Phosphatidate phosphatase, a key regulator of lipid homeostasis

Abstract

Yeast Pah1p phosphatidate phosphatase (PAP) catalyzes the penultimate step in the synthesis of triacylglycerol. PAP plays a crucial role in lipid homeostasis by controlling the relative proportions of its substrate phosphatidate and its product diacylglycerol. The cellular amounts of these lipid intermediates influence the synthesis of triacylglycerol and the pathways by which membrane phospholipids are synthesized. Physiological functions affected by PAP activity include phospholipid synthesis gene expression, nuclear/endoplasmic reticulum membrane growth, lipid droplet formation, and vacuole homeostasis and fusion. Yeast lacking Pah1p PAP activity are acutely sensitive to fatty acid-induced toxicity and exhibit respiratory deficiency. PAP is distinguished in its cellular location, catalytic mechanism, and physiological functions from Dpp1p and Lpp1p lipid phosphate phosphatases that utilize a variety of substrates that include phosphatidate. Phosphorylation/dephosphorylation is a major mechanism by which Pah1p PAP activity is regulated. Pah1p is phosphorylated by cytosolic-associated Pho85p-Pho80p, Cdc28p-cyclin B, and protein kinase A and is dephosphorylated by the endoplasmic reticulum-associated Nem1p-Spo7p phosphatase. The dephosphorylation of Pah1p stimulates PAP activity and facilitates the association with the membrane/phosphatidate allowing for its reaction and triacylglycerol synthesis. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Fig. 1

Lipid synthesis in yeast. The pathways shown for the synthesis of TAG and phospholipids include the relevant steps discussed in this review. PAP (highlighted in blue) catalyzes the dephosphorylation of PA to form DAG in the penultimate step in TAG synthesis. The PAP substrate PA and product DAG are utilized for the synthesis of phospholipids via the CDP-DAG pathway and Kennedy pathway, respectively. Gro-3-P, glycerol-3-phosphate; LysoPA, lysophosphatidate; PA, phosphatidate; DAG, diacylglycerol; TAG, triacylglycerol; CDP-DAG, CDP-diacylglycerol; PI, phosphatidylinositol; PS, phosphatidylserine; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PG, phosphatidylglycerol; CL, cardiolipin.

Fig. 2

PAP and LPP enzymes are localized in different cellular compartments. The cartoon shows the integral membrane enzymes Dpp1p and Lpp1p in the vacuole and Golgi, respectively. Pah1p in the cytosol is phosphorylated on multiple sites (symbols decorating enzyme). At the ER membrane, Pah1p is dephosphorylated by the Nem1p-Spo7p phosphatase complex, which allows for its interaction with the membrane where its substrate PA resides. The dephosphorylated form of Pah1p catalyzes the dephosphorylation of PA to generate DAG for the synthesis of TAG that is stored in lipid droplets.

Fig. 3

Distinguishing characteristics of PAP and LPP enzymes. The basic characteristics of the yeast PAP (Pah1p) and LPP (Dpp1p and Lpp1p) enzymes, including their catalytic motifs and phosphorylation sites, are summarized in the figure. H, amphipathic helix; HAD, haloacid dehalogenase; , approximate positions of the sites (Ser¹¹⁰, Ser¹¹⁴, Ser¹⁶⁸, Ser⁶⁰², Thr⁷²³, Ser⁷⁴⁴, and Ser⁷⁴⁸) phosphorylated by Pho85p-Pho80p; ★, approximate positions of the sites (Ser⁶⁰², Thr⁷²³, and Ser⁷⁴⁴) phosphorylated by Cdc28p-cyclin B; , approximate positions of the sites (Ser¹⁰, Ser⁶⁷⁷, Ser⁷⁷³, Ser⁷⁷⁴, and Ser⁷⁸⁸) phosphorylated by protein kinase A. The diagrams of each protein are not drawn to scale.

Fig. 4

Summary of pah1Δ mutant phenotypes. The reactions catalyzed by Pah1p PAP and Dgk1p DAG kinase control the balance of PA and DAG. The pah1Δ phenotypes suppressed and not suppressed by the dgk1Δ mutation are listed in the figure. Phosphatidate Phosphatase, a Key Regulator of Lipid Homeostasis Florencia Pascual and George M. Carman*