Phosphatidate phosphatase regulates membrane phospholipid synthesis via phosphatidylserine synthase

Abstract

The yeast Saccharomyces cerevisiae serves as a model eukaryote to elucidate the regulation of lipid metabolism. In exponentially growing yeast, a diverse set of membrane lipids are synthesized from the precursor phosphatidate via the liponucleotide intermediate CDP-diacylglycerol. As cells exhaust nutrients and progress into the stationary phase, phosphatidate is channeled via diacylglycerol to the synthesis of triacylglycerol. The CHO1-encoded phosphatidylserine synthase, which catalyzes the committed step in membrane phospholipid synthesis via CDP-diacylglycerol, and the PAH1-encoded phosphatidate phosphatase, which catalyzes the committed step in triacylglycerol synthesis are regulated throughout cell growth by genetic and biochemical mechanisms to control the balanced synthesis of membrane phospholipids and triacylglycerol. The loss of phosphatidate phosphatase activity (e.g., pah1Δ mutation) increases the level of phosphatidate and its conversion to membrane phospholipids by inducing Cho1 expression and phosphatidylserine synthase activity. The regulation of the CHO1 expression is mediated through the inositol-sensitive upstream activation sequence (UAS_INO), a cis-acting element for the phosphatidate-controlled Henry (Ino2-Ino4/Opi1) regulatory circuit. Consequently, phosphatidate phosphatase activity regulates phospholipid synthesis through the transcriptional regulation of the phosphatidylserine synthase enzyme.

Keywords: Diacylglycerol; Phosphatidic acid; Phosphatidylserine; Phospholipid; Triacylglycerol; Yeast.

Fig. 1

Reaction catalyzed by yeast PAP and the domain structure and phosphorylation sites in Pah1. A, The figure shows the structures of phosphatidate (PA) and diacylglycerol (DAG) and the reaction catalyzed by PAP. B, The diagram shows the positions of the amphipathic helix (AH, pink) required for endoplasmic reticulum membrane interaction (Karanasios et al., 2010), the NLIP (green) and HAD-like (yellow) domains that are required for PAP activity (Han et al., 2007), the acidic tail (AT) required for interaction with Nem1-Spo7 (Karanasios et al., 2013), and the serine (S) and threonine (T) residues that are phosphorylated by Pho85-Pho80 (Choi et al., 2012), Cdc28-cyclin B (Choi et al., 2011), protein kinase A (Su et al., 2012), protein kinase C (Su et al., 2014a), and casein kinase II (Hsieh et al., 2016).

Fig. 2

Reaction catalyzed by yeast PSS and the domain structure and phosphorylation sites in Cho1. A, The figure shows the structures of CDP-diacylglycerol (CDP-DAG) and phosphatidylserine (PS) and the reaction catalyzed by PSS. B, The diagram shows the positions of the CDP-alcohol phosphotransferase motif (yellow) and the serine (S) residues that are phosphorylated by protein kinase A (Choi et al., 2010).

Fig. 3. Lipid synthesis in yeast

The pathways shown for the synthesis of lipids include the relevant steps discussed in this review. A more comprehensive figure for the synthesis of triacylglycerol and membrane phospholipids via the CDP-diacylglycerol and Kennedy pathways may be found in reference (Henry et al., 2012). The CDP-diacylglycerol pathway of phospholipid synthesis is highlighted in pink, whereas the Kennedy pathway is shown in grey to indicate its minor role in phospholipid synthesis in cells grown without choline (Cho) or ethanolamine (Etn). The reactions catalyzed by the CHO1-encoded PSS, PAH1-encoded PAP, and DGK1-encoded diacylglycerol kinase are indicated. Abbreviations: PA, phosphatidate; DAG, diacylglycerol; TAG, triacylglycerol; CDP-DAG, CDP-diacylglycerol; PS, phosphatidylserine; PE, phosphatidylethanolamine; PC, phosphatidylcholine; PI, phosphatidylinositol.

