Rat C1-tetrahydrofolate synthase. cDNA isolation, tissue-specific levels of the mRNA, and expression of the protein in yeast

The First Structure of Human MTHFD2L and Its Implications for the Development of Isoform-Selective Inhibitors

Methylenetetrahydrofolate dehydrogenase 2 (MTHFD2) is a mitochondrial 1-carbon metabolism enzyme, which is an attractive anticancer drug target as it is highly upregulated in cancer but is not expressed in healthy adult cells. Selective MTHFD2 inhibitors could therefore offer reduced side-effects during treatment, which are common with antifolate drugs that target other 1C-metabolism enzymes. This task is challenging however, as MTHFD2 shares high sequence identity with the constitutively expressed isozymes cytosolic MTHFD1 and mitochondrial MTHFD2L. In fact, one of the most potent MTHFD2 inhibitors reported to date, TH7299, is actually more active against MTHFD1 and MTHFD2L. While structures of MTHFD2 and MTHFD1 exist, no MTHFD2L structures are available. We determined the first structure of MTHFD2L and its complex with TH7299, which reveals the structural basis for its highly potent MTHFD2L inhibition. Detailed analysis of the MTHFD2L structure presented here clearly highlights the challenges associated with developing truly isoform-selective MTHFD2 inhibitors.

Molecular structure of a 5,10-methylenetetrahydrofolate dehydrogenase from the silkworm Bombyx mori

The enzyme 5,10‐methylenetetrahydrofolate dehydrogenase (MTHFD) is essential for the production of certain amino acids (glycine, serine, and methionine) and nucleic acids (thymidylate and purine). Here, we identified a cDNA encoding this enzyme from the silkworm Bombyx mori. The recombinant B. mori MTHFD (bmMTHFD) expressed in Escherichia coli recognized 5,10‐methylenetetrahydrofolate and 5,10‐methenyltetrahydrofolate as substrate in the presence of NADP⁺ as well as NAD⁺. The bmMTHFD structure was determined at a resolution of 1.75 Å by X‐ray crystallography. Site‐directed mutagenesis indicated that the amino acid residue Tyr49 contributed to its catalytic activity. Our findings provide insight into the mechanism underlying the activity of MTHFD from B. mori and potentially other insects and may therefore facilitate the development of inhibitors specific to MTHFD as insecticides.

Crystal Structure of the Emerging Cancer Target MTHFD2 in Complex with a Substrate-Based Inhibitor

To sustain their proliferation, cancer cells become dependent on one-carbon metabolism to support purine and thymidylate synthesis. Indeed, one of the most highly upregulated enzymes during neoplastic transformation is MTHFD2, a mitochondrial methylenetetrahydrofolate dehydrogenase and cyclohydrolase involved in one-carbon metabolism. Because MTHFD2 is expressed normally only during embryonic development, it offers a disease-selective therapeutic target for eradicating cancer cells while sparing healthy cells. Here we report the synthesis and preclinical characterization of the first inhibitor of human MTHFD2. We also disclose the first crystal structure of MTHFD2 in complex with a substrate-based inhibitor and the enzyme cofactors NAD⁺ and inorganic phosphate. Our work provides a rationale for continued development of a structural framework for the generation of potent and selective MTHFD2 inhibitors for cancer treatment. Cancer Res; 77(4); 937-48. ©2017 AACR.

Mitochondrial MTHFD2L is a dual redox cofactor-specific methylenetetrahydrofolate dehydrogenase/methenyltetrahydrofolate cyclohydrolase expressed in both adult and embryonic tissues

Mammalian mitochondria are able to produce formate from one-carbon donors such as serine, glycine, and sarcosine. This pathway relies on the mitochondrial pool of tetrahydrofolate (THF) and several folate-interconverting enzymes in the mitochondrial matrix. We recently identified MTHFD2L as the enzyme that catalyzes the oxidation of 5,10-methylenetetrahydrofolate (CH2-THF) in adult mammalian mitochondria. We show here that the MTHFD2L enzyme is bifunctional, possessing both CH2-THF dehydrogenase and 5,10-methenyl-THF cyclohydrolase activities. The dehydrogenase activity can use either NAD(+) or NADP(+) but requires both phosphate and Mg(2+) when using NAD(+). The NADP(+)-dependent dehydrogenase activity is inhibited by inorganic phosphate. MTHFD2L uses the mono- and polyglutamylated forms of CH2-THF with similar catalytic efficiencies. Expression of the MTHFD2L transcript is low in early mouse embryos but begins to increase at embryonic day 10.5 and remains elevated through birth. In adults, MTHFD2L is expressed in all tissues examined, with the highest levels observed in brain and lung.

