Glucosyl-3-phosphoglycerate synthase (GpgS) is a key enzyme that catalyses the first glucosylation step in methylglucose lipopolysaccharide biosynthesis in Mycobacterium spp. Here, the crystallization and preliminary crystallographic analysis of GpgS from M. tuberculosis and of its complex with UDP are reported.
Glucosyl-3-phosphoglycerate synthase (GpgS) is a key enzyme that catalyses the first glucosylation step in methylglucose lipopolysaccharide biosynthesis in mycobacteria. These important molecules are believed to be involved in the regulation of fatty-acid and mycolic acid synthesis. The enzyme belongs to the recently defined GT81 family of retaining glycosyltransferases (CAZy, Carbohydrate-Active Enzymes Database; see http://www.cazy.org). Here, the purification, crystallization and preliminary crystallographic analysis are reported of GpgS from Mycobacterium tuberculosis and of its complex with UDP. GpgS crystals belonged to space group I4, with unit-cell parameters a = 98.85, b = 98.85, c = 127.64 Å, and diffracted to 2.6 Å resolution. GpgS–UDP complex crystals belonged to space group I4, with unit-cell parameters a = 98.32, b = 98.32, c = 127.96 Å, and diffracted to 3.0 Å resolution.
glycosyltransferases; methylglucose lipopolysaccharides; Mycobacterium tuberculosis
Mycobacteria synthesize intracellular methylglucose lipopolysaccharides (MGLP) proposed to regulate fatty acid synthesis. Although their structures have been elucidated, the identity of most biosynthetic genes remains unknown. The first step in MGLP biosynthesis is catalyzed by a glucosyl-3-phosphoglycerate synthase (GpgS, Rv1208 in Mycobacterium tuberculosis H37Rv). However, a typical glucosyl-3-phosphoglycerate phosphatase (GpgP, EC184.108.40.206) for dephosphorylation of glucosyl-3-phosphoglycerate to glucosylglycerate, was absent from mycobacterial genomes. We purified the native GpgP from Mycobacterium vanbaalenii and identified the corresponding gene deduced from amino acid sequences by mass spectrometry. The M. tuberculosis ortholog (Rv2419c), annotated as a putative phosphoglycerate mutase (PGM, EC220.127.116.11), was expressed and functionally characterized as a new GpgP. Regardless of the high specificity for glucosyl-3-phosphoglycerate, the mycobacterial GpgP is not a sequence homolog of known isofunctional GpgPs. The assignment of a new function in M. tuberculosis genome expands our understanding of this organism's genetic repertoire and of the early events in MGLP biosynthesis.
A single-step pathway for the synthesis of the compatible solute glucosylglycerate (GG) is proposed based on the activity of a recombinant glucosylglycerate synthase (Ggs) from Persephonella marina. The corresponding gene encoded a putative glycosyltransferase that was part of an operon-like structure which also contained the genes for glucosyl-3-phosphoglycerate synthase (GpgS) and glucosyl-3-phosphoglycerate phosphatase (GpgP), the enzymes that lead to the synthesis of GG through the formation of glucosyl-3-phosphoglycerate. The putative glucosyltransferase gene was expressed in Escherichia coli, and the recombinant product catalyzed the synthesis of GG in one step from ADP-glucose and d-glycerate, with Km values at 70°C of 1.5 and 2.2 mM, respectively. This glucosylglycerate synthase (Ggs) was also able to use GDP- and UDP-glucose as donors to form GG, but the efficiencies were lower. Maximal activity was observed at temperatures between 80 and 85°C, and Mg2+ or Ca2+ was required for catalysis. Ggs activity was maximal and remained nearly constant at pH values between 5.5 and pH 8.0, and the half-lives for inactivation were 74 h at 85°C and 8 min at 100°C. This is the first report of an enzyme catalyzing the synthesis of GG in one step and of the existence of two pathways for GG synthesis in the same organism.
