Phosphatidylserine decarboxylase from Escherichia coli

Maturation of the malarial phosphatidylserine decarboxylase is mediated by high affinity binding to anionic phospholipids

Decarboxylation of phosphatidylserine (PS) to form phosphatidylethanolamine by PS decarboxylases (PSDs) is an essential process in most eukaryotes. Processing of a malarial PSD proenzyme into its active alpha and beta subunits is by an autoendoproteolytic mechanism regulated by anionic phospholipids, with PS serving as an activator and phosphatidylglycerol (PG), phosphatidylinositol, and phosphatidic acid acting as inhibitors. The biophysical mechanism underlying this regulation remains unknown. We used solid phase lipid binding, liposome-binding assays, and surface plasmon resonance to examine the binding specificity of a processing-deficient Plasmodium PSD (PkPSDS308A) mutant enzyme and demonstrated that the PSD proenzyme binds strongly to PS and PG but not to phosphatidylethanolamine and phosphatidylcholine. The equilibrium dissociation constants (Kd) of PkPSD with PS and PG were 80.4 nM and 66.4 nM, respectively. The interaction of PSD with PS is inhibited by calcium, suggesting that the binding mechanism involves ionic interactions. In vitro processing of WT PkPSD proenzyme was also inhibited by calcium, consistent with the conclusion that PS binding to PkPSD through ionic interactions is required for the proenzyme processing. Peptide mapping identified polybasic amino acid motifs in the proenzyme responsible for binding to PS. Altogether, the data demonstrate that malarial PSD maturation is regulated through a strong physical association between PkPSD proenzyme and anionic lipids. Inhibition of the specific interaction between the proenzyme and the lipids can provide a novel mechanism to disrupt PSD enzyme activity, which has been suggested as a target for antimicrobials, and anticancer therapies.

Proteolytic Control of Lipid Metabolism

Synthesis and regulation of lipid levels and identities is critical for a wide variety of cellular functions, including structural and morphological properties of organelles, energy storage, signaling, and stability and function of membrane proteins. Proteolytic cleavage events regulate and/or influence some of these lipid metabolic processes and as a result help modulate their pleiotropic cellular functions. Proteins involved in lipid regulation are proteolytically cleaved for the purpose of their relocalization, processing, turnover, and quality control, among others. The scope of this review includes proteolytic events governing cellular lipid dynamics. After an initial discussion of the classic example of sterol regulatory element-binding proteins, our focus will shift to the mitochondrion, where a range of proteolytic events are critical for normal mitochondrial phospholipid metabolism and enforcing quality control therein. Recently, mitochondrial phospholipid metabolic pathways have been implicated as important for the proliferative capacity of cancers. Thus, the assorted proteases that regulate, monitor, or influence the activity of proteins that are important for phospholipid metabolism represent attractive targets to be manipulated for research purposes and clinical applications.

Synthetic Minimal Cell: Self-Reproduction of the Boundary Layer

A critical aspect in the bottom-up construction of a synthetic minimal cell is to develop an entity that is capable of self-reproduction. A key role in this process is the expansion and division of the boundary layer that surrounds the compartment, a process in which content loss has to be avoided and the barrier function maintained. Here, we describe the latest developments regarding self-reproduction of a boundary layer with a focus on the growth and division of phospholipid-based membranes in the context of a synthetic minimal cell.

