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J Biol Chem. 1999 Jul 16;274(29):20206-14.

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

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  • 1Departments of Medicine, University of Washington, Seattle, Washington 98195, USA.


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.

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