Tissue-specific expression of the Na,K-ATPase beta 3 subunit - The presence of beta 3 in lung and liver addresses the problem of the missing subunit

The Na,R-ATPase belongs to a family of P-type ion-translocating ATPases sharing homologous catalytic subunits (alpha) that traverse the membrane several times and contain the binding sites for ATP and cations, In this family, only Na,K- and H,K-ATPases have been shown to have a second subunit, a single-span glycoprotein called beta. Recently a new isoform (beta 3) has been identified in mammals. Here we describe structural features and tissue distribution of the beta 3 protein, utilizing an antiserum specific for its N terminus, beta 3 was the only beta detected in Na,K-ATPase purified from C6 glioma, Treatment with N-glycosidase F confirmed that beta 3 is a glycoprotein containing N-linked carbohydrate chains, Molecular masses of the glycosylated protein and core protein were estimated to be 42 and 35 kDa, respectively, which are different from those of the beta 1 and beta 2 subunits. Detection of beta subunits has historically been difficult in certain tissues, Sensitivity was improved by deglycosylating, and expression was evaluated by obtaining estimates of beta 3/alpha ratio. The proportion of beta 3 protein in the rat was highest in lung and testis, It was also present in liver and skeletal muscle, whereas kidney, heart, and brain contained it only as a minor component of the Na,K-ATPase. In P7 rat, we found skeletal muscle and lung Na,K-ATPase to be the most enriched in beta 3 subunit, whereas expression in liver was very low, illustrating developmentally regulated changes in expression. The substantial expression in lung and adult liver very likely explains long-standing puzzles about an apparent paucity of beta subunit in membranes or in discrete cellular or subcellular structures.