Biosynthesis of Membrane Lipids

The pathways in Escherichia coli and (largely by analogy) S. enterica remain the paradigm of bacterial lipid synthetic pathways, although recently considerable diversity among bacteria in the specific areas of lipid synthesis has been demonstrated. The structural biology of the fatty acid synthetic proteins is essentially complete. However, the membrane-bound enzymes of phospholipid synthesis remain recalcitrant to structural analyses. Recent advances in genetic technology have allowed the essentialgenes of lipid synthesis to be tested with rigor, and as expected most genes are essential under standard growth conditions. Conditionally lethal mutants are available in numerous genes, which facilitates physiological analyses. The array of genetic constructs facilitates analysis of the functions of genes from other organisms. Advances in mass spectroscopy have allowed very accurate and detailed analyses of lipid compositions as well as detection of the interactions of lipid biosynthetic proteins with one another and with proteins outside the lipid pathway. The combination of these advances has resulted in use of E. coli and S. enterica for discovery of new antimicrobials targeted to lipid synthesis and in deciphering the molecular actions of known antimicrobials. Finally,roles for bacterial fatty acids other than as membrane lipid structural components have been uncovered. For example, fatty acid synthesis plays major roles in the synthesis of the essential enzyme cofactors, biotin and lipoic acid. Although other roles for bacterial fatty acids, such as synthesis of acyl-homoserine quorum-sensing molecules, are not native to E. coli introduction of the relevant gene(s) synthesis of these foreign molecules readily proceeds and the sophisticated tools available can used to decipher the mechanisms of synthesis of these molecules.

From Protease to Decarboxylase: THE MOLECULAR METAMORPHOSIS OF PHOSPHATIDYLSERINE DECARBOXYLASE

Phosphatidylserine decarboxylase (PSDs) play a central role in the synthesis of phosphatidylethanolamine in numerous species of prokaryotes and eukaryotes. PSDs are unusual decarboxylase containing a pyruvoyl prosthetic group within the active site. The covalently attached pyruvoyl moiety is formed in a concerted reaction when the PSD proenzyme undergoes an endoproteolytic cleavage into a large β-subunit, and a smaller α-subunit, which harbors the prosthetic group at its N terminus. The mechanism of PSD proenzyme cleavage has long been unclear. Using a coupled in vitro transcription/translation system with the soluble Plasmodium knowlesi enzyme (PkPSD), we demonstrate that the post-translational processing is inhibited by the serine protease inhibitor, phenylmethylsulfonyl fluoride. Comparison of PSD sequences across multiple phyla reveals a uniquely conserved aspartic acid within an FFXRX6RX12PXD motif, two uniquely conserved histidine residues within a PXXYHXXHXP motif, and a uniquely conserved serine residue within a GS(S/T) motif, suggesting that PSDs belong to the D-H-S serine protease family. The function of the conserved D-H-S residues was probed using site-directed mutagenesis of PkPSD. The results from these mutagenesis experiments reveal that Asp-139, His-198, and Ser-308 are all essential for endoproteolytic processing of PkPSD, which occurs in cis. In addition, within the GS(S/T) motif found in all PSDs, the Gly-307 residue is also essential, but the Ser/Thr-309 is non-essential. These results define the mechanism whereby PSDs begin their biochemical existence as proteases that execute one autoendoproteolytic cleavage reaction to give rise to a mature PSD harboring a pyruvoyl prosthetic group.

Lipids of mitochondria

A unique organelle for studying membrane biochemistry is the mitochondrion whose functionality depends on a coordinated supply of proteins and lipids. Mitochondria are capable of synthesizing several lipids autonomously such as phosphatidylglycerol, cardiolipin and in part phosphatidylethanolamine, phosphatidic acid and CDP-diacylglycerol. Other mitochondrial membrane lipids such as phosphatidylcholine, phosphatidylserine, phosphatidylinositol, sterols and sphingolipids have to be imported. The mitochondrial lipid composition, the biosynthesis and the import of mitochondrial lipids as well as the regulation of these processes will be main issues of this review article. Furthermore, interactions of lipids and mitochondrial proteins which are highly important for various mitochondrial processes will be discussed. Malfunction or loss of enzymes involved in mitochondrial phospholipid biosynthesis lead to dysfunction of cell respiration, affect the assembly and stability of the mitochondrial protein import machinery and cause abnormal mitochondrial morphology or even lethality. Molecular aspects of these processes as well as diseases related to defects in the formation of mitochondrial membranes will be described.

