Subunit isoform selectivity in assembly of Na,K-ATPase α-β heterodimers

Abstract

To catalyze ion transport, the Na,K-ATPase must contain one α and one β subunit. When expressed by transfection in various expression systems, each of the four α subunit isoforms can assemble with each of the three β subunit isoforms and form an active enzyme, suggesting the absence of selective α-β isoform assembly. However, it is unknown whether in vivo conditions the α-β assembly is random or isoform-specific. The α2-β2 complex was selectively immunoprecipitated by both anti-α2 and anti-β2 antibodies from extracts of mouse brain, which contains cells co-expressing multiple Na,K-ATPase isoforms. Neither α1-β2 nor α2-β1 complexes were detected in the immunoprecipitates. Furthermore, in MDCK cells co-expressing α1,β1, and β2 isoforms, a greater fraction of the β2 subunits was unassembled with α1 as compared with that of the β1 subunits, indicating preferential association of the α1 isoform with the β1 isoform. In addition, the α1- β2 complex was less resistant to various detergents than the α1-β1 complex isolated from MDCK cells or the α2-β2 complex isolated from mouse brain. Therefore, the diversity of the α-β 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 α and βsubunit isoforms.