Molecular dissection of the S-adenosylmethionine-binding site of phosphatidylethanolamine N-methyltransferase

A Narrative Review on Maternal Choline Intake and Liver Function of the Fetus and the Infant; Implications for Research, Policy, and Practice

Dietary choline is needed to maintain normal health, including normal liver function in adults. Fatty liver induced by a choline-deficient diet has been consistently observed in human and animal studies. The effect of insufficient choline intake on hepatic fat accumulation is specific and reversible when choline is added to the diet. Choline requirements are higher in women during pregnancy and lactation than in young non-pregnant women. We reviewed the evidence on whether choline derived from the maternal diet is necessary for maintaining normal liver function in the fetus and breastfed infants. Studies have shown that choline from the maternal diet is actively transferred to the placenta, fetal liver, and human milk. This maternal-to-child gradient can cause depletion of maternal choline stores and increase the susceptibility of the mother to fatty liver. Removing choline from the diet of pregnant rats causes fatty liver both in the mother and the fetus. The severity of fatty liver in the offspring was found to correspond to the severity of fatty liver in the respective mothers and to the duration of feeding the choline-deficient diet to the mother. The contribution of maternal choline intake in normal liver function of the offspring can be explained by the role of phosphatidylcholine in lipid transport and as a component of cell membranes and the function of choline as a methyl donor that enables synthesis of phosphatidylcholine in the liver. Additional evidence is needed on the effect of choline intake during pregnancy and lactation on health outcomes in the fetus and infant. Most pregnant and lactating women are currently not achieving the adequate intake level of choline through the diet. Therefore, public health policies are needed to ensure sufficient choline intake through adding choline to maternal multivitamin supplements.

Induction and Aggravation of the Endoplasmic-Reticulum Stress by Membrane-Lipid Metabolic Intermediate Phosphatidyl-N-Monomethylethanolamine

Phosphatidylcholine (PC) is produced via two distinct pathways in both hepatocytes and yeast, Saccharomyces cerevisiae. One of these pathways involves the sequential methylation of phosphatidylethanolamine (PE). In yeast cells, the methyltransferase, Cho2, converts PE to phosphatidylmonomethylethanolamine (PMME), which is further modified to PC by another methyltransferase, Opi3. On the other hand, free choline is utilized for PC production via the Kennedy pathway. The blockage of PC production is well known to cause endoplasmic reticulum (ER) stress and activate the ER-stress sensor, Ire1, to induce unfolded protein response (UPR). Here, we demonstrate that even when free choline is sufficiently supplied, the opi3Δ mutation, but not the cho2 Δ mutation, induces the UPR. The UPR was also found to be induced by CHO2 overexpression. Further, monomethylethanolamine, which is converted to PMME probably through the Kennedy pathway, caused or potentiated ER stress in both mammalian and yeast cells. We thus deduce that PMME per se is an ER-stressing molecule. Interestingly, spontaneously accumulated PMME seemed to aggravate ER stress in yeast cells. Collectively, our findings demonstrate the multiple detrimental effects of the low-abundance phospholipid species, PMME.

The topology of the ER-resident phospholipid methyltransferase Opi3 of Saccharomyces cerevisiae is consistent with in trans catalysis

Phospholipid N-methyltransferases (PLMTs) synthesize phosphatidylcholine by methylating phosphatidylethanolamine using S-adenosylmethionine as a methyl donor. Eukaryotic PLMTs are integral membrane enzymes located in the endoplasmic reticulum (ER). Recently Opi3, a PLMT of the yeast Saccharomyces cerevisiae was proposed to perform in trans catalysis, i.e. while localized in the ER, Opi3 would methylate lipid substrates located in the plasma membrane at membrane contact sites. Here, we tested whether the Opi3 active site is located at the cytosolic side of the ER membrane, which is a prerequisite for in trans catalysis. The membrane topology of Opi3 (and its human counterpart, phosphatidylethanolamine N-methyltransferase, expressed in yeast) was addressed by topology prediction algorithms and by the substituted cysteine accessibility method. The results of these analyses indicated that Opi3 (as well as phosphatidylethanolamine N-methyltransferase) has an N-out C-in topology and contains four transmembrane domains, with the fourth forming a re-entrant loop. On the basis of the sequence conservation between the C-terminal half of Opi3 and isoprenyl cysteine carboxyl methyltransferases with a solved crystal structure, we identified amino acids critical for Opi3 activity by site-directed mutagenesis. Modeling of the structure of the C-terminal part of Opi3 was consistent with the topology obtained by the substituted cysteine accessibility method and revealed that the active site faces the cytosol. In conclusion, the location of the Opi3 active site identified here is consistent with the proposed mechanism of in trans catalysis, as well as with conventional catalysis in cis.

Genome wide characterization of phospholipase A & C families and pattern of lysolipids and diacylglycerol changes under abiotic stresses in Brassica napus L

Plant phospholipase A (PLA) and C (PLC) families are least explored in terms of structure, diversity and their roles in membrane lipid remodeling under stress conditions. In this study, we performed gene family analysis, determined gene expression in different tissues and monitored transcriptional regulation of patatin-related PLA family and PLC family in oil crop Brassica napus under dehydration, salt, abscisic acid and cold stress. The identified 29 BnapPLA genes and 40 BnaPLC genes shared high similarities with Arabidopsis pPLAs and PLCs, respectively. This study highlighted the expression pattern of BnapPLAs and BnaPLCs in different tissues and their expression in response to abiotic stresses in Brassica napus. The results revealed that several members of BnapPLA3, PI-PLC1/2 and NPC1 were actively regulated by abiotic stresses. Lipid changes at different time points under stress conditions were also measured. Lipid profiling revealed that the level of lysophospholipids and diacylglycerol (DAG) showed a varied pattern of changes under different abiotic stress treatments. The change of lipids correlated with the transcriptional regulation of a few specific members of pPLA and PLC families. Our study suggested that A and C-type phospholipases in Brassica napus may have diverse physiological and regulatory roles in abiotic stress response and tolerance.

