NOVEL 13A ANTIGEN IS AN INTEGRAL PROTEIN OF THE BASOLATERAL MEMBRANE OF RAT GLOMERULAR PODOCYTES

BACKGROUND: We described recently the 13A monoclonal antibody recogniz ing a 120 kilodalton protein located at the bases of podocyte foot pro cesses in rat glomeruli. The antigen was extracellular, either a compo nent of the glomerular basement membrane or an integral membrane prote in. As only few markers exist for the basal domain of the podocyte mem branes, we wanted to characterize the antigen further. EXPERIMENTAL DE SIGN: The distribution of the 13A antigen in rat tissues and cultured cells was studied by immunofluorescence and immunoelectron microscopy. Cultured cells were also used for its biochemical and functional char acterization. RESULTS: The antigen was detected in several rat epithel ial and smooth muscle tissues. In polarized epithelia, it was restrict ed to the basolateral membranes, and in stratified epithelia, to the b asal cell layer. In contrast to its limited distribution in vivo, the antigen was detected in vitro in several cultured fibroblastoid or epi thelial rat cell lines, and in cultured mesangial cells. In nonpolariz ed cells, it had a diffuse granular distribution at the cell surface, and at the ventral surface, it colocalized with vinculin in areas rese mbling focal adhesions, as shown by double immunofluorescence staining . In polarized epithelial cells, the 13A antigen was concentrated at t he basolateral membranes. By immunoelectron microscopy, it was often p resent at the tips of cell extensions and at adhesion sites. Pretreatm ent of cryostat sections or cultured cells with trypsin partially inhi bited antibody binding, whereas detergents abolished it totally. The a ntigen of cultured cells could not be identified by Western blotting o r immunoprecipitation techniques. The antibodies did not seem to affec t cell growth or adhesion. CONCLUSIONS: The 13A antigen is an integral membrane protein of several rat epithelial tissues and cultured cells , and is particularly abundant in the podocyte foot processes. Althoug h its identity and function remain unknown, the 13A protein is a valua ble marker for the basal membrane domain of the podocyte.