Practice question

Temporal control coat association ensures vesicle budding but permits subsequent fusion because SNAREs hidden under coat cannot engage target. After detachment coat must be shed within seconds exposing v-SNAREs for tethering. Distinct uncoating mechanisms evolved: COPII shedding triggered by Sar1 GTP hydrolysis stimulated by Sec23 GAP plus Sec13-31 destabilization; COPI shedding by ARF GAP1/2 ASAP stimulating ARF1 hydrolysis plus curvature mismatch. Clathrin uncoating for AP2 and AP1/GGA vesicles requires chaperone system unique: DnaJ co-chaperone auxilin 1/2 specifically recognizes assembled clathrin lattice at vertices via clathrin binding motif and J domain recruiting Hsc70 ATPase 70 kDa heat shock cognate via ATPase domain. Hsc70 ATP bound initial interaction converts after auxilin-stimulated ATP hydrolysis to ADP tight binding prying apart heavy and light chain interactions dissociating triskelia into soluble pool within seconds post-fission. Sar1 initiates COPII budding not uncoating, Sec61 ribosome-associated ER translocon for protein import not vesicle coat factor, clathrin cannot auto-disassemble. Hsc70 therefore dedicated ATP-dependent uncoating enzyme recycling coats for successive rounds maintaining flux around 100 vesicles per minute per cell at steady state.