An engineered mutant of a host phospholipid synthesis gene inhibits viral replication without compromising host fitness

Viral infections universally rely on numerous hijacked host factors to be successful. It is therefore possible to control viral infections by manipulating host factors that are critical for viral replication. Given that host genes may play essential roles in certain cellular processes, any successful manipulations for virus control should cause no or mild effects on host fitness. We previously showed that a group of positive-strand RNA viruses enrich phosphatidylcholine (PC) at the sites of viral replication. Specifically, brome mosaic virus (BMV) replication protein 1a interacts with and recruits a PC synthesis enzyme, phosphatidylethanolamine methyltransferase, Cho2p, to the viral replication sites that are assembled on the perinuclear endoplasmic reticulum (ER) membrane. Deletion of the CHO2 gene inhibited BMV replication by 5-fold; however, it slowed down host cell growth as well. Here, we show that an engineered Cho2p mutant supports general PC synthesis and normal cell growth but blocks BMV replication. This mutant interacts and colocalizes with BMV 1a but prevents BMV 1a from localizing to the perinuclear ER membrane. The mislocalized BMV 1a fails to induce the formation of viral replication complexes. Our study demonstrates an effective antiviral strategy in which a host lipid synthesis gene is engineered to control viral replication without comprising host growth.

Identification and Characterization of Two New S-Adenosylmethionine-Dependent Methyltransferase Encoding Genes Suggested Their Involvement in Stipe Elongation of Flammulina velutipes

Two new SAM-dependent methyltransferase encoding genes (fvsmt1 and fvsmt2) were identified from the genome of Flammulina velutipes. In order to make a comprehensive characterization of both genes, we performed in silico analysis of both genes and used qRT-PCR to reveal their expression patterns during the development of F. velutipes. There are 4 and 6 exons with total length of 693 and 978 bp in fvsmt2 and fvsmt1, respectively. The deduced proteins, i.e., FVSMT1 and FVSMT2 contained 325 and 230 amino acids with molecular weight 36297 and 24894 Da, respectively. Both proteins contained a SAM-dependent catalytic domain with signature motifs (I, p-I, II, and III) defining the SAM fold. SAM-dependent catalytic domain is located either in the middle or at the N-terminal of FVSMT2 and FVSMT1, respectively. Alignment and phylogenic analysis showed that FVSMT1 is a homolog to a protein-arginine omega-N-methyltransferase, while FVSMT2 is of cinnamoyl CoA O-methyltransferase type and predicted subcellular locations of these proteins are mitochondria and cytoplasm, respectively. qRT-PCR showed that fvsmt1 and fvsmt2 expression was regulated in different developmental stages. The maximum expression levels of fvsmt1 and fvsmt2 were observed in stipe elongation, while no difference was found in mycelium and pileus. These results positively demonstrate that both the methyltransferase encoding genes are involved in the stipe elongation of F. velutipes.

Mammalian Sulfur Amino Acid Metabolism: A Nexus Between Redox Regulation, Nutrition, Epigenetics, and Detoxification

SIGNIFICANCE

Transsulfuration allows conversion of methionine into cysteine using homocysteine (Hcy) as an intermediate. This pathway produces S-adenosylmethionine (AdoMet), a key metabolite for cell function, and provides 50% of the cysteine needed for hepatic glutathione synthesis. The route requires the intake of essential nutrients (e.g., methionine and vitamins) and is regulated by their availability. Transsulfuration presents multiple interconnections with epigenetics, adenosine triphosphate (ATP), and glutathione synthesis, polyol and pentose phosphate pathways, and detoxification that rely mostly in the exchange of substrates or products. Major hepatic diseases, rare diseases, and sensorineural disorders, among others that concur with oxidative stress, present impaired transsulfuration. Recent Advances: In contrast to the classical view, a nuclear branch of the pathway, potentiated under oxidative stress, is emerging. Several transsulfuration proteins regulate gene expression, suggesting moonlighting activities. In addition, abnormalities in Hcy metabolism link nutrition and hearing loss.

CRITICAL ISSUES

Knowledge about the crossregulation between pathways is mostly limited to the hepatic availability/removal of substrates and inhibitors. However, advances regarding protein-protein interactions involving oncogenes, identification of several post-translational modifications (PTMs), and putative moonlighting activities expand the potential impact of transsulfuration beyond methylations and Hcy.

FUTURE DIRECTIONS

Increasing the knowledge on transsulfuration outside the liver, understanding the protein-protein interaction networks involving these enzymes, the functional role of their PTMs, or the mechanisms controlling their nucleocytoplasmic shuttling may provide further insights into the pathophysiological implications of this pathway, allowing design of new therapeutic interventions. Antioxid. Redox Signal. 29, 408-452.

