Psr1p/Psr2p, two plasma membrane phosphatases with an essential DXDX(T/V) motif required for sodium stress response in yeast

Regulation of intracellular ion concentration is an essential function of a ll cells. In this study, we report the identification of two previously unc haracterized genes, PSR1 and PSR2, that perform an essential function under conditions of sodium ion stress in the yeast Saccharomyces cerevisiae. Psr 1p and Psr2p are highly homologous and were identified through their homolo gy with the endoplasmic reticulum membrane protein Nem1p, Localization and biochemical fractionation studies show that Psr1p is associated with the pl asma membrane via a short amino-terminal sequence also present in Psr2p. Gr owth of the psr1psr2 mutant is severely inhibited under conditions of sodiu m but not potassium ion or sorbitol stress. This growth defect is due to th e inability of the psr1psr2 mutant to properly induce transcription of ENA1 /PMR2, the major sodium extrusion pump of yeast cells. We provide genetic e vidence that this regulation is independent of the phosphatase calcineurin, previously implicated in the sodium stress response in yeast. We show that Psr1p contains a DXDX(T/V) phosphatase motif essential for its function in vivo and that a Psr1p-PtA fusion purified from yeast extracts exhibits pho sphatase activity. Based on these data, we suggest that Psr1p/Psr2p, member s of an emerging class of eukaryotic phosphatases, are novel regulators of salt stress response in yeast.