The expression, purification, crystallization and preliminary X-ray diffraction analysis of CTP:inositol-1-phosphate cytidylyltransferase from A. fulgidus is described.
Archaeoglobus fulgidus, a hyperthermophilic archaeon, accumulates di-myo-inositol phosphate (DIP) in response to heat stress. Recently, the pathway for biosynthesis of DIP has been elucidated in this organism and involves a bifunctional enzyme that contains two domains: CTP:inositol-1-phosphate cytidylyltransferase (IPCT) as a soluble domain and di-myo-inositol-1,3′-phosphate-1-phosphate synthase (DIPPS) as a membrane domain. Here, the expression, purification, crystallization and preliminary X-ray diffraction analysis of the IPCT domain from A. fulgidus in the apo form are reported. The crystals diffracted to 2.4 Å resolution using a synchrotron source and belonged to the orthorhombic space group P21212, with unit-cell parameters a = 154.7, b = 83.9, c = 127.7 Å.
CTP:inositol-1-phosphate cytidylyltransferase; Archaeoglobus fulgidus; compatible solutes; CDP-inositol; di-myo-inositol phosphate
The pathway for the synthesis of di-myo-inositol-phosphate (DIP) was recently elucidated on the basis of the detection of the relevant activities in cell extracts of Archaeoglobus fulgidus and structural characterization of products by nuclear magnetic resonance (NMR) (N. Borges, L. G. Gonçalves, M. V. Rodrigues, F. Siopa, R. Ventura, C. Maycock, P. Lamosa, and H. Santos, J. Bacteriol. 188:8128-8135, 2006). Here, a genomic approach was used to identify the genes involved in the synthesis of DIP. Cloning and expression in Escherichia coli of the putative genes for CTP:l-myo-inositol-1-phosphate cytidylyltransferase and DIPP (di-myo-inositol-1,3′-phosphate-1′-phosphate, a phosphorylated form of DIP) synthase from several (hyper)thermophiles (A. fulgidus, Pyrococcus furiosus, Thermococcus kodakaraensis, Aquifex aeolicus, and Rubrobacter xylanophilus) confirmed the presence of those activities in the gene products. The DIPP synthase activity was part of a bifunctional enzyme that catalyzed the condensation of CTP and l-myo-inositol-1-phosphate into CDP-l-myo-inositol, as well as the synthesis of DIPP from CDP-l-myo-inositol and l-myo-inositol-1-phosphate. The cytidylyltransferase was absolutely specific for CTP and l-myo-inositol-1-P; the DIPP synthase domain used only l-myo-inositol-1-phosphate as an alcohol acceptor, but CDP-glycerol, as well as CDP-l-myo-inositol and CDP-d-myo-inositol, were recognized as alcohol donors. Genome analysis showed homologous genes in all organisms known to accumulate DIP and for which genome sequences were available. In most cases, the two activities (l-myo-inositol-1-P cytidylyltransferase and DIPP synthase) were fused in a single gene product, but separate genes were predicted in Aeropyrum pernix, Thermotoga maritima, and Hyperthermus butylicus. Additionally, using l-myo-inositol-1-phosphate labeled on C-1 with carbon 13, the stereochemical configuration of all the metabolites involved in DIP synthesis was established by NMR analysis. The two inositol moieties in DIP had different stereochemical configurations, in contradiction of previous reports. The use of the designation di-myo-inositol-1,3′-phosphate is recommended to facilitate tracing individual carbon atoms through metabolic pathways.
Archaeoglobus fulgidus accumulates di-myo-inositol phosphate (DIP) and diglycerol phosphate (DGP) in response to heat and osmotic stresses, respectively, and the level of glycero-phospho-myo-inositol (GPI) increases primarily when the two stresses are combined. In this work, the pathways for the biosynthesis of these three compatible solutes were established based on the detection of the relevant enzymatic activities and characterization of the intermediate metabolites by nuclear magnetic resonance analysis. The synthesis of DIP proceeds from glucose-6-phosphate via four steps: (i) glucose-6-phosphate was converted into l-myo-inositol 1-phosphate by l-myo-inositol 1-phosphate synthase; (ii) l-myo-inositol 1-phosphate was activated to CDP-inositol at the expense of CTP; this is the first demonstration of CDP-inositol synthesis in a biological system; (iii) CDP-inositol was coupled with l-myo-inositol 1-phosphate to yield a phosphorylated intermediate, 1,1′-di-myo-inosityl phosphate 3-phosphate (DIPP); (iv) finally, DIPP was dephosphorylated into DIP by the action of a phosphatase. The synthesis of the two other polyol-phosphodiesters, DGP and GPI, proceeds via the condensation of CDP-glycerol with the respective phosphorylated polyol, glycerol 3-phosphate for DGP and l-myo-inositol 1-phosphate for GPI, yielding the respective phosphorylated intermediates, 1X,1′X-diglyceryl phosphate 3-phosphate (DGPP) and 1-(1X-glyceryl) myo-inosityl phosphate 3-phosphate (GPIP), which are subsequently dephosphorylated to form the final products. The results disclosed here represent an important step toward the elucidation of the regulatory mechanisms underlying the differential accumulation of these compounds in response to heat and osmotic stresses.
