Predicted location and limited accessibility of protein kinase A phosphorylation site on Na-K-ATPase

Regulation of Na-K-ATPase by cAMP-dependent protein kinase occurs in a vari ety of tissues. Phosphorylation of the enzyme's catalytic subunit at a clas sical phosphorylation consensus motif has been observed with purified enzym e. Demonstration of phosphorylation at the same site in normal living cells or tissues has been more difficult, however, making it uncertain that the Na-K-ATPase is a direct physiological substrate of the kinase. Recently, th e structure of the homologous sarco( endo) plasmic reticulum Ca-ATPase (SER CA1a) has been determined at 2.6 Angstrom resolution (Toyoshima C, Nakasako M, Nomura H, and Ogawa H. Nature 405: 647-655, 2000.), and the Na-K-ATPase should have the same fold. Here, the Na-K-ATPase sequence has been aligned with the Ca-ATPase structure to examine the predicted disposition of the p hosphorylation site. The location is close to the membrane and partially bu ried by adjacent loops, and the site is unlikely to be accessible to the ki nase in this conformation. Conditions that may expose the site or further b ury it are discussed to highlight the issues facing future research on regu lation of Na-K-ATPase by cAMP-dependent pathways.