Molecular architecture of synaptic vesicles

Abstract

Synaptic vesicles (SVs) store and transport neurotransmitters to the presynaptic active zone for release by exocytosis. After release, SV proteins and excess membrane are recycled via endocytosis, and new SVs are formed in a clathrin-dependent manner. This process maintains the morphology and complex molecular composition of SVs through multiple recycling rounds. Previous studies explored the molecular composition of SVs through proteomic analysis and fluorescent microscopy, proposing a model for an average SV 1,2 . However, the structural heterogeneity and molecular architecture of individual SVs are not well described. Here we used cryo-electron tomography to visualize morphological and molecular details of SVs isolated from mouse brains and inside cultured neurons. We describe several classes of small proteins on the SV surface and long proteinaceous densities inside SVs. We identified V-ATPases, determined a structure using subtomogram average, and showed them forming a complex with the membrane-embedded protein synaptophysin. Our bioluminescence assay revealed pairwise interactions between VAMP2 and synaptophysin and V-ATPase Voe1 domains. Interestingly, V-ATPases were randomly distributed on the surface of SVs irrespective of vesicle sizes. A subpopulation of isolated vesicles and vesicles inside neurons contained a partially assembled clathrin coat with a soccer-ball symmetry. We observed a V-ATPase under clathrin cage in several isolated clathrin-coated vesicles. Additionally, from isolated SV preparations and within hippocampal neurons we identified clathrin baskets without vesicles. We determined their preferential location in proximity to the cell membrane. Our analysis advances the understanding of individual SVs’ diversity and their molecular architecture.