PRIMARY SEQUENCE AND FUNCTIONAL EXPRESSION OF A NOVEL BETA-SUBUNIT OF THE P-ATPASE GENE FAMILY

The cortical collecting tubule (CCT) of the mammalian kidney reabsorbs sodium and potassium, processes that are mediated by Na/K-ATPase and Na/K-ATPase. CCT is also an important site for proton secretion, which is driven, in part, by H/K-ATPase. Na/K-ATPase and H/K-ATPase are members of the ion-motive P-ATPase gene family. They ate closely related plasma membrane proteins which consist of alpha beta heterodimers. The urinary bladder of the toad Bufo marinus is the amphibian counterpart of mammalian CCT. We have previously characterized a ouabain-resistant Na/K-ATPase [see ref. 17], from TBM cells, a clonal cell line derived from the toad bladder, which expresses transepithelial sodium transport. In the present study, we report the primary sequence and functional expression of a novel beta subunit (beta(bladder) = beta(b1)) isolated from a toad bladder epithelial cell cDNA Library. The deduced polypeptide is 299 amino acids in length and has a predicted molecular mass of 33 kDa. The beta(b1) protein exhibits 35% amino acid identity to the previously characterized beta(1) of B. marinus Na/K-ATPase and 39% identity with beta(3) of B. marinus Na/K-ATPase. It shares 38% identity with the mammalian beta gastric WK-ATPase and 52% with the mammalian beta(2) Na/K-ATPase. Northern blot analysis shows that a 1.4 X 10(3)-base mRNA is expressed at a high level in bladder epithelial cells and eye and at a trace level in kidney; it is not detectable in significant amounts in the stomach, colon and small intestine. The beta(b1) subunit can associate with the alpha(1) subunit of B. marinus Na/K-ATPase to form a functional sodium pump in the Xenopus laevis oocyte. Our data indicate that, in addition to the known beta(1) and beta(3) isoforms, a third distinct isoform of the beta subunit is present in the bladder epithelium. This new isoform could be functionally associated with alpha subunits of either Na/K- or H/K-ATPase.