A novel ubiquitous form of Munc-18 interacts with multiple syntaxins. Use of the yeast two-hybrid system to study interactions between proteins involved in membrane traffic.

Munc-18-1 is a 67-kDa neuronal protein that binds tightly to syntaxin 1 and functions in synaptic vesicle exocytosis (Hata, Y., Slaughter, C.A., and Südhof, T.C. (1993a) Nature 366, 347-351). We have now characterized a new Munc-18 isoform, Munc-18-2, that exhibits 63% amino acid sequence identity with Munc-18-1. Munc-18-2 is expressed in most tissues, whereas Munc-18-1 is primarily expressed in brain. Using recombinant Munc-18-1 and Munc-18-2 produced in COS cells, we show that both forms of Munc-18 bind tightly to syntaxins 1A, 2, and 3 but not to syntaxin 4. In an independent approach to study the binding specificities of Munc-18-1 and Munc-18-2, we used the yeast two-hybrid system. This assay system depends on protein-protein interactions in the cell nucleus. We validated its utility for studying membrane trafficking proteins by testing well characterized interactions between cytosolic proteins that are known to be physiologically important in exocytosis. Strong interactions, such as the binding of syntaxins 1-4 with SNAP-25, were effectively detected by the yeast two-hybrid assay, but weak binding, such as the binding of syntaxins to synaptotagmin or of synaptotagmin to neurexins, was not. Studies on full-length and truncated forms of Munc-18s by the yeast two-hybrid system confirmed their interactions with syntaxins. Both the N and the C terminus of Munc-18 were essential for binding. Munc-18-1 and Munc-18-2 bind only to syntaxins 1A, 2, and 3 but not 4 and 5 by yeast-two hybrid system assays. Our studies demonstrate that neural and non-neural tissues have distinct forms of Munc-18, which may function in different types of exocytosis. The lack of specificity of the interactions between syntaxins and Munc-18s indicates that specificity of membrane trafficking reactions is not dependent on this interaction.