Yeast plasma membranes contain a small 55 amino acid hydrophobic polypeptide, Pmp3p, which has high sequence similarity to a novel family of plant polypeptides that are overexpressed under high salt concentration or low temperature treatment. The PMP3 gene is not essential under normal growth conditions. However, its deletion increases the plasma membrane potential and confers sensitivity to cytotoxic cations, such as Na+ and hygromycin B. Interestingly, the disruption of PMP3 exacerbates the NaCl sensitivity phenotype of a mutant strain lacking the Pmr2p/Enap Na+-ATPases and the Nha1p Na+/H+ antiporter, and suppresses the potassium dependency of a strain lacking the K+ transporters, Trk1p and Trk2p. All these phenotypes could be reversed by the addition of high Ca2+ concentration to the medium. These genetic interactions indicate that the major effect of the PMP3 deletion is a hyperpolarization of the plasma membrane potential that probably promotes a non-specific influx of monovalent cations. Expression of plant RCI2A in yeast could substitute for the loss of Pmp3p, indicating a common role for Pmp3p and the plant homologue
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