Oligomerization of the Na,K-ATPase in Cell Membranes

Abstract

The higher order oligomeric state of the Na,K-ATPase alphabeta heterodimer in cell membranes is the subject of controversy. We have utilized the baculovirus-infected insect cell system to express Na,K-ATPase with alpha-subunits bearing either His(6) or FLAG epitopes at the carboxyl terminus. Each of these constructs produced functional Na,K-ATPase alphabeta heterodimers that were delivered to the plasma membrane (PM). Cells were simultaneously co-infected with viruses encoding alpha-His/beta and alpha-FLAG/beta Na,K-ATPases. Co-immunoprecipitation of the His-tagged alpha-subunit in the endoplasmic reticulum (ER) and PM fractions of co-infected cells by the anti-FLAG antibody demonstrates that protein-protein associations exist between these heterodimers. This suggests the Na,K-ATPase is present in cell membranes in an oligomeric state of at least (alphabeta)(2) composition. Deletion of 256 amino acid residues from the central cytoplasmic loop of the alpha-subunit results in the deletion alpha-4,5-loop-less (alpha-4,5LL), which associates with beta but is confined to the ER. Co-immunoprecipitation demonstrates that when this inactive alpha-4,5LL/beta heterodimer is co-expressed with wild-type alphabeta, oligomers of wild-type alphabeta and alpha-4,5LL/beta form in the ER, but the alpha-4,5LL mutant remains retained in the ER, and the wild-type protein is still delivered to the PM. We conclude that the Na,K-ATPase is present as oligomers of the monomeric alphabeta heterodimer in native cell membranes.