Fig. 4

Model for the PAP-mediated regulation of PSS expression and lipid synthesis during growth. The diagram shows the Henry regulatory circuit that includes the repressor Opi1, the Ino2-Ino4 activator complex, and the UAS_INO element in CHO1 (Henry et al., 2012), and the bifurcation of phosphatidate for the synthesis of triacylglycerol and phospholipids via the CDP-diacylglycerol pathway. Under growth conditions (e.g., exponential phase, left) whereby Pah1 expression (green highlight) and PAP activity are low (Pascual et al., 2013), the level of phosphatidate is elevated and the Opi1 repressor is tethered to the nuclear/endoplasmic reticulum membrane via its interactions with phosphatidate and Scs2. This allows for the transcriptional activation (bold arrow) of CHO1 by the Ino2-Ino4 complex and the inductions of Cho1 (pink highlight) and PSS activity for increased phospholipid synthesis (large letters) via the CDP-diacylglycerol pathway. The reduced rate of triacylglycerol synthesis in the exponential phase of growth (Pascual et al., 2013) is indicated with small grey letters. Under growth conditions (e.g., stationary phase, right) whereby Pah1 expression (pink highlight) and PAP activity are high (Pascual et al., 2013), the synthesis of triacylglycerol is elevated (large letters) and the phosphatidate level is reduced (small grey letters). This allows for the dissociation of Opi1 from the nuclear/endoplasmic reticulum membrane and its entry into the nucleus where it represses the transcriptional activation of CHO1 (thin grey arrow and letters) by inhibiting the function of the Ino2-Ino4 activator complex through its binding to Ino2. The repression of Cho1 (green highlight) and PSS activity results in the reduction of phospholipid synthesis (small grey letters). Abbreviations: PA, phosphatidate; DAG, diacylglycerol; TAG, triacylglycerol; CDP-DAG, CDP-diacylglycerol; PS, phosphatidylserine; PC, phosphatidylcholine; PI, phosphatidylinositol. The figure was taken from Han and Carman (2017).

Fig. 4

Model for the PAP-mediated regulation of PSS expression and lipid synthesis during growth. The diagram shows the Henry regulatory circuit that includes the repressor Opi1, the Ino2-Ino4 activator complex, and the UAS_INO element in CHO1 (Henry et al., 2012), and the bifurcation of phosphatidate for the synthesis of triacylglycerol and phospholipids via the CDP-diacylglycerol pathway. Under growth conditions (e.g., exponential phase, left) whereby Pah1 expression (green highlight) and PAP activity are low (Pascual et al., 2013), the level of phosphatidate is elevated and the Opi1 repressor is tethered to the nuclear/endoplasmic reticulum membrane via its interactions with phosphatidate and Scs2. This allows for the transcriptional activation (bold arrow) of CHO1 by the Ino2-Ino4 complex and the inductions of Cho1 (pink highlight) and PSS activity for increased phospholipid synthesis (large letters) via the CDP-diacylglycerol pathway. The reduced rate of triacylglycerol synthesis in the exponential phase of growth (Pascual et al., 2013) is indicated with small grey letters. Under growth conditions (e.g., stationary phase, right) whereby Pah1 expression (pink highlight) and PAP activity are high (Pascual et al., 2013), the synthesis of triacylglycerol is elevated (large letters) and the phosphatidate level is reduced (small grey letters). This allows for the dissociation of Opi1 from the nuclear/endoplasmic reticulum membrane and its entry into the nucleus where it represses the transcriptional activation of CHO1 (thin grey arrow and letters) by inhibiting the function of the Ino2-Ino4 activator complex through its binding to Ino2. The repression of Cho1 (green highlight) and PSS activity results in the reduction of phospholipid synthesis (small grey letters). Abbreviations: PA, phosphatidate; DAG, diacylglycerol; TAG, triacylglycerol; CDP-DAG, CDP-diacylglycerol; PS, phosphatidylserine; PC, phosphatidylcholine; PI, phosphatidylinositol. The figure was taken from Han and Carman (2017).