THE ALPHA-SUBUNIT OF THE NA,K-ATPASE SPECIFICALLY AND STABLY ASSOCIATES INTO OLIGOMERS

The Na,K-ATPase is a heterodimer consisting of an alpha and a beta subunit. Both Na,K-ATPase subunits are encoded by multigene families. Several isoforms for the alpha (alpha 1, alpha 2, and alpha 3) and beta (beta 1, beta 2, and beta 3) subunits have been identified. All these isoforms are capable of forming functionally active enzyme. Although there is general agreement that the Na,K-ATPase consists of alpha and beta subunits in equimolar amounts, the quaternary structure of the Na,K-ATPase and its functional significance is unknown. Several studies have demonstrated that the enzyme exists within the plasma membrane as an oligomer of alpha beta dimers. However, because the alpha beta protomer seems to be catalytically competent, the possibility exists that higher oligomers are irrelevant to function. The ability to express different a isoforms in insect cells and the availability of isoform-specific antibodies has provided the opportunity to test for the existence of stable and specific associations among alpha subunits. By coexpressing different alpha-subunit isoforms in cultured cells, we demonstrate that the Na,K-ATPase alpha subunits specifically and stably associate into oligomeric complexes. This same association among alpha-subunit isoforms was demonstrated in the native enzyme from rat brain. The interaction between Na,K-ATPase alpha subunits is highly specific. When the Na,K-ATPase alpha subunit is coexpressed with the alpha subunit from the H,M-ATPase, the H,K subunit does not associate with the Na,K subunit. Moreover, expression of the truncated alpha 1T isoform with the full-length alpha subunit demonstrates that the C-terminal portion of the polypeptide is important in the alpha-subunit association. Although these results do not clarify the functional role of alpha alpha associations, they do establish their highly specific nature and suggest that oligomerization of alpha beta protomers may be important to the stability and physiological regulation of the enzyme.