Processing and topology of the yeast mitochondrial phosphatidylserine decarboxylase 1

The inner mitochondrial membrane plays a crucial role in cellular lipid homeostasis through biosynthesis of the non-bilayer-forming lipids phosphatidylethanolamine and cardiolipin. In the yeast Saccharomyces cerevisiae, the majority of cellular phosphatidylethanolamine is synthesized by the mitochondrial phosphatidylserine decarboxylase 1 (Psd1). The biogenesis of Psd1 involves several processing steps. It was speculated that the Psd1 precursor is sorted into the inner membrane and is subsequently released into the intermembrane space by proteolytic removal of a hydrophobic sorting signal. However, components involved in the maturation of the Psd1 precursor have not been identified. We show that processing of Psd1 involves the action of the mitochondrial processing peptidase and Oct1 and an autocatalytic cleavage at a highly conserved LGST motif yielding the α- and β-subunit of the enzyme. The Psd1 β-subunit (Psd1β) forms the membrane anchor, which binds the intermembrane space-localized α-subunit (Psd1α). Deletion of a transmembrane segment in the β-subunit results in mislocalization of Psd1 and reduced enzymatic activity. Surprisingly, autocatalytic cleavage does not depend on proper localization to the inner mitochondrial membrane. In summary, membrane integration of Psd1 is crucial for its functionality and for maintenance of mitochondrial lipid homeostasis.

Identification of gene encoding Plasmodium knowlesi phosphatidylserine decarboxylase by genetic complementation in yeast and characterization of in vitro maturation of encoded enzyme

The 23-megabase genome of Plasmodium falciparum, the causative agent of severe human malaria, contains ∼5300 genes, most of unknown function or lacking homologs in other organisms. Identification of these gene functions will help in the discovery of novel targets for the development of antimalarial drugs and vaccines. The P. falciparum genome is unusually A+T-rich, which hampers cloning and expressing these genes in heterologous systems for functional analysis. The large repertoire of genetic tools available for Saccharomyces cerevisiae makes this yeast an ideal system for large scale functional complementation analyses of parasite genes. Here, we report the construction of a cDNA library from P. knowlesi, which has a lower A+T content compared with P. falciparum. This library was applied in a yeast complementation assay to identify malaria genes involved in the decarboxylation of phosphatidylserine. Transformation of a psd1Δpsd2Δdpl1Δ yeast strain, defective in phosphatidylethanolamine synthesis, with the P. knowlesi library led to identification of a new parasite phosphatidylserine decarboxylase (PkPSD). Unlike phosphatidylserine decarboxylase enzymes from other eukaryotes that are tightly associated with membranes, the PkPSD enzyme expressed in yeast was equally distributed between membrane and soluble fractions. In vitro studies reveal that truncated forms of PkPSD are soluble and undergo auto-endoproteolytic maturation in a phosphatidylserine-dependent reaction that is inhibited by other anionic phospholipids. This study defines a new system for probing the function of Plasmodium genes by library-based genetic complementation and its usefulness in revealing new biochemical properties of encoded proteins.

Phosphatidylserine decarboxylases, key enzymes of lipid metabolism

Phosphatidylserine decarboxylases (PSDs) (E.C. 4.1.1.65) are enzymes which catalyze the formation of phosphatidylethanolamine (PtdEtn) by decarboxylation of phosphatidylserine (PtdSer). This enzymatic activity has been identified in both prokaryotic and eukaryotic organisms. PSDs occur as two types of proteins depending on their localization and the sequence of a conserved motif. Type I PSDs include enzymes of eukaryotic mitochondria and bacterial origin which contain the amino acid sequence LGST as a characteristic motif. Type II PSDs are found in the endomembrane system of eukaryotes and contain a typical GGST motif. These characteristic motifs are considered as autocatalytic cleavage sites where proenzymes are split into alpha- and beta-subunits. The S-residue set free by this cleavage serves as an attachment site of a pyruvoyl group which is required for the activity of the enzymes. Moreover, PSDs harbor characteristic binding sites for the substrate PtdSer. Substrate supply to eukaryotic PSDs requires lipid transport because PtdSer synthesis and decarboxylation are spatially separated. Targeting of PSDs to their proper locations requires additional intramolecular domains. Mitochondrially localized type I PSDs are directed to the inner mitochondrial membrane by N-terminal targeting sequences. Type II PSDs also contain sequences in their N-terminal extensions which might be required for subcellular targeting. Lack of PSDs causes various defects in different cell types. The physiological relevance of these findings and the central role of PSDs in lipid metabolism will be discussed in this review.