Many of the marine microorganisms which are adapted to grow at temperatures above 80°C accumulate di-myo-inositol phosphate (DIP) in response to heat stress. This led to the hypothesis that the solute plays a role in thermoprotection, but there is a lack of definitive experimental evidence. Mutant strains of Thermococcus kodakarensis (formerly Thermococcus kodakaraensis), manipulated in their ability to synthesize DIP, were constructed and used to investigate the involvement of DIP in thermoadaptation of this archaeon. The solute pool of the parental strain comprised DIP, aspartate, and α-glutamate. Under heat stress the level of DIP increased 20-fold compared to optimal conditions, whereas the pool of aspartate increased 4.3-fold in response to osmotic stress. Deleting the gene encoding the key enzyme in DIP synthesis, CTP:inositol-1-phosphate cytidylyltransferase/CDP-inositol:inositol-1-phosphate transferase, abolished DIP synthesis. Conversely, overexpression of the same gene resulted in a mutant with restored ability to synthesize DIP. Despite the absence of DIP in the deletion mutant, this strain exhibited growth parameters similar to those of the parental strain, both at optimal (85°C) and supraoptimal (93.7°C) temperatures for growth. Analysis of the respective solute pools showed that DIP was replaced by aspartate. We conclude that DIP is part of the strategy used by T. kodakarensis to cope with heat stress, and aspartate can be used as an alternative solute of similar efficacy. This is the first study using mutants to demonstrate the involvement of compatible solutes in the thermoadaptation of (hyper)thermophilic organisms.
Biosynthesis of di-myo-inositol-1,1′-phosphate (DIP) is proposed to occur with myo-inositol and myo-inositol 1-phosphate (I-1-P) used as precursors. Activation of the I-1-P with CTP and condensation of the resultant CDP-inositol (CDP-I) with myo-inositol then generates DIP. The sole known biosynthetic pathway of inositol in all organisms is the conversion of d-glucose-6-phosphate to myo-inositol. This conversion requires two key enzymes: l-I-1-P synthase and I-1-P phosphatase. Enzymatic assays using 31P nuclear magnetic resonance spectroscopy as well as a colorimetric assay for inorganic phosphate have confirmed the occurrence of l-I-1-P synthase and a moderately specific I-1-P phosphatase. The enzymatic reaction that couples CDP-I with myo-inositol to generate DIP has also been detected in Methanococcus igneus. 13C labeling studies with [2,3-13C]pyruvate and [3-13C]pyruvate were used to examine this pathway in M. igneus. Label distribution in DIP was consistent with inositol units formed from glucose-6-phosphate, but the label in the glucose moiety was scrambled via transketolase and transaldolase activities of the pentose phosphate pathway.
We examined the accumulation of organic solutes under optimum growth conditions in 12 species of thermophilic and hyperthermophilic Archaea belonging to the Crenarchaeota and Euryarchaeota. Pyrobaculum aerophilum, Thermoproteus tenax, Thermoplasma acidophilum, and members of the order Sulfolobales accumulated trehalose. Pyrococcus furiosus accumulated di-myo-inositol-1,1(prm1)(3,3(prm1))-phosphate and (beta)-mannosylglycerate, Methanothermus fervidus accumulated cyclic-2,3-bisphosphoglycerate and (beta)-mannosylglycerate, while the only solute detected in Pyrodictium occultum was di-myo-inositol-1,1(prm1)(3,3(prm1))-phosphate. Methanopyrus kandleri accumulated large concentrations of cyclic-2,3-bisphosphoglycerate. On the other hand, Archaeoglobus fulgidus accumulated three phosphorylated solutes; prominent among them was a compound identified as di-glycerol-phosphate. This solute increased in concentration as the salinity of the medium and the growth temperature were raised, suggesting that this compound serves as a general stress solute. Di-myo-inositol-1,1(prm1)(3,3(prm1))-phosphate accumulated at supraoptimal temperature only. The relationship between the accumulation of unusual solutes and high temperatures is also discussed.
