Yeast Nem1-Spo7 Protein Phosphatase Activity on Pah1 Phosphatidate Phosphatase Is Specific for the Pho85-Pho80 Protein Kinase Phosphorylation Sites

Background: Pah1 phosphatidate phosphatase translocates to the membrane through its dephosphorylation by the membrane-associated Nem1-Spo7 phosphatase complex. Results: Nem1-Spo7 phosphatase was characterized for its enzymological, kinetic, and regulatory properties with phosphorylated forms of Pah1. Conclusion: Nem1-Spo7 phosphatase exhibits the highest specificity for Pah1 phosphorylated by the Pho85-Pho80 protein kinase complex. Significance: The Nem1-Spo7-mediated dephosphorylation regulates the function of Pah1 in lipid metabolism. Pah1 is the phosphatidate phosphatase in the yeast Saccharomyces cerevisiae that produces diacylglycerol for triacylglycerol synthesis and concurrently controls the levels of phosphatidate used for phospholipid synthesis. Phosphorylation and dephosphorylation of Pah1 regulate its subcellular location and phosphatidate phosphatase activity. Compared with its phosphorylation by multiple protein kinases, Pah1 is dephosphorylated by a protein phosphatase complex consisting of Nem1 (catalytic subunit) and Spo7 (regulatory subunit). In this work, we characterized the Nem1-Spo7 phosphatase complex for its enzymological, kinetic, and regulatory properties with phosphorylated Pah1. The dephosphorylation of Pah1 by Nem1-Spo7 phosphatase resulted in the stimulation (6-fold) of phosphatidate phosphatase activity. For Pah1 phosphorylated by the Pho85-Pho80 kinase complex, maximum Nem1-Spo7 phosphatase activity required Mg2+ ions (8 mm) and Triton X-100 (0.25 mm) at pH 5.0. The energy of activation for the reaction was 8.4 kcal/mol, and the enzyme was thermally labile at temperatures above 40 degrees C. The enzyme activity was inhibited by sodium vanadate, sodium fluoride, N-ethylmaleimide, and phenylglyoxal but was not significantly affected by lipids or nucleotides. Nem1-Spo7 phosphatase activity was dependent on the concentrations of Pah1 phosphorylated by Pho85-Pho80, Cdc28-cyclin B, PKA, and PKC with k(cat) and K-m values of 0.29 s(-1) and 81 nm, 0.11 s(-1) and 127 nm, 0.10 s(-1) and 46 nm, and 0.02 s(-1) and 38 nm, respectively. Its specificity constant (k(cat)/K-m) for Pah1 phosphorylated by Pho85-Pho80 was 1.6-, 4-, and 6-fold higher, respectively, than that phosphorylated by PKA, Cdc28-cyclin B, and PKC.