The pathway for the synthesis of glucosylglycerate (GG) in the thermophilic bacterium Persephonella marina is proposed based on the activities of recombinant glucosyl-3-phosphoglycerate (GPG) synthase (GpgS) and glucosyl-3-phosphoglycerate phosphatase (GpgP). The sequences of gpgS and gpgP from the cold-adapted bacterium Methanococcoides burtonii were used to identify the homologues in the genome of P. marina, which were separately cloned and overexpressed as His-tagged proteins in Escherichia coli. The recombinant GpgS protein of P. marina, unlike the homologue from M. burtonii, which was specific for GDP-glucose, catalyzed the synthesis of GPG from UDP-glucose, GDP-glucose, ADP-glucose, and TDP-glucose (in order of decreasing efficiency) and from d-3-phosphoglycerate, with maximal activity at 90°C. The recombinant GpgP protein, like the M. burtonii homologue, dephosphorylated GPG and mannosyl-3-phosphoglycerate (MPG) to GG and mannosylglycerate, respectively, yet at high temperatures the hydrolysis of GPG was more efficient than that of MPG. Gel filtration indicates that GpgS is a dimeric protein, while GpgP is monomeric. This is the first characterization of genes and enzymes for the synthesis of GG in a thermophile.
The biosynthetic pathway for the rare compatible solute mannosylglucosylglycerate (MGG) accumulated by Rhodopirellula baltica, a marine member of the phylum Planctomycetes, has been elucidated. Like one of the pathways used in the thermophilic bacterium Petrotoga mobilis, it has genes coding for glucosyl-3-phosphoglycerate synthase (GpgS) and mannosylglucosyl-3-phosphoglycerate (MGPG) synthase (MggA). However, unlike Ptg. mobilis, the mesophilic R. baltica uses a novel and very specific MGPG phosphatase (MggB). It also lacks a key enzyme of the alternative pathway in Ptg. mobilis – the mannosylglucosylglycerate synthase (MggS) that catalyses the condensation of glucosylglycerate with GDP-mannose to produce MGG. The R. baltica enzymes GpgS, MggA, and MggB were expressed in E. coli and characterized in terms of kinetic parameters, substrate specificity, temperature and pH dependence. This is the first characterization of genes and enzymes for the synthesis of compatible solutes in the phylum Planctomycetes and for the synthesis of MGG in a mesophile.
The compatible solute mannosylglucosylglycerate (MGG), recently identified in Petrotoga miotherma, also accumulates in Petrotoga mobilis in response to hyperosmotic conditions and supraoptimal growth temperatures. Two functionally connected genes encoding a glucosyl-3-phosphoglycerate synthase (GpgS) and an unknown glycosyltransferase (gene Pmob_1143), which we functionally characterized as a mannosylglucosyl-3-phosphoglycerate synthase and designated MggA, were identified in the genome of Ptg. mobilis. This enzyme used the product of GpgS, glucosyl-3-phosphoglycerate (GPG), as well as GDP-mannose to produce mannosylglucosyl-3-phosphoglycerate (MGPG), the phosphorylated precursor of MGG. The MGPG dephosphorylation was determined in cell extracts, and the native enzyme was partially purified and characterized. Surprisingly, a gene encoding a putative glucosylglycerate synthase (Ggs) was also identified in the genome of Ptg. mobilis, and an active Ggs capable of producing glucosylglycerate (GG) from ADP-glucose and d-glycerate was detected in cell extracts and the recombinant enzyme was characterized, as well. Since GG has never been identified in this organism nor was it a substrate for the MggA, we anticipated the existence of a nonphosphorylating pathway for MGG synthesis. We putatively identified the corresponding gene, whose product had some sequence homology with MggA, but it was not possible to recombinantly express a functional enzyme from Ptg. mobilis, which we named mannosylglucosylglycerate synthase (MggS). In turn, a homologous gene from Thermotoga maritima was successfully expressed, and the synthesis of MGG was confirmed from GDP-mannose and GG. Based on the measurements of the relevant enzyme activities in cell extracts and on the functional characterization of the key enzymes, we propose two alternative pathways for the synthesis of the rare compatible solute MGG in Ptg. mobilis.