NADP(H) phosphatase has not been identified in eubacteria and eukaryotes. In archaea, MJ0917 of hyperthermophilic Methanococcus jannaschii is a fusion protein comprising NAD kinase and an inositol monophosphatase homologue that exhibits high NADP(H) phosphatase activity (S. Kawai, C. Fukuda, T. Mukai, and K. Murata, J. Biol. Chem. 280:39200-39207, 2005). In this study, we showed that the other archaeal inositol monophosphatases, MJ0109 of M. jannaschii and AF2372 of hyperthermophilic Archaeoglobus fulgidus, exhibit NADP(H) phosphatase activity in addition to the already-known inositol monophosphatase and fructose-1,6-bisphosphatase activities. Kinetic values for NADP+ and NADPH of MJ0109 and AF2372 were comparable to those for inositol monophosphate and fructose-1,6-bisphosphate. This implies that the physiological role of the two enzymes is that of an NADP(H) phosphatase. Further, the two enzymes showed inositol polyphosphate 1-phosphatase activity but not 3′-phosphoadenosine 5′-phosphate phosphatase activity. The inositol polyphosphate 1-phosphatase activity of archaeal inositol monophosphatase was considered to be compatible with the similar tertiary structures of inositol monophosphatase, fructose-1,6-bisphosphatase, inositol polyphosphate 1-phosphatase, and 3′-phosphoadenosine 5′-phosphate phosphatase. Based on this fact, we found that 3′-phosphoadenosine 5′-phosphate phosphatase (CysQ) of Escherichia coli exhibited NADP(H) phosphatase and fructose-1,6-bisphosphatase activities, although inositol monophosphatase (SuhB) and fructose-1,6-bisphosphatase (Fbp) of E. coli did not exhibit any NADP(H) phosphatase activity. However, the kinetic values of CysQ and the known phenotype of the cysQ mutant indicated that CysQ functions physiologically as 3′-phosphoadenosine 5′-phosphate phosphatase rather than as NADP(H) phosphatase.
Inositol monophosphatase (EC 22.214.171.124) plays a pivotal role in the biosynthesis of di-myo-inositol-1,1′-phosphate, an osmolyte found in hyperthermophilic archaea. Given the sequence homology between the MJ109 gene product of Methanococcus jannaschii and human inositol monophosphatase, the MJ109 gene was cloned and expressed in Escherichia coli and examined for inositol monophosphatase activity. The purified MJ109 gene product showed inositol monophosphatase activity with kinetic parameters (Km = 0.091 ± 0.016 mM; Vmax = 9.3 ± 0.45 μmol of Pi min−1 mg of protein−1) comparable to those of mammalian and E. coli enzymes. Its substrate specificity, Mg2+ requirement, Li+ inhibition, subunit association (dimerization), and heat stability were studied and compared to those of other inositol monophosphatases. The lack of inhibition by low concentrations of Li+ and high concentrations of Mg2+ and the high rates of hydrolysis of glucose-1-phosphate and p-nitrophenylphosphate are the most pronounced differences between the archaeal inositol monophosphatase and those from other sources. The possible causes of these kinetic differences are discussed, based on the active site sequence alignment between M. jannaschii and human inositol monophosphatase and the crystal structure of the mammalian enzyme.
In addition to di-myo-inositol-1,3′-phosphate (DIP), a compatible solute widespread in hyperthermophiles, the organic solute pool of Thermotoga maritima comprises 2-(O-β-d-mannosyl)-di-myo-inositol-1,3′-phosphate (MDIP) and 2-(O-β-d-mannosyl-1,2-O-β-d-mannosyl)-di-myo-inositol-1,3′-phosphate (MMDIP), two newly identified β-1,2-mannosides. In cells grown under heat stress, MDIP was the major solute, accounting for 43% of the total pool; MMDIP and DIP accumulated to similar levels, each corresponding to 11.5% of the total pool. The synthesis of MDIP involved the transfer of the mannosyl group from GDP-mannose to DIP in a single-step reaction catalyzed by MDIP synthase. This enzyme used MDIP as an acceptor of a second mannose residue, yielding the di-mannosylated compound. Minor amounts of the tri-mannosylated form were also detected. With a genomic approach, putative genes for MDIP synthase were identified in the genome of T. maritima, and the assignment was confirmed by functional expression in Escherichia coli. Genes with significant sequence identity were found only in the genomes of Thermotoga spp., Aquifex aeolicus, and Archaeoglobus profundus. MDIP synthase of T. maritima had maximal activity at 95°C and apparent Km values of 16 mM and 0.7 mM for DIP and GDP-mannose, respectively. The stereochemistry of MDIP was characterized by isotopic labeling and nuclear magnetic resonance (NMR): DIP selectively labeled with carbon 13 at position C1 of the l-inositol moiety was synthesized and used as a substrate for MDIP synthase. This β-1,2-mannosyltransferase is unrelated to known glycosyltransferases, and within the domain Bacteria, it is restricted to members of the two deepest lineages, i.e., the Thermotogales and the Aquificales. To our knowledge, this is the first β-1,2-mannosyltransferase characterized thus far.
