Biosynthesis of Mannophospholipids by Mycobacterium phlei

Development in the Concept of Bacterial Polysaccharide Repeating Unit-Based Antibacterial Conjugate Vaccines

The surface of cells is coated with a dense layer of glycans, known as the cell glycocalyx. The complex glycans in the glycocalyx are involved in various biological events, such as bacterial pathogenesis, protection of bacteria from environmental stresses, etc. Polysaccharides on the bacterial cell surface are highly conserved and accessible molecules, and thus they are excellent immunological targets. Consequently, bacterial polysaccharides and their repeating units have been extensively studied as antigens for the development of antibacterial vaccines. This Review surveys the recent developments in the synthetic and immunological investigations of bacterial polysaccharide repeating unit-based conjugate vaccines against several human pathogenic bacteria. The major challenges associated with the development of functional carbohydrate-based antibacterial conjugate vaccines are also considered.

The thick waxy coat of mycobacteria, a protective layer against antibiotics and the host's immune system

Tuberculosis, caused by the pathogenic bacterium Mycobacterium tuberculosis (Mtb), is the leading cause of death from an infectious disease, with a mortality rate of over a million people per year. This pathogen's remarkable resilience and infectivity is largely due to its unique waxy cell envelope, 40% of which comprises complex lipids. Therefore, an understanding of the structure and function of the cell wall lipids is of huge indirect clinical significance. This review provides a synopsis of the cell envelope and the major lipids contained within, including structure, biosynthesis and roles in pathogenesis.

Structural basis of phosphatidyl-myo-inositol mannosides biosynthesis in mycobacteria

Phosphatidyl-myo-inositol mannosides (PIMs) are glycolipids of unique chemical structure found in the inner and outer membranes of the cell envelope of all Mycobacterium species. The PIM family of glycolipids comprises phosphatidyl-myo-inositol mono-, di-, tri-, tetra-, penta-, and hexamannosides with different degrees of acylation. PIMs are considered not only essential structural components of the cell envelope but also the precursors of lipomannan and lipoarabinomannan, two major lipoglycans implicated in host-pathogen interactions. Since the description of the complete chemical structure of PIMs, major efforts have been committed to defining the molecular bases of its biosynthetic pathway. The structural characterization of the integral membrane phosphatidyl-myo-inositol phosphate synthase (PIPS), and that of three enzymes working at the protein-membrane interface, the phosphatidyl-myo-inositol mannosyltransferases A and B, and the acyltransferase PatA, established the basis of the early steps of the PIM pathway at the molecular level. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Structural basis for selective recognition of acyl chains by the membrane-associated acyltransferase PatA

The biosynthesis of phospholipids and glycolipids are critical pathways for virtually all cell membranes. PatA is an essential membrane associated acyltransferase involved in the biosynthesis of mycobacterial phosphatidyl-myo-inositol mannosides (PIMs). The enzyme transfers a palmitoyl moiety from palmitoyl–CoA to the 6-position of the mannose ring linked to 2-position of inositol in PIM₁/PIM₂. We report here the crystal structures of PatA from Mycobacterium smegmatis in the presence of its naturally occurring acyl donor palmitate and a nonhydrolyzable palmitoyl–CoA analog. The structures reveal an α/β architecture, with the acyl chain deeply buried into a hydrophobic pocket that runs perpendicular to a long groove where the active site is located. Enzyme catalysis is mediated by an unprecedented charge relay system, which markedly diverges from the canonical HX₄D motif. Our studies establish the mechanistic basis of substrate/membrane recognition and catalysis for an important family of acyltransferases, providing exciting possibilities for inhibitor design.

PatA is a membrane-associated acyltransferase that is essential for the biosynthesis of mycobacterial glycolipids. Here, Albesa-Jové et al. describe structures of PatA from Mycobacterium smegmatis in complex with acyl donors and show that catalysis occurs by an unusual charge-relay mechanism.

