INTERACTION OF MUNC-18-2 WITH SYNTAXIN-3 CONTROLS THE ASSOCIATION OF APICAL SNARES IN EPITHELIAL-CELLS

The docking/fusion of transport vesicles mediated by the soluble NSF a ttachment protein receptors (SNAREs) is thought to be regulated by Sec 1-related proteins. Munc-18-2, a member of this family, is predominant ly expressed in the epithelial cells of several tissues. We demonstrat e here that Munc-18-2 colocalizes with syntaxin 3 at the apical plasma membrane of intestinal epithelium and Caco-2 cells. The presence of a physical complex of the two proteins is verified by 2-way coimmunopre cipitation. The quantity of the complex is reduced by treatment of Cac o-2 cells with the alkylating agent N-ethylmaleimide which also has an inhibitory effect on the ability of Munc-18-2 to associate with synta xin 3 in vitro. The amount of Munc-18-2 in the complex increases upon treatment of the cells with the protein kinase C activator phorbol myr istate acetate, indicating a functional connection between the complex and cell signalling. Increasing the amount of Munc-18-2 bound to synt axin 3 by overexpression results in a marked decrease in the SNARE pro teins SNAP-23 and cellubrevin bound to the syntaxin, These results def ine a novel functional complex of Munc-18-2 and syntaxin 3 involved in the regulation of apical membrane transport.