When nerve cells communicate, vesicles from one neuron fuse with the presynaptic membrane releasing chemicals that signal to the next. Similarly, when insulin binds its receptor on adipocytes or muscle, glucose transporter-4 vesicles fuse with the cell membrane, allowing glucose to be imported. These essential processes require the interaction of SNARE proteins on vesicle and cell membranes, and the presence of the enigmatic protein Munc18 that binds the SNARE protein Syntaxin. The specific regulatory role of Munc18 proteins has been hotly debated, with some favouring a negative regulatory role, and others supporting a positive regulatory role. 

We addressed this question by applying complementary approaches - SAXS, SANS, contrast matching, chemical cross-linking, mass spectrometry and modeling - to investigate the structures of Munc18:Syntaxin complexes from both the neuronal and the adipocyte system (1). Data for the Munc18-1:Syntaxin1a complex in the absence of a short stretch of Syntaxin N-terminal residues indicates that this complex adopts an inhibitory closed binding mode, as exemplified by a previous crystal structure of the complex. However, in the presence of the N-terminal residues, both Syntaxin1a and Syntaxin4 adopt extended conformations in complexes with their respective Munc18 partners.

Overall, our results indicate that the neuronal Munc18-1:Syntaxin1a proteins can adopt two alternate and functionally distinct binding modes, closed and open, depending on the presence of the N-terminal Syntaxin1a residues. In contrast, the adipocyte Munc18c:Syntaxin4 complex adopts only the open, activating, binding mode. This work highlights the importance of short peptide tags in regulating protein interactions, the role of dynamic protein interactions in membrane trafficking and the power of complementary methods to probe these dynamic states.

1. MP Christie, AE Whitten, GJ King, S-H Hu, RR Jarrott, KE Chen, AP Duff, P Callow, BM Collins, DE James and JL Martin (2012) Proc Natl Acad Sci USA 109(25):9816-21