Formaldehyde in brain: an overlooked player in neurodegeneration?

Formaldehyde is an environmental pollutant that is also generated in substantial amounts in the human body during normal metabolism. This aldehyde is a well-established neurotoxin that affects memory, learning, and behavior. In addition, in several pathological conditions, including Alzheimer's disease, an increase in the expression of formaldehyde-generating enzymes and elevated levels of formaldehyde in brain have been reported. This article gives an overview on the current knowledge on the generation and metabolism of formaldehyde in brain cells as well as on formaldehyde-induced alterations in metabolic processes. Brain cells have the potential to generate and to dispose formaldehyde. In culture, both astrocytes and neurons efficiently oxidize formaldehyde to formate which can be exported or further oxidized. Although moderate concentrations of formaldehyde are not acutely toxic for brain cells, exposure to formaldehyde severely affects their metabolism as demonstrated by the formaldehyde-induced acceleration of glycolytic flux and by the rapid multidrug resistance protein 1-mediated export of glutathione from both astrocytes and neurons. These formaldehyde-induced alterations in the metabolism of brain cells may contribute to the impaired cognitive performance observed after formaldehyde exposure and to the neurodegeneration in diseases that are associated with increased formaldehyde levels in brain.

Mammalian MTHFD2L encodes a mitochondrial methylenetetrahydrofolate dehydrogenase isozyme expressed in adult tissues

Previous studies in our laboratory showed that isolated, intact adult rat liver mitochondria are able to oxidize the 3-carbon of serine and the N-methyl carbon of sarcosine to formate without the addition of any other cofactors or substrates. Conversion of these 1-carbon units to formate requires several folate-interconverting enzymes in mitochondria. The enzyme(s) responsible for conversion of 5,10-methylene-tetrahydrofolate (CH(2)-THF) to 10-formyl-THF in adult mammalian mitochondria are currently unknown. A new mitochondrial CH(2)-THF dehydrogenase isozyme, encoded by the MTHFD2L gene, has now been identified. The recombinant protein exhibits robust NADP(+)-dependent CH(2)-THF dehydrogenase activity when expressed in yeast. The enzyme is localized to mitochondria when expressed in CHO cells and behaves as a peripheral membrane protein, tightly associated with the matrix side of the mitochondrial inner membrane. The MTHFD2L gene is subject to alternative splicing and is expressed in adult tissues in humans and rodents. This CH(2)-THF dehydrogenase isozyme thus fills the remaining gap in the pathway from CH(2)-THF to formate in adult mammalian mitochondria.

Compartmentalization of Mammalian folate-mediated one-carbon metabolism

The recognition that mitochondria participate in folate-mediated one-carbon metabolism grew out of pioneering work beginning in the 1950s from the laboratories of D.M. Greenberg, C.G. Mackenzie, and G. Kikuchi. These studies revealed mitochondria as the site of oxidation of one-carbon donors such as serine, glycine, sarcosine, and dimethylglycine. Subsequent work from these laboratories and others demonstrated the participation of folate coenzymes and folate-dependent enzymes in these mitochondrial processes. Biochemical and molecular genetic approaches in the 1980s and 1990s identified many of the enzymes involved and revealed an interdependence of cytoplasmic and mitochondrial one-carbon metabolism. These studies led to the development of a model of eukaryotic one-carbon metabolism that comprises parallel cytosolic and mitochondrial pathways, connected by one-carbon donors such as serine, glycine, and formate. Sequencing of the human and other mammalian genomes has facilitated identification of the enzymes that participate in this intercompartmental one-carbon metabolism, and animal models are beginning to clarify the roles of the cytoplasmic and mitochondrial isozymes of these enzymes. Identifying the mitochondrial transporters for the one-carbon donors and elucidating how flux through these pathways is controlled are two areas ripe for exploration.

