The Na-K-Cl cotransport protein of shark rectal gland. I. Development of monoclonal antibodies, immunoaffinity purification, and partial biochemical characterization.

The Na-K-Cl cotransporter mediates the coupled transport of Na, K, and Cl across the plasma membrane of many animal cell membranes. It is inhibited by loop diuretics such as furosemide, bumetanide, and benzmetanide. We have developed a panel of monoclonal antibodies directed against the 195-kDa shark rectal gland Na-K-Cl cotransport protein. Four representative antibodies (J3, J4, J7, and J25), each of which recognizes a discrete structural domain, were selected for detailed characterization. When a radiolabeled loop diuretic is bound to the cotransporter prior to solubilization, each antibody immunoprecipitates the same diuretic-protein complex. Of the four antibodies, J4 favors the native protein over the denatured one and does not bind well to proteolytic fragments; in contrast, J7 recognizes the cotransporter only after it has been solubilized. J3, J7, and J25 each recognize a unique ensemble of proteolytic fragments of the 195-kDa protein; analysis of the patterns of recognition has yielded a tentative assignment of the approximate location of the epitopes within the peptide. When the cotransport protein is treated with N-glycanase to remove N-linked oligosaccharides, its apparent mass decreases to approximately 135 kDa. The deglycosylated form is recognized by each of the antibodies except J25; this suggests that the J25 epitope is within the oligosaccharide component or in a peptide domain whose folding is disturbed by carbohydrate removal. An immunoaffinity matrix constructed with the J4 antibody permits single-step purification of the 195-kDa protein; other proteins copurify with the large glycoprotein, but none of these appear to be subunits of a stoichiometric complex. The amino acid sequence of four fragments of the 195-kDa cotransport protein is reported. Immunofluorescence and immunoelectron microscopy demonstrates, in agreement with physiological evidence, that the 195-kDa protein is distributed along the basolateral membrane and excluded from the apical membrane of the rectal gland secretory cell.