The role of the yeast Sec1/Munc18 protein, Vps45p, in the assembly of its cognate snare complex

Abstract

Members of the SNARE (soluble NSF attachment protein receptor) superfamily of proteins are indispensable players in all intracellular transport pathways. Sec1/Munc18 (SM) proteins are also essential for membrane trafficking, but a model encompassing universal function(s) for SM proteins has been difficult to formulate. The yeast SM protein Vps45p regulates traffic from the trans-Golgi network (TGN) to late endosomes and facilitates the assembly of its cognate SNARE proteins (Tlg2p, Tlg1p, Vti1p, and Snc2p) into complexes. The first section of this work addresses the interaction(s) between Vps45p and its cognate SNARE partners. The second area of investigation in this work addresses the functional role(s) of these interactions. Analysis of one mode of interaction between Vps45p and its cognate syntaxin, Tlg2p, reveals that this mode of interaction is dispensable for Vps45p function. A dominant negative version of Vps45p is exploited to uncover an interaction between Vps45p and SNARE complexes that is mediated via a second, distinct mode. An investigation of the ability of Vps45p to interact with each of its individual cognate SNARE proteins also uncovers an interaction between Vps45p and its cognate v-SNARE, Snc2p. Collectively, these results suggest that Vps45p executes its function(s) through multiple modes of interaction with its cognate SNARE proteins, and support a model in which Vps45p undertakes a series of distinct interactions with specific intermediates of the SNARE cycle. Such a model would allow Vps45p to play multiple roles in the SNARE cycle and would also help contribute to a unifying hypothesis to describe SM protein function