Vesicular trafficking, the movement of vesicles between organelles and the plasma membrane for secretion, consists of multiple highly regulated processes. Many protein families function as specificity and regulatory determinants to ensure correct vesicle targeting and timing of trafficking events. The SNARE proteins dock and fuse vesicles to their target membranes. Sec1/Munc18 (SM) proteins regulate membrane fusion through interactions with the SNAREs—SM proteins have been shown to act as both inhibitors and stimulators of SNARE assembly and membrane fusion. However, the details of these SM protein functions are not understood.

Constructing a model of SM protein function has been challenging due to the various modes of interactions reported between SM proteins and their SNAREs. SM proteins interact with their cognate SNAREs and SNARE complexes through several distinct modes. The most conserved mode is an interaction with the syntaxin N-peptide; other modes of binding, such as the syntaxin closed conformation, are hypothesized to be specific for specialized cell types. In order to elucidate the general function of SM proteins, I investigated the function of the endosomal SM protein Vps45p by analyzing its interactions with its cognate syntaxin Tlg2p and its role in SNARE assembly.

I had two main hypotheses: that the Tlg2p N-peptide does not solely mediate the interaction between Vps45p and Tlg2p; and that Vps45p functions to stimulate SNARE complex assembly. I systematically mapped the interaction between Vps45p and Tlg2p using various Tlg2p truncations containing the different domains of Tlg2p and discovered a second binding site on Tlg2p that corresponds to the closed conformation. The neuronal SM-syntaxin pair interacts in a similar manner, indicating that this interaction mode is conserved. To characterize the closed conformation binding mode further, and determine its relationship to the N-peptide binding mode, I developed a quantitative fluorescent electrophoretic mobility shift assay. Results indicate that these two sites do not bind simultaneously and that the N-peptide binding modulates the closed conformation affinity. Furthermore, I monitored the effect of Vps45p on SNARE complex assembly using size exclusion chromatography. Under the conditions tested, Vps45p did not appear to stimulate SNARE complex assembly. The work presented here addresses several puzzling issues in the field and significantly contributes to the construction of a new mechanistic model for SM protein function. In this new model, the SM protein is recruited to the membrane by its interaction with the syntaxin N-peptide. The SM protein then binds the syntaxin closed conformation thus inhibiting SNARE complex assembly. Upon dissociation of the SM protein from the closed conformation, an event perhaps regulated by the SM protein, syntaxin opens and interacts with the other SNAREs to form a SNARE complex. Fusion ensues, stimulated by the SM protein.