Evolution of the Phosphatidylcholine Biosynthesis Pathways in Green Algae: Combinatorial Diversity of Methyltransferases

Phosphatidylcholine (PC) is one of the most common phospholipids in eukaryotes, although some green algae such as Chlamydomonas reinhardtii are known to lack PC. Recently, we detected PC in four species in the genus Chlamydomonas: C. applanata NIES-2202, C. asymmetrica NIES-2207, C. debaryana NIES-2212, and C. sphaeroides NIES-2242. To reveal the PC biosynthesis pathways in green algae and the evolutionary scenario involved in their diversity, we analyzed the PC biosynthesis genes in these four algae using draft genome sequences. Homology searches suggested that PC in these species is synthesized by phosphoethanolamine-N-methyltransferase (PEAMT) and/or phosphatidylethanolamine-N-methyltransferase (PEMT), both of which are absent in C. reinhardtii. Recombinant PEAMTs from these algae showed methyltransferase activity for phosphoethanolamine but not for monomethyl phosphoethanolamine in vitro, in contrast to land plant PEAMT, which catalyzes the three methylations from phosphoethanolamine to phosphocholine. This suggested an involvement of other methyltransferases in PC biosynthesis. Here, we characterized the putative phospholipid-N-methyltransferase (PLMT) genes of these species by genetic and phylogenetic analysis. Complementation assays using a PC biosynthesis-deficient yeast suggested that the PLMTs of these algae can synthesize PC from phosphatidylethanolamine. These results indicated that the PC biosynthesis pathways in green algae differ from those of land plants, although the enzymes involved are homologous. Phylogenetic analysis suggested that the PEAMTs and PLMTs in these algae were inherited from the common ancestor of green algae. The absence of PC biosynthesis in many Chlamydomonas species is likely a result of parallel losses of PEAMT and PLMT in this genus.

Role of Aramchol in steatohepatitis and fibrosis in mice

Nonalcoholic steatohepatitis (NASH) is the advanced form of nonalcoholic fatty liver disease (NAFLD) that sets the stage for further liver damage. The mechanism for the progression of NASH involves multiple parallel hits, including oxidative stress, mitochondrial dysfunction, inflammation, and others. Manipulation of any of these pathways may be an approach to prevent NASH development and progression. Arachidyl‐amido cholanoic acid (Aramchol) is presently in a phase IIb NASH study. The aim of the present study was to investigate Aramchol's mechanism of action and its effect on fibrosis using the methionine‐ and choline‐deficient (MCD) diet model of NASH. We collected liver and serum from mice fed an MCD diet containing 0.1% methionine (0.1MCD) for 4 weeks; these mice developed steatohepatitis and fibrosis. We also collected liver and serum from mice receiving a control diet, and metabolomes and proteomes were determined for both groups. The 0.1MCD‐fed mice were given Aramchol (5 mg/kg/day for the last 2 weeks), and liver samples were analyzed histologically. Aramchol administration reduced features of steatohepatitis and fibrosis in 0.1MCD‐fed mice. Aramchol down‐regulated stearoyl‐coenyzme A desaturase 1, a key enzyme involved in triglyceride biosynthesis and the loss of which enhances fatty acid β‐oxidation. Aramchol increased the flux through the transsulfuration pathway, leading to a rise in glutathione (GSH) and the GSH/oxidized GSH ratio, the main cellular antioxidant that maintains intracellular redox status. Comparison of the serum metabolomic pattern between 0.1MCD‐fed mice and patients with NAFLD showed a substantial overlap. Conclusion: Aramchol treatment improved steatohepatitis and fibrosis by 1) decreasing stearoyl‐coenyzme A desaturase 1 and 2) increasing the flux through the transsulfuration pathway maintaining cellular redox homeostasis. We also demonstrated that the 0.1MCD model resembles the metabolic phenotype observed in about 50% of patients with NAFLD, which supports the potential use of Aramchol in NASH treatment. (Hepatology Communications 2017;1:911–927)

Characterization of a Bvg-regulated fatty acid methyl-transferase in Bordetella pertussis

The whooping cough agent Bordetella pertussis controls the expression of its large virulence regulon in a coordinated manner through the two-component signal transduction system BvgAS. In addition to the genes coding for bona fide virulence factors, the Bvg regulon comprises genes of unknown function. In this work, we characterized a new Bvg-activated gene called BP2936. Homologs of BP2936 are found in other pathogenic Bordetellae and in several other species, including plant pathogens and environmental bacteria. We showed that the gene product of BP2936 is a membrane-associated methyl-transferase of free fatty acids. We thus propose to name it FmtB, for fatty acid methyl-transferase of Bordetella. The role of this protein was tested in cellular and animal models of infection, but the loss of BP2936 did not appear to affect host-pathogen interactions in those assays. The high level of conservation of BP2936 among B. pertussis isolates nevertheless argues that it probably plays a role in the life cycle of this pathogen.

Mutational analysis of the integral membrane methyltransferase isoprenylcysteine carboxyl methyltransferase (ICMT) reveals potential substrate binding sites

The eukaryotic integral membrane enzyme isoprenylcysteine carboxyl methyltransferase (ICMT) methylates the carboxylate of a lipid-modified cysteine at the C terminus of its protein substrates. This is the final post-translational modification of proteins containing a CAAX motif, including the oncoprotein Ras, and therefore, ICMT may serve as a therapeutic target in cancer development. ICMT has no discernible sequence homology with soluble methyltransferases, and aspects of its catalytic mechanism are unknown. For example, how both the methyl donor S-adenosyl-l-methionine (AdoMet), which is water-soluble, and the methyl acceptor isoprenylcysteine, which is lipophilic, are recognized within the same active site is not clear. To identify regions of ICMT critical for activity, we combined scanning mutagenesis with methyltransferase assays. We mutated nearly half of the residues of the ortholog of human ICMT from Anopheles gambiae and observed reduced or undetectable catalytic activity for 62 of the mutants. The crystal structure of a distantly related prokaryotic methyltransferase (Ma Mtase), which has sequence similarity with ICMT in its AdoMet binding site but methylates different substrates, provides context for the mutational analysis. The data suggest that ICMT and Ma MTase bind AdoMet in a similar manner. With regard to residues potentially involved in isoprenylcysteine binding, we identified numerous amino acids within transmembrane regions of ICMT that dramatically reduced catalytic activity when mutated. Certain substitutions of these caused substrate inhibition by isoprenylcysteine, suggesting that they contribute to the isoprenylcysteine binding site. The data provide evidence that the active site of ICMT spans both cytosolic and membrane-embedded regions of the protein.