The pathway for the synthesis of the organic solute glucosylglycerate (GG) is proposed based on the activities of the recombinant glucosyl-3-phosphoglycerate synthase (GpgS) and glucosyl-3-phosphoglycerate phosphatase (GpgP) from Methanococcoides burtonii. A mannosyl-3-phosphoglycerate phosphatase gene homologue (mpgP) was found in the genome of M. burtonii (http://www.jgi.doe.gov), but an mpgS gene coding for mannosyl-3-phosphoglycerate synthase (MpgS) was absent. The gene upstream of the mpgP homologue encoded a putative glucosyltransferase that was expressed in Escherichia coli. The recombinant product had GpgS activity, catalyzing the synthesis of glucosyl-3-phosphoglycerate (GPG) from GDP-glucose and d-3-phosphoglycerate, with a high substrate specificity. The recombinant MpgP protein dephosphorylated GPG to GG and was also able to dephosphorylate mannosyl-3-phosphoglycerate (MPG) but no other substrate tested. Similar flexibilities in substrate specificity were confirmed in vitro for the MpgPs from Thermus thermophilus, Pyrococcus horikoshii, and “Dehalococcoides ethenogenes.” GpgS had maximal activity at 50°C. The maximal activity of GpgP was at 50°C with GPG as the substrate and at 60°C with MPG. Despite the similarity of the sugar donors GDP-glucose and GDP-mannose, the enzymes for the synthesis of GPG or MPG share no amino acid sequence identity, save for short motifs. However, the hydrolysis of GPG and MPG is carried out by phosphatases encoded by homologous genes and capable of using both substrates. To our knowledge, this is the first report of the elucidation of a biosynthetic pathway for glucosylglycerate.
Mycobacteria produce two unique families of cytoplasmic polymethylated polysaccharides - the methylglucose lipopolysaccharides (MGLPs) and the methylmannose polysaccharides (MMPs) - the physiological functions of which are still poorly defined. Towards defining the roles of these polysaccharides in mycobacterial physiology, we generated knock-out mutations of genes in their putative biosynthetic pathways.
We report here on the characterization of the Rv1208 protein of Mycobacterium tuberculosis and its ortholog in Mycobacterium smegmatis (MSMEG_5084) as the enzymes responsible for the transfer of the first glucose residue of MGLPs. Disruption of MSMEG_5084 in M. smegmatis resulted in a dramatic decrease in MGLP synthesis directly attributable to the almost complete abolition of glucosyl-3-phosphoglycerate synthase activity in this strain. Synthesis of MGLPs in the mutant was restored upon complementation with wild-type copies of the Rv1208 gene from M. tuberculosis or MSMEG_5084 from M. smegmatis.
This is the first evidence linking Rv1208 to MGLP biosynthesis. Thus, the first step in the initiation of MGLP biosynthesis in mycobacteria has been defined, and subsequent steps can be inferred.
Tuberculosis constitutes today a serious threat to human health worldwide, aggravated by the increasing number of identified multi-resistant strains of Mycobacterium tuberculosis, its causative agent, as well as by the lack of development of novel mycobactericidal compounds for the last few decades. The increased resilience of this pathogen is due, to a great extent, to its complex, polysaccharide-rich, and unusually impermeable cell wall. The synthesis of this essential structure is still poorly understood despite the fact that enzymes involved in glycosidic bond synthesis represent more than 1% of all M. tuberculosis ORFs identified to date. One of them is GpgS, a retaining glycosyltransferase (GT) with low sequence homology to any other GTs of known structure, which has been identified in two species of mycobacteria and shown to be essential for the survival of M. tuberculosis. To further understand the biochemical properties of M. tuberculosis GpgS, we determined the three-dimensional structure of the apo enzyme, as well as of its ternary complex with UDP and 3-phosphoglycerate, by X-ray crystallography, to a resolution of 2.5 and 2.7 Å, respectively. GpgS, the first enzyme from the newly established GT-81 family to be structurally characterized, displays a dimeric architecture with an overall fold similar to that of other GT-A-type glycosyltransferases. These three-dimensional structures provide a molecular explanation for the enzyme's preference for UDP-containing donor substrates, as well as for its glucose versus mannose discrimination, and uncover the structural determinants for acceptor substrate selectivity. Glycosyltransferases constitute a growing family of enzymes for which structural and mechanistic data urges. The three-dimensional structures of M. tuberculosis GpgS now determined provide such data for a novel enzyme family, clearly establishing the molecular determinants for substrate recognition and catalysis, while providing an experimental scaffold for the structure-based rational design of specific inhibitors, which lay the foundation for the development of novel anti-tuberculosis therapies.
