Questions: Endomembrane System Integration and Vesicular Transport

Question 1

A drug completely blocks all SNARE protein function in a cell. What is the most direct consequence for protein trafficking?

• A. Proteins accumulate in the ER because COPII vesicles cannot bud from ER exit sites
• B. Vesicles form and travel to correct compartments but cannot fuse with their target membranes
• C. Proteins are degraded immediately after synthesis because the ER quality-control system fails
• D. The Golgi apparatus disperses because COPI retrograde vesicles maintain its structural integrity

Answer: (B)

SNARE proteins are the fusion machinery, not the budding machinery. Coat proteins (COPII, COPI, clathrin) handle vesicle budding and cargo selection; Rab GTPases guide vesicles to their targets; SNAREs execute membrane fusion at the final step. Blocking SNAREs means vesicles can form, travel to correct compartments, and tether — but cannot complete fusion. Cargo accumulates in vesicles that cannot unload. Option A describes a COPII block at ER exit sites, not a SNARE block.

Question 2

Which coat protein mediates retrograde transport, returning escaped ER-resident proteins from the Golgi back to the ER?

• A. COPII — the primary coat for all ER-Golgi transport in both directions
• B. Clathrin — handles all intracellular vesicle budding events
• C. COPI — coats vesicles budding from Golgi cisternae for retrograde transport back to the ER
• D. Sar1 — the GTPase that directly forms the retrograde coat

Answer: (C)

COPII mediates anterograde transport (ER → Golgi). COPI mediates retrograde transport (Golgi → ER), retrieving escaped ER-resident proteins bearing retrieval signals such as the KDEL sequence for lumenal proteins. Clathrin handles budding at the trans-Golgi network for lysosomal targeting and at the plasma membrane during endocytosis. Sar1 (option D) is a GTPase that recruits COPII coat assembly — it is involved in COPII, not COPI, vesicle formation.

Question 3

COPI-coated vesicles carry cargo in the anterograde direction — from the ER through Golgi cisternae toward the trans-Golgi network.

• T. True
• F. False

Answer: False

COPI mediates retrograde transport — from the Golgi back to the ER, and between Golgi cisternae in the cis direction. COPII handles anterograde transport (ER → Golgi). The two coat systems serve opposite directions: COPII moves newly synthesized cargo forward in the secretory pathway, while COPI retrieves escaped resident proteins to maintain each compartment's distinct molecular identity.

Question 4

The endomembrane system is a one-way secretory pipeline: proteins flow from ER to Golgi to destination and are not recycled.

• T. True
• F. False

Answer: False

The endomembrane system is a closed loop, not a one-way pipeline. COPI vesicles continuously return material from Golgi to ER; endocytosis retrieves plasma membrane added during exocytosis; early endosomes sort receptors back to the cell surface while routing other cargo to lysosomes. This recycling is essential for maintaining the distinct protein composition of each compartment and for balancing plasma membrane area — without retrograde flow, the ER would deplete and the plasma membrane would grow uncontrollably.

Question 5

Why is SNARE protein specificity crucial for accurate protein delivery in the endomembrane system?

Model answer: SNARE specificity ensures vesicles fuse only with their correct target membrane. v-SNAREs on the vesicle and t-SNAREs on the target must be cognate pairs to zipper together and drive membrane fusion. Without this specificity, a vesicle carrying lysosomal hydrolases could fuse with the plasma membrane and secrete destructive enzymes extracellularly, or secretory cargo could be delivered to the wrong compartment. SNARE pairing acts as the final proofreading checkpoint after coat proteins select cargo and Rab GTPases guide targeting.

The three-layer targeting system — coat proteins select cargo, Rab GTPases guide to destination, SNAREs execute fusion — provides redundancy that allows a cell to run thousands of simultaneous trafficking events with high fidelity. SNARE specificity is the last checkpoint. This architecture explains how organelle identity is maintained despite continuous membrane flux: each compartment's t-SNARE repertoire ensures only the correct vesicles can dock and fuse, regardless of accidental proximity.