p90(RSK) is a serum-stimulated Na+/H+ exchanger isoform-1 kinase - Regulatory phosphorylation of serine 703 of Na+/H+ exchanger isoform-1

The Na+/H+ exchanger isoform-1 (NHE-1) is the key member of a family of exc hangers that regulates intracellular pH and cell volume. Activation of NHE- 1 by growth factors is rapid, correlates with increased NHE-1 phosphorylati on and cell alkalinization, and plays a role in cell cycle progression. By two-dimensional tryptic peptide mapping of immunoprecipitated NHE-1, we ide ntify serine 703 as the major serum-stimulated amino acid, Mutation of seri ne 703 to alanine had no effect on acid-stimulated Na+/H+ exchange but comp letely prevented the growth factor-mediated increase in NHE-1 affinity for HC. In addition, we show that p90 ribosomal S6 kinase (p90(RSK)) is a key N HE-1 kinase since p90(RSK) phosphorylates NHE-1 serine 703 stoichiometrical ly in vitro, and transfection with kinase inactive p90(RSK) inhibits serum- induced phosphorylation of NHE-1 serine 703 in transfected 293 cells. These findings establish p90(RSK) as a serum-stimulated NHE-1 kinase and a media tor of increased Na+/H+ exchange in vivo.