Purification, characterization, sequencing and molecular cloning of a novel cysteine methyltransferase that regulates trehalose-6-phosphate synthase from Saccharomyces cerevisiae

BACKGROUND

In Saccharomyces cerevisiae methylation at cysteine residue displayed enhanced activity of trehalose-6-phosphate synthase (TPS).

METHODS

The cysteine methyltransferase (CMT) responsible for methylating TPS was purified and characterized. The amino acid sequence of the enzyme protein was determined by a combination of N-terminal sequencing and MALDI-TOF/TOF analysis. The nucleotide sequence of the CMT gene was determined, isolated from S. cerevisiae and expressed in E. coli. Targeted disruption of the CMT gene by PCR based homologous recombination in S. cerevisiae was followed by metabolite characterization in the mutant.

RESULTS

The purified enzyme was observed to enhance the activity of TPS by a factor of 1.76. The 14kDa enzyme was found to be cysteine specific. The optimum temperature and pH of enzyme activity was calculated as 30°C and 7.0 respectively. The Km Vmax and Kcat against S-adenosyl-l-methionine (AdoMet) were 4.95μM, 3.2U/mg and 6.4s(-1) respectively. Competitive inhibitor S-Adenosyl-l-homocysteine achieved a Ki as 10.9μM against AdoMet. The protein sequence contained three putative AdoMet binding motifs. The purified recombinant CMT activity exhibited similar physicochemical characteristics with the native counterpart. The mutant, Mataα, cmt:: kan(r) exhibited almost 50% reduction in intracellular trehalose concentration.

CONCLUSION

A novel cysteine methyltransferase is purified, which is responsible for enhanced levels of trehalose in S. cerevisiae.

GENERAL SIGNIFICANCE

This is the first report about a cysteine methyltransferase which performs S methylation at cysteine residue regulating TPS activity by 50%, which resulted in an increase of the intercellular stress sugar, trehalose.

Phospholipid methylation in mammals: from biochemistry to physiological function

Phosphatidylcholine is made in the liver via the CDP-choline pathway and via the conversion of phosphatidylethanolamine to phosphatidylcholine by 3 transmethylation reactions from AdoMet catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). PEMT is a 22.3kDa integral transmembrane protein of the endoplasmic reticulum and mitochondria-associated membranes. The only tissue with quantitatively significant PEMT activity is liver; however, low levels of PEMT in adipocytes have been implicated in lipid droplet formation. PEMT activity is regulated by the concentration of substrates (phosphatidylethanolamine and AdoMet) as well as the ratio of AdoMet to AdoHcy. Transcription of PEMT is enhanced by estrogen whereas the transcription factor Sp1 is a negative regulator of PEMT transcription. Studies with mice that lack PEMT have provided novel insights into the function of this enzyme. PEMT activity is required to maintain hepatic membrane integrity and for the formation of choline when dietary choline supply is limited. PEMT is required for normal secretion of very low-density lipoproteins. The lack of PEMT protects against diet-induced atherosclerosis in two mouse models. Most unexpectedly, mice that lack PEMT are protected from diet-induced obesity and insulin resistance. Moreover, mice lacking PEMT have increased susceptibility to diet-induced fatty liver and steatohepatitis. This article is part of a Special Issue entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.

Plasma membrane--endoplasmic reticulum contact sites regulate phosphatidylcholine synthesis

Synthesis of phospholipids, sterols and sphingolipids is thought to occur at contact sites between the endoplasmic reticulum (ER) and other organelles because many lipid-synthesizing enzymes are enriched in these contacts. In only a few cases have the enzymes been localized to contacts in vivo and in no instances have the contacts been demonstrated to be required for enzyme function. Here, we show that plasma membrane (PM)--ER contact sites in yeast are required for phosphatidylcholine synthesis and regulate the activity of the phosphatidylethanolamine N-methyltransferase enzyme, Opi3. Opi3 activity requires Osh3, which localizes to PM-ER contacts where it might facilitate in trans catalysis by Opi3. Thus, membrane contact sites provide a structural mechanism to regulate lipid synthesis.

Physiological roles of phosphatidylethanolamine N-methyltransferase

Phosphatidylethanolamine N-methyltransferase (PEMT) catalyzes the methylation of phosphatidylethanolamine to phosphatidylcholine (PC). This 22.3 kDa protein is localized to the endoplasmic reticulum and mitochondria associated membranes of liver. The supply of the substrates AdoMet and phosphatidylethanolamine, and the product AdoHcy, can regulate the activity of PEMT. Estrogen has been identified as a positive activator, and Sp1 as a negative regulator, of transcription of the PEMT gene. Targeted inactivation of the PEMT gene produced mice that had a mild phenotype when fed a chow diet. However, when Pemt(-/-) mice were fed a choline-deficient diet steatohepatitis and liver failure developed after 3 days. The steatohepatitis was due to a decreased ratio of PC to phosphatidylethanolamine that caused leakage from the plasma membrane of hepatocytes. Pemt(-/-) mice exhibited attenuated secretion of very low-density lipoproteins and homocysteine. Pemt(-/-) mice bred with mice that lacked the low-density lipoprotein receptor, or apolipoprotein E were protected from high fat/high cholesterol-induced atherosclerosis. Surprisingly, Pemt(-/-) mice were protected from high fat diet-induced obesity and insulin resistance compared to wildtype mice. If the diet were supplemented with additional choline, the protection against obesity/insulin resistance in Pemt(-/-) mice was eliminated. Humans with a Val-to-Met substitution in PEMT at residue 175 may have increased susceptibility to nonalcoholic liver disease. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