The biosynthesis of archaeal ether-type glycolipids was investigated in vitro using Methanothermobacter thermautotrophicus cell-free homogenates. The sole sugar moiety of glycolipids and phosphoglycolipids of the organism is the β-d-glucosyl-(1→6)-d-glucosyl (gentiobiosyl) unit. The enzyme activities of archaeol:UDP-glucose β-glucosyltransferase (monoglucosylarchaeol [MGA] synthase) and MGA:UDP-glucose β-1,6-glucosyltransferase (diglucosylarchaeol [DGA] synthase) were found in the methanoarchaeon. The synthesis of DGA is probably a two-step glucosylation: (i) archaeol + UDP-glucose → MGA + UDP, and (ii) MGA + UDP-glucose → DGA + UDP. Both enzymes required the addition of K+ ions and archaetidylinositol for their activities. DGA synthase was stimulated by 10 mM MgCl2, in contrast to MGA synthase, which did not require Mg2+. It was likely that the activities of MGA synthesis and DGA synthesis were carried out by different proteins because of the Mg2+ requirement and their cellular localization. MGA synthase and DGA synthase can be distinguished in cell extracts greatly enriched for each activity by demonstrating the differing Mg2+ requirements of each enzyme. MGA synthase preferred a lipid substrate with the sn-2,3 stereostructure of the glycerol backbone on which two saturated isoprenoid chains are bound at the sn-2 and sn-3 positions. A lipid substrate with unsaturated isoprenoid chains or sn-1,2-dialkylglycerol configuration exhibited low activity. Tetraether-type caldarchaetidylinositol was also actively glucosylated by the homogenates to form monoglucosyl caldarchaetidylinositol and a small amount of diglucosyl caldarchaetidylinositol. The addition of Mg2+ increased the formation of diglucosyl caldarchaetidylinositol. This suggested that the same enzyme set synthesized the sole sugar moiety of diether-type glycolipids and tetraether-type phosphoglycolipids.
Lewis x (Lex) and sialyl Lewis x (SLex)-containing glycans play important roles in numerous physiological and pathological processes. The key enzyme for the final step formation of these Lewis antigens is α1-3-fucosyltransferase. Here we report molecular cloning and functional expression of a novel Helicobacter hepaticus α1-3-fucosyltransferase (HhFT1) which shows activity towards both non-sialylated and sialylated Type II oligosaccharide acceptor substrates. It is a promising catalyst for enzymatic and chemoenzymatic synthesis of Lex, sialyl Lex and their derivatives. Unlike all other α1-3/4-fucosyltransferases characterized so far which belong to Carbohydrate Active Enzyme (CAZy, http://www.cazy.org/) glycosyltransferase family GT10, the HhFT1 shares protein sequence homology with α1-2-fucosyltransferases and belongs to CAZy glycosyltransferase family GT11. The HhFT1 is thus the first α1-3-fucosyltransferase identified in the GT11 family.
cloning; fucosyltransferase; Helicobacter hepaticus; Lewis x; sialyl Lewis x
The Carbohydrate-Active Enzymes database (CAZy; http://www.cazy.org) provides online and continuously updated access to a sequence-based family classification linking the sequence to the specificity and 3D structure of the enzymes that assemble, modify and breakdown oligo- and polysaccharides. Functional and 3D structural information is added and curated on a regular basis based on the available literature. In addition to the use of the database by enzymologists seeking curated information on CAZymes, the dissemination of a stable nomenclature for these enzymes is probably a major contribution of CAZy. The past few years have seen the expansion of the CAZy classification scheme to new families, the development of subfamilies in several families and the power of CAZy for the analysis of genomes and metagenomes. This article outlines the changes that have occurred in CAZy during the past 5 years and presents our novel effort to display the resolution and the carbohydrate ligands in crystallographic complexes of CAZymes.