Uptake and utilization of lyso-phosphatidylethanolamine by Saccharomyces cerevisiae

Phosphatidylethanolamine (PtdEtn) is synthesized by multiple pathways located in different subcellular compartments in yeast. Strains defective in the synthesis of PtdEtn via phosphatidylserine (PtdSer) synthase/decarboxylase are auxotrophic for ethanolamine, which must be transported into the cell and converted to phospholipid by the cytidinediphosphate-ethanolamine-dependent Kennedy pathway. We now demonstrate that yeast strains with psd1Delta psd2Delta mutations, devoid of PtdSer decarboxylases, import and acylate exogenous 1-acyl-2-hydroxyl-sn-glycero-3-phosphoethanolamine (lyso-PtdEtn). Lyso-PtdEtn supports growth and replaces the mitochondrial pool of PtdEtn much more efficiently than and independently of PtdEtn derived from the Kennedy pathway. Deletion of both the PtdSer decarboxylase and Kennedy pathways yields a strain that is a stringent lyso-PtdEtn auxotroph. Evidence for the specific uptake of lyso-PtdEtn by yeast comes from analysis of strains harboring deletions of the aminophospholipid translocating P-type ATPases (APLTs). Elimination of the APLTs, Dnf1p and Dnf2p, or their noncatalytic beta-subunit, Lem3p, blocked the import of radiolabeled lyso-PtdEtn and resulted in growth inhibition of lyso-PtdEtn auxotrophs. In cell extracts, lyso-PtdEtn is rapidly converted to PtdEtn by an acyl-CoA-dependent acyltransferase. These results now provide 1) an assay for APLT function based on an auxotrophic phenotype, 2) direct demonstration of APLT action on a physiologically relevant substrate, and 3) a genetic screen aimed at finding additional components that mediate the internalization, trafficking, and acylation of exogenous lyso-phospholipids.

Characterization of a non-mitochondrial type I phosphatidylserine decarboxylase in Plasmodium falciparum

In search of key enzymes in Plasmodium phospholipid metabolism, we demonstrate the presence of a parasite-encoded phosphatidylserine decarboxylase (PSD) in the membrane fraction of Plasmodium falciparum-infected erythrocytes. PSD cDNA, encoding phosphatidylserine decarboxylase (PfPSD), was cloned by screening a directional cDNA library derived from the trophozoite erythrocytic stage. The corresponding PfPSD gene is located on chromosome 9 of P. falciparum, contains one intron of 938 nucleotides and is transcribed into a 3.7 kb mRNA. PfPSD cDNA encodes a putative protein of 362 amino acids, with a predicted molecular mass of 42.6 kDa, which clearly belongs to the type I PSD family. Only a 35 kDa polypeptide was detected in the parasite using a specific rabbit antiserum. PfPSD has a 314VGSS317 sequence near its carboxyl-terminus that is related to the Escherichia coli, yeast and human LGST motif, which is the site of proenzyme processing. PSD enzyme was expressed in E. coli with a KM of 63 +/- 19 microM and a VMAX of 680 +/- 49 nmol of phosphatidylethanolamine formed h-1 mg-1 protein. Site-directed mutagenesis of the VGSS active site demonstrated that the PfPSD proenzyme was processed into two non-identical subunits (alpha and beta) and revealed the crucial role played by each residue in enzyme processing and activity. Using indirect immunofluorescence, PfPSD labelling was co-localized with an endoplasmic reticulum marker, but not with a mitochondrial vital dye. This P. falciparum PSD is the first type I PSD identified in the endoplasmic reticulum compartment.