Lipoarabinomannan and related glycoconjugates: structure, biogenesis and role in Mycobacterium tuberculosis physiology and host-pathogen interaction

Approximately one third of the world's population is infected with Mycobacterium tuberculosis, the causative agent of tuberculosis. This bacterium has an unusual lipid-rich cell wall containing a vast repertoire of antigens, providing a hydrophobic impermeable barrier against chemical drugs, thus representing an attractive target for vaccine and drug development. Apart from the mycolyl–arabinogalactan–peptidoglycan complex, mycobacteria possess several immunomodulatory constituents, notably lipomannan and lipoarabinomannan. The availability of whole-genome sequences of M. tuberculosis and related bacilli over the past decade has led to the identification and functional characterization of various enzymes and the potential drug targets involved in the biosynthesis of these glycoconjugates. Both lipomannan and lipoarabinomannan possess highly variable chemical structures, which interact with different receptors of the immune system during host–pathogen interactions, such as Toll-like receptors-2 and C-type lectins. Recently, the availability of mutants defective in the synthesis of these glycoconjugates in mycobacteria and the closely related bacterium, Corynebacterium glutamicum, has paved the way for host–pathogen interaction studies, as well as, providing attenuated strains of mycobacteria for the development of new vaccine candidates. This review provides a comprehensive account of the structure, biosynthesis and immunomodulatory properties of these important glycoconjugates.

Lipoarabinomannan biosynthesis in Corynebacterineae: the interplay of two α(1→2)-mannopyranosyltransferases MptC and MptD in mannan branching

Lipomannan (LM) and lipoarabinomannan (LAM) are key Corynebacterineae glycoconjugates that are integral components of the mycobacterial cell wall, and are potent immunomodulators during infection. LAM is a complex heteropolysaccharide synthesized by an array of essential glycosyltransferase family C (GT-C) members, which represent potential drug targets. Herein, we have identified and characterized two open reading frames from Corynebacterium glutamicum that encode for putative GT-Cs. Deletion of NCgl2100 and NCgl2097 in C. glutamicum demonstrated their role in the biosynthesis of the branching α(1→2)-Manp residues found in LM and LAM. In addition, utilizing a chemically defined nonasaccharide acceptor, azidoethyl 6-O-benzyl-α-D-mannopyranosyl-(1→6)-[α-D-mannopyranosyl-(1→6)]₇-D-mannopyranoside, and the glycosyl donor C₅₀-polyprenol-phosphate-[¹⁴C]-mannose with membranes prepared from different C. glutamicum mutant strains, we have shown that both NCgl2100 and NCgl2097 encode for novel α(1→2)-mannopyranosyltransferases, which we have termed MptC and MptD respectively. Complementation studies and in vitro assays also identified Rv2181 as a homologue of Cg-MptC in Mycobacterium tuberculosis. Finally, we investigated the ability of LM and LAM from C. glutamicum, and C. glutamicumΔmptC and C. glutamicumΔmptD mutants, to activate Toll-like receptor 2. Overall, our study enhances our understanding of complex lipoglycan biosynthesis in Corynebacterineae and sheds further light on the structural and functional relationship of these classes of polysaccharides.

Molecular basis of phosphatidyl-myo-inositol mannoside biosynthesis and regulation in mycobacteria

Phosphatidyl-myo-inositol mannosides (PIMs) are unique glycolipids found in abundant quantities in the inner and outer membranes of the cell envelope of all Mycobacterium species. They are based on a phosphatidyl-myo-inositol lipid anchor carrying one to six mannose residues and up to four acyl chains. PIMs are considered not only essential structural components of the cell envelope but also the structural basis of the lipoglycans (lipomannan and lipoarabinomannan), all important molecules implicated in host-pathogen interactions in the course of tuberculosis and leprosy. Although the chemical structure of PIMs is now well established, knowledge of the enzymes and sequential events leading to their biosynthesis and regulation is still incomplete. Recent advances in the identification of key proteins involved in PIM biogenesis and the determination of the three-dimensional structures of the essential phosphatidyl-myo-inositol mannosyltransferase PimA and the lipoprotein LpqW have led to important insights into the molecular basis of this pathway.

