Question 1

How do COPI and COPII vesicles differ from clathrin-coated vesicles in pinching off?

Accepted Answer

COPI and COPII vesicles pinch off spontaneously when membrane leaflets fuse together at the budding site, requiring no additional proteins. Clathrin-coated vesicles, by contrast, require dynamin, a cytosolic GTPase that assembles as a helical collar around the vesicle neck. Dynamin's GTP hydrolysis drives conformational changes that twist the helix and mechanically pinch off the bud from the plasma membrane.

Question 2

What role does dynamin play in clathrin-coated vesicle formation?

Accepted Answer

Dynamin is a 100-kDa GTPase that assembles into helical spirals at vesicle bud necks. It contains a phosphatidylinositol 4,5-bisphosphate binding domain anchoring it to the membrane and GTPase domains that couple GTP hydrolysis to conformational changes. The energy from GTP hydrolysis drives twisting of the helix, generating the mechanical force that pinches the clathrin-coated vesicle from the membrane.

Question 3

How does phosphatidylinositol 4,5-bisphosphate regulate clathrin coat stability?

Accepted Answer

Phosphatidylinositol 4,5-bisphosphate anchors dynamin to the membrane during vesicle pinching. Once the bud pinches off, PIP phosphatase depletes phosphatidylinositol 4,5-bisphosphate from the vesicle membrane. This depletion weakens adaptor protein binding and destabilizes the clathrin coat, initiating rapid uncoating of the newly formed vesicle.

Question 4

What proteins are responsible for disassembling the clathrin coat after vesicle budding?

Accepted Answer

Hsp70, an ATPase chaperone protein, and auxilin, its co-chaperone, work together to disassemble clathrin coats. Auxilin binds Hsp70 and recruits it to the clathrin lattice, stimulating ATP hydrolysis. This energy-dependent process uncoats the entire vesicle by dissociating clathrin triskelions, allowing rapid coat shedding after the vesicle pinches off.

Question 5

Why is rapid clathrin coat shedding important for vesicle function?

Accepted Answer

Rapid coat shedding after pinching off exposes the vesicle membrane, enabling it to dock and fuse with target organelles. Tight control of coat formation and shedding facilitates precise timing of vesicle budding and fusion, ensuring efficient cargo delivery through membrane traffic pathways and maintaining proper cellular organization.

Question 6

How does dynamin's conformational change mechanically separate a vesicle from the membrane?

Accepted Answer

Dynamin assembles as a helical collar around the vesicle neck and undergoes a GTP hydrolysis-driven conformational change. This change causes the helix to twist, generating mechanical tension that stretches and constricts the membrane neck. The twisting motion ultimately pinches off the vesicle bud, severing it from the donor organelle membrane.

Question 7

What happens to adaptor proteins when phosphatidylinositol 4,5-bisphosphate is depleted from the vesicle?

Accepted Answer

Adaptor proteins like AP2 bind to the membrane through phosphatidylinositol 4,5-bisphosphate interactions. When PIP phosphatase depletes phosphatidylinositol 4,5-bisphosphate after pinching off, adaptor protein binding weakens significantly. This destabilization of adaptor protein attachment triggers disassembly of the clathrin coat and enables the vesicle to proceed with its delivery function.

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