Regulation of membrane fusion by the membrane-proximal coil of the t-SNARE during zippering of SNAREpins

We utilize structurally targeted peptides to identify a "t_c fusion switch" inherent to the coil domains of the neuronal t-SNARE that pairs with the cognate v-SNARE. The t_c fusion switch is located in the membrane-proximal portion of the t-SNARE and controls the rate at which the helical bundle that forms the SNARE-pin can zip up to drive bilayer fusion. When the fusion switch is "off" (the intrinsic state of the t-SNARE), zippering of the helices from their membrane-distal ends is impeded and fusion is slow. When the t_c fusion switch is "on," fusion is much faster. The t_c fusion switch can be thrown by a peptide that corresponds to the membrane-proximal half of the cognate v-SNARE, and binds reversibly to the cognate region of the t-SNARE. This structures the coil in the membrane-proximal domain of the t-SNARE and accelerates fusion, implying that the intrinsically unstable coil in that region is a natural impediment to the completion of zippering, and thus, fusion. Proteins that stabilize or destabilize one or the other state of the t_c fusion switch would exert fine temporal control over the rate of fusion after SNAREs have already partly zippered up.