ALDH1L2 is the mitochondrial homolog of 10-formyltetrahydrofolate dehydrogenase

Cytosolic 10-formyltetrahydrofolate dehydrogenase (FDH, ALDH1L1) is an abundant enzyme of folate metabolism. It converts 10-formyltetrahydrofolate to tetrahydrofolate and CO(2) in an NADP(+)-dependent reaction. We have identified a gene at chromosome locus 12q24.11 of the human genome, the product of which has 74% sequence similarity with cytosolic FDH. This protein has an extra N-terminal sequence of 22 amino acid residues, predicted to be a mitochondrial translocation signal. Transfection of COS-7 or A549 cell lines with a construct in which green fluorescent protein was introduced between the leader sequence and the rest of the putative mitochondrial FDH (mtFDH) has demonstrated mitochondrial localization of the fusion protein, suggesting that the identified gene encodes a mitochondrial enzyme. Purified pig liver mtFDH displayed dehydrogenase/hydrolase activities similar to cytosolic FDH. Real-time PCR performed on an array of human tissues has shown that although cytosolic FDH mRNA is highest in liver, kidney, and pancreas, mtFDH mRNA is most highly expressed in pancreas, heart, and brain. In contrast to the cytosolic enzyme, which is not detectable in cancer cells, the presence of mtFDH was demonstrated in several human cancer cell lines by conventional and real-time PCR and by Western blot. Analysis of genomes of different species indicates that the mitochondrial enzyme is a later evolutionary product when compared with the cytosolic enzyme. We propose that this novel mitochondrial enzyme is a likely source of CO(2) production from 10-formyltetrahydrofolate in mitochondria and plays an essential role in the distribution of one-carbon groups between the cytosolic and mitochondrial compartments of the cell.

Mitochondrial C1-tetrahydrofolate synthase (MTHFD1L) supports the flow of mitochondrial one-carbon units into the methyl cycle in embryos

Mitochondrial folate-dependent one-carbon (1-C) metabolism converts 1-C donors such as serine and glycine to formate, which is exported and incorporated into the cytoplasmic tetrahydrofolate (THF) 1-C pool. Developing embryos depend on this mitochondrial pathway to provide 1-C units for cytoplasmic process such as de novo purine biosynthesis and the methyl cycle. This pathway is composed of sequential methylene-THF dehydrogenase, methenyl-THF cyclohydrolase, and 10-formyl-THF synthetase activities. In embryonic mitochondria, the bifunctional MTHFD2 enzyme catalyzes the dehydrogenase and cyclohydrolase reactions, but the enzyme responsible for the mitochondrial synthetase reaction has not been identified in embryos. A monofunctional 10-formyl-THF synthetase (MTHFD1L gene product) functions in adult mitochondria and is a likely candidate for the embryonic activity. Here we show that the MTHFD1L enzyme is present in mitochondria from normal embryonic tissues and embryonic fibroblast cell lines, and embryonic mitochondria possess the ability to synthesize formate from glycine. The MTHFD1L transcript was detected at all stages of mouse embryogenesis examined. In situ hybridizations showed that MTHFD1L was expressed ubiquitously throughout the embryo but with localized regions of higher expression. The spatial pattern of MTHFD1L expression was virtually indistinguishable from that of MTHFD2 and MTHFD1 (cytoplasmic C(1)-THF synthase) in embryonic day 9.5 mouse embryos, suggesting coordinated regulation. Finally, we show using stable isotope labeling that in an embryonic mouse cell line, greater than 75% of 1-C units entering the cytoplasmic methyl cycle are mitochondrially derived. Thus, a complete pathway of enzymes for supplying 1-C units from the mitochondria to the methyl cycle in embryonic tissues is established.