Phospholipid-assisted refolding of an integral membrane protein. Minimum structural features for phosphatidylethanolamine to act as a molecular chaperone

Escherichia coli-derived phosphatidylethanolamine (PE) or PE with fully saturated fatty acids was able to correct in vitro a defect in folding in the lipid-dependent epitope 4B1 of lactose permease (LacY) resulting from in vivo assembly in the absence of PE. PE plasmalogen, PE with two unsaturated fatty acids, and lyso-PE, which all do not favor bilayer organization, did not support proper refolding. Proper refolding occurred when these latter lipids were mixed with a bilayer-forming lipid (phosphatidylglycerol), which alone could not support refolding. L-Phosphatidylserine (PS; natural diastereomer) did support proper refolding. PE derivatives of increasing degrees of methylation were progressively less effective in supporting refolding, with phosphatidylcholine being completely ineffective. Therefore, the properties of nonmethylated aminophospholipids capable of organization into a bilayer configuration are essential for the recovery of the native state of epitope 4B1 after misassembly in vivo in the absence of PE. Neither D-PS (sn-glycero-1-phosphate backbone) nor P-D-S (D-serine in the head group) is competent in supporting proper refolding unless used in binary mixtures with phosphatidylglycerol. The detailed characterization of phospholipid-assisted refolding reported here further supports a specific rather than nonspecific role for PE in structural maturation of lactose permease in vivo (Bogdanov, M., and Dowhan, W. (1998) EMBO J. 17, 5255-5264).

Rapid transmembrane movement of newly synthesized phosphatidylethanolamine across the inner membrane of Escherichia coli

For the first time the transmembrane movement of an endogenously synthesized phospholipid across the inner membrane of E. coli is reported. [14C]phosphatidylethanolamine (PE) was biosynthetically introduced into inner membrane vesicles from the PE-deficient strain AD93, by reconstitution with the enzyme phosphatidylserine (PS) synthetase. Upon addition of wild type cell lysate containing PS synthetase, and the metabolic substrates CTP and [14C]serine to inside-out vesicles from AD93, [14C]PS was synthesized, which was for the most part converted into [14C]PE. [14C]PE was introduced in right-side out vesicles by enclosing PS synthetase and CTP in the vesicle lumen and adding [14C]serine. The newly synthesized [14C]PE immediately equilibrated over both membrane leaflets (t1/2 less than one min), as determined by its accessibility toward the amino-reactive chemical fluorescamine. In both inside- out and right-side out vesicles, a 35-65% distribution was found of the newly synthesized PE over the cytoplasmic and periplasmic leaflet, respectively. The transport process of PE was not influenced by the presence of ATP or the proton motive force in inside out vesicles. Pretreatment of both types of vesicles with sulfhydryl reagents, or of right-side out vesicles with proteinase K, did not affect the rate and extent of the transmembrane distribution of the newly synthesized PE.

Phosphatidylserine decarboxylase

Phosphatidylserine decarboxylase (PSD) is an important enzyme in the synthesis of phosphatidylethanolamine in both prokaryotes and eukaryotes. The cloned bacterial gene encodes an integral membrane protein that is first made as a proenzyme, and subsequently proteolyzed to an alpha subunit, containing a pyruvoyl prosthetic group, and a beta subunit. Two types of decarboxylases are found in yeast, PSD1 and PSD2, that localize to the inner mitochondrial membrane and the Golgi/vacuole membrane, respectively. The mammalian enzyme is also found in the inner mitochondrial membrane. The yeast genes and mammalian cDNA have been cloned and sequenced. The yeast genes contain 5' sequences associated with regulation of expression by inositol and choline. The yeast PSD1 and the mammalian PSD both contain an LGST amino acid motif that identifies the site of proteolysis and pyruvoyl prosthetic group attachment in the bacterial enzyme. The yeast PSD1 and mammalian PSD also have mitochondrial targeting and inner membrane sorting sequences. Processing intermediates have been defined in the mammalian enzyme that correspond to the sequential removal of the mitochondrial targeting and inner membrane sorting sequence, followed by formation of the alpha and beta subunits. In contrast, the PSD2 enzyme contains a putative Golgi localization/retention sequence and a C2 homology domain, in addition to predicted alpha and beta subunits. The transport requirements for substrate access to the PSD enzymes have provided important information about lipid trafficking, and the availability of yeast mutants is likely to provide important new genetic selections in the future.

