Protein kinase A phosphorylates the Nem1-Spo7 protein phosphatase complex that regulates the phosphorylation state of the phosphatidate phosphatase Pah1 in yeast.

The Nem1-Spo7 protein phosphatase plays a role in lipid synthesis by controlling the membrane localization of Pah1, the diacylglycerol-producing phosphatidate (PA) phosphatase that is crucial for the synthesis of triacylglycerol in the yeast Saccharomyces cerevisiae By dephosphorylating Pah1, Nem1-Spo7 facilitates its translocation to the nuclear/endoplasmic reticulum membrane for catalytic activity. Like its substrate Pah1, Nem1-Spo7 is phosphorylated in the cell, but the specific protein kinases involved remain to be identified. In this study, we demonstrate that the Nem1-Spo7 complex is phosphorylated by protein kinase A (PKA), which is associated with active cell growth, metabolic activity, and membrane phospholipid synthesis. In vitro phosphorylation of purified Nem1-Spo7 and of their synthetic peptides revealed that both subunits of the phosphatase complex are PKA substrates. Using phosphoamino acid and phosphopeptide-mapping analyses coupled with site-directed mutagenesis, we identified Ser-140 and Ser-210 of Nem1 and Ser-28 of Spo7 as PKA-targeted phosphorylation sites. Immunodetection of the phosphatase complex from the cell with anti-PKA substrate antibody confirmed the in vivo phosphorylations of Nem1 and Spo7 on the serine residues. Lipid-labeling analysis of cells bearing phosphorylation-deficient alleles of NEM1 and SPO7 indicated that the PKA phosphorylation of the phosphatase complex stimulates phospholipid synthesis and attenuates the synthesis of triacylglycerol. This work advances the understanding of how PKA-mediated posttranslational modifications of Nem1 and Spo7 regulate lipid synthesis in yeast.