Mitochondrial methylenetetrahydrofolate dehydrogenase, methenyltetrahydrofolate cyclohydrolase, and formyltetrahydrofolate synthetases

Folate-mediated metabolism involves enzyme-catalyzed reactions that occur in the cytoplasmic, mitochondrial, and nuclear compartments in mammalian cells. Which of the folate-dependent enzymes are expressed in these compartments depends on the stage of development, cell type, cell cycle, and whether or not the cell is transformed. Mitochondria become formate-generating organelles in cells and tissues expressing the MTHFD2 and MTHFD1L genes. The products of these nuclear genes were derived from trifunctional precursor proteins, expressing methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase, and formyltetrahydrofolate synthetase activities. The MTHFD2 protein is a bifunctional protein with dehydrogenase and cyclohydrolase activities that arose from a trifunctional precursor through the loss of the synthetase domain and a novel adaptation to NAD rather than NADP specificity for the dehydrogenase. The MTHFD1L protein retains the size of its trifunctional precursor, but through the mutation of critical residues, both the dehydrogenase and cyclohydrolase activities have been silenced. MTHFD1L is thus a monofunctional formyltetrahydrofolate synthetase. This review discusses the properties and functions of these mitochondrial proteins and their role in supporting cytosolic purine synthesis during embryonic development and in cells undergoing rapid growth.

The expression of mitochondrial methylenetetrahydrofolate dehydrogenase-cyclohydrolase supports a role in rapid cell growth

Deletion of the gene encoding NAD-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase (NMDMC) in mice was demonstrated previously to result in failure to establish definitive erythropoiesis in the developing liver. We examined the expression pattern of nmdmc to look for evidence that would support a tissue specific role for this activity. However, whole mount in situ hybridization revealed ubiquitous expression of nmdmc in the tissues of E9.5 and E10.5 embryos suggesting a broader role. Analysis of chimeras demonstrated that nmdmc-/- cells can survive in liver and other tissues of chimeras establishing that the null defect can be rescued by metabolites supplied by surrounding normal cells. Both the expression pattern and metabolite rescue support the proposal that mitochondrial NMDMC provides one-carbon units for purine synthesis during embryogenesis. The elevated expression of NMDMC in tumour cells, but not in surrounding normal cells, is predicted to result in significant differences in folate-mediated support for purine synthesis in the two cell types.

Characterization of the rat cytoplasmic C1-tetrahydrofolate synthase gene and analysis of its expression in liver regeneration and fetal development

The eukaryotic trifunctional enzyme, C(1)-tetrahydrofolate (THF) synthase, interconverts folic acid derivatives between various oxidation states and is critical for normal cellular function, growth, and differentiation. Using a rat C(1)-THF synthase cDNA and synthetic oligonucleotides, the rat C(1)-THF synthase gene was isolated and characterized. The gene consists of 28 exons and spans 67.5 kbp. Primer extension, RNase protection, and rapid amplification of cDNA ends (RACE) experiments indicate the presence of multiple transcription start points (tsp) within a 250-bp window located between 50 and 300 bp upstream from the start codon. The 5' flanking region is devoid of a TATA consensus sequence motif, but putative regulatory elements, including NF-kappabeta, HNF-4alpha1, RARalpha1, C/EBP, and PPAR are present in the promoter region. The 5' flanking region also contains two sets of tetranucleotide repeats and two short interspersed nuclear elements (SINES). The initial 2500 bp of 5' flanking sequences of the rat and mouse cytoplasmic C(1)-THF synthase genes share 70% identity. However, comparison with the human gene from the Human Genome Data Bank revealed no significant homology in the 5' flanking region. The gene structure characterization led to the identification of a pseudogene that is 94% identical to the C(1)-THF synthase gene and probably diverged 10-12 million years ago. In addition, the gene expression patterns of C(1)-THF synthase were investigated during liver regeneration and liver and kidney organogenesis, two highly regulated events. In both processes, C(1)-THF synthase expression correlated with increased nucleotide metabolism. This pattern suggests that the gene is regulated in response to changes in the demand for folate-dependent one-carbon units.

Mammalian fibroblasts lacking mitochondrial NAD+-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase are glycine auxotrophs