Phospholipid metabolism of serine in Plasmodium-infected erythrocytes involves phosphatidylserine and direct serine decarboxylation

Erythrocytes infected with Plasmodium falciparum or Plasmodium knowlesi efficiently incorporated radioactive serine into phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn) and phosphatidylcholine (PtdCho). Serine was also metabolized into ethanolamine (Etn) and phosphorylethanolamine (P-Etn) via direct serine decarboxylation; this is a major phenomenon since together these metabolites represent 60% of total radioactive water-soluble metabolites. They were identified by reverse-phase HPLC and two TLC-type analyses and confirmed by alkaline phosphatase treatment, which depleted the radioactive P-Etn peak completely with a concomitant increase in that of Etn. In the presence of 5 microM labelled serine, radioactivity appeared in Etn and P-Etn after a 25 min lag period, and isotopic equilibrium was reached at 40 and 95 min respectively. There was a similar lag period for PtdEtn formation, which accumulated steadily for at least 180 min. Incorporation of serine into phospholipids and water-soluble metabolites increased in the presence of up to 500 microM external serine. An apparent plateau was then reached for all metabolites except intracellular serine and Etn. Exogenous Etn (at 20 microM) induced a concomitant dramatic decrease in serine incorporation into P-Etn and all phospholipids, but not into Etn. Increasing exogenous serine to 100 microM decreased the incorporation of radioactive Etn into PtdEtn by only 30%, and the PtdCho level was not affected. 2-Hydroxyethylhydrazine significantly decreased serine incorporation into P-Etn and PtdEtn, whereas Etn was accumulated. No concomitant inhibition of PtdSer or PtdCho labelling from serine occurred, even when PtdEtn formation was decreased by 95%. This indicates that the PtdEtn pool derived from direct serine decarboxylation differed from that derived from PtdSer decarboxylation, and the latter appeared to be preferentially used for PtdCho biosynthesis. Hydroxylamine also inhibited phosphorylation of serine-derived Etn but not that of exogenous Etn. The rate of PtdSer synthesis from 10 microM L-serine was 3.1+/-0.5 and 2.95+/-1.3 nmol/5 h per 10(10) infected cells, whereas L-serine decarboxylation accounted for 7.1+/-1.5 and 9.9+/-3 nmol/5 h per 10(10) infected cells for P. falciparum and P. knowlesi respectively (means+/-S.E.M.). The serine decarboxylating reaction was not detected in other higher eukaryotic cells such as mouse fibroblasts and human lymphocytes. Finally, these results also indicate compartmentalization of phospholipid metabolism in Plasmodium-infected erythrocytes.

Post-translational processing of the phosphatidylserine decarboxylase gene product in Chinese hamster ovary cells