The hyperthermophilic archaeon Archaeoglobus fulgidus strain 7324 has been shown to grow on starch and sulfate and thus represents the first sulfate reducer able to degrade polymeric sugars. The enzymes involved in starch degradation to glucose 6-phosphate were studied. In extracts of starch-grown cells the activities of the classical starch degradation enzymes, α-amylase and amylopullulanase, could not be detected. Instead, evidence is presented here that A. fulgidus utilizes an unusual pathway of starch degradation involving cyclodextrins as intermediates. The pathway comprises the combined action of an extracellular cyclodextrin glucanotransferase (CGTase) converting starch to cyclodextrins and the intracellular conversion of cyclodextrins to glucose 6-phosphate via cyclodextrinase (CDase), maltodextrin phosphorylase (Mal-P), and phosphoglucomutase (PGM). These enzymes, which are all induced after growth on starch, were characterized. CGTase catalyzed the conversion of starch to mainly β-cyclodextrin. The gene encoding CGTase was cloned and sequenced and showed highest similarity to a glucanotransferase from Thermococcus litoralis. After transport of the cyclodextrins into the cell by a transport system to be defined, these molecules are linearized via a CDase, catalyzing exclusively the ring opening of the cyclodextrins to the respective maltooligodextrins. These are degraded by a Mal-P to glucose 1-phosphate. Finally, PGM catalyzes the conversion of glucose 1-phosphate to glucose 6-phosphate, which is further degraded to pyruvate via the modified Embden-Meyerhof pathway.
The addition of L-serine to inositol-containing growth medium repressed membrane-associated CDPdiacylglycerol synthase (CTP:phosphatidate cytidylyltransferase, EC 126.96.36.199) and phosphatidylserine synthase (CDPdiacylglycerol:L-serine O-phosphatidyltransferase, EC 188.8.131.52) activities and subunit levels in wild-type Saccharomyces cerevisiae. Enzyme activities and subunit levels were not repressed when inositol was absent from the growth medium. The addition of L-serine to the growth medium did not affect the phospholipid composition of wild-type cells. CDPdiacylglycerol synthase and phosphatidylserine synthase were not regulated in the S. cerevisiae inositol biosynthesis ino2, ino4, and opi1 regulatory mutants, suggesting that regulation by inositol plus L-serine is coupled to inositol synthesis. Inositol and L-serine did not affect the activities of purified CDPdiacylglycerol synthase and phosphatidylserine synthase. The addition of compounds structurally related to L-serine to the growth medium of wild-type cells also resulted in a repression of CDPdiacylglycerol synthase and phosphatidylserine synthase but only in the presence of inositol. Phosphatidylinositol synthase (CDPdiacylglycerol:myo-inositol 3-phosphatidyltransferase, EC 184.108.40.206) was not regulated by inositol plus L-serine.
myo-Inositol (inositol) is an essential nutrient that is used for building phosphatidylinositol and its derivatives in eukaryotes and even in some eubacteria such as the mycobacteria. As a consequence, fungal, protozoan and mycobacterial pathogens must be able to acquire inositol in order to proliferate and cause infection in their hosts. There are two primary mechanisms for acquiring inositol. One is to synthesize inositol from glucose 6-phosphate using two sequentially acting enzymes: inositol-3-phosphate synthase (Ino1p) converts glucose 6-phosphate to inositol 3-phosphate, and then inositol monophosphatase (IMPase) dephosphorylates inositol 3-phosphate to generate inositol. The other mechanism is to import inositol from the environment via inositol transporters. Inositol is readily abundant in the bloodstream of mammalian hosts, providing a source from which many pathogens could potentially import inositol. However, despite this abundance of inositol in the host, some pathogens such as the bacterium Mycobacterium tuberculosis and the protist parasite Trypanosoma brucei must be able to make inositol de novo in order to cause disease (M. tuberculosis) or even grow (T. brucei). Other pathogens such as the fungus Candida albicans are equally adept at causing disease by importing inositol or by making it de novo. The role of inositol acquisition in the biology and pathogenesis of the parasite Leishmania and the fungus Cryptococcus are being explored as well. The specific strategies used by these pathogens to acquire inositol while in the host are discussed in relation to each pathogen's unique metabolic requirements.
