Abstract: The Sec1/Munc18 (SM) protein family members perform an essential, albeit poorly understood function in association with SNARE complexes in membrane fusion. The Saccharomyces cerevisiae Sec1p has a C-terminal tail that is missing in its mammalian homologues. Here we show that deletion of the Sec1p tail (amino acids 658-724) renders cells temperature-sensitive for growth, reduces sporulation efficiency, causes a secretion defect and abolishes Sec1p-SNARE component coimmunoprecipitation. The results show that the Sec1p tail binds preferentially ternary Sso1p-Sec9p-Snc2p complexes and it enhances ternary SNARE complex formation in vitro. The Bimolecular fluorescence complementation (BiFC) assay results suggest that in the SNARE deficient sso2-1 Deltasso1 cells, Mso1p, a Sec1p binding protein, helps to target Sec1p(1-657) lacking the C-terminal tail to the sites of secretion. The results suggest that the Mso1p C-terminus is important for Sec1p(1-657) targeting. We show, that in addition to Sec1p, Mso1p can bind the Rab-GTPase Sec4p in vitro. The BiFC results suggest that Mso1p acts in close association with Sec4p on intracellular membranes in the bud. This association depends on the Sec4p GEF Sec2p. Our results reveal a novel binding mode between the Sec1p C-terminal tail and the SNARE complex, and suggest a role for Mso1p as an effector of Sec4p.