Psr1p/Psr2p, Two Plasma Membrane Phosphatases with an Essential DXDX(T/V) Motif Required for Sodium Stress Response in Yeast

Abstract

Regulation of intracellular ion concentration is an essential function of all cells. In this study, we report the identification of two previously uncharacterized genes, PSR1 and PSR2, that perform an essential function under conditions of sodium ion stress in the yeast Saccharomyces cerevisiae. Psr1p and Psr2p are highly homologous and were identified through their homology with the endoplasmic reticulum membrane protein Nem1p. Localization and biochemical fractionation studies show that Psr1p is associated with the plasma membrane via a short amino-terminal sequence also present in Psr2p. Growth of the psr1psr2 mutant is severely inhibited under conditions of sodium but not potassium ion or sorbitol stress. This growth defect is due to the inability of the psr1psr2 mutant to properly induce transcription of ENA1/PMR2, the major sodium extrusion pump of yeast cells. We provide genetic evidence that this regulation is independent of the phosphatase calcineurin, previously implicated in the sodium stress response in yeast. We show that Psr1p contains a DX-DX(T/V) phosphatase motif essential for its function in vivo and that a Psr1p-PtA fusion purified from yeast extracts exhibits phosphatase activity. Based on these data, we suggest that Psr1p/Psr2p, members of an emerging class of eukaryotic phosphatases, are novel regulators of salt stress response in yeast.Response and adaptation to rapidly changing environmental conditions is a fundamental feature of all living cells. Because of the knowledge of its sequenced genome and the availability of powerful genetic approaches, budding yeast Saccharomyces cerevisiae has been a particularly valuable model system for studying such responses. Of particular significance for this unicellular eukaryote is the response to changes in osmolarity of the medium. Yeast cells are able to detect and respond to changes in osmolarity by two independent osmosensors, Sln1p and Sho1p (1, 2). These membrane-bound proteins activate the so-called high osmolarity glycerol (HOG) 1 mitogen-activated protein kinase cascade (1, 3), which in turn mediates the transcriptional activation of several genes required for adaptation to high osmolarity mostly through the stress response elements. These include several general osmotic stress genes, like GPD1 (glycerol-phosphate dehydrogenase) required for prevention of water loss, heat shock proteins like HSP12, or CTT1 (cytosolic catalase) among others (4 -6). The HOG pathway is also responsible for transmitting stress signals emerging from high concentrations of certain ions, such as lithium or sodium (7). High salinity media can be toxic for yeast cells because they lead to loss of turgor pressure and block several metabolic reactions. In this case, an essential event required for the survival of yeast cells is the transcriptional activation of the ENA1/PMR2 gene. ENA1/PMR2 encodes for a P-type ATPase required for the efflux of sodium ions (8, 9). Although yeast cells contain a tandem array of nearly identical genes encoding ion pumps involved in sod...

The TORC1 effector kinase Npr1 fine tunes the inherent activity of the Mep2 ammonium transport protein

“…Furthermore, we identify plasma membrane Psr1 and Psr2 as redundant phosphatases required for the Mep2 dephosphorylation. These data reveal a new role for the Psr proteins in the regulation of ammonium transport in addition to their role in sodium and stress response 40,41 .…”

“…Consistently, double psr1D psr2D-mutant cells were fully protected against glutamine-induced S457 dephosphorylation of Mep2. Psr1 and Psr2 are two plasma membrane phosphatases so far mainly described for their role in sodium resistance and Msn2/4 activation 40,41 . Our data reveal Psr1 and Psr2 as two redundant phosphatases required for glutamine-induced Mep2 dephosphorylation.…”

Evolutionary Genomics of the HAD Superfamily: Understanding the Structural Adaptations and Catalytic Diversity in a Superfamily of Phosphoesterases and Allied Enzymes

“…86 This family has diversified into seven subfamilies in the eukaryotes and their viruses ( Table 1). The most widespread of these is the Psr1p subfamily which is conserved throughout the eukaryotes and is typified by a conserved N-terminal module required for membrane localization, 87,88 and a conserved cysteine (Psr1p _ Sc in Figure 9). Members of this subfamily are slow evolving and are likely to be the principal CTD phosphatases of eukaryotes, and ancient components of the nuclear membrane.…”

Alkali Metal Cation Transport and Homeostasis in Yeasts

“…Other gene products, such as Psr1 and Psr2 (two similar membrane proteins with phosphatase activity [263]), Hal9 (a putative transcription factor containing a zinc finger that, when overexpressed, was able to confer increased tolerance to sodium or lithium ions [166]), or the nuclear phosphoprotein Lic4/Atc1 (related to cation homeostasis several years ago [105]) have been proposed to modulate saline tolerance by means of the control of ENA1 expression, although no precise mechanism is known.…”

“…Moreover, maintenance of a high intracellular K ϩ /Na ϩ ratio has been shown to be very important for plant growth in soils with increased salinity (147,151,202). Most of the plasma membrane and vacuolar antiporters characterized so far in different plants (263) belong to the ScNhx1 subfamily (151), although ScKha1 homologs (forming the CHX subfamily) have also been identified in plant chloroplast and endosomal membranes (45). Several members of the Nhx1 family have been shown to be crucial for plant salt tolerance.…”