Primary fibroblasts established from embryos of NAD-dependent mitochondrial methylenetetrahydrofolate dehydrogenase-cyclohydrolase (NMDMC) knockout mice were spontaneously immortalized or transformed with SV40 Large T antigen. Mitotracker Red CMXRos staining of the cells indicates the presence of intact mitochondria with a membrane potential. The nmdmc(-/-) cells are auxotrophic for glycine, demonstrating that NMDMC is the only methylenetetrahydrofolate dehydrogenase normally expressed in the mitochondria of these cell lines. Growth of null mutant but not wild type cells on complete medium with dialyzed serum is stimulated about 2-fold by added formate or hypoxanthine. Radiolabeling experiments demonstrated a 3-10 x enhanced incorporation of radioactivity into DNA from formate relative to serine by nmdmc(-/-) cells. The generation of one-carbon units by mitochondria in nmdmc(-/-) cells is completely blocked, and the cytoplasmic folate pathways alone are insufficient for optimal purine synthesis. The results demonstrate a metabolic role for NMDMC in supporting purine biosynthesis. Despite the recognition of these metabolic defects in the mutant cell lines, the phenotype of nmdmc(-/-) embryos that begin to die at E13.5 is not improved when pregnant dams are given a glycine-rich diet or daily injections of sodium formate.

New insights into the rat spermatogonial proteome: identification of 156 additional proteins

Despite the essential role played by spermatogonia in testicular function, little is known about these cells. To improve our understanding of their biology, our group recently identified a set of 53 spermatogonial proteins using two-dimensional (2-D) gel electrophoresis and mass spectrometry. To continue this work, we investigated a subset of the spermatogonial proteome using narrow range immobilized pH gradients to favor the detection of less abundant proteins. A 2-D reference map of spermatogonia in the pH range 4-9 was created, and protein entities fractionated in a pH 5-6 2-D gel were further processed for protein identification. A new set of 156 polypeptides was identified by peptide mass fingerprinting and tandem mass spectrometry. These polypeptides corresponded to 102 different proteins, which reflect the complexity of post-translational modifications. Seventy-nine of these proteins were identified for the first time in spermatogonia. All identified proteins were classified into functional groups. This work represents a first step toward the establishment of a systematic spermatogonia protein database.

Mitochondrial NAD-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase is essential for embryonic development

Folate-dependent enzymes are compartmentalized between the cytoplasm and mitochondria of eukaryotes. The role of mitochondrial folate-dependent metabolism and the extent of its contribution to cytoplasmic processes are areas of active investigation. NAD-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase (NMDMC) catalyzes the interconversion of 5,10-methylenetetrahydrofolate and 10-formyltetrahydrofolate in mitochondria of mammalian cells, but its metabolic role is not yet clear. Its expression in embryonic tissues but not in most adult tissues as well as its stringent transcriptional regulation led us to postulate that it may play a role in embryonic development. To investigate the metabolic role of NMDMC, we used a knockout approach to delete the nmdmc gene in mice. Heterozygous mice appear healthy, but homozygous NMDMC knockout mice die in utero. At embryonic day 12.5 (E12.5), homozygous null embryos exhibit no obvious developmental defects but are smaller and pale and die soon thereafter. Mutant fetal livers contain fewer nucleated cells and lack the characteristic redness of wild-type or heterozygous livers. The frequencies of CFU-erythroid (CFU-E) and burst-forming unit-erythroid (BFU-E) from fetal livers of E12.5 null mutants were not reduced compared with those of wild-type or heterozygous embryos. It has been assumed that initiation of protein synthesis in mitochondria requires a formylated methionyl-tRNA(fmet). One role postulated for NMDMC is to provide 10-formyltetrahydrofolate as a formyl group donor for the synthesis of this formylmethionyl-tRNA(fmet). To determine if the loss of NMDMC impairs protein synthesis and thus could be a cause of embryonic lethality, mitochondrial translation products were examined in cells in culture. Mitochondrial protein synthesis was unaffected in NMDMC-null mutant cell lines compared with the wild type. These results show that NMDMC is not required to support initiation of protein synthesis in mitochondria in isolated cells but instead demonstrate an essential role for mitochondrial folate metabolism during embryonic development.