New insights into the early steps of phosphatidylinositol mannoside biosynthesis in mycobacteria: PimB' is an essential enzyme of Mycobacterium smegmatis

Phosphatidyl-myo-inositol mannosides (PIMs) are key glycolipids of the mycobacterial cell envelope. They are considered not only essential structural components of the cell but also important molecules implicated in host-pathogen interactions. Although their chemical structures are well established, knowledge of the enzymes and sequential events leading to their biosynthesis is still incomplete. Here we show for the first time that although both mannosyltransferases PimA and PimB' (MSMEG_4253) recognize phosphatidyl-myo-inositol (PI) as a lipid acceptor, PimA specifically catalyzes the transfer of a Manp residue to the 2-position of the myo-inositol ring of PI, whereas PimB' exclusively transfers to the 6-position. Moreover, whereas PimB' can catalyze the transfer of a Manp residue onto the PI-monomannoside (PIM1) product of PimA, PimA is unable in vitro to transfer Manp onto the PIM1 product of PimB'. Further assays using membranes from Mycobacterium smegmatis and purified PimA and PimB' indicated that the acylation of the Manp residue transferred by PimA preferentially occurs after the second Manp residue has been added by PimB'. Importantly, genetic evidence is provided that pimB' is an essential gene of M. smegmatis. Altogether, our results support a model wherein Ac1PIM2, a major form of PIMs produced by mycobacteria, arises from the consecutive action of PimA, followed by PimB', and finally the acyltransferase MSMEG_2934. The essentiality of these three enzymes emphasizes the interest of novel anti-tuberculosis drugs targeting the initial steps of PIM biosynthesis.

Identification of a novel alpha(1-->6) mannopyranosyltransferase MptB from Corynebacterium glutamicum by deletion of a conserved gene, NCgl1505, affords a lipomannan- and lipoarabinomannan-deficient mutant

Mycobacterium tuberculosis and Corynebacterium glutamicum share a similar cell wall structure and orthologous enzymes involved in cell wall assembly. Herein, we have studied C. glutamicum NCgl1505, the orthologue of putative glycosyltransferases Rv1459c from M. tuberculosis and MSMEG3120 from Mycobacterium smegmatis. Deletion of NCgl1505 resulted in the absence of lipomannan (Cg-LM-A), lipoarabinomannan (Cg-LAM) and a multi-mannosylated polymer (Cg-LM-B) based on a 1,2-di-O-C₁₆/C_18:1-(α-D-glucopyranosyluronic acid)-(1→3)-glycerol (GlcAGroAc₂) anchor, while syntheses of triacylated-phosphatidyl-myo-inositol dimannoside (Ac₁PIM₂) and Man₁GlcAGroAc₂ were still abundant in whole cells. Cell-free incubation of C. glutamicum membranes with GDP-[¹⁴C]Man established that C. glutamicum synthesized a novel α(1→6)-linked linear form of Cg-LM-A and Cg-LM-B from Ac₁PIM₂ and Man₁GlcAGroAc₂ respectively. Furthermore, deletion of NCgl1505 also led to the absence of in vitro synthesized linear Cg-LM-A and Cg-LM-B, demonstrating that NCgl1505 was involved in core α(1→6) mannan biosynthesis of Cg-LM-A and Cg-LM-B, extending Ac₁PI[¹⁴C]M₂ and [¹⁴C]Man₁GlcAGroAc₂ primers respectively. Use of the acceptor α-D-Manp-(1→6)-α-D-Manp-O-C₈ in an in vitro cell-free assay confirmed NCgl1505 as an α(1→6) mannopyranosyltransferase, now termed MptB. While Rv1459c and MSMEG3120 demonstrated similar in vitroα(1→6) mannopyranosyltransferase activity, deletion of the Rv1459c homologue in M. smegmatis did not result in loss of mycobacterial LM/LAM, indicating a functional redundancy for this enzyme in mycobacteria.

Structural characterization of a partially arabinosylated lipoarabinomannan variant isolated from a Corynebacterium glutamicum ubiA mutant

Arabinan polysaccharide side-chains are present in both Mycobacterium tuberculosis and Corynebacterium glutamicum in the heteropolysaccharide arabinogalactan (AG), and in M. tuberculosis in the lipoglycan lipoarabinomannan (LAM). This study shows by quantitative sugar and glycosyl linkage analysis that C. glutamicum possesses a much smaller LAM version, Cg-LAM, characterized by single t-Araf residues linked to the α(1→6)-linked mannan backbone. MALDI-TOF MS showed an average molecular mass of 13 800–15 400 Da for Cg-LAM. The biosynthetic origin of Araf residues found in the extracytoplasmic arabinan domain of AG and LAM is well known to be provided by decaprenyl-monophosphoryl-d-arabinose (DPA). However, the characterization of LAM in a C. glutamicum : : ubiA mutant devoid of prenyltransferase activity and devoid of DPA-dependent arabinan deposition into AG revealed partial formation of LAM, albeit with a slightly altered molecular mass. These data suggest that in addition to DPA utilization as an Araf donor, alternative pathways exist in Corynebacterianeae for Araf delivery, possibly via an unknown sugar nucleotide.

