Surface Charges of the Membrane Crucially Affect Regulation of Na,K-ATPase by Phospholemman (FXYD1)

The human alpha(1)/His(10)-beta(1) isoform of Na,K-ATPase has been reconstituted as a complex with and without FXYD1 into proteoliposomes of various lipid compositions in order to study the effect of the regulatory subunit on the half-saturating Na+ concentration (K (1/2)) of Na+ ions for activation of the ion pump. It has been shown that the fraction of negatively charged lipid in the bilayer crucially affects the regulatory properties. At low concentrations of the negatively charged lipid DOPS (< 10 %), FXYD1 increases K (1/2) of Na+ ions for activation of the ion pump. Phosphorylation of FXYD1 by protein kinase A at Ser68 abrogates this effect. Conversely, for proteoliposomes made with high concentrations of DOPS (> 10 %), little or no effect of FXYD1 on the K (1/2) of Na+ ions is observed. Depending on ionic strength and lipid composition of the proteoliposomes, FXYD1 can alter the K (1/2) of Na+ ions by up to twofold. We propose possible molecular mechanisms to explain the regulatory effects of FXYD1 and the influence of charged lipid and protein phosphorylation. In particular, the positively charged C-terminal helix of FXYD1 appears to be highly mobile and may interact with the cytoplasmic N domain of the alpha-subunit, the interaction being strongly affected by phosphorylation at Ser68 and the surface charge of the membrane.