Structure of membrane-associated neuronal SNARE complex: implication in neurotransmitter release

To enable fusion between biological membranes, t-SNAREs and v-SNARE present in opposing bilayers, interact and assemble in a circular configuration forming ring-complexes, which establish continuity between the opposing membranes, in presence of calcium ions. The size of a t-/v-SNARE ring complex is dictated by the curvature of the opposing membrane. Hence smaller vesicles form small SNARE-ring complexes, as opposed to large vesicles. Neuronal communication depends on the fusion of 40–50 nm in diameter membrane-bound synaptic vesicles containing neurotransmitters at the nerve terminal. At the presynaptic membrane, 12–17 nm in diameter cup-shaped neuronal porosomes are present where synaptic vesicles transiently dock and fuse. Studies demonstrate the presence of SNAREs at the porosome base. Atomic force microscopy (AFM), electron microscopy (EM), and electron density measurement studies demonstrate that at the porosome base, where synaptic vesicles dock and transiently fuse, proteins, possibly comprised of t-SNAREs, are found assembled in a ring conformation. To further determine the structure and arrangement of the neuronal t-/v-SNARE complex, 50 nm t-and v-SNARE proteoliposomes were mixed, allowing t-SNARE-vesicles to interact with v-SNARE vesicles, followed by detergent solubilization and imaging of the resultant t-/v-SNARE complexes formed using both AFM and EM. Our results demonstrate formation of 6–7 nm membrane-directed self-assembled t-/v-SNARE ring complexes, similar to, but twice as large as the ring structures present at the base of neuronal porosomes. The smaller SNARE ring at the porosome base may reflect the 3–4 nm base diameter, where 40–50 nm in diameter v-SNARE-associated synaptic vesicle transiently dock and fuse to release neurotransmitters.