The cell wall of Mycobacterium tuberculosis contains a wide range of phosphatidyl inositol-based glycolipids that play critical structural roles and, in part, govern pathogen-host interactions. Synthesis of phosphatidyl inositol is dependent on free myo-inositol, generated through dephosphorylation of myo-inositol-1-phosphate by inositol monophosphatase (IMPase). Human IMPase, the putative target of lithium therapy, has been studied extensively, but the function of four IMPase-like genes in M. tuberculosis is unclear.
We determined the crystal structure, to 2.6 Å resolution, of the IMPase M. tuberculosis SuhB in the apo form, and analysed self-assembly by analytical ultracentrifugation. Contrary to the paradigm of constitutive dimerization of IMPases, SuhB is predominantly monomeric in the absence of the physiological activator Mg2+, in spite of a conserved fold and apparent dimerization in the crystal. However, Mg2+ concentrations that result in enzymatic activation of SuhB decisively promote dimerization, with the inhibitor Li+ amplifying the effect of Mg2+, but failing to induce dimerization on its own.
The correlation of Mg2+-driven enzymatic activity with dimerization suggests that catalytic activity is linked to the dimer form. Current models of lithium inhibition of IMPases posit that Li+ competes for one of three catalytic Mg2+ sites in the active site, stabilized by a mobile loop at the dimer interface. Our data suggest that Mg2+/Li+-induced ordering of this loop may promote dimerization by expanding the dimer interface of SuhB. The dynamic nature of the monomer-dimer equilibrium may also explain the extended concentration range over which Mg2+ maintains SuhB activity.
The membrane lipids of archaea are characterized by unique isoprenoid biochemistry, which typically is based on two core lipid structures, sn-2,3-diphytanylglycerol diether (archaeol) and sn-2,3-dibiphytanyldiglycerol tetraether (caldarchaeol). The biosynthetic pathway for the tetraether lipid entails unprecedented head-to-head coupling of isoprenoid intermediates by an unknown mechanism involving unidentified enzymes. To investigate the isoprenoid ether lipid biosynthesis pathway of the hyperthermophilic archaeon, Archaeoglobus fulgidus, its lipid synthesis machinery was reconstructed in an engineered E. coli strain in an effort to demonstrate, for the first time, efficient isoprenoid ether lipid biosynthesis for the production of the intermediate, digeranylgeranylglyceryl phosphate (DGGGP). The biosynthesis of DGGGP was verified using a LC/MS/MS technique and was accomplished by cloning and expressing the native E. coli gene for IPP isomerase (idi), along with the A. fulgidus genes for G1P dehydrogenase (egsA) and GGPP synthase (gps), under the control of the lac promoter. The A. fulgidus genes for GGGP synthase (GGGPS) and DGGGP synthase (DGGGPS), under the control of the araBAD promoter, were then introduced and expressed to enable DGGGP biosynthesis in vivo. This investigation established roles for four A. fulgidus genes in the isoprenoid ether lipid pathway for DGGGP biosynthesis and provides a platform useful for identification of subsequent, currently unknown, steps in tetraether lipid biosynthesis proceeding from DGGGP, which is the presumed substrate for the head-to-head coupling reaction yielding unsaturated caldarchaeol.
