Practice question

Assembly of heteromeric SNARE core provides mechanical work for fusion in neurons, endocrine cells and constitutive secretion. Proteins classified into Q-SNAREs supplying glutamine at central zero ionic layer and R-SNAREs supplying arginine. Target plasma membrane harbors two Q proteins: syntaxin1 integral with single Q helix preceded by Habc regulatory domain clamped by Munc18 and SNAP-25 peripheral anchored via palmitoylated linker contributing two Q helices lacking transmembrane domain together contributing three Q motifs collectively t-SNARE. Vesicle membrane supplies R protein synaptobrevin VAMP2 with single R helix. Upon calcium entry triggering synaptotagmin complexin clamp released allowing N-to-C zippering forming parallel four-helix bundle extremely tight SDS resistant releasing about 65 kT energy deforming membranes into fusion pore. SNAP-25 does not act as kinase phosphorylating cargo nor initiate coat disassembly nor degrade misfolded proteins; role purely structural providing two helices. Botulinum toxins A, C, E cleave SNAP-25 or VAMP abolishing regulated exocytosis proving absolute requirement as t-SNARE component essential for neurotransmission.