Roles of phosphatidate phosphatase enzymes in lipid metabolism

Abstract

Phosphatidate phosphatase (PAP) enzymes catalyze the dephosphorylation of phosphatidate, yielding diacylglycerol and inorganic phosphate. In eukaryotic cells, PAP activity has a central role in the synthesis of phospholipids and triacylglycerol through its product diacylglycerol, and it also generates and/or degrades lipid-signaling molecules that are related to phosphatidate. There are two types of PAP enzyme, Mg(2+) dependent (PAP1) and Mg(2+) independent (PAP2), but only genes encoding PAP2 enzymes had been identified until recently, when a gene (PAH1) encoding a PAP1 enzyme was found in Saccharomyces cerevisiae. This discovery has revealed a molecular function of the mammalian protein lipin, a deficiency of which causes lipodystrophy in mice. With molecular information now available for both types of PAP, the specific roles of these enzymes in lipid metabolism are being clarified.

Figure 1

PAP catalyzes the dephosphorylation of PtdOH to yield DAG. The structures of the substrate and product are shown with fatty acyl groups containing 18 carbon atoms and one double bond.

Figure 2

PAP1 is important in the synthesis of lipids in yeast. The product DAG is used for the synthesis of PtdEtn and PtdCho and for the synthesis of TAG. The substrate PtdOH is used for the synthesis all phospholipids and PtdIns-derived sphingolipids through the intermediate CDP-DAG. The UAS_INO-containing genes that are subject to inositol-mediated regulation are indicated in red. Complete details of the lipid biosynthetic pathways in yeast can be found elsewhere [12,13]. PtdOH, phosphatidate; DAG, diacylglycerol; TAG, triacylglycerol; Glc6P, glucose-6-phosphate; Ins3P, inositol-3-phosphate; Ins, inositol; PtdIns, phosphatidylinositol; PtdInsPs, phosphatidylinositol phosphates; PtdGro-P, phosphatidylglycerophosphate; PtdGro, phosphatidylglycerol; PtdSer, phosphatidylserine; PtdEtn, phosphatidylethanolamine; PtdCho, phosphatidylcholine; Etn, ethanolamine; P-Etn, phosphoethanolamine; Cho, choline, P-Cho, phosphocholine.

Figure 3

PAP1 and PAP2 type enzymes have different catalytic motifs. (a) The reaction catalyzed by the PAP1 type enzyme uses a DxDxT motif present in a HAD-like domain. The evolutionarily conserved N-terminal region and HAD-like domain in yeast Pah1p and the human lipin proteins are indicated in yellow. Asterisk indicates the conserved aspartate residue responsible for phosphate binding in the phosphatase reaction. (b) The reaction catalyzed by the PAP2 type enzyme uses a three-domain lipid phosphatase motif. Asterisk indicates the amino acids that are conserved in each domain and required for phosphatase activity.

Figure 4

Model of the role of PAP1 in the transcriptional regulation of UAS_INO-containing genes by inositol. (a) UAS_INO-containing genes (e.g. INO1) are maximally expressed (thick arrow) when wild-type yeast is grown in the absence of inositol. Expression of INO1 is dependent on interaction of the Ino2p–Ino4p complex with the UAS_INO element in the gene promoter. Under this growth condition, the repressor Opi1p is associated with the nuclear/ER membrane through interactions with PtdOH and Scs2p (a vesicle-associated protein homolog). The PAP1 enzyme is phosphorylated and has reduced catalytic activity. (b) When inositol is added to the growth medium, transcription of INO1 is attenuated (thin arrow) by the interaction of Opi1p with Ino2p. Dissociation of Opi1p from the nuclear/ER membrane and its translocation into the nucleus are caused by a decrease in PtdOH concentration, which is mediated by the stimulation of PAP1 activity after dephosphorylation of the enzyme.