The use of a spectrophotometric assay to study the interaction of S-adenosylmethionine synthetase with methionine analogues

A dual regulatory circuit consisting of S-adenosylmethionine decarboxylase protein and its reaction product controls expression of the paralogous activator prozyme in Trypanosoma brucei

Polyamines are essential for cell growth of eukaryotes including the etiologic agent of human African trypanosomiasis (HAT), Trypanosoma brucei. In trypanosomatids, a key enzyme in the polyamine biosynthetic pathway, S-adenosylmethionine decarboxylase (TbAdoMetDC) heterodimerizes with a unique catalytically-dead paralog called prozyme to form the active enzyme complex. In higher eukaryotes, polyamine metabolism is subject to tight feedback regulation by spermidine-dependent mechanisms that are absent in trypanosomatids. Instead, in T. brucei an alternative regulatory strategy based on TbAdoMetDC prozyme has evolved. We previously demonstrated that prozyme protein levels increase in response to loss of TbAdoMetDC activity. Herein, we show that prozyme levels are under translational control by monitoring incorporation of deuterated leucine into nascent prozyme protein. We furthermore identify pathway factors that regulate prozyme mRNA translation. We find evidence for a regulatory feedback mechanism in which TbAdoMetDC protein and decarboxylated AdoMet (dcAdoMet) act as suppressors of prozyme translation. In TbAdoMetDC null cells expressing the human AdoMetDC enzyme, prozyme levels are constitutively upregulated. Wild-type prozyme levels are restored by complementation with either TbAdoMetDC or an active site mutant, suggesting that TbAdoMetDC possesses an enzyme activity-independent function that inhibits prozyme translation. Depletion of dcAdoMet pools by three independent strategies: inhibition/knockdown of TbAdoMetDC, knockdown of AdoMet synthase, or methionine starvation, each cause prozyme upregulation, providing independent evidence that dcAdoMet functions as a metabolic signal for regulation of the polyamine pathway in T. brucei. These findings highlight a potential regulatory paradigm employing enzymes and pseudoenzymes that may have broad implications in biology.

Trypanosoma brucei is a single-celled eukaryotic pathogen and the causative agent of human African trypanosomiasis (HAT). Polyamines are organic polycations that are essential for growth in T. brucei to facilitate protein translation and to maintain redox homeostasis. The pathway is the target of eflornithine, a current frontline therapy for treatment of HAT. Polyamine biosynthetic enzymes are regulated at multiple levels in mammals (e.g. transcription, translation and protein turnover), but in contrast, T. brucei lacks these mechanisms. Instead in T. brucei a central enzyme in polyamine metabolism called AdoMetDC must form a complex with a sister protein (termed a pseudoenzyme) to be active. Herein, we show that cellular levels of this sister protein we call prozyme are in turn feedback regulated by both AdoMetDC and by its reaction product in response to cell treatments that reduce pathway output. This regulatory paradigm highlights how pseudoenzymes can evolve to play an important role in metabolic pathway regulation and in organismal fitness.

Effects of High Toxic Boron Concentration on Protein Profiles in Roots of Two Citrus Species Differing in Boron-Tolerance Revealed by a 2-DE Based MS Approach

Citrus are sensitive to boron (B)-toxicity. In China, B-toxicity occurs in some citrus orchards. So far, limited data are available on B-toxicity-responsive proteins in higher plants. Thirteen-week-old seedlings of "Sour pummelo" (Citrus grandis) and "Xuegan" (Citrus sinensis) was fertilized every other day until dripping with nutrient solution containing 10 μM (control) or 400 μM (B-toxicity) H₃BO₃ for 15 weeks. The typical B-toxic symptom only occurred in 400 μM B-treated C. grandis leaves, and that B-toxicity decreased root dry weight more in C. grandis seedlings than in C. sinensis ones, demonstrating that C. sinensis was more tolerant to B-toxicity than C. grandis. Using a 2-dimensional electrophoresis (2-DE) based MS approach, we identified 27 up- and four down-accumulated, and 28 up- and 13 down-accumulated proteins in B-toxic C. sinensis and C. grandis roots, respectively. Most of these proteins were isolated only from B-toxic C. sinensis or C. grandis roots, only nine B-toxicity-responsive proteins were shared by the two citrus species. Great differences existed in B-toxicity-induced alterations of protein profiles between C. sinensis and C. grandis roots. More proteins related to detoxification were up-accumulated in B-toxic C. grandis roots than in B-toxic C. sinensis roots to meet the increased requirement for the detoxification of the more reactive oxygen species and other toxic compounds such as aldehydes in the former. For the first time, we demonstrated that the active methyl cycle was induced and repressed in B-toxic C. sinensis and C. grandis roots, respectively, and that C. sinensis roots had a better capacity to keep cell wall and cytoskeleton integrity than C. grandis roots in response to B-toxicity, which might be responsible for the higher B-tolerance of C. sinensis. In addition, proteins involved in nucleic acid metabolism, biological regulation and signal transduction might play a role in the higher B-tolerance of C. sinensis.

