Synaptophysin Chaperones the Assembly of 12 SNAREpins under each Ready-Release Vesicle

The synaptic vesicle protein Synaptophysin has long been known to form a complex with the v-SNARE VAMP, but a more specific molecular function or mechanism of action in exocytosis has been lacking because gene knockouts have minimal effects. Utilizing fully-defined reconstitution and single-molecule measurements, we now report that Synaptophysin functions as a chaperone that determines the number of SNAREpins assembling between a ready-release vesicle and its target membrane bilayer. Specifically, Synaptophysin directs the assembly of 12 {+/-} 1 SNAREpins under each docked vesicle, even in the face of an excess of SNARE proteins. The SNAREpins assemble in successive waves of 6 {+/-} 1 and 5 {+/-} 2 SNAREpins, respectively, tightly linked to oligomerization of and binding to the vesicle Ca++ sensor Synaptotagmin. Templating of 12 SNAREpins by Synaptophysin is likely the direct result of its hexamer structure and its binding of VAMP2 dimers, both of which we demonstrate in detergent extracts and lipid bilayers. Significance StatementSynaptophysin is the most abundant protein and a unique constituent of synaptic vesicles, yet it has no known function, due to minimal genetic phenotypes and the lack of biochemical assays. Here, we directly establish using two independent methods that the synaptic vesicle protein Synaptophysin forms a hexameric complex containing 12 copies of the v-SNARE VAMP2. These v-SNAREs assemble into SNAREpins as ready-release vesicles are formed in a fully-defined cell-free system, and do so in two equal waves organized by oligomerization of the Ca++ sensor Synaptotagmin. In the absence of Synaptophysin, two waves are also observed, but the number of SNAREpins in each varies widely. We suggest that a single Synaptophysin hexamer in each vesicle symmetrically organizes 6 pairs of peripheral and central SNAREpins, the latter being directly bound to the Synaptotagmin ring. This gives rise to the symmetrical ring-like arrangement of densities observed by cryo-EM tomography under each synaptic vesicle (1, 2).