Subunit Isoform Selectivity in Assembly of Na,K-ATPase α-β Heterodimers

To catalyze ion transport, the Na,K-ATPase must contain one alpha and one beta subunit. When expressed by transfection in various expression systems, each of the four alpha subunit isoforms can assemble with each of the three beta subunit isoforms and form an active enzyme, suggesting the absence of selective alpha-beta isoform assembly. However, it is unknown whether in vivo conditions the alpha-beta assembly is random or isoform-specific. The alpha(2)-beta(2) complex was selectively immunoprecipitated by both anti-alpha(2) and anti-beta(2) antibodies from extracts of mouse brain, which contains cells co-expressing multiple Na,K-ATPase isoforms. Neither alpha(1)-beta(2) nor alpha(2)-beta(1) complexes were detected in the immunoprecipitates. Furthermore, in MDCK cells co-expressing alpha(1), beta(1), and beta(2) isoforms, a greater fraction of the beta(2) subunits was unassembled with alpha(1) as compared with that of the beta(1) subunits, indicating preferential association of the alpha(1) isoform with the beta(1) isoform. In addition, the alpha(1)-beta(2) complex was less resistant to various detergents than the alpha(1)-beta(1) complex isolated from MDCK cells or the alpha(2)-beta(2) complex isolated from mouse brain. Therefore, the diversity of the alpha-beta Na,K-ATPase heterodimers in vivo is determined not only by cell-specific co-expression of particular isoforms, but also by selective association of the alpha and beta subunit isoforms.