S-adenosylmethionine-binding properties of a bacterial phospholipid N-methyltransferase

The presence of the membrane lipid phosphatidylcholine (PC) in the bacterial membrane is critically important for many host-microbe interactions. The phospholipid N-methyltransferase PmtA from the plant pathogen Agrobacterium tumefaciens catalyzes the formation of PC by a three-step methylation of phosphatidylethanolamine via monomethylphosphatidylethanolamine and dimethylphosphatidylethanolamine. The methyl group is provided by S-adenosylmethionine (SAM), which is converted to S-adenosylhomocysteine (SAH) during transmethylation. Despite the biological importance of bacterial phospholipid N-methyltransferases, little is known about amino acids critical for binding to SAM or phospholipids and catalysis. Alanine substitutions in the predicted SAM-binding residues E58, G60, G62, and E84 in A. tumefaciens PmtA dramatically reduced SAM-binding and enzyme activity. Homology modeling of PmtA satisfactorily explained the mutational results. The enzyme is predicted to exhibit a consensus topology of the SAM-binding fold consistent with cofactor interaction as seen with most structurally characterized SAM-methyltransferases. Nuclear magnetic resonance (NMR) titration experiments and (14)C-SAM-binding studies revealed binding constants for SAM and SAH in the low micromolar range. Our study provides first insights into structural features and SAM binding of a bacterial phospholipid N-methyltransferase.

Docosahexaenoic acid in plasma phosphatidylcholine may be a potential marker for in vivo phosphatidylethanolamine N-methyltransferase activity in humans

BACKGROUND

Choline is an essential nutrient for humans, and part of this requirement is met by endogenous synthesis catalyzed by hepatic phosphatidylethanolamine N-methyltransferase (PEMT). PEMT activity is difficult to estimate in humans because it requires a liver biopsy. Previously, we showed that mice that lack functional PEMT have dramatically reduced concentrations of docosahexaenoic acid (DHA; 22:6n-3) in plasma and of liver phosphatidylcholine (PtdCho)-a phospholipid formed by PEMT.

OBJECTIVE

The objective was to evaluate plasma PtdCho-DHA concentrations as a noninvasive marker of liver PEMT activity in humans.

DESIGN

Plasma PtdCho-DHA concentrations were measured in 72 humans before and after they consumed a low-choline diet, and correlations were analyzed in relation to estrogen status, PEMT polymorphism rs12325817, the ratio of plasma S-adenosylmethionine (AdoMet) to S-adenosylhomocysteine (AdoHcy), and dietary choline intake; all of these factors are associated with changes in liver PEMT activity. PtdCho-DHA and PEMT activity were also measured in human liver specimens.

RESULTS

At baseline, the portion of PtdCho species containing DHA (pmol PtdCho-DHA/nmol PtdCho) was higher in premenopausal women than in men and postmenopausal women (P < 0.01). This ratio was lower in premenopausal women with the rs12325817 polymorphism in the PEMT gene (P < 0.05), and PtdCho-DHA concentration and PEMT activity were lower in human liver samples from women who were homozygous for PEMT rs12325817 (P < 0.05). The ratio of DHA-PtdCho to PtdCho in plasma was directly correlated with the ratio of AdoMet to AdoHcy (P = 0.0001). The portion of PtdCho species containing DHA in plasma was altered in subjects who consumed a low-choline diet.

CONCLUSION

PtdCho-DHA may be useful as a surrogate marker for in vivo hepatic PEMT activity in humans. This trial was registered at clinicaltrials.gov as NCT00065546.

Sequential synthesis and methylation of phosphatidylethanolamine promote lipid droplet biosynthesis and stability in tissue culture and in vivo

Triacylglycerols are stored in eukaryotic cells within lipid droplets (LD). The LD core is enwrapped by a phospholipid monolayer with phosphatidylcholine (PC), the major phospholipid, and phosphatidylethanolamine (PE), a minor component. We demonstrate that the onset of LD formation is characterized by a change in cellular PC, PE, and phosphatidylserine (PS). With induction of differentiation of 3T3-L1 fibroblasts into adipocytes, the cellular PC/PE ratio decreased concomitant with LD formation, with the most pronounced decline between confluency and day 5. The mRNA for PS synthase-1 (forms PS from PC) and PS decarboxylase (forms PE from PS) increased after day 5. Activity and protein of PE N-methyltransferase (PEMT), which produces PC by methylation of PE, are absent in 3T3-L1 fibroblasts but were induced at day 5. High fat challenge induced PEMT expression in mouse adipose tissue. PE, produced via PS decarboxylase, was the preferred substrate for methylation to PC. A PEMT-GFP fusion protein decorated the periphery of LD. PEMT knockdown in 3T3-L1 adipocytes correlated with increased basal triacylglycerol hydrolysis. Pemt(-/-) mice developed desensitization against adenosine-mediated inhibition of basal hydrolysis in adipose tissue, and adipocyte hypotrophy was observed in Pemt(-/-) animals on a high fat diet. Knock-out of PEMT in adipose tissue down-regulated PS synthase-1 mRNA, suggesting coordination between PE supply and converting pathways during LD biosynthesis. We conclude that two consecutive processes not previously related to LD biogenesis, (i) PE production via PS and (ii) PE conversion via PEMT, are implicated in LD formation and stability.

