Regulation of Membrane Fusion in Exocytosis

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Project Summary/Abstract How can virtually the same SNARE machine operate at dramatically different speeds depending on context, often far faster than a single SNAREpin? This is one of the central questions driving the field today, and the problem it embodies stands in boldest relief at the neuronal synapse, so it is here that we focus on the structures, biophysics, and physiological properties of the key protein machinery. Our overall hypothesis is that multiple SNAREpins released synchronously, each already close to the point of triggering fusion, co-operate to achieve fusion dramatically faster than any one alone. During the current period of support we discovered that the calcium sensor Synaptotagmin (normally anchored in synaptic vesicle) can self-assemble in vitro into Ca2+- sensitive, ring-like oligomers ~30 nm in diameter and have suggested that such rings forming between the synaptic vesicle (or insulin secretory vesicle) and the plasma membrane would prevent release until they are disrupted by Ca2+. Our specific hypothesis is that such ring oligomers of Synaptotagmin (Syt) are a central organizing principle for exocytosis, enabling the clamping and rapid synchronous release of multiple SNAREpins. This hypothesis is strongly supported by recent experiments in which a targeted mutation (F349A) that de-stabilizes Syt1 rings dramatically increases spontaneous and evoked release and in hippocampal neurons, and dramatically reduces the synchronicity of release with the action potential. We propose to 1) Test the hypothesis that ring-like oligomers of Synaptotagmins regulate exocytosis; 2) Test the hypothesis that Syt1 and Syt7 play distinct structural and functional roles in synchronous and asynchronous release from the same docked vesicles; 3) Elucidate the dynamics and topology of Munc13 and its proposed oligomers and the posited dual roles as vesicle tether and outer ring chaperone templating SNAREpins; and 4) Obtain by single particle cryo-EM and cryo EM tomography high resolution structures of functional release sites in vitro and in situ trapped in defined functional states. Similar machinery mediates neuroendocrine secretory physiology, including pancreatic insulin secretion, so we expect the answers will be highly relevant to the mission of NIDDK. Further, there is little doubt in the post-leptin era of the key role of the nervous system in metabolic balance and diseases.