AdoMet analog synthesis and utilization: current state of the art

S-Adenosyl-l-methionine (AdoMet) is an essential enzyme cosubstrate in fundamental biology with an expanding range of biocatalytic and therapeutic applications. In recent years, technologies enabling the synthesis and utilization of novel functional AdoMet surrogates have rapidly advanced. Developments highlighted within this brief review include improved syntheses of AdoMet analogs, unique S-adenosyl-l-methionine isosteres with enhanced stability, and corresponding applications in epigenetics, proteomics and natural product/small molecule diversification ('alkylrandomization').

Lysine acetylation regulates the activity of Escherichia coli S-adenosylmethionine synthase

Lysine acetylation is one of the most abundant post-translational modifications. However, physiological roles of this modification in bacteria are largely unknown. Previous protein acetylome analysis showed that Escherichia coli adenosylmethionine synthase (MAT) undergoes acetylation in vivo, but the biological functions of this modification still need to be uncovered. In this study, MAT of E. coli was over-expressed and purified. Subsequent mass spectrometry analysis showed that 12 lysine residues of the protein were acetylated. Site-directed mutagenesis analysis was performed and the results showed that acetylated lysine residues play important roles in the enzymatic activity of MAT. Next, deacetylation assay was performed by using CobB as the deacetylase, and the results showed that CobB could deacetylate MAT in vitro In addition, the enzymatic activities of acetylated and deacetylated MAT were compared in vitro, and results showed that acetylation led to a decrease in its enzymatic activity, which could be reversed by CobB deacetylation. Altogether, our data suggest that CobB modulates the enzymatic activity of E. coli MAT in vitro.

Alternative substrates selective for S-adenosylmethionine synthetases from pathogenic bacteria

S-adenosyl-l-methionine (AdoMet) synthetase catalyzes the production of AdoMet, the major biological methyl donor and source of methylene, amino, ribosyl, and aminopropyl groups in the metabolism of all known organism. In addition to these essential functions, AdoMet can also serve as the precursor for two different families of quorum sensing molecules that trigger virulence in Gram-negative human pathogenic bacteria. The enzyme responsible for AdoMet biosynthesis has been cloned, expressed and purified from several of these infectious bacteria. AdoMet synthetase (MAT) from Neisseria meningitidis shows similar kinetic parameters to the previously characterized Escherichia coli enzyme, while the Pseudomonas aeruginosa enzyme has a decreased catalytic efficiency for its MgATP substrate. In contrast, the more distantly related MAT from Campylobacter jejuni has an altered quaternary structure and possesses a higher catalytic turnover than the more closely related family members. Methionine analogs have been examined to delineate the substrate specificity of these enzyme forms, and several alternative substrates have been identified with the potential to block quorum sensing while still serving as precursors for essential methyl donation and radical generation reactions.

A new tool for biotechnology: AdoMet-dependent methyltransferases

AdoMet-dependent methyltransferases catalyze highly specific methyl group transfers from the ubiquitous cofactor S-adenosyl-L-methionine to a multitude of biological targets in the cell. Recently, DNA methyltransferases have been used for the sequence-specific, covalent attachment of larger chemical groups to plasmid and bacteriophage DNA using two classes of synthetic AdoMet analogs. These synthetic cofactors, in combination with the myriad AdoMet-dependent methyltransferases available in nature, provide new molecular tools for precise, targeted functionalization and labeling of large natural DNAs and, in all likelihood, RNAs and proteins. This paves the way for numerous novel applications in the functional analysis of biological methylation, biotechnology and medical diagnostics.

Enhanced expression of S-adenosylmethionine synthetase causes overproduction of actinorhodin in Streptomyces coelicolor A3(2)

We found that a 46-kDa protein is highly expressed in an actinorhodin-overproducing Streptomyces coelicolor A3(2) mutant (KO-179), which exhibited a low-level resistance to streptomycin. The protein was identified as S-adenosylmethionine (SAM) synthetase, which is a product of the metK gene. Enzyme assay revealed that SAM synthetase activity in strain KO-179 was 5- to 10-fold higher than in wild-type cells. The elevation of SAM synthetase activity was found to be associated with an increase in the level of intracellular SAM. RNase protection assay revealed that the metK gene was transcribed from two distinct promoters (p1 and p2) and that enhanced expression of the MetK protein in the mutant strain KO-179 was attributed to elevated transcription from metKp2. Strikingly, the introduction of a high-copy-number plasmid containing the metK gene into wild-type cells resulted in a precocious hyperproduction of actinorhodin. Furthermore, the addition of SAM to the culture medium induced Act biosynthesis in wild-type cells. Overexpression of metK stimulated the expression of the pathway-specific regulatory gene actII-ORF4, as demonstrated by the RNase protection assay. The addition of SAM also caused hyperproduction of streptomycin in Streptomyces griseus. These findings implicate the significant involvement of intracellular SAM in initiating the onset of secondary metabolism in STREPTOMYCES:

Vinyl sulfonium as novel proteolytic enzyme inhibitor

Vinyl sulfoniums were synthesized from vinyl sulfides by methylation, and inhibited the proteolytic enzyme papain. Inhibition studies suggest a mechanism by which the vinyl sulfonium inhibitor covalently and irreversibly modifies the enzyme.

