AN ATYPICAL PKC DIRECTLY ASSOCIATES AND COLOCALIZES AT THE EPITHELIALTIGHT JUNCTION WITH ASIP, A MAMMALIAN HOMOLOG OF CAENORHABDITIS-ELEGANS POLARITY PROTEIN PAR-3

Cell polarity is fundamental to differentiation and function of most c ells. Studies in mammalian epithelial cells have revealed that the est ablishment and maintenance of cell polarity depends upon cell adhesion , signaling networks, the cytoskeleton, and protein transport. Atypica l protein kinase C (PKC) isotypes PKC zeta and PKC lambda have been im plicated in signaling through lipid metabolites including phosphatidyl inositol 3-phosphates, but their physiological role remains elusive. I n the present study we report the identification of a protein, ASIP (a typical PKC isotype-specific interacting protein), that binds to aPKCs , and show that it colocalizes with PKC lambda to the cell junctional complex in cultured epithelial MD CKII cells and rat intestinal epithe lia. In addition, immunoelectron microscopy revealed that ASIP localiz es to tight junctions in intestinal epithelial cells. Furthermore, ASI P shows significant sequence similarity to Caenorhabditis elegans PAR- 3. PAR-3 protein is localized to the anterior periphery of the one-cel l embryo, and is required for the establishment of cell polarity in ea rly embryos. ASIP and PAR-3 share three PDZ domains, and can both bind to aPKCs. Taken together, our results suggest a role for a protein co mplex containing ASIP and aPKC in the establishment and/or maintenance of epithelial cell polarity. The evolutionary conservation of the pro tein complex and its asymmetric distribution in polarized cells from w orm embryo to mammalian-differentiated cells may mean that the complex functions generally in the organization of cellular asymmetry.