N-Acetylglucosamine 1-phosphate uridyltransferase (GlmU) from M. tuberculosis H37Rv has been crystallized and preliminary X-ray crystallographic analysis has been performed. GlmU is a bi-domained bifunctional enzyme that is involved in the biosynthesis of UDP-N-acetylglucosamine, a precursor in peptidoglycan biosynthesis in M. tuberculosis.
The gene product of open reading frame Rv1018c from Mycobacterium tuberculosis is annotated as encoding a probable N-acetylglucosamine 1-phosphate uridylyltransferase (MtbGlmU), an enzyme that catalyzes the biosynthesis of UDP-N-acetylglucosamine, a precursor common to lipopolysaccharide and peptidoglycan biosynthesis. Following overexpression in Escherichia coli, the enzyme was purified and crystallized using the hanging-drop vapour-diffusion method. Native diffraction data were collected from crystals belonging to space group R32 and processed to a resolution of 2.2 Å.
Mycobacterium tuberculosis H37Rv; Rv1018c; N-acetylglucosamine 1-phosphate uridyltransferase; peptidoglycan metabolism; GlmU
The small regulatory subunit of acetohydroxylate synthase (IlvH) from M. tuberculosis has been crystallized and preliminary crystallographic analysis has been performed.
Branched amino-acid biosynthesis is important to bacterial pathogens such as Mycobacterium tuberculosis (Mtb), a microorganism that presently causes more deaths in humans than any other prokaryotic pathogen (http://www.who.int/tb). In this study, the molecular cloning, expression, purification, crystallization and preliminary crystallographic analysis of recombinant IlvH, the small regulatory subunit of acetohydroxylic acid synthase (AHAS) in Mtb, are reported. AHAS carries out the first common reaction in the biosynthesis of valine, leucine and isoleucine. AHAS is an essential enzyme in Mtb and its inactivation leads to a lethal phenotype [Sassetti et al. (2001 ▶), Proc. Natl Acad. Sci. USA, 98, 12712–12717]. Thus, inhibitors of AHAS could potentially be developed into novel anti-Mtb therapies.
acetolactate synthase regulatory subunit; branched amino-acid biosynthesis; Mycobacterium tuberculosis; IlvH
M. tuberculosis tetrahydrodipicolinate-N-succinyltransferase, the enzyme that catalyses the fifth reaction step of the lysine-biosynthesis pathway, has been cloned, expressed, purified and crystallized.
Tetrahydrodipicolinate-N-succinyltransferase from Mycobacterium tuberculosis (DapD, Rv1201c) has been cloned, heterologously expressed in Escherichia coli, purified using standard chromatographic techniques and crystallized in the cubic space group I23 or I213. Preliminary diffraction data analysis indicates the presence of five molecules per asymmetric unit. Furthermore, the data exhibit icosahedral point-group symmetry. One possible explanation for this is that the enzyme assembles into a 60-mer exhibiting 235 point-group symmetry and crystallizes as such in space group I23. In this case, the combination of crystallographic and noncrystallographic symmetry elements results in an arrangement of the icosahedrons in the cubic crystal with one pentamer in the asymmetric unit. Another explanation is that the packing of the molecules itself mimics icosahedral symmetry. In this case both space groups I23 and I213 would be possible.
tetrahydrodipicolinate-N-succinyltransferase; Mycobacterium tuberculosis; DapD
Since its inception, the carbohydrate-active enzymes database (CAZy; http://www.cazy.org) has described the families of enzymes that cleave or build complex carbohydrates, namely the glycoside hydrolases (GH), the polysaccharide lyases (PL), the carbohydrate esterases (CE), the glycosyltransferases (GT) and their appended non-catalytic carbohydrate-binding modules (CBM). The recent discovery that members of families CBM33 and family GH61 are in fact lytic polysaccharide monooxygenases (LPMO), demands a reclassification of these families into a suitable category.
Because lignin is invariably found together with polysaccharides in the plant cell wall and because lignin fragments are likely to act in concert with (LPMO), we have decided to join the families of lignin degradation enzymes to the LPMO families and launch a new CAZy class that we name “Auxiliary Activities” in order to accommodate a range of enzyme mechanisms and substrates related to lignocellulose conversion. Comparative analyses of these auxiliary activities in 41 fungal genomes reveal a pertinent division of several fungal groups and subgroups combining their phylogenetic origin and their nutritional mode (white vs. brown rot).
