Roles of phosphatidate phosphatase enzymes in lipid metabolism

Lipid phosphate phosphatases from Saccharomyces cerevisiae

DPP1-encoded and LPP1-encoded lipid phosphate phosphatases are integral membrane proteins in the yeast Saccharomyces cerevisiae. They catalyze the Mg(2+)-independent dephosphorylation of bioactive lipid phosphate molecules such as diacylglycerol pyrophosphate and phosphatidate. These enzymes possess a three-domain lipid phosphatase motif that is localized to the hydrophilic surface of the membrane. The lipid phosphate phosphatase activities of DPP1-encoded and LPP1-encoded enzymes are measured by following the release of water-soluble radioactive inorganic phosphate from chloroform-soluble radioactive lipid phosphate substrate following a chloroform/methanol/water phase partition. The DPP1-encoded enzyme, commonly referred to as diacylglycerol pyrophosphate phosphatase, is purified from wild-type S. cerevisiae membranes by detergent solubilization with Triton X-100 followed by chromatography with DEAE-cellulose (DE53), Affi-Gel blue, hydroxylapatite, and Mono Q. The purification scheme yields an essentially homogeneous enzyme preparation that is stable for several years upon storage at -80 degrees . The properties of the DPP1-encoded and LPP1-encoded lipid phosphate phosphatase enzymes are summarized.

The lipin protein family: dual roles in lipid biosynthesis and gene expression

The prevalence of obesity in the western world has focused attention on factors that influence triglyceride biosynthesis, storage, and utilization. Members of the lipin protein family have a newly discovered enzymatic role in triglyceride and phospholipid biosynthesis as a phosphatidate phosphatase, and also act as an inducible transcriptional coactivator in conjunction with peroxisome proliferator-activated receptor gamma (PPAR gamma) coactivator-1 alpha and PPAR alpha. Through these activities, the founding member of the family, lipin-1, influences lipid metabolism and glucose homeostasis in diverse tissues including adipose tissue, skeletal muscle, and liver. The physiological roles of lipin-2 and lipin-3 are less well defined, but are likely to carry out similar functions in glycerolipid biosynthesis and gene expression in a distinct tissue distribution.

The cellular functions of the yeast lipin homolog PAH1p are dependent on its phosphatidate phosphatase activity

The Saccharomyces cerevisiae PAH1-encoded Mg2+-dependent phosphatidate phosphatase (PAP1, 3-sn-phosphatidate phosphohydrolase, EC 3.1.3.4) catalyzes the dephosphorylation of phosphatidate to yield diacylglycerol and Pi. This enzyme plays a major role in the synthesis of triacylglycerols and phospholipids in S. cerevisiae. PAP1 contains the DXDX(T/V) catalytic motif (DIDGT at residues 398-402) that is shared by the mammalian fat-regulating protein lipin 1 and the superfamily of haloacid dehalogenase-like proteins. The yeast enzyme also contains a conserved glycine residue (Gly80) that is essential for the fat-regulating function of lipin 1 in a mouse model. In this study, we examined the roles of the putative catalytic motif and the conserved glycine for PAP1 activity by a mutational analysis. The PAP1 activities of the D398E and D400E mutant enzymes were reduced by >99.9%, and the activity of the G80R mutant enzyme was reduced by 98%. The mutant PAH1 alleles whose products lacked PAP1 activity were nonfunctional in vivo and failed to complement the pah1Delta mutant phenotypes of temperature sensitivity, respiratory deficiency, nuclear/endoplasmic reticulum membrane expansion, derepression of INO1 expression, and alterations in lipid composition. These results demonstrated that the PAP1 activity of the PAH1 gene product is essential for its roles in lipid metabolism and cell physiology.

Respiratory deficiency mediates the regulation of CHO1-encoded phosphatidylserine synthase by mRNA stability in Saccharomyces cerevisiae

The CHO1-encoded phosphatidylserine synthase (CDP-diacylglycerol:l-serine O-phosphatidyltransferase, EC 2.7.8.8) is one of the most highly regulated phospholipid biosynthetic enzymes in the yeast Saccharomyces cerevisiae. CHO1 expression is regulated by nutrient availability through a regulatory circuit involving a UAS(INO) cis-acting element in the CHO1 promoter, the positive transcription factors Ino2p and Ino4p, and the transcriptional repressor Opi1p. In this work, we examined the post-transcriptional regulation of CHO1 by mRNA stability. CHO1 mRNA was stabilized in mutants defective in deadenylation (ccr4Delta), mRNA decapping (dcp1), and the 5'-3'-exonuclease (xrn1), indicating that the CHO1 transcript is primarily degraded through the general 5'-3' mRNA decay pathway. In respiratory-sufficient cells, the CHO1 transcript was moderately stable with a half-life of 12 min. However, the CHO1 transcript was stabilized to a half-life of >45 min in respiratory-deficient (rho(-) and rho(o)) cells, the cox4Delta mutant defective in the cytochrome c oxidase, and wild type cells treated with KCN (a cytochrome c oxidase inhibitor). The increased CHO1 mRNA stability in response to respiratory deficiency caused increases in CHO1 mRNA abundance, phosphatidylserine synthase protein and activity, and the synthesis of phosphatidylserine in vivo. Respiratory deficiency also caused increases in the activities of CDP-diacylglycerol synthase, phosphatidylserine decarboxylase, and the phospholipid methyltransferases. Phosphatidylinositol synthase and choline kinase activities were not affected by respiratory deficiency. This work advances our understanding of phosphatidylserine synthase regulation and underscores the importance of mitochondrial respiration to the regulation of phospholipid synthesis in S. cerevisiae.

