo accumulates in donor compartment, disrupting
downstream trafficking.
10. How does quality control in the ER ensure only properly folded proteins proceed?
Solution:
Chaperones (BiP, calnexin) assist folding.
Misfolded proteins are tagged with ubiquitin, extracted to cytosol by ERAD, and
degraded by proteasomes.
✅ Summary Table
Question Key Concepts
1. Vesicle necessity Compartmentalized transport
2. Coat protein roles Directionality in vesicle trafficking
, 3. KDEL retrieval ER retention mechanism
4. Docking/fusion Rab GTPases & SNAREs roles
5. M6P sorting Lysosomal targeting from Golgi
6. Signal peptides ER targeting & translocation
7. Energetics GTP/ATP in budding/uncoating
8. Trafficking pathways Alternate lysosomal routes
9. v-SNARE mutation Fusion blockade
effect
10. ER quality control Folding surveillance & degradation
11. What is the role of dynamin in vesicle formation?
Answer:
Dynamin is a GTPase that pinches off clathrin-coated vesicles from the plasma membrane or
Golgi by constricting the vesicle neck through GTP hydrolysis.
12. How do Rab GTPases ensure vesicle targeting specificity?
Solution:
Rabs bind specific effector proteins on target membranes → tether vesicles to correct
compartments → ensure spatial and temporal accuracy of vesicle fusion.
13. What happens if the Sar1 GTPase is non-functional in the ER?
Answer:
COPII vesicle formation fails → no anterograde transport from ER to Golgi → proteins
accumulate in the ER → possible activation of the unfolded protein response (UPR).
14. Why are lysosomal hydrolases tagged with mannose-6-phosphate (M6P)?
Solution:
M6P tags bind M6P receptors in the trans-Golgi → direct hydrolases to the lysosome → failure
to tag = secretion of hydrolases into extracellular space (I-cell disease).