The new class introduced in the CAZy database extends the traditional CAZy families, and provides a better coverage of the full extent of the lignocellulose breakdown machinery.
CAZy database; Evolution of lignocellulose breakdown; Ligninolytic enzymes; Lytic polysaccharide monooxygenases
Mannosyl-3-phosphoglycerate synthase (MpgS) is a key enzyme in the biosynthesis of MG. Here, the purification, crystallization and preliminary crystallographic characterization of apo MpgS from Thermus thermophilus HB27 are reported.
Mannosylglycerate (MG) is a compatible solute that is widespread in marine organisms that are adapted to hot environments, with its intracellular pool generally increasing in response to osmotic stress. These observations suggest that MG plays a relevant role in osmoadaptation and thermoadaptation. The pathways for the synthesis of MG have been characterized in a number of thermophilic and hyperthermophilic organisms. Mannosyl-3-phosphoglycerate synthase (MpgS) is a key enzyme in the biosynthesis of MG. Here, the purification, crystallization and preliminary crystallographic characterization of apo MpgS from Thermus thermophilus HB27 are reported. The addition of Zn2+ to the crystallization buffer was essential in order to obtain crystals. The crystals belonged to one of the enantiomorphic tetragonal space groups P41212 or P43212, with unit-cell parameters a = b = 113, c = 197 Å. Diffraction data were obtained to a resolution of 2.97 Å.
mannosyl-3-phosphoglycerate synthase; Thermus thermophilus HB27
GlycoPep Grader (GPG) is a freely-available software tool designed to accelerate the process of accurately determining glycopeptide composition from tandem mass spectrometric data. GPG relies on the identification of unique dissociation patterns shown for high mannose, hybrid, and complex N-linked glycoprotein types, including patterns specific to those structures containing fucose or sialic acid residues. The novel GPG scoring algorithm scores potential candidate compositions of the same nominal mass against MS/MS data through evaluation of the Y1 ion and other peptide-containing product ions, across multiple charge states, when applicable. In addition to evaluating the peptide portions of a given glycopeptide, the GPG algorithm predicts and scores product ions that result from unique neutral losses of terminal glycans. GPG has been applied to a variety of glycoproteins, including RNase B, asialofetuin and transferrin, and the HIV envelope glycoprotein, CON-S gp140 CFI. The GPG software is implemented predominantly in PostgreSQL, with PHP as the presentation tier, and is publically accessible online. Thus far, the algorithm has identified the correct compositional assignment from multiple candidate N-glycopeptides in all tests performed.
The acyl-CoA carboxylase β subunit (ACCD6) of M. tuberculosis has been crystallized and preliminary X-ray crystallographic analysis has been performed.
Mycobacterium tuberculosis (Mtb) acyl-CoA carboxylase is involved in the biosynthesis of mycolic acids, which are a key component of the bacillus cell wall. The Mtb genome encodes six acyl-CoA carboxylase β subunits (ACCD1–6), three of which (ACCD4–6) are essential for survival of the pathogen on minimal medium. Mtb ACCD6 has been expressed, purified and crystallized. The two forms of Mtb ACCD6 crystals belonged to space groups P41212 and P212121 and diffracted to 2.9 and 2.5 Å resolution, respectively, at a synchrotron-radiation source.