Archaeoglobus fulgidus; isoprenoid; ether lipid; DGGGP
myo-Inositol monophosphatase (IMP) is an essential enzyme in the myo-inositol metabolic pathway where it primarily dephosphorylates myo-inositol 1-phosphate to maintain the cellular inositol pool which is important for many metabolic and signalling pathways in plants. The stress-induced increased accumulation of inositol has been reported in a few plants including chickpea; however, the role and regulation of IMP is not well defined in response to stress. In this work, it has been shown that IMP activity is distributed in all organs in chickpea and was noticeably enhanced during environmental stresses. Subsequently, using degenerate oligonucleotides and RACE strategy, a full-length IMP cDNA (CaIMP) was cloned and sequenced. Biochemical study revealed that CaIMP encodes a lithium-sensitive phosphatase enzyme with broad substrate specificity, although maximum activity was observed with the myo-inositol 1-phosphate and l-galactose 1-phosphate substrates. Transcript analysis revealed that CaIMP is differentially expressed and regulated in different organs, stresses and phytohormones. Complementation analysis in Arabidopsis further confirmed the role of CaIMP in l-galactose 1-phosphate and myo-inositol 1-phosphate hydrolysis and its participation in myo-inositol and ascorbate biosynthesis. Moreover, Arabidopsis transgenic plants over-expressing CaIMP exhibited improved tolerance to stress during seed germination and seedling growth, while the VTC4/IMP loss-of-function mutants exhibited sensitivity to stress. Collectively, CaIMP links various metabolic pathways and plays an important role in improving seed germination and seedling growth, particularly under stressful environments.
Ascorbate; gluconeogenesis; inositol; multifunctional; phosphatase; seed germination; stress tolerance.
Bluensomycin (glebomycin) is an aminocyclitol antibiotic that differs structurally from dihydrostreptomycin in having bluensidine (1D-1-O-carbamoyl-3-guanidinodeoxy-scyllo-inositol) rather than streptidine (1,3-diguanidino-1,3-dideoxy-scyllo-inositol) as its aminocyclitol moiety. Extracts of the bluensomycin producer Streptomyces hygroscopicus form glebosus ATCC 14607 (S. glebosus) were found to have aminodeoxy-scyllo-inositol kinase activity but to lack 1D-1-guanidino-3-amino-1,3-dideoxy-scyllo-inositol kinase activity, showing for the first time that these two reactions in streptomycin producers must be catalyzed by different enzymes. S. glebosus extracts therefore possess the same five enzymes required for synthesis of guanidinodeoxy-scyllo-inositol from myo-inositol that are found in streptomycin producers but lack the next three of the four enzymes found in streptomycin producers that are required to synthesize the second guanidino group of streptidine-P. In place of a second guanidino group, S. glebosus extracts were found to catalyze a Mg2(+)-dependent carbamoylation of guanidinodeoxy-scyllo-inositol to form bluensidine, followed by a phosphorylation to form bluensidine-P. The novel carbamoyl-P:guanidinodeoxy-scyllo-inositol O-carbamoyltransferase and ATP:bluensidine phosphotransferase activities were not detected in streptomycin producers or in S. glebosus during its early rapid growth phase. Free bluensidine appears to be a normal intermediate in bluensomycin biosynthesis, in contrast to the case of streptomycin biosynthesis; in the latter, although exogenous streptidine can enter the pathway via streptidine-P, free streptidine is not an intermediate in the endogenous biosynthetic pathway. Comparison of the streptomycin and bluensomycin biosynthetic pathways provides a unique opportunity to evaluate those proposed mechanisms for the evolutionary acquisition of new biosynthetic capabilities that involve gene duplication and subsequent mutational changes in one member of the pair. In this model, there are at least five pairs of enzymes catalyzing analogous reactions that can be analyzed for homology at both the protein and DNA levels, including two putative pairs of inositol kinases detected in this study.
myo-Inositol is a precursor for cell wall components, is used as a backbone of myo-inositol trisphosphate (Ins(1,4,5)P3) and phosphatidylinositol phosphate signaling molecules, and is debated about whether it is also a precursor in an alternate ascorbic acid synthesis pathway. Plants control inositol homeostasis by regulation of key enzymes involved in myo-inositol synthesis and catabolism. Recent transcriptional profiling data indicate up-regulation of the myo-inositol oxygenase (MIOX) genes under conditions in which energy or nutrients are limited. To test whether the MIOX genes are required for responses to low energy, we first examined MIOX2 and MIOX4 gene expression regulation by energy/nutrient conditions. We found that both MIOX2 and MIOX4 expression are suppressed by exogenous glucose addition in the shoot, but not in the root. Both genes were abundantly expressed during low energy/nutrient conditions. Loss-of-function mutants in MIOX genes contain alterations in myo-inositol levels and growth changes in the root. Miox2 mutants can be complemented with a MIOX2:green fluorescent protein fusion. Further we show here that MIOX2 is a cytoplasmic protein, while MIOX4 is present mostly in the cytoplasm, but also occasionally in the nucleus. Together, these data suggest that MIOX catabolism in the shoot may influence root growth responses during low energy/nutrient conditions.