Molecular genetic analysis of the gene encoding the trifunctional enzyme MTHFD (methylenetetrahydrofolate-dehydrogenase, methenyltetrahydrofolate-cyclohydrolase, formyltetrahydrofolate synthetase) in patients with neural tube defects

It is now well recognized that periconceptional folic acid or folic acid containing multivitamin supplementation reduces the risk of neural tube defects (NTDs). Recently we were able to show that homozygosity for a thermolabile variant of the enzyme methylenetetrahydrofolate reductase is associated with an increased risk for spina bifida in patients recruited from the Dutch population. However, this genetic risk factor could not account for all folic acid preventable NTDs. In an attempt to identify additional folate related enzymes that contribute to NTD etiology we now studied the methylenetetrahydrofolate dehydrogenase gene on chromosome 14q24 which encodes a single protein with three catalytic properties important in the folate metabolism. The cDNA sequence of 38 familial and 79 sporadic patients was screened for the presence of mutations by single strand conformation polymorphism (SSCP) analysis followed by sequencing. Two amino acid substitutions were identified. The first one (R293H) was detected in a patient with familial spina bifida and not in 300 control individuals. The mutation was inherited from the unaffected maternal grandmother and was also present in two younger brothers of the index patient, one of them displaying spina bifida occulta and the other being unaffected. The second change turned out to be an amino acid polymorphism (R653Q) that was present in both patients and controls with similar frequencies. Our results so far provide no evidence for a major role of the methylenetetrahydrofolate-dehydrogenase (MTHFD) gene in NTD etiology. However, the identification of a mutation in one family suggests that this gene can act as a risk factor for human NTD.

Genetic and physiologic analysis of a formyl-tetrahydrofolate synthetase mutant of Streptococcus mutans

Previously we reported that transposon Tn917 mutagenesis of Streptococcus mutans JH1005 yielded an isolate detective in its normal ability to produce a mutacin (P. J. Crowley, J. D. Hillman, and A. S. Bleiweis, abstr. D55, p. 258 in Abstracts of the 95th General Meeting of the American Society for Microbiology 1995, 1995). In this report we describe the recovery of the mutated gene by shotgun cloning. Sequence analysis of insert DNA adjacent to Tn917 revealed homology to the gene encoding formyl-tetrahydrofolate synthetase (Fhs) from both prokaryotic and eukaryotic sources. In many bacteria, Fhs catalyzes the formation of 10-formyl-tetrahydrofolate, which is used directly in purine biosynthesis and formylation of Met-tRNA and indirectly in the biosynthesis of methionine, serine, glycine, and thymine. Analysis of the fhs mutant grown anaerobically in a minimal medium demonstrated that the mutant had an absolute dependency only for adenine, although addition of methionine was necessary for normal growth. Coincidently it was discovered that the mutant was sensitive to acidic pH; it grew more slowly than the parent strain on complex medium at pH 5. Complementation of the mutant with an integration vector harboring a copy of fhs restored its ability to grow in minimal medium and at acidic pH as well as to produce mutacin. This represents the first characterization of Fhs in Streptococcus.

Site-directed mutagenesis of putative catalytic and nucleotide binding sites in N10-formyltetrahydrofolate synthetase

To determine the importance of specific amino-acid residues in catalysis and substrate binding by N10-formylH4 folate synthetase, one lysine and three histidine residues in the enzyme from Clostridium cylindrosporum were mutated to glutamine and serine residues, respectively. These residues, Lys-71, His-125, His-131, and His-268, are conserved in four bacterial and five eukaryotic proteins for which the amino-acid sequences are known. Previous evidence indicated that a histidine residue may play a role in catalysis and it has been proposed that Lys-71 could be a member of a putative nucleotide binding consenus sequence. The histidine mutations, H125S, H131S, and H268S, produced proteins that were unstable and were proteolytically degraded to different extents. No activity of purified H268S could be detected and the 240 kDa native tetramer was also absent. Activities of the H125S and H131S mutants could be measured and the Km values of the substrates were similar to those for the wild-type enzyme. It is concluded that the mutations resulted in monomers that do not fold properly and/or do not associate to the active tetramer and, as a consequence, are susceptible to intracellular proteolytic digestion. On the other hand, the K71Q mutation did not produce proteolyzed material. The resulting protein had a kcat value which was reduced by a factor of 3.3 x 10(-4). Km values of the substrates were not affected, nor were the affinty constants for MgATP and H4PteG3. CD and fluorescence spectra demonstrated that little change in the tertiary structure of the protein had occurred as a result of the mutation. The monomer form of K71Q was less stable than the monomer of the wild-type enzyme and reassociated less efficiently than the wild-type. From these results it is suggested that Lys-71 plays a critical role in catalysis by N10-formylH4 folate synthetase and that this residue may reside at an intersubunit interface.

