Predicted alterations in tertiary structure of the N-terminus of Na(+)/K(+)-ATPase alpha-subunit caused by phosphorylation or acidic replacement of the PKC phosphorylation site Ser-23.

The protein kinase C (PKC)-mediated phosphorylation of the Na(+)/K(+)-ATPase alpha-subunit has been shown to play an important role in regulation of the Na(+)/K(+)-ATPase activity. In the rat alpha1-subunit, phosphorylation occurs at Ser-23 and results in inhibition of the transport function of the Na(+)/K(+)-ATPase, which is mimicked by replacing the Ser-23 by the negatively charged glutamic acid or by aspartic acid. Using comparative molecular modeling, we investigated whether phosphorylation or acidic replacement at position 23 causes a dramatic change in the molecular electrostatic potential at position 23 as a result of insertion of a negative charge of the phosphoryl group or Glu per se, or whether, alternatively, the modification causes larger-scale conformational changes in the N-terminus of the alpha-subunit. The results predict a considerable conformational change of the 30-residue stretch around Ser-23 when mutated to the residues carrying a net negative charge or being phosphorylated. The structural rearrangements occur within the N-terminal helix-loop-helix motif with a set of charged residues. This motif has structural homology with one in the Ca(2+)-ATPase and may form a function-related structural site in the P-type ATPases. Comparative molecular modeling indicates a lengthening of the interhelical loop and an order-to-disorder transition by disrupting a helix at position 23 because of posphorylation.