Functional analysis of two isoforms of phosphatidylethanolamine N-methyltransferase

The enzyme catalysing the conversion of PE (phosphatidylethanolamine) into PC (phosphatidylcholine), PEMT (PE N-methyltransferase), exists as two isoforms, PEMT-L (longer isoform of PEMT) and PEMT-S (shorter isoform of PEMT). In the present study, to compare the functions of the two isoforms of PEMT, we established HEK (human embryonic kidney)-293 cell lines stably expressing PEMT-L and PEMT-S. Both PEMT-L and PEMT-S were localized in the ER (endoplasmic reticulum). PEMT-L, but not PEMT-S, was N-glycosylated with high-mannose oligosaccharides. The enzymatic activity of PEMT-S was much higher than that of PEMT-L. By using novel enzymatic assays for measuring PC and PE, we showed that PEMT-L and PEMT-S expression remarkably increased the cellular PC content, whereas the PE content was decreased by PEMT-S expression, but was hardly affected by PEMT-L expression. The cellular content of phosphatidylserine was also reduced by the expression of PEMT-L or PEMT-S. MS analyses demonstrated that the expression of PEMT-S led to more increases in the molecular species of PC and PC-O (ether-linked PC) with longer polyunsaturated chains than that of PEMT-L, whereas the PC-O species with shorter chains were increased more by PEMT-L expression than by PEMT-S expression, suggesting a difference in the substrate specificity of PEMT-L and PEMT-S. On the other hand, various PE and PE-O species were decreased by PEMT-S expression. In addition, PEMT-L and PEMT-S expression promoted the proliferation of HEK-293 cells. Based upon these findings, we propose a model in which the enzymatic activity and substrate specificity are regulated by the glycosylated N-terminal region of PEMT-L localized in the ER lumen.

The Kennedy pathway--De novo synthesis of phosphatidylethanolamine and phosphatidylcholine

The glycerophospholipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE) account for greater than 50% of the total phospholipid species in eukaryotic membranes and thus play major roles in the structure and function of those membranes. In most eukaryotic cells, PC and PE are synthesized by an aminoalcoholphosphotransferase reaction, which uses sn-1,2-diradylglycerol and either CDP-choline or CDP-ethanolamine, respectively. This is the last step in a biosynthetic pathway known as the Kennedy pathway, so named after Eugene Kennedy who elucidated it over 50 years ago. This review will cover various aspects of the Kennedy pathway including: each of the biosynthetic steps, the functions and roles of the phospholipid products PC and PE, and how the Kennedy pathway has the potential of being a chemotherapeutic target against cancer and various infectious diseases.

Phosphatidylcholine biosynthesis and its significance in bacteria interacting with eukaryotic cells

Phosphatidylcholine (PC), a typical eukaryotic membrane phospholipid, is present in only about 10% of all bacterial species, in particular in bacteria interacting with eukaryotes. A number of studies revealed that PC plays a fundamental role in symbiotic and pathogenic microbe-host interactions. Agrobacterium tumefaciens mutants lacking PC are unable to elicit plant tumors. The human pathogens Brucella abortus and Legionella pneumophila require PC for full virulence. The plant symbionts Bradyrhizobium japonicum and Sinorhizobium meliloti depend on wild-type levels of PC to establish an efficient root nodule symbiosis. Two pathways for PC biosynthesis are known in bacteria, the methylation pathway and the phosphatidylcholine synthase (Pcs) pathway. The methylation pathway involves a three-step methylation of phosphatidylethanolamine by at least one phospholipid N-methyltransferase to yield phosphatidylcholine. In the Pcs pathway, choline is condensed directly with CDP-diacylglycerol to form PC. This review focuses on the biosynthetic pathways and the significance of PC in bacteria with an emphasis on plant-microbe interactions.

Functional characterization of phospholipid N-methyltransferases from Arabidopsis and soybean

Phospholipid N-methyltransferase (PLMT) enzymes catalyze the S-adenosylmethionine-dependent methylation of ethanolamine-containing phospholipids to produce the abundant membrane lipid phosphatidylcholine (PtdCho). In mammals and yeast, PLMT activities are required for the de novo synthesis of the choline headgroup found in PtdCho. PLMT enzyme activities have also been reported in plants, yet their roles in PtdCho biosynthesis are less clear because most plants can produce the choline headgroup entirely via soluble substrates, initiated by the methylation of free ethanolamine-phosphate. To gain further insights into the function of PLMT enzymes in plants, we isolated PLMT cDNAs from Arabidopsis and soybean (Glycine max) based upon primary amino acid sequence homology to the rat PLMT, phosphatidylethanolamine N-methyltransferase. Using a heterologous yeast expression system, it was shown that plant PLMTs methylate phosphatidylmonomethylethanolamine and phosphatidyldimethylethanolamine but cannot utilize phosphatidylethanolamine as a substrate. Identification of an Arabidopsis line containing a knock-out dissociator transposon insertion within the single copy AtPLMT gene allowed us to investigate the consequences of loss of PLMT function. Although the accumulation of the PLMT substrates phosphatidylmonomethylethanolamine and phosphatidyldimethylethanolamine was considerably elevated in the atplmt knock-out line, PtdCho levels remained normal, and no obvious differences were observed in plant morphology or development under standard growth conditions. However, because the metabolic routes through which PtdCho is synthesized in plants vary greatly among differing species, it is predicted that the degree with which PtdCho synthesis is dependent upon PLMT activities will also vary widely throughout the plant kingdom.