myo-inositol; ascorbic acid; nutrient response; energy
Phytases hydrolyse phytate (myo-inositol hexakisphosphate), the principal form of phosphate stored in plant seeds to produce phosphate and lower phosphorylated myo-inositols. They are used extensively in the feed industry, and have been characterised biochemically and structurally with a number of structures in the PDB. They are divided into four distinct families: histidine acid phosphatases (HAP), β-propeller phytases, cysteine phosphatases and purple acid phosphatases and also split into three enzyme classes, the 3-, 5- and 6-phytases, depending on the position of the first phosphate in the inositol ring to be removed. We report identification, cloning, purification and 3D structures of 6-phytases from two bacteria, Hafnia alvei and Yersinia kristensenii, together with their pH optima, thermal stability, and degradation profiles for phytate. An important result is the structure of the H. alvei enzyme in complex with the substrate analogue myo-inositol hexakissulphate. In contrast to the only previous structure of a ligand-bound 6-phytase, where the 3-phosphate was unexpectedly in the catalytic site, in the H. alvei complex the expected scissile 6-phosphate (sulphate in the inhibitor) is placed in the catalytic site.
Inositol pyrophosphates (such as IP7 and IP8) are multifunctional signaling molecules that regulate diverse cellular activities. Inositol pyrophosphates have ‘high-energy’ phosphoanhydride bonds, so their enzymatic synthesis requires that a substantial energy barrier to the transition state be overcome. Additionally, inositol pyrophosphate kinases can show stringent ligand specificity, despite the need to accommodate the steric bulk and intense electronegativity of nature’s most concentrated three-dimensional array of phosphate groups. Here we examine how these catalytic challenges are met by describing the structure and reaction cycle of an inositol pyrophosphate kinase at the atomic level. We obtained crystal structures of the kinase domain of human PPIP5K2 complexed with nucleotide cofactors and either substrates, product or a MgF3− transition-state mimic. We describe the enzyme’s conformational dynamics, its unprecedented topological presentation of nucleotide and inositol phosphate, and the charge balance that facilitates partly associative in-line phosphoryl transfer.
A synthetic pathway has been constructed for the production of glucuronic and glucaric acids from glucose in Escherichia coli. Coexpression of the genes encoding myo-inositol-1-phosphate synthase (Ino1) from Saccharomyces cerevisiae and myo-inositol oxygenase (MIOX) from mice led to production of glucuronic acid through the intermediate myo-inositol. Glucuronic acid concentrations up to 0.3 g/liter were measured in the culture broth. The activity of MIOX was rate limiting, resulting in the accumulation of both myo-inositol and glucuronic acid as final products, in approximately equal concentrations. Inclusion of a third enzyme, uronate dehydrogenase (Udh) from Pseudomonas syringae, facilitated the conversion of glucuronic acid to glucaric acid. The activity of this recombinant enzyme was more than 2 orders of magnitude higher than that of Ino1 and MIOX and increased overall flux through the pathway such that glucaric acid concentrations in excess of 1 g/liter were observed. This represents a novel microbial system for the biological production of glucaric acid, a “top value-added chemical” from biomass.
(sup13)C and (sup1)H nuclear magnetic resonance spectroscopy was used to identify and quantify organic solutes accumulated by the hyperthermophilic archaeon Pyrococcus furiosus in response to temperature and salinity. Di-myo-inositol-phosphate and 2-O-(beta)-mannosylglycerate were the major organic solutes accumulated in these cells. The total intracellular organic solutes increased significantly in response either to an increase in temperature or to an increase in salinity, but (beta)-mannosylglycerate accumulated mainly at high salinities, whereas the concentration of di-myo-inositol-phosphate increased dramatically at supraoptimal growth temperatures. Glutamate was present at concentrations detectable by nuclear magnetic resonance only in cells grown in low-salinity media. The intracellular levels of K(sup+) are clearly dependent on the salinity of the medium, and the concentrations of this cation are high enough to counterbalance the negative charges of (beta)-mannosylglycerate and di-myo-inositol-phosphate in the cell. The results presented here together with those previously reported for Pyrococcus woesei (S. Scholz, J. Sonnenbichler, W. Schafer, and R. Hensel, FEBS Lett. 306:239-242, 1992) strongly support a role for di-myo-inositol-phosphate in thermoprotection.