acyl-CoA carboxylase; ACCD6; Rv2247
Nothofagin is a major antioxidant of redbush herbal tea and represents a class of bioactive flavonoid-like C-glycosidic natural products. We developed an efficient enzymatic synthesis of nothofagin based on a one-pot coupled glycosyltransferase-catalyzed transformation that involves perfectly selective 3′-C-β-d-glucosylation of naturally abundant phloretin and applies sucrose as expedient glucosyl donor. C-Glucosyltransferase from Oryza sativa (rice) was used for phloretin C-glucosylation from uridine 5′-diphosphate (UDP)-glucose, which was supplied continuously in situ through conversion of sucrose and UDP catalyzed by sucrose synthase from Glycine max (soybean). In an evaluation of thermodynamic, kinetic, and stability parameters of the coupled enzymatic reactions, poor water solubility of the phloretin acceptor substrate was revealed as a major bottleneck of conversion efficiency. Using periodic feed of phloretin controlled by reaction progress, nothofagin concentrations (45 mM; 20 g l−1) were obtained that vastly exceed the phloretin solubility limit (5–10 mM). The intermediate UDP-glucose was produced from catalytic amounts of UDP (1.0 mM) and was thus recycled 45 times in the process. Benchmarked against comparable glycosyltransferase-catalyzed transformations (e.g., on quercetin), the synthesis of nothofagin has achieved intensification in glycosidic product formation by up to three orders of magnitude (μM→mM range). It thus makes a strong case for the application of Leloir glycosyltransferases in biocatalytic syntheses of glycosylated natural products as fine chemicals.
carbohydrates; C-glycosides; glycosyltransferases; natural products; UDP-glucose recycling
Acetyl coenzyme A carboxylase (ACC) is a key enzyme providing a substrate for mycolic acid biosynthesis. Although in vitro studies have demonstrated that the protein encoded by accD6 (Rv2247) may be a functional carboxyltransferase subunit of ACC in Mycobacterium tuberculosis, the in vivo function and regulation of accD6 in slow- and fast-growing mycobacteria remain elusive. Here, directed mutagenesis demonstrated that although accD6 is essential for M. tuberculosis, it can be deleted in Mycobacterium smegmatis without affecting its cell envelope integrity. Moreover, we showed that although it is part of the type II fatty acid synthase operon, the accD6 gene of M. tuberculosis, but not that of M. smegmatis, possesses its own additional promoter (Pacc). The expression level of accD6Mtb placed only under the control of Pacc is 10-fold lower than that in wild-type M. tuberculosis but is sufficient to sustain cell viability. Importantly, this limited expression level affects growth, mycolic acid content, and cell morphology. These results provide the first in vivo evidence for AccD6 as a key player in the mycolate biosynthesis of M. tuberculosis, implicating AccD6 as the essential ACC subunit in pathogenic mycobacteria and an excellent target for new antitubercular compounds. Our findings also highlight important differences in the mechanism of acetyl carboxylation between pathogenic and nonpathogenic mycobacterial species.
The regulatory domain of M. tuberculosis aspartokinase, the enzyme which catalyses the first reaction step in the biosynthesis of the amino acids lysine, methionine and threonine, has been cloned, expressed, purified and crystallized. Preliminary X-ray diffraction analysis of several crystals revealed the presence of five distinct crystal forms.
The regulatory domain of Mycobacterium tuberculosis aspartokinase (Mtb-AK, Mtb-Ask, Rv3709c) has been cloned, heterologously expressed in Escherichia coli and purified using standard chromatographic techniques. Screening for initial crystallization conditions using the regulatory domain (AK-β) in the presence of the potential feedback inhibitor threonine identified four conditions which yielded crystals suitable for X-ray diffraction analysis. From these four conditions five different crystal forms of Mtb-AK-β resulted, three of which belonged to the orthorhombic system, one to the tetragonal system and one to the monoclinic system. The highest resolution (1.6 Å) was observed for a crystal form belonging to space group P212121, with unit-cell parameters a = 53.70, b = 63.43, c = 108.85 Å and two molecules per asymmetric unit.
aspartokinase; Rv3709c; Mycobacterium tuberculosis; tuberculosis
The 'global public good' (GPG) concept has gained increasing attention, in health as well as development circles. However, it has suffered in finding currency as a general tool for global resource mobilisation, and is at risk of being attached to almost anything promoting development. This overstretches and devalues the validity and usefulness of the concept. This paper first defines GPGs and describes the policy challenge that they pose. Second, it identifies two key areas, health R&D and communicable disease control, in which the GPG concept is clearly relevant and considers the extent to which it has been applied. We point out that that, while there have been many new initiatives, it is not clear that additional resources from non-traditional sources have been forthcoming. Yet achieving this is, in effect, the entire purpose of applying the GPG concept in global health. Moreover, the proliferation of disease-specific programs associated with GPG reasoning has tended to promote vertical interventions at the expense of more general health sector strengthening. Third, we examine two major global health policy initiatives, the Global Fund against AIDS, Tuberculosis and Malaria (GFATM) and the bundling of long-standing international health goals in the form of Millennium Development Goals (MDG), asking how the GPG perspective has contributed to defining objectives and strategies. We conclude that both initiatives are best interpreted in the context of traditional development assistance and, one-world rhetoric aside, have little to do with the challenge posed by GPGs for health. The paper concludes by considering how the GPG concept can be more effectively used to promote global health.
