The yeast Pah1p phosphatidate phosphatase, which catalyzes the penultimate step in the synthesis of triacylglycerol and plays a role in the transcriptional regulation of phospholipid synthesis genes, is a cytosolic enzyme that associates with the nuclear/endoplasmic reticulum membrane to catalyze the dephosphorylation of phosphatidate to yield diacylglycerol. Pah1p is phosphorylated on seven (Ser-110, Ser-114, Ser-168, Ser-602, Thr-723, Ser-744, and Ser-748) sites that are targets for proline-directed protein kinases. In this work, we showed that the seven sites are phosphorylated by Pho85p-Pho80p, a protein kinase-cyclin complex known to regulate a variety of cellular processes. The phosphorylation of recombinant Pah1p was time- and dose-dependent and dependent on the concentrations of ATP (3.7 μm) and Pah1p (0.25 μm). Phosphorylation reduced (6-fold) the catalytic efficiency (V(max)/K(m)) of Pah1p and reduced (3-fold) its interaction (K(d)) with liposomes. Alanine mutations of the seven sites ablated the inhibitory effect that Pho85p-Pho80p had on Pah1p activity and on the interaction with liposomes. Analysis of pho85Δ mutant cells, phosphate-starved wild type cells, and cells expressing phosphorylation-deficient forms of Pah1p indicated that loss of Pho85p-Pho80p phosphorylation reduced Pah1p abundance. In contrast, lack of Nem1p-Spo7p, the phosphatase complex that dephosphorylates Pah1p at the nuclear/endoplasmic reticulum membrane, stabilized Pah1p abundance. Although loss of phosphorylation caused a decrease in abundance, a greater amount of Pah1p was associated with membranes when compared with phosphorylated enzyme, and the loss of phosphorylation allowed bypass of the Nem1p-Spo7p requirement for Pah1p function in the synthesis of triacylglycerol.

• PMID: 22334681
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Journal Article | Research Support, N.I.H., Extramural

Authors

Choi HS, Su WM, Han GS, Plote D, Xu Z, Carman GM

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