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

Abstract

SNARE (Soluble N-ethyl-maleimide sensitive factor Attachment protein Receptor) proteins assemble in tight core complexes, which promote fusion of carrier vesicles with target compartments. Members of this class of proteins are expressed in all eukaryotic cells and are distributed in distinct subcellular 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 membrane, but rather is retained intracellularly when overexpressed in MDCK and 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-SNARE. The interaction between syntaxin 1A and munc-18 occurs in the biosynthetic pathway and is required to promote the exit of the t-SNARE from the Golgi complex. This enabled us to investigate the targeting of syntaxin1A in polarized cells. Confocal analysis of polarized monolayers demonstrates that syntaxin1A is delivered to both the apical and basolateral domains independently of the munc-18 proteins used in the cotranfection experiments. In search 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 represent limiting but essential factors in the transport of syntaxin1A from the Golgi complex to the epithelial cell surface. They also suggest the presence of codominant apical and basolateral sorting signals in the syntaxin1A sequence.