In vitro characterization of the enzyme properties of the phospholipid N-methyltransferase PmtA from Agrobacterium tumefaciens

Agrobacterium tumefaciens requires phosphatidylcholine (PC) in its membranes for plant infection. The phospholipid N-methyltransferase PmtA catalyzes all three transmethylation reactions of phosphatidylethanolamine (PE) to PC via the intermediates monomethylphosphatidylethanolamine (MMPE) and dimethylphosphatidylethanolamine (DMPE). The enzyme uses S-adenosylmethionine (SAM) as the methyl donor, converting it to S-adenosylhomocysteine (SAH). Little is known about the activity of bacterial Pmt enzymes, since PC biosynthesis in prokaryotes is rare. In this article, we present the purification and in vitro characterization of A. tumefaciens PmtA, which is a monomeric protein. It binds to PE, the intermediates MMPE and DMPE, the end product PC, and phosphatidylglycerol (PG) and phosphatidylinositol. Binding of the phospholipid substrates precedes binding of SAM. We used a coupled in vitro assay system to demonstrate the enzymatic activity of PmtA and to show that PmtA is inhibited by the end products PC and SAH and the antibiotic sinefungin. The presence of PG stimulates PmtA activity. Our study provides insights into the catalysis and control of a bacterial phospholipid N-methyltransferase.

Photoaffinity Labeling and Mutational Analysis of 24‐C‐Sterol Methyltransferase Defines the AdoMet Binding Site

Photolabeling and site-directed mutagenesis were performed on recombinant Saccharomyces cerevisiae 24-C-sterol methyltransferase (SMT) to elucidate the location and role of active site residues involved in AdoMet binding and catalysis. Bioinformatic analysis of the SMT revealed a ten amino acid segment, conserved between L124 and P133, associated with the Rossmann-like fold belonging to AdoMet-dependent methyltransferases. Irradiation of the SMT in the presence of [methyl-3H3]AdoMet directly photolabeled the protein. The specificity of photolabeling was demonstrated by inactivation experiments with structural analogs of AdoMet, including sinefungin. Trypsin digestion of the [methyl-3H3]AdoMet photolabeled Erg6p afforded a single radioactive band in SDS-PAGE gel of 4 kDa. HPLC purification of this material generated a single radioactive fraction. The corresponding 3H-AdoMet-peptide adduct was subjected to Edman sequencing and the first fifteen residues gave a sequence Gly120-Asp-Leu-Val-Leu-Asp-Val-Gly-Cys-Gly-Val-Gly-Gly-Pro-Ala134 that contained the predicted AdoMet binding site. Amino acid residues in the tryptic digest fragment considered to bind covalently with the AdoMet at Asp125, Cys128, Pro133 and Tyr153 were replaced with leucine and analyzed kinetically and by photolabeling inactivation experiments. The results indicate that one or both of Cys128 and Pro133 are covalently bound to AdoMet.

Successful expression and purification of human CIAPIN1 in baculovirus-insect cell system and application of this system to investigation of its potential methyltransferase activity

CIAPIN1 is a newly identified anti-apoptosis molecule which plays an important role in definitive haematopoiesis in mouse fetal liver and confers multidrug resistance in gastric cancer cells. However, the biophysical function of CIAPIN1 is far from elucidated. Bioinformatics predicts that CIAPIN1 may contain a generic methyltransferase motif and a Zn-ribbon-like motif. Based on these data, we postulated that CIAPIN1 might be a DNA or RNA methyltransferase. To substantiate this proposal, recombinant human CIAPIN1 (rhCIAPIN1) was expressed by a baculovirus-insect cell system and purified by Ni-NTA affinity chromatography. In vitro DNA and RNA methyltransferring tests, DNA demethylation test and S-adenosyl-l-[methyl-3H]methionine (3H-AdoMet) binding test were carried out. Our experiments failed to demonstrate that rhCIAPIN1 had any DNA, RNA methyltransferase activity, DNA demethylase activity, or had the capability of binding AdoMet in vitro. Further studies are needed to definitely clarify whether CIAPIN1 has methyltransferase activity.

Phosphoethanolamine N-methyltransferase (PMT-1) catalyses the first reaction of a new pathway for phosphocholine biosynthesis in Caenorhabditis elegans

The development of nematicides targeting parasitic nematodes of animals and plants requires the identification of biochemical targets not found in host organisms. Recent studies suggest that Caenorhabditis elegans synthesizes phosphocholine through the action of PEAMT (S-adenosyl-L-methionine:phosphoethanolamine N-methyltransferases) that convert phosphoethanolamine into phosphocholine. Here, we examine the function of a PEAMT from C. elegans (gene: pmt-1; protein: PMT-1). Our analysis shows that PMT-1 only catalyses the conversion of phosphoethanolamine into phospho-monomethylethanolamine, which is the first step in the PEAMT pathway. This is in contrast with the multifunctional PEAMT from plants and Plasmodium that perform multiple methylations in the pathway using a single enzyme. Initial velocity and product inhibition studies indicate that PMT-1 uses a random sequential kinetic mechanism and is feedback inhibited by phosphocholine. To examine the effect of abrogating PMT-1 activity in C. elegans, RNAi (RNA interference) experiments demonstrate that pmt-1 is required for worm growth and development and validate PMT-1 as a potential target for inhibition. Moreover, providing pathway metabolites downstream of PMT-1 reverses the RNAi phenotype of pmt-1. Because PMT-1 is not found in mammals, is only distantly related to the plant PEAMT and is conserved in multiple parasitic nematodes of humans, animals and crop plants, inhibitors targeting it may prove valuable in human and veterinary medicine and agriculture.