The principal neutralization determinant (PND) of human immunodeficiency virus type 1 envelope glycoprotein gp120 contains a conserved GPG sequence. The effects of a 29-amino-acid deletion of most of the PND, a 3-amino-acid deletion in the GPG sequence, and 16 single-amino-acid substitutions in the GPG sequence were determined in a transient expression assay. All mutant envelope glycoproteins were expressed at levels comparable to that of the wild-type envelope, and mutations in the GPG sequence did not affect processing to gp120 or, except for the 29-amino-acid deletion, binding to CD4. Of all of the mutants, only the GHG and GFG mutants induced formation of syncytia similar in size and number to those induced by the wild-type envelope. When the envelope expression level was increased 10-fold or more, several additional mutants (APG, GAG, GSG, GQG, GVG, and GPF) also induced syncytium formation. Transfection with infectious proviral molecular clones containing the GHG, GFG, APG, GAG, GSG, or GPF mutations induced production of viral particles; however, only the GPG, GHG, and GFG viruses produced active infections in CD4-bearing cells. Furthermore, whereas the wild-type virus was efficiently neutralized by PND polyclonal and monoclonal antibodies, the GHG- and GFG-containing viruses were not. These results show that mutations in the GPG sequence found within the PND do not affect envelope expression and do not significantly affect CD4 binding or production of viral particles but that they do affect the ability of the envelope to induce syncytia and those of the viral particles to infect CD4 cells and be neutralized by PND antibodies.
Brassinosteroids (BRs) are signaling molecules that play essential roles in the spatial regulation of plant growth and development. In contrast to other plant hormones BRs act locally, close to the sites of their synthesis, and thus homeostatic mechanisms must operate at the cellular level to equilibrate BR concentrations. Whilst it is recognized that levels of bioactive BRs are likely adjusted by controlling the relative rates of biosynthesis and by catabolism, few factors, which participate in these regulatory events, have as yet been identified. Previously we have shown that the UDP-glycosyltransferase UGT73C5 of Arabidopsis thaliana catalyzes 23-O-glucosylation of BRs and that glucosylation renders BRs inactive. This study identifies the closest homologue of UGT73C5, UGT73C6, as an enzyme that is also able to glucosylate BRs in planta.
In a candidate gene approach, in which homologues of UGT73C5 were screened for their potential to induce BR deficiency when over-expressed in plants, UGT73C6 was identified as an enzyme that can glucosylate the BRs CS and BL at their 23-O-positions in planta. GUS reporter analysis indicates that UGT73C6 shows over-lapping, but also distinct expression patterns with UGT73C5 and YFP reporter data suggests that at the cellular level, both UGTs localize to the cytoplasm and to the nucleus. A liquid chromatography high-resolution mass spectrometry method for BR metabolite analysis was developed and applied to determine the kinetics of formation and the catabolic fate of BR-23-O-glucosides in wild type and UGT73C5 and UGT73C6 over-expression lines. This approach identified novel BR catabolites, which are considered to be BR-malonylglucosides, and provided first evidence indicating that glucosylation protects BRs from cellular removal. The physiological significance of BR glucosylation, and the possible role of UGT73C6 as a regulatory factor in this process are discussed in light of the results presented.
The present study generates essential knowledge and molecular and biochemical tools, that will allow for the verification of a potential physiological role of UGT73C6 in BR glucosylation and will facilitate the investigation of the functional significance of BR glucoside formation in plants.
arabidopsis; brassinosteroids; glycosylation; homeostasis; malonylation; steroids