Radiation-resistant bacteria have garnered a great deal of attention from scientists seeking to expose the mechanisms underlying their incredible survival abilities. Recent analyses showed that the resistance to ionizing radiation (IR) in the archaeon Halobacterium salinarum is dependent upon Mn-antioxidant complexes responsible for the scavenging of reactive oxygen species (ROS) generated by radiation. Here we examined the role of the compatible solutes trehalose, mannosylglycerate, and di-myo-inositol phosphate in the radiation resistance of aerobic and anaerobic thermophiles. We found that the IR resistance of the thermophilic bacteria Rubrobacter xylanophilus and Rubrobacter radiotolerans was highly correlated to the accumulation of high intracellular concentration of trehalose in association with Mn, supporting the model of Mn2+-dependent ROS scavenging in the aerobes. In contrast, the hyperthermophilic archaea Thermococcus gammatolerans and Pyrococcus furiosus did not contain significant amounts of intracellular Mn, and we found no significant antioxidant activity from mannosylglycerate and di-myo-inositol phosphate in vitro. We therefore propose that the low levels of IR-generated ROS under anaerobic conditions combined with highly constitutively expressed detoxification systems in these anaerobes are key to their radiation resistance and circumvent the need for the accumulation of Mn-antioxidant complexes in the cell.
The addition of ethanolamine or choline to inositol-containing growth medium resulted in a reduction of CTP:phosphatidate cytidylyltransferase (CDP-diacylglycerol synthase; EC 220.127.116.11) activity in Saccharomyces cerevisiae. The reduction of activity did not occur in the absence of inositol. CDP-diacylglycerol synthase activity was not regulated in a S. cerevisiae mutant strain (opi1; an inositol biosynthesis regulatory mutant) by the addition of phospholipid precursors to the growth medium.
PH domains represent one of the most common domains in the human proteome. These domains are recognized as important mediators of protein-phosphoinositide and protein-protein interactions. Phosphoinositides are lipid components of the membrane that function as signaling molecules by targeting proteins to their sites of action. Phosphoinositide based signaling pathways govern a diverse range of important cellular processes including membrane remodeling, differentiation, proliferation and survival. Myo-Inositol phosphates are soluble signaling molecules that are structurally similar to the head groups of phosphoinositides. These molecules have been proposed to function, at least in part, by regulating PH domain-phosphoinositide interactions. Given the structural similarity of inositol phosphates we were interested in examining the specificity of PH domains towards the family of myo-inositol pentakisphosphate isomers.
In work reported here we demonstrate that the C-terminal PH domain of pleckstrin possesses the specificity required to discriminate between different myo-inositol pentakisphosphate isomers. The structural basis for this specificity was determined using high-resolution crystal structures. Moreover, we show that while the PH domain of Grp1 does not possess this high degree of specificity, the PH domain of protein kinase B does.
These results demonstrate that some PH domains possess enough specificity to discriminate between myo-inositol pentakisphosphate isomers allowing for these molecules to differentially regulate interactions with phosphoinositides. Furthermore, this work contributes to the growing body of evidence supporting myo-inositol phosphates as regulators of important PH domain-phosphoinositide interactions. Finally, in addition to expanding our knowledge of cellular signaling, these results provide a basis for developing tools to probe biological pathways.
Sorbitol (aldose reductase) pathway flux in diabetes perturbs intracellular metabolism by two putative mechanisms: reciprocal osmoregulatory depletion of other organic osmolytes e.g., myo-inositol, and alterations in NADPH/NADP+ and/or NADH/NAD+. The "osmolyte" and "redox" hypotheses predict secondary elevations in CDP-diglyceride, the rate-limiting precursor for phosphatidylinositol synthesis, but through different mechanisms: the "osmolyte" hypothesis via depletion of intracellular myo-inositol (the cosubstrate for phosphatidylinositol-synthase) and the "redox" hypothesis through enhanced de novo synthesis from triose phosphates. The osmolyte hypothesis predicts diminished phosphoinositide-derived arachidonyl-diacylglycerol, while the redox hypothesis predicts increased total diacylglycerol and phosphatidic acid. In high aldose reductase expressing retinal pigment epithelial cells, glucose-induced, aldose reductase inhibitor-sensitive CDP-diglyceride accumulation and inhibition of 32P-incorporation into phosphatidylinositol paralleled myo-inositol depletion (but not cytoplasmic redox, that was unaffected by glucose) and depletion of arachidonyl-diacylglycerol. 3 mM pyruvate added to the culture medium left cellular redox unaltered, but stimulated Na(+)-dependent myo-inositol uptake, accumulation, and incorporation into phosphatidylinositol. These results favor myo-inositol depletion rather than altered redox as the primary cause of glucose-induced aldose reductase-related defects in phospholipid metabolism in cultured retinal pigment epithelial cells.