Role of PAH1 ‐encoded PA Phosphatase in Triacylglycerol Synthesis in Yeast

The PAH1 gene in Saccharomyces cerevisiae encodes the Mg 2+ ‐dependent phosphatidate (PA) phosphatase. PA phosphatase catalyzes the dephosphorylation of PA yielding diacylglycerol and P i* This diacylglycerol may be used for the synthesis of triacylglycerol, as well as the synthesis of phospholipids via the Kennedy pathway. The accumulation of triacylglycerol in the stationary phase of growth has been attributed to increased PA phosphatase activity. However, the exact gene/enzyme responsible has been unknown. The expression of PAH1 mRNA and PA phosphatase protein was maximal in the exponential phase, and both declined as cells entered the stationary phase. In vivo pulse‐labeling experiments showed that the rate of triacylglycerol synthesis was high in exponentially growing cells and continued throughout growth. However, the loss of PAH1 ‐encoded PA phosphatase activity resulted in almost a complete loss of triacylglycerol synthesis. Partial loss of triacylglycerol synthesis was observed in nem1 Δ mutant cells that lack the Nem1p‐Spo7p phosphatase complex that dephosphorylates PA phosphatase at the nuclear/ER membrane. As expected, the amount of triacylglycerol in stationary phase of pah1 Δ cells was reduced by >90%. Supported by NIH grant GM 28140.