Syntaxin1A and Munc18-1 play essential roles in exocytosis. However, the molecular mechanism and the functional roles of their interaction in insulin secretion remain to be explored. Using membrane capacitance measurement, we examine effect of overexpressing Mune18-1 on exocytosis in pancreatic beta cells. The results show that Munc18-1 negatively regulates vesicle fusion. To probe the interaction between Munc18-1 and Syntaxin1A, Munc18-1-Tdimer2 and EGFP-Syntaxin1A were co-transfected into INS-1 cells. FRET measurement confirmed that Munc18-1 interacted with wild type Syntaxin1A, but not the constitutively open form (DM) of Syntaxin1A. Overexpressing DM in primary pancreatic beta cells augmented insulin secretion, and this effect can overcome the inhibitory effect of Munc18-1 overexpression. We propose that Munc18-1 inhibitis the SNARE complex assembly by stabilizing Syntaxin1A in a closed conformation in vesicle priming process, therefore negatively regulates insulin secretion. (c) 2007 Elsevier Inc. All rights reserved.