Isoform-specific interaction of the cytoplasmic domains of Na,K-ATPase

The Na,K-ATPase is a heterodimer consisting of an alpha and a beta subunit, which exchanges intracellular Na+ for extracellular K+ using the energy of ATP hydrolysis. Several studies have demonstrated that the enzyme exists as an (alphabeta)2 heterotetramer, an oligomer of alphabeta dimers within the cell membrane, at least during some portion of the transport cycle although its functional significance is unknown. In our study, we employed the yeast two-hybrid system to identify the cytoplasmic domains of the Na,K-ATPase which might be involved in intersubunit and/or intrasubunit interactions to form higher order oligmers. Our data demonstrate that the N-terminus and the cytoplasmic loop 1 of the alpha2 subunit interact with each other, while those of the alpha1 subunit do not, suggesting that the interaction is isoform-specific. Therefore, the N-terminal and the cytoplasmic loop 1 might be the regions where the alpha2 subunit, which are involved in alpha alpha interactions, stabilize Na,K-ATPase as alphabeta protomer, diprotomer, or higher order oligomer because the interaction can be intrasubunit as well as intersubunit interactions. Our study suggests that there may be an isoform-specific difference in the alpha-alpha interaction and that the isoform-specific interaction may contribute significantly to the differences of the physiological function and regulation among the alpha isoforms.