Munc13 and SNAP25 dependent tethering plays a key role in synaptic vesicle priming

Synaptic vesicle tethering, priming, and neurotransmitter release require a coordinated action of multiple protein complexes. While physiological experiments, interaction data, and structural studies of purified systems were essential for our understanding of the function of the individual complexes involved, they cannot combine high structural detail with the unperturbed organization of complexes within cells to resolve how the actions of individual complexes integrate. We employed cryo-electron tomography to simultaneously image multiple presynaptic protein complexes and lipids at molecular resolution in their native composition, conformation and environment. Our results argue that tethers comprising proteins Munc13 and SNAP25 differentially and spatially confine vesicles with single nanometer precision, define vesicle tethering states, and provide molecular mechanisms that guide vesicles towards fusion, which includes molecular priming by conversion to SNARE complex-dependent tethers. These findings present an example of a cellular function performed by an extended molecular assembly comprising multiple, molecularly diverse complexes.