Know when to Hold ‘Em, Know when to Fold ‘Em; SM Proteins as Templates for SNARE Assembly

Abstract

A major focus of our research is the machinery that guides the fusion of vesicles during intracellular transport. These vesicles deliver their cargo via membrane fusion reactions executed by membrane-bridging SNARE complexes. The formation of SNARE complexes generally requires that four different SNARE proteins, anchored in two different membranes, undergo a coupled folding and assembly reaction during which the SNARE motifs zipper up into a parallel four-helix bundle. This complicated process is inefficient in vitro, and is certain to be even more challenging in vivo, where it must compete with the formation of various non-cognate and off-pathway SNARE complexes. Consequently, we hypothesize that SNARE complex assembly reactions in the cell are orchestrated by a set of ‘topologically aware’ chaperones called multisubunit tethering complexes (MTCs). We furthermore propose that the key task of catalyzing four-helix bundle formation falls to the Sec1/Munc18 (SM) proteins, working together with - and sometimes as integral subunits of - the MTCs. Therefore, the goal of our work is to achieve an improved structural and mechanistic understanding of MTC and SM function in the assembly of fusogenic SNARE complexes. My presentation will focus on the SM proteins, and on our X-ray crystallographic and biochemical efforts to understand how they catalyze four-helix bundle formation.