Dominant suppressor mutation bypasses the sphingolipid requirement for growth of Saccharomyces cells at low pH: role of the CWP2 gene

Strains of Saccharomyces cerevisiae termed sphingolipid compensatory (SLC) do not grow at low pH when the cells lack sphingolipids. To begin to unders tand why sphingolipids are required for growth at low pH, we isolated deriv atives of SLC strains, termed low pH resistant (Lpr(R)), carrying the LPR s uppressor gene that allows growth at pH 4.1 when cells lack sphingolipids. Suppression is due to mutation of a single nuclear gene. The LPR suppressor gene functions, at least in part, by enhancing the ability of cells lackin g sphingolipids to generate a net efflux of protons in suspension fluid wit h a pH range of 4.0-6.0. The LPR suppressor gene also enables cells lacking sphingolipids to maintain their intracellular pH near neutrality when the pH of the suspension fluid is low, unlike cells lacking the suppressor gene , which cannot maintain their intracellular pH in the face of a low externa l pH. These results demonstrate that some functions(s) of sphingolipids nec essary for growth at low pH can be bypassed by a suppressor mutation. Attem pts to clone the LPR suppressor gene were not successful, but they led to t he isolation of the CWP2 gene, which encodes a major mannoprotein component of the outer cell wall. It was isolated because an increased copy number h as the unusual property of increasing the frequency at which LprR strains a rise. As we show here, part of the reason for this effect is that the CWP2 gene is essential for generating a net efflux of protons and for controllin g intracellular pH in LprR strains that lack sphingolipids. These results s uggest new cellular functions for the Cwp2 protein.