Practice question

Maintenance of available fusion-competent SNAREs requires constant recycling machinery whose failure halts secretion rapidly. After vesicle fusion, v- and t-SNAREs intertwined as cis complex on acceptor membrane representing low-energy product. Soluble alpha-SNAP 35 kDa oligomerizes along complex as three to four copies per bundle recognizing acidic surface patches. Hexameric NSF ATPase binds SNAP coat via N-domains forming 20S particle originally observed in detergent extracts. Upon ATP hydrolysis D1 ring undergoes large movements pulling SNAREs through central pore with mechanical force unfolding coiled coil into individual helices. Released syntaxin may associate with SM proteins Munc18 or Sly1 preventing promiscuous assembly, synaptobrevin sorted into budding vesicles for retrograde return. Rab GTPases confer tether specificity but do not unfold SNAREs; clathrin and dynamin mediate budding and fission, not reuse. Studies in squid giant presynaptic terminal and Drosophila comatose mutants lacking NSF show massive accumulation of high molecular weight 20S particles and immediate block of transmitter release illustrating recycling by NSF essential housekeeping for repeated fusion events throughout life.