SNARE proteins and Sec1p/Munc18-family (SM) proteins constitute the core molecular machines that mediate nearly all intracellular membrane fusion. Other proteins regulate the core machinery to enable fusion at the right time and location. Dysfunction of these proteins has been linked to neurological and immunological disorders, cancers, diabetes, and other diseases. Decades of research have established that SNAREs couple their ordered folding and assembly to membrane fusion in a SM protein-dependent manner. However, it remains unclear what mechanistic role SM proteins plays in SNARE assembly and how SNARE assembly is coupled to membrane fusion. Using high-resolution optical tweezers, we recently found that neuronal SM protein Munc18-1 catalyzes step-wise assembly of three synaptic SNAREs (syntaxin, VAMP2, and SNAP-25) into a four-helix bundle, a process essential for neurotransmission and insulin secretion. Importantly, Munc18-1 serves as a template to guide directional SNARE assembly along a new pathway. In this application, we plan to first establish that the template complex is a conserved intermediate for SNARE-SM fusion machineries and a key target for other proteins to regulate SNARE assembly and membrane fusion. We will examine effects of key regulators involved in calcium-triggered exocytosis (Munc13-1, complexin, synaptotagmin, NSF, and alpha-SNAP) and phosphorylation of SNARE and SM proteins on SNARE assembly and disassembly. Then, we will develop new assays to simultaneous detect step-wise SNARE assembly and state-wise membrane fusion using large nanodiscs and trapped GUVs. A major goal is to reconstitute the calcium-triggered membrane fusion under controlled experimental conditions and to understand their working mechanism. Finally, we will extend our methodologies to pinpoint the molecular mechanisms of membrane binding and lipid exchange by extended synaptotagmins (E-Syts) and of mechanosensation by ion channel NOMPC. Our long-term goal is to develop a general approach to elucidate stability, folding, and dynamics of membrane proteins.
SNARE-SM proteins and extended synaptotagmins generate force to mediate membrane fusion and lipid exchange, respectively, while NOMPC senses mechanical force to conduct ions. Mechanistic understanding of these proteins are limited by our ability to measure and apply forces at a single- molecule level. We will use optical tweezers, fluorescence microscopy, and other complementary approaches to elucidate their working mechanisms.