Plastidic phosphatidic acid phosphatases identified in a distinct subfamily of lipid phosphate phosphatases with prokaryotic origin

Plastidic phosphatidic acid phosphatase (PAP) dephosphorylates phosphatidic acid to yield diacylglycerol, which is a precursor for galactolipids, a primary and indispensable component of photosynthetic membranes. Despite its functional importance, the molecular characteristics and phylogenetic origin of plastidic PAP were unknown because no potential homologs have been found. Here, we report the isolation and characterization of plastidic PAPs in Arabidopsis that belong to a distinct lipid phosphate phosphatase (LPP) subfamily with prokaryotic origin. Because no homolog of mammalian LPP was found in cyanobacteria, we sought an LPP ortholog in a more primitive organism, Chlorobium tepidum, and its homologs in cyanobacteria. Arabidopsis had five homologs of cyanobacterial LPP, three of which (LPP gamma, LPP epsilon 1, and LPP epsilon 2) localized to chloroplasts. Complementation of yeast Delta dpp1 Delta lpp1 Delta pah1 by plastidic LPPs rescued the relevant phenotype in vitro and in vivo, suggesting that they function as PAPs. Of the three LPPs, LPP gamma activity best resembled the native activity. The three plastidic LPPs were differentially expressed both in green and nongreen tissues, with LPP gamma expressed the highest in shoots. A knock-out mutant for LPP gamma could not be obtained, although a lpp epsilon 1 lpp epsilon 2 double knock-out showed no significant changes in lipid composition. However, lpp gamma homozygous mutant was isolated only under ectopic overexpression of LPP gamma, suggesting that loss of LPP gamma may cause lethal effect on plant viability. Thus, in Arabidopsis, there are three isoforms of plastidic PAP that belong to a distinct subfamily of LPP, and LPP gamma may be the primary plastidic PAP.

A signal from inside the peroxisome initiates its division by promoting the remodeling of the peroxisomal membrane

We define the dynamics of spatial and temporal reorganization of the team of proteins and lipids serving peroxisome division. The peroxisome becomes competent for division only after it acquires the complete set of matrix proteins involved in lipid metabolism. Overloading the peroxisome with matrix proteins promotes the relocation of acyl-CoA oxidase (Aox), an enzyme of fatty acid beta-oxidation, from the matrix to the membrane. The binding of Aox to Pex16p, a membrane-associated peroxin required for peroxisome biogenesis, initiates the biosynthesis of phosphatidic acid and diacylglycerol (DAG) in the membrane. The formation of these two lipids and the subsequent transbilayer movement of DAG initiate the assembly of a complex between the peroxins Pex10p and Pex19p, the dynamin-like GTPase Vps1p, and several actin cytoskeletal proteins on the peroxisomal surface. This protein team promotes membrane fission, thereby executing the terminal step of peroxisome division.

A conserved phosphatase cascade that regulates nuclear membrane biogenesis

A newly emerging family of phosphatases that are members of the haloacid dehalogenase superfamily contains the catalytic motif DXDX(T/V). A member of this DXDX(T/V) phosphatase family known as Dullard was recently shown to be a potential regulator of neural tube development in Xenopus [Satow R, Chan TC, Asashima M (2002) Biochem Biophys Res Commun 295:85-91]. Herein, we demonstrate that human Dullard and the yeast protein Nem1p perform similar functions in mammalian cells and yeast cells, respectively. In addition to similarity in primary sequence, Dullard and Nem1p possess similar domains and show similar substrate preferences, and both localize to the nuclear envelope. Additionally, we show that human Dullard can rescue the aberrant nuclear envelope morphology of nem1Delta yeast cells, functionally replacing Nem1p. Finally, Nem1p, has been shown to deposphorylate the yeast phosphatidic acid phosphatase Smp2p [Santos-Rosa H, Leung J, Grimsey N, Peak-Chew S, Siniossoglou S (2005) EMBO J 24:1931-1941], and we show that Dullard dephosphorylates the mammalian phospatidic acid phosphatase, lipin. Therefore, we propose that Dullard participates in a unique phosphatase cascade regulating nuclear membrane biogenesis, and that this cascade is conserved from yeast to mammals.

Triacylglycerol metabolism in adipose tissue

Triacylglycerol (TAG) in adipose tissue serves as the major energy storage form in higher eukaryotes. Obesity, resulting from excess white adipose tissue, has increased dramatically in recent years resulting in a serious public health problem. Understanding of adipocyte-specific TAG synthesis and hydrolysis is critical to the development of strategies to treat and prevent obesity and its closely associated diseases, for example, Type 2 diabetes, hypertension and atherosclerosis. In this review, we present an overview of the major enzymes in TAG synthesis and lipolysis, including the recent discovery of a novel adipocyte TAG hydrolase.