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 exchangers that regulates intracellular pH and cell volume. Activation of NHE-1 by growth factors is rapid, correlates with increased NHE-1 phosphorylation and cell alkalinization, and plays a role in cell cycle progression. By two-dimensional tryptic peptide mapping of immunoprecipitated NHE-1, we identify serine 703 as the major serum-stimulated amino acid. Mutation of serine 703 to alanine had no effect on acid-stimulated Na⁺/H⁺ exchange but completely prevented the growth factor-mediated increase in NHE- 1 affinity for H⁺. In addition, we show that p90 ribosomal S6 kinase (p90(RSK)) is a key NHE-1 kinase since p90(RSK) phosphorylates NHE-1 serine 703 stoichiometrically 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 mediator of increased Na⁺/H⁺ exchange in vivo.