Identification of an alpha(1-->6) mannopyranosyltransferase (MptA), involved in Corynebacterium glutamicum lipomanann biosynthesis, and identification of its orthologue in Mycobacterium tuberculosis

Corynebacterium glutamicum and Mycobacterium tuberculosis share a similar cell wall architecture, and the availability of their genome sequences has enabled the utilization of C. glutamicum as a model for the identification and study of, otherwise essential, mycobacterial genes involved in lipomannan (LM) and lipoarabinomannan (LAM) biosynthesis. We selected the putative glycosyltransferase-Rv2174 from M. tuberculosis and deleted its orthologue NCgl2093 from C. glutamicum. This resulted in the formation of a novel truncated lipomannan (Cg-t-LM) and a complete ablation of LM/LAM biosynthesis. Purification and characterization of Cg-t-LM revealed an overall decrease in molecular mass, a reduction of alpha(1-->6) and alpha(1-->2) glycosidic linkages illustrating a reduced degree of branching compared with wild-type LM. The deletion mutant's biochemical phenotype was fully complemented by either NCgl2093 or Rv2174. Furthermore, the use of a synthetic neoglycolipid acceptor in an in vitro cell-free assay utilizing the sugar donor beta-D-mannopyranosyl-1-monophosphoryl-decaprenol together with the neoglycolipid acceptor alpha-D-Manp-(1-->6)-alpha-D-Manp-O-C8 as a substrate, confirmed NCgl2093 and Rv2174 as an alpha(1-->6) mannopyranosyltransferase (MptA), involved in the latter stages of the biosynthesis of the alpha(1-->6) mannan core of LM. Altogether, these studies have identified a new mannosyltransferase, MptA, and they shed further light on the biosynthesis of LM/LAM in Corynebacterianeae.

Molecular recognition and interfacial catalysis by the essential phosphatidylinositol mannosyltransferase PimA from mycobacteria

Mycobacterial phosphatidylinositol mannosides (PIMs) and metabolically derived cell wall lipoglycans play important roles in host-pathogen interactions, but their biosynthetic pathways are poorly understood. Here we focus on Mycobacterium smegmatis PimA, an essential enzyme responsible for the initial mannosylation of phosphatidylinositol. The structure of PimA in complex with GDP-mannose shows the two-domain organization and the catalytic machinery typical of GT-B glycosyltransferases. PimA is an amphitrophic enzyme that binds mono-disperse phosphatidylinositol, but its transferase activity is stimulated by high concentrations of non-substrate anionic surfactants, indicating that the early stages of PIM biosynthesis involve lipid-water interfacial catalysis. Based on structural, calorimetric, and mutagenesis studies, we propose a model wherein PimA attaches to the membrane through its N-terminal domain, and this association leads to enzyme activation. Our results reveal a novel mode of phosphatidylinositol recognition and provide a template for the development of potential antimycobacterial compounds.

Inactivation of Corynebacterium glutamicum NCgl0452 and the role of MgtA in the biosynthesis of a novel mannosylated glycolipid involved in lipomannan biosynthesis

