Acts as a magnesium-dependent phosphatidate phosphatase enzyme which catalyzes the conversion of phosphatidic acid to diacylglycerol during triglyceride, phosphatidylcholine and phosphatidylethanolamine biosynthesis in the reticulum endoplasmic membrane (PubMed:17158099). Plays important roles in controlling the metabolism of fatty acids at different levels. Acts also as a nuclear transcriptional coactivator for PPARGC1A to modulate lipid metabolism. a 1,2-diacyl-sn-glycero-3-phosphate + H2O = a 1,2-diacyl-sn-glycerol + phosphate Inhibited by N-ethylmaleimide. Translocates from cytosol to endoplasmic reticulum membrane with increasing levels of oleate. Expressed at high level in liver and to some extend in lung, kidney, placenta, spleen, thymus, lymph node, prostate, testes, small intestine, and colon. Expressed also in circulating red blood cells and site of lymphopoiesis. By fasting in hepatocytes. Up-regulated in fld/fld (defect in LPIN1) mice. Up-regulated at protein level but not at transcript level in ob/ob and db/db mice, two obese mice models. Contains 1 Asp-Xaa-Asp-Xaa-Thr (DXDXT) motif, a catalytic motif known to be essential for phosphatidate phosphatase activity. Contains one Leu-Xaa-Xaa-Ile-Leu (LXXIL) motif, a motif known to be a transcriptional binding motif. Belongs to the lipin family.