Methylenetetrahydrofolate dehydrogenase-cyclohydrolase from Photobacterium phosphoreum shares properties with a mammalian mitochondrial homologue

The marine bioluminescent bacterium Photobacterium phosphoreum expresses a bifunctional methylenetetrahydrofolate dehydrogenase-cyclohydrolase with dual cofactor specificity. An investigation of the kinetic parameters of the P. phosphoreum enzyme indicate that its utilization of dinucleotide cofactors shares similarities with the human mitochondrial dehydrogenase-cyclohydrolase. Both enzymes exhibit dual cofactor specificity and the NAD(+)-dependent dehydrogenase activities from both enzymes can be activated by inorganic phosphate. Furthermore, an analysis of multiply aligned dehydrogenase-cyclohydrolase sequences from 11 species revealed that bacterial and mitochondrial enzymes are more closely related to each other than to the dehydrogenase-cyclohydrolase domains from eukaryotic trifunctional enzymes, and that the bacterial and mitochondrial enzymes share a common point of divergence. Since the NADP+ cofactor is kinetically favoured by a factor of 18 over NAD+, and is therefore likely to be the preferred in vivo cofactor, we propose that the P. phosphoreum enzyme and the human mitochondrial enzyme evolved from a common ancestral dehydrogenase-cyclohydrolase with dual cofactor specificity, but that cofactor preference in these two enzymes diverged in response to different metabolic requirements.

The N-terminal, dehydrogenase/cyclohydrolase domain of yeast cytoplasmic trifunctional C1-tetrahydrofolate synthase requires the C-terminal, synthetase domain for the catalytic activity in vitro

The yeast ADE3(1-333) gene which encodes a truncated protein containing the N-terminal 5,10-methylene-tetrahydrofolate (THF) dehydrogenase (D)/5,10-methyl-THF cyclohydrolase (C) domain of cytoplasmic trifunctional C1-THF synthase is able to complement all the phenotypes associated with ade3 mutations in vivo. However, expression of the ADE3(1-333) gene in an ade3 strain does not retain any D activity in vitro. Expression in a yeast ade3 strain of the ADE3(1-333) fused to the Escherichia coli lacZ gene or to the yeast SER2 gene allows detection of D and C activities in vitro. These results indicate that the N-terminal D/C domain of C1-THF synthase requires the C-terminal 10-formyl-THF synthetase domain for stable catalytic activity in vitro.

NAD-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase is targeted to the cytoplasm in insect cell lines

Cytosolic NADP-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase synthetase and the mitochondrial NAD-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase (NMDMC) are differentially expressed during insect development although both enzymes are detectable at all stages. In contrast, cell lines derived from a variety of insect species express high levels of NMDMC but undetectable levels of the NADP-dependent enzyme. Northern analysis indicates the NMDMC message is expressed at levels 50-100 times higher in a Drosophila cell line compared to adult flies. RNase protection showed the predominance of shortened transcripts that require initiation at a downstream AUG producing a truncated protein that lacks a mitochondrial targeting sequence. These changes in expression effectively exchange the cytosolic NADP-dependent dehydrogenase for one with NAD specificity.

Primary structure of a folate-dependent trifunctional enzyme from Spodoptera frugiperda

The dehydrogenase and synthetase activities of the NADP-dependent methylenetetra-hydrofolate dehydrogenase-cyclohydrolase-synthetase are undetectable in extracts of the Spodoptera frugiperda cell line, Sf9. However, a single cDNA encoding this protein was isolated from a library and sequenced. The deduced amino acid sequence codes for a protein of 933 amino acids in length that shows 59% identity to the human enzyme. The cDNA inserted in the yeast expression vector pVT102-U complements a purine auxotrophic yeast strain lacking this enzyme.

T-protein of the glycine decarboxylase multienzyme complex: evidence for partial similarity to formyltetrahydrofolate synthetase

We have isolated and sequenced cDNA clones encoding T-protein of the glycine decarboxylase complex from three plant species, Flaveria pringlei, Solanum tuberosum and Pisum sativum. The predicted amino acid sequences of these clones are at least 87% identical and all are similar to the predicted sequences of the bovine, human, chicken and Escherichia coli T-proteins. Alignment of all these sequences revealed conserved domains, one of which showed a significant similarity to a part of the formyltetrahydrofolate synthetases from procaryotes and eucaryotes. This suggests that the T-protein sequence is not as unique as previously thought.

