VIP36, a novel component of glycolipid rafts and exocytic carrier vesicles in epithelial cells

In simple epithelial cells, apical and basolateral proteins and lipids in transit to the cell surface are sorted in the trans-Golgi network. We have recently isolated detergent-insoluble complexes from Madin-Darby canine kidney cells that are enriched in glycosphingolipids, apical cargo and a subset of the proteins of the exocytic carrier vesicles. The vesicular proteins are thought to be involved in protein sorting and include VIP21-caveolin. The vesicular protein VIP36 (36 kDa vesicular integral membrane protein) has been purified from a CHAPS-insoluble residue and a cDNA encoding VIP36 has been isolated. The N-terminal 31 kDa luminal/exoplasmic domain of the encoded protein shows homology to leguminous plant lectins. The transiently expressed protein is localized to the Golgi apparatus, endosomal and vesicular structures and the plasma membrane, as predicted for a protein involved in transport between the Golgi and the cell surface. It is diffusely localized on the plasma membrane but can be redistributed by antibody modulation into caveolae and clathrin-coated pits. We speculate that VIP36 binds to sugar residues of glycosphingolipids and/or glycosylphosphatidyl-inositol anchors and might provide a link between the extracellular/luminal face of glycolipid rafts and the cytoplasmic protein segregation machinery.