Syntaxin 1A is delivered to the apical and basolateral domains of epithelial cells: the role of munc-18 proteins

SNARE (Soluble N-ethyl-maleimide sensitive factor Attachment protein Recept or) proteins assemble in tight core complexes, which promote fusion of carr ier vesicles with target compartments. Members of this class of proteins ar e expressed in all eukaryotic cells and are distributed in distinct subcell ular compartments. The molecular mechanisms underlying sorting of SNAREs to their physiological sites of action are still poorly understood. Here have we analyzed the transport of syntaxin1A in epithelial cells. In line with previous data we found that syntaxin1A is not transported to the plasma mem brane, but rather is retained intracellularly when overexpressed in MDCK an d Caco-2 cells. Its delivery to the cell surface is recovered after munc-18 -1 cotransfection. Furthermore, overexpression of the ubiquitous isoform of munc-18, munc-18-2, is also capable of rescuing the transport of the t-SNA RE. The interaction between syntaxin 1A and munc-18 occurs in the biosynthe tic pathway and is required to promote the exit of the t-SNARE from the Gol gi complex. This enabled us to investigate the targeting of syntaxin1A in p olarized cells. Confocal analysis of polarized monolayers; demonstrates tha t syntaxin1A is delivered to both the apical and basolateral domains indepe ndently of the munc-18 proteins used in the cotranfection experiments. In s earch of the mechanisms underlying syntaxin 1A sorting to the cell surface, we found that a portion of the protein is included in non-ionic detergent insoluble complexes. Our results indicate that the munc-18 proteins represe nt limiting but essential factors in the transport of syntaxin1A from the G olgi complex to the epithelial cell surface. They also suggest the presence of codominant apical and basolateral sorting signals in the syntaxin1A seq uence.