The α subunit of the Na,K-ATPase specifically and stably associates into oligomers

The Na,K-ATPase is a heterodimer consisting of an α and a β subunit. Both Na,K-ATPase subunits are encoded by multigene families. Several isoforms for the α (α1, α2, and α3) and β (β1, β2, and β3) 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 α and β 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 αβ dimers. However, because the αβ protomer seems to be catalytically competent, the possibility exists that higher oligomers are irrelevant to function. The ability to express different α 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 α subunits. By coexpressing different α-subunit isoforms in cultured cells, we demonstrate that the Na,K-ATPase α subunits specifically and stably associate into oligomeric complexes. This same association among α-subunit isoforms was demonstrated in the native enzyme from rat brain. The interaction between Na,K-ATPase α subunits is highly specific. When the Na,K-ATPase α subunit is coexpressed with the α subunit from the H,K- ATPase, the H,K subunit does not associate with the Na,K subunit. Moreover, expression of the truncated α1T isoform with the full-length α subunit demonstrates that the C-terminal portion of the polypeptide is important in the α-subunit association. Although these results do not clarify the functional role of αα associations, they do establish their highly specific nature and suggest that oligomerization of αβ protomers may be important to the stability and physiological regulation of the enzyme.