Phospholipase C interacts with Sgd1p and is required for expression of GPD1 and osmoresistance in Saccharomyces cerevisiae

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Abstract
The Saccharomyces cerevisiae PLC1 gene encodes a homolog of the δ isoform of mammalian phosphoinositide-specific phospholipase C. Cells deleted for PLC1 (plc1Δ) are viable, but display several phenotypes, including osmotic, temperature, and nocodazole sensitivity. We have used a two-hybrid screen to identify Plc1p-interacting proteins. One of the interacting proteins found was Sgd1p, a recently identified, essential, nuclear protein. The SGD1 gene was originally cloned by complementation of an osmostress-sensitive mutant. The Plc1p-Sgd1p interaction was confirmed biochemically by affinity chromatography. SGD1 interacts genetically with both PLC1 and HOG1 (which encodes an osmosensing mitogen-activated protein kinase). Overexpression of Sgd1p suppresses the temperature sensitivity of cells bearing the plc1-4 allele, and the double mutant strain plc1Δ sgd1-1 displays enhanced temperature and nocodazole sensitivity. The plc1Δ hog1Δ strain displays increased osmosensitivity, and has a synthetic defect in glycerol synthesis and the expression of GPD1 (which encodes the enzyme glycerol 3-phosphate dehydrogenase that is involved in glycerol biosynthesis), suggesting that Plc1p and Hog1p function in independent pathways. The hog1Δ sgd1-1 double mutant displays enhanced osmosensitivity relative to that of either single mutant. The triple mutant plc1Δ hog1Δ sgd1-1 is inviable, while the plc1Δ hog1Δ sgd1-2 strain grows extremely slowly and is more osmosensitive than the plc1Δ hog1Δ or hog1Δ sgd1-2 strain. These results are consistent with a model in which Plc1p and Hog1p function in parallel pathways affecting osmoregulation, and signals from both these pathways converge, at least partly, on Sgd1p.

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