The isolation and characterization of a Drosophila gene encoding a putative NAD-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase

Mammalian NAD-dependent 5,10-methylenetetrahydrofolate dehydrogenase-5,10-methenyltetrahydrofolate cyclohydrolase is a bifunctional mitochondrial enzyme expressed in most established cell lines but only in developing normal tissues. We report the cloning and molecular characterization of a Drosophila gene (DNMDMC) that encodes a protein with 56% identity to the mammalian bifunctional protein. Like the mammalian bifunctional proteins, the Drosophila protein contains a putative mitochondrial targeting sequence and its transcripts are expressed in developing tissues. Unlike its mammalian homologs, DNMDMC is expressed at high levels in adult tissues. DNMDMC maps to polytene chromosome band 85C, is encoded in three exons, and is closely flanked by two additional genes.

Sequence and expression of the gene for N10-formyltetrahydrofolate synthetase from Clostridium cylindrosporum

Sau3 A and Hind III restriction fragments of Clostridium cylindrosporum genomic DNA were used to isolate clones containing 80% of the N10-H4folate synthetase gene in a 5' fragment and the remaining 20% of the gene in the 3' fragment. These fragments were joined at a common SnaB I restriction site and expressed in Escherichia coli at a level equivalent to what is normally found in C. cylindrosporum. Sequence comparisons show a large degree of homology with genes from two other clostridial species, including a thermophile. Certain conserved sequences found in the three clostridial proteins and in the N10-H4folate synthetase portion of eukaryotic C1-H4folate synthases may represent consensus sequences for nucleotide and H4folate binding.

Transcriptional regulation of murine NADP(+)-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase-synthetase

The cytosolic NADP(+)-dependent methylenetetrahydrofolate dehydrogenase-methenyltetrahydrofolate cyclohydrolase-formyltetrahydrofolate synthetase is ubiquitously expressed in all mouse tissues and cell lines examined. Northern analyses of the RNA indicated that there is an extensive variation in the levels of mRNA in different tissues. However, the gene is refractory to induction by serum, phorbol esters or growth factors in cultured fibroblasts. The mRNA of the NADP(+)-dependent trifunctional enzyme is stabilized post-transcriptionally by insulin-like growth factor-1.

Deficient synthesis of MTHFD, a trifunctional folate-dependent enzyme, in the CHO Ade E mutant

MTHFD is a folate-dependent trifunctional protein comprised of three activities: N5,N10-methylenetetrahydrofolate dehydrogenase, N5,N10-methenyltetrahydrofolate cyclohydrolase, and N10-formyltetrahydrofolate synthetase. The enzymes catalyze sequential interconversion of tetrahydrofolate derivatives required for purine, methionine, and thymidylate synthesis. A Chinese hamster ovary cell line (Ade-E), reported to have reduced cyclohydrolase activity, was studied to characterize the nature of the mutation. Enzymatic assays showed reduced activities of all three enzymes of the polypeptide. Immunoblotting and immunoprecipitation of radiolabeled cell extracts indicated that MTHFD protein was greatly reduced or absent in the mutant. Northern analysis of a clonal derivative of Ade-E revealed normal levels of MTHFD mRNA. These results suggest that the mutation affects a posttranscriptional process in the synthesis of the trifunctional enzyme.

Immunolocalization of C1-tetrahydrofolate synthase in the rat kidney

The distribution of the trifunctional enzyme C1-tetrahydrofolate synthase (C1-THF synthase) was examined in the rat kidney by immunolocalization with anti-C1-THF synthase serum using the peroxidase-antiperoxidase method. C1-THF synthase immunoreactivity was detected in both distal and proximal epithelial cells. Staining of the distal tubule epithelia was more intense and granular whereas staining of the proximal tubule epithelia was diffuse. All cells of the cortical collecting duct showed positive granular staining. In the outer medullary collecting duct, the intercalated cells showed intense granular cytoplasmic staining and the principal cells were either negative or weakly positive. The ascending thick limb of Henle's loop was also positive. Glomeruli and the inner medulla showed no staining for C1-THF synthase.