Methionine interference in rapamycin production involves repression of demethylrapamycin methyltransferase and S-adenosylmethionine synthetase

In a chemically defined medium, L-methionine decreased production of rapamycin and increased that of demethylrapamycin. Growth with L-methionine yielded cells with a lower ability to convert demethylrapamycin to rapamycin and decreased the level of S-adenosylmethionine synthetase and S-adenosylmethionine. Thus, methionine represses at least one methyltransferase of rapamycin biosynthesis and S-adenosylmethionine synthetase.

Methionine deprivation and methionine analogs inhibit cell proliferation and growth of human xenografted gliomas

Growth of numerous malignant tumors depends on an exogenous methionine (MET) supply, while endogenously synthesized MET supports normal cell proliferation. Because an antitumor effect should be obtained by aggravating the altered MET metabolism in gliomas, MET dependency of human xenografted gliomas was evaluated and a therapeutic approach using MET deprivation or MET analogs to induce MET starvation was applied. In vitro proliferation inhibition of glioma cell lines by MET deprivation and two MET analogs, ethionine (ETH) and trifluoromethylhomocysteine (TFH), was measured. Proliferation of 7 human glioma cell lines tested was inhibited in MET-free medium, and was poorly or not reversed by homocysteine (HCY). ETH or TFH (concentration range: 0.005-2 mg/ml) inhibited proliferation of all cell lines tested. MET analog-induced inhibition was abolished by MET and enhanced by HCY. Cell-cycle alterations due to MET deprivation were optimally assessed after 30 h of culture and bromodeoxyuridine incorporation. In MET- medium, cells were arrested in the G1-phase. ETH induced a dramatic accumulation of cells in the G2-phase. ATP contents were reduced by MET analogs only in HCY+ medium, suggesting complementary effects of MET analogs and HCY. Human glioma bearing nude mice were fed an amino acid-substituted MET- HCY-supplemented diet (MET-HCY+) and/or treated with MET analogs, injected intraperitoneally daily. Using two human xenografted tumors derived from gliomas, antitumor effects were obtained by subjecting tumor-bearing nude mice to MET starvation. TG-1-MA was more sensitive to MET depletion (40% of growth inhibition, P < 0.10) than TG-8-OZ (no growth inhibition). Antitumor effects of a MET-HCY+ diet and 200 mg/kg of ETH were potentiated when co-administered to glioma-bearing mice (77% GI, P < 0.025 and 67%, P < 0.0057 to TG-1-MA and TG-8-OZ respectively). A dose-response effect with no toxicity was obtained when the ETH dose was increased 10 fold. Potentiation of the effects of ETH and a MET-free diet indicates that they probably act on the same pathway but not the same target. In conclusion, experimentally induced MET deprivation and MET-analog treatment retarded the growth of human gliomas. Combination of MET-analog therapy with MET substitution by HCY enhanced their respective effects.

Growth of methionine-dependent human prostate cancer (PC-3) is inhibited by ethionine combined with methionine starvation

Methionine (MET) is required for cell metabolism. MET endogenously synthesized from homocysteine (HCY) supports the proliferation of normal cells, but not that of numerous malignant cells, as shown previously. MET starvation should have an anti-tumour effect, and its deleterious effects on the hosts might be prevented by HCY. Anti-tumour effects of MET starvation must be reinforced by ethionine (ETH), a MET analogue. MET dependency of PC-3, a human prostate cancer cell line, was studied in vitro. Proliferation of PC-3 cells, cultivated in MET-free medium, was 29% compared with growth in MET+HCY- medium. Addition of HCY to MET-free medium increased the proliferation rate to 56%. The concentration of ETH required to decrease the PC-3 cell proliferation rate to 50% (IC50) was 0.5 mg ml(-1) in MET-HCY- medium. ETH-induced inhibition was abolished by MET addition and was reinforced by HCY. PC-3 cell cycle was blocked in the S-G2-phase after 30 h culture in the absence of MET; this blockage was not reversed by addition of HCY. ETH at the IC50 in MET-HCY+ medium blocked DNA replication. Apoptotic cells appeared after 30 h incubation in MET-HCY+ medium only when ETH was added. ATP pools were decreased after 15 h of culture in MET-free medium. In vivo, MET starvation was obtained by feeding tumour-bearing mice a diet containing a synthetic amino acid mixture as the protein supply, in which HCY replaced MET. Given to nude mice bearing xenografted PC-3, from day 1 after grafting and for 3 weeks, this diet inhibited tumour growth (34% on day 20, P < 0.007); this effect was potentiated by ETH (200 mg kg(-1) day(-1) i.p.) (56% on day 20, P < 5 x 10(-5)). The differences between the effects of these two treatments were significant (P < 0.017) and optimal on day 20. These data showed that combination of ETH and HCY slowed the proliferation of prostate cancer cells in vitro and in vivo, decreased ATP synthesis and caused cell cycle arrest and apoptosis. Experimental therapy based on cancer cell MET metabolism deficiency could be efficient for treating advanced prostate cancers refractory to current therapies.