We have isolated a full-length cDNA clone of the Chinese hamster ovary (CHO) pssC gene, which encodes mitochondrial phosphatidylserine decarboxylase. The cDNA clone is capable of increasing phosphatidylserine decarboxylase activity to 11-fold in CHO-K1 cells. The pssC gene product predicted from the cDNA sequence is composed of 409 amino acid residues. In an in vitro translation system coupled with in vitro transcription, the cDNA clone directs the formation of a protein with an apparent molecular mass of 46 kDa. In CHO-K1 cells, the cDNA clone leads to the production of two major peptides with apparent molecular masses of 38 and 34 kDa, as determined by Western blotting with an antibody raised against a recombinant pssC protein. When CHO-K1 cells transfected with the cDNA clone are labelled with [35S]methionine for a short period, proteins immunoprecipitated with the antibody lack radioactive 38 and 34 kDa peptides, but contain two radioactive peptides with apparent molecular masses of 46 and 42 kDa instead. The pssC gene product predicted from the cDNA sequence has, near its C-terminus, a unique Leu-Gly-Ser-Thr sequence which is known as a processing site for Escherichia coli phosphatidylserine decarboxylase. A mutant pssC cDNA clone, in which Ser378 in the conserved sequence is replaced by Ala, leads to overproduction of 46, 42 and 38 kDa peptides, but not a 34 kDa peptide. This mutant clone is incapable of increasing phosphatidylserine decarboxylase activity, in contrast to the wild-type clone. These results indicate that the processing at the Leu-Gly-Ser-Thr sequence is essential for formation of the active enzyme. Thus, the pssC gene product is converted into mature phosphatidylserine decarboxylase through multiple steps of post-translational processing.

A novel phosphatidylserine-binding peptide motif defined by an anti-idiotypic monoclonal antibody. Localization of phosphatidylserine-specific binding sites on protein kinase C and phosphatidylserine decarboxylase

A monoclonal anti-idiotypic antibody, Id8F7, previously shown to bind to a phosphatidylserine (PS)-specific binding site on protein kinase C (PKC) has been used to identify a 12-amino acid consensus sequence shared by PKC and phosphatidylserine decarboxylase (PSD). The 14-amino acid synthetic peptide derived from the corresponding region of PSD (amino acids 351-364 of the enzyme from Chinese hamster ovary cells) bound effectively and specifically to PS, and that derived from rat PKC gamma (amino acids 227-240) bound weakly but specifically to PS. Analysis of binding of Id8F7 to various synthetic peptides revealed that the consensus sequence motif, FXFXLKXXXKXR, is responsible for the interaction with both Id8F7 and PS. The results suggest that the conserved amino acid residues represent a basic structural motif for the specific interaction with PS, and the corresponding regions of PKC and PSD form the PS-specific binding sites of these enzymes.

Regulation of phospholipid biosynthesis in Saccharomyces cerevisiae by CTP

In the yeast Saccharomyces cerevisiae, the major membrane phospholipid phosphatidylcholine is synthesized by the CDP-diacylglycerol and CDP-choline pathways. We examined the regulation of phosphatidylcholine synthesis by CTP. The cellular concentration of CTP was elevated (2.4-fold) by overexpressing CTP synthetase, the enzyme responsible for the synthesis of CTP. The overexpression of CTP synthetase resulted in a 2-fold increase in the utilization of the CDP-choline pathway for phosphatidylcholine synthesis. The increase in CDP-choline pathway usage was not due to an increase in the expression of any of the enzymes in this pathway. CDP-choline, the product of the phosphocholine cytidylyltransferase reaction, was the limiting intermediate in the CDP-choline pathway. The apparent Km of CTP (1.4 mM) for phosphocholine cytidylyltransferase was 2-fold higher than the cellular concentration of CTP (0.7 mM) in control cells. This provided an explanation of why the overexpression of CTP synthetase caused an increase in the cellular concentration of CDP-choline. Phosphatidylserine synthase activity was reduced in cells overexpressing CTP synthetase. This was not due to a transcriptional repression mechanism. Instead, the decrease in phosphatidylserine synthase activity was due, at least in part, to a direct inhibition of activity by CTP. These results show that CTP plays a role in the regulation of the pathways by which phosphatidylcholine is synthesized. This regulation includes the supple of CTP for the phosphocholine cytidylyltransferase reaction in the CDP-choline pathway and the inhibition of the phosphatidylserine synthase reaction in the CDP-diacylglycerol pathway.