Methyl balance and transmethylation fluxes in humans

Various questions have been raised about labile methyl balance and total transmethylation fluxes, and further discussion has been encouraged. This report reviews and discusses some of the relevant evidence now available. The fact that, if needed, labile methyl balance is maintained by methylneogenesis appears to be established, but several aspects of transmethylation remain uncertain: definitive measurements of the rate of total transmethylation in humans of both sexes on various diets and at various ages; the extent to which synthesis of phosphatidylcholine has been underestimated; and the relative contributions of the 2 pathways for the formation of sarcosine (ie, N-methylglycine). The available evidence indicates that the quantitatively most important pathways for S-adenosylmethionine-dependent transmethylation in mammals are the syntheses of creatine by guanidinoacetate methyltransferase, of phosphatidylcholine by phosphatidylethanolamine methyltransferase, and of sarcosine by glycine N-methyltransferase. Data presented in this report show that S-adenosylmethionine and methionine accumulate abnormally in the plasma of humans with glycine N-methyltransferase deficiency but not of those with guanidinoacetate N-methyltransferase deficiency or in the plasma or livers of mice devoid of phosphatidylethanolamine N-methyltransferase activity. The absence of such accumulations in the latter 2 conditions may be due to removal of S-adenosylmethionine by synthesis of sarcosine. Steps that may help clarify the remaining issues include the determination of the relative rates of synthesis of sarcosine, creatine, and phosphatidylcholine by rapid measurement of the rates of radiolabel incorporation into these compounds from L-[methyl-3H]methionine administered intraportally to an experimental animal; clarification of the intracellular hepatic isotope enrichment value during stable-isotope infusion studies to enhance the certainty of methyl flux estimates during such studies; and definitive measurement of the dietary betaine intake from various diets.

Localization-independent regulation of homocysteine secretion by phosphatidylethanolamine N-methyltransferase

Genetic ablation of phosphatidylethanolamine N-methyltransferase (PEMT) in mice causes a 50% reduction in plasma homocysteine (Hcy) levels. Because hyperhomocysteinemia is an independent risk factor for cardiovascular disease, resolution of the molecular basis for this reduction is of significant clinical interest. The PEMT pathway is a metabolically channeled process localized to the endoplasmic reticulum (ER). To assess the importance of PEMT localization for Hcy homeostasis, we identified and ablated the minimal ER targeting motif. Mutagenesis of a conserved, C-terminal lysine residue (197) relocalized the enzyme to the Golgi, demonstrating that Lys-197 is essential for targeting PEMT to the ER. To evaluate the functional significance of PEMT localization, hepatoma cell lines were generated that stably expressed either ER- or Golgi-localized PEMT only. Intriguingly, stable expression of PEMT in either the ER or the Golgi caused increased Hcy secretion. Moreover, PEMT-mediated Hcy secretion correlated with the methyltransferase activity of the enzyme, independently of subcellular localization. Thus, our data suggest that Hcy homeostasis is regulated concomitantly with PEMT activity but independently of PEMT localization.

Analysis of the localization and topology of nurim, a polytopic protein tightly associated with the inner nuclear membrane

Nurim is an inner nuclear membrane (INM) protein that was first isolated in a visual screen for nuclear envelope-localizing proteins. Nurim lacks an N-terminal domain characteristic of other INM proteins examined to date and may represent a class of proteins that localize to the INM by a distinct mechanism. To further characterize this protein, we constructed nurim-green fluorescent protein fusions and analyzed aspects of localization, biochemistry, and membrane topology. Results from immunoprobing and protease protection assays together with other analyses indicate that nurim (total length of 262 residues) is a six transmembrane-spanning protein and contains a hairpin turn in its C-terminal transmembrane domain, resulting in the N and C termini residing on the same side of the membrane. A loop region between the fourth and fifth transmembrane domains is exposed toward the nucleoplasm and contains a region accessible for site-specific endoproteinase cleavage. In biochemical fractionation, nurim remained extremely tightly bound to nuclear fractions and was released in significant quantities only in the presence of 4 m urea. Under conditions in which nuclear lamins were completely extracted, a significant population of nurim remained resistant to solubilization. This tight binding requires the C-terminal region of the protein. DNase treatment only marginally influenced its retention characteristics in nuclei. Results from consideration of sequence alignments and identification of specific topological features of nurim indicate that it may possess enzymic function. These results are discussed with reference to the retention mechanism and possible nuclear function of nurim.

Post-translational Formylglycine Modification of Bacterial Sulfatases by the Radical S-Adenosylmethionine Protein AtsB

C(alpha)-Formylglycine (FGly) is the catalytic residue of sulfatases. FGly is generated by post-translational modification of a cysteine (prokaryotes and eukaryotes) or serine (prokaryotes) located in a conserved (C/S)XPXR motif. AtsB of Klebsiella pneumoniae is directly involved in FGly generation from serine. AtsB is predicted to belong to the newly discovered radical S-adenosylmethionine (SAM) superfamily. By in vivo and in vitro studies we show that SAM is the critical co-factor for formation of a functional AtsB.SAM.sulfatase complex and for FGly formation by AtsB. The SAM-binding site of AtsB involves (83)GGE(85) and possibly also a juxtaposed FeS center coordinated by Cys(39) and Cys(42), as indicated by alanine scanning mutagenesis. Mutation of these and other conserved cysteines as well as treatment with metal chelators fully impaired FGly formation, indicating that all three predicted FeS centers are crucial for AtsB function. It is concluded that AtsB oxidizes serine to FGly by a radical mechanism that is initiated through reductive cleavage of SAM, thereby generating the highly oxidizing deoxyadenosyl radical, which abstracts a hydrogen from the serine-C(beta)H(2)-OH side chain.