Functional role and immunocytochemical localization of the gamma(a) and gamma(b) forms of the Na,K-ATPase gamma subunit

The gamma subunit of the Na,K-ATPase is a member of the FXYD family of type 2 transmembrane proteins that probably function as regulators of ion trans port. Rat gamma is present primarily in the kidney as two main splice varia nts, gamma (a) and gamma (b), which differ only at their extracellular N te rmini (TELSANH and MDRWYL, respectively; Kuster, B., Shainskaya, A., Pu, H. X., Goldshleger, R., Blostein, R., Mann, M., and Karlish, S. J. D. (2000) J. Biol. Chem. 275, 18441-18446). Expression in cultured cells indicates th at both variants affect catalytic properties, without a detectable differen ce between gamma (a) and gamma (b). At least two singular effects are seen, irrespective of whether the variants are expressed in HeLa or rat alpha1-t ransfected HeLa cells, i.e. (i) an increase in apparent affinity for ATP, p robably secondary to a left shift in E-1 <-> E-2 conformational. equilibriu m and (ii) an increase in K+ antagonism of cytoplasmic Na+ activation. Anti bodies against the C terminus common to both variants (anti-gamma) abrogate the first effect but not the second. In contrast, gamma (a) and gamma (b) show differences in their localization along the kidney tubule. Using anti- gamma (C-terminal) and antibodies to the rat alpha subunit as well as antib odies to identify cell types, double immunofluorescence showed gamma in the basolateral membrane of several tubular segments. Highest expression is in the medullary portion of the thick ascending limb (TAL), which contains bo th gamma (a) and gamma (b). In fact, TAL is the only positive tubular segme nt in the medulla. In the cortex, most tubules express gamma but at lower l evels. Antibodies specific for gamma (a) and gamma (b) showed differences i n their cortical location; gamma (a) is specific for cells in the macula de nsa and principal cells of the cortical collecting duct but not cortical TA L. In contrast, gamma (b) but not gamma (a) is present in the cortical TAL only. Thus, the importance of gamma (a) and gamma (b) may be related to the ir partially overlapping but distinct expression patterns and tissue-specif ic functions of the pump that these serve.