Phosphorylation of Phospholemman (FXYD1) by Protein Kinases A and C Modulates Distinct Na,K-ATPase Isozymes

Abstract Phospholemman (FXYD1), mainly expressed in heart and skeletal muscle, is a member of the FXYD protein family, which has been shown to decrease the apparent K+ and Na+ affinity of Na,K-ATPase ( Crambert, G., Fuzesi, M., Garty, H., Karlish, S., and Geering, K. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 11476-11481 ). In this study, we use the Xenopus oocyte expression system to study the role of phospholemman phosphorylation by protein kinases A and C in the modulation of different Na,K-ATPase isozymes present in the heart. Phosphorylation of phospholemman by protein kinase A has no effect on the maximal transport activity or on the apparent K+ affinity of Na,K-ATPase α1/β1 and α2/β1 isozymes but increases their apparent Na+ affinity, dependent on phospholemman phosphorylation at Ser68. Phosphorylation of phospholemman by protein kinase C affects neither the maximal transport activity of α1/β1 isozymes nor the K+ affinity of α1/β1 and α2/β1 isozymes. However, protein kinase C phosphorylation of phospholemman increases the maximal Na,K-pump current of α2/β1 isozymes by an increase in their turnover number. Thus, our results indicate that protein kinase A phosphorylation of phospholemman has similar functional effects on Na,K-ATPase α1/β and α2/β isozymes and increases their apparent Na+ affinity, whereas protein kinase C phosphorylation of phospholemman modulates the transport activity of Na,K-ATPase α2/β but not of α1/β isozymes. The complex and distinct regulation of Na,K-ATPase isozymes by phosphorylation of phospholemman may be important for the efficient control of heart contractility and excitability.