Mycobacterium tuberculosis PimB has been demonstrated to catalyze the addition of a mannose residue from GDP-mannose to a monoacylated phosphatidyl-myo-inositol mannoside (Ac(1)PIM(1)) to generate Ac(1)PIM(2). Herein, we describe the disruption of its probable orthologue Cg-pimB and the chemical analysis of glycolipids and lipoglycans isolated from wild type Corynebacterium glutamicum and the C. glutamicum::pimB mutant. Following a careful analysis, two related glycolipids, Gl-A and Gl-X, were found in the parent strain, but Gl-X was absent from the mutant. The biosynthesis of Gl-X was restored in the mutant by complementation with either Cg-pimB or Mt-pimB. Subsequent chemical analyses established Gl-X as 1,2-di-O-C(16)/C(18:1)-(alpha-d-mannopyranosyl)-(1-->4)-(alpha-d-glucopyranosyluronic acid)-(1-->3)-glycerol (ManGlcAGroAc(2)) and Gl-A as the precursor, GlcAGroAc(2). In addition, C. glutamicum::pimB was still able to produce Ac(1)PIM(2), suggesting that Cg-PimB catalyzes the synthesis of ManGlcAGroAc(2) from GlcAGroAc(2). Isolation of lipoglycans from C. glutamicum led to the identification of two related lipoglycans. The larger lipoglycan possessed a lipoarabinomannan-like structure, whereas the smaller lipoglycan was similar to lipomannan (LM). The absence of ManGlcA-GroAc(2) in C. glutamicum::pimB led to a severe reduction in LM. These results suggested that ManGlcAGroAc(2) was further extended to an LM-like molecule. Complementation of C. glutamicum::pimB with Cg-pimB and Mt-pimB led to the restoration of LM biosynthesis. As a result, Cg-PimB, which we have assigned as MgtA, is now clearly defined as a GDP-mannose-dependent alpha-mannosyltransferase from our in vitro analyses and is involved in the biosynthesis of ManGlcAGroAc(2).

Biosynthesis of mycobacterial lipoarabinomannan: role of a branching mannosyltransferase

Lipoarabinomannan (LAM), one of the few known bacterial glycosylphosphoinositides (GPIs), occurs in various structural forms in Mycobacterium species. It has been implicated in key aspects of the physiology of Mycobacterium tuberculosis and the immunology and pathogenesis of tuberculosis. Yet, little is known of the biosynthesis of LAM. A bioinformatics approach identified putative integral membrane proteins, MSMEG4250 in Mycobacterium smegmatis and Rv2181 in M. tuberculosis, with 10 predicted transmembrane domains and a glycosyltransferase (GT) motif (DID), features that are common to eukaryotic mannosyltransferases (ManTs) of the GT-C superfamily that rely on polyprenyl-linked rather than nucleotide-linked sugar donors. Inactivation of M. smegmatis MSMEG4250 by allelic exchange resulted in altered growth and inability to synthesize lipomannan (LM) but accumulation of a previously uncharacterized, truncated LAM. MALDI-TOF/MS and NMR indicated a structure lower in molecular weight than the native molecule, a preponderance of 6-linked Manp residues, and the absence of 2,6-linked and terminal Manp. Complementation of the mutant with the corresponding ortholog of M. tuberculosis H37Rv restored normal LM/LAM synthesis. The data suggest that MSMEG4250 and Rv2181 are ManTs that are responsible for the addition of alpha(1-->2) branches to the mannan core of LM/LAM and that arrest of this branching in the mutant deters formation of native LAM. The results allow for the presentation of a unique model of LM and LAM biosynthesis. The generation of mutants defective in the synthesis of LM/LAM will help define the role of these GPIs in the immunology and pathogenesis of mycobacterial infections and physiology of the organism.

PimE is a polyprenol-phosphate-mannose-dependent mannosyltransferase that transfers the fifth mannose of phosphatidylinositol mannoside in mycobacteria

Phosphatidylinositol mannosides (PIMs) are a major class of glycolipids in all mycobacteria. AcPIM2, a dimannosyl PIM, is both an end product and a precursor for polar PIMs, such as hexamannosyl PIM (AcPIM6) and the major cell wall lipoglycan, lipoarabinomannan (LAM). The mannosyltransferases that convert AcPIM2 to AcPIM6 or LAM are dependent on polyprenol-phosphate-mannose (PPM), but have not yet been characterized. Here, we identified a gene, termed pimE that is present in all mycobacteria, and is required for AcPIM6 biosynthesis. PimE was initially identified based on homology with eukaryotic PIG-M mannosyltransferases. PimE-deleted Mycobacterium smegmatis was defective in AcPIM6 synthesis, and accumulated the tetramannosyl PIM, AcPIM4. Loss of PimE had no affect on cell growth or viability, or the biosynthesis of other intracellular and cell wall glycans. However, changes in cell wall hydrophobicity and plasma membrane organization were detected, suggesting a role for AcPIM6 in the structural integrity of the cell wall and plasma membrane. These defects were corrected by ectopic expression of the pimE gene. Metabolic pulse-chase radiolabeling and cell-free PIM biosynthesis assays indicated that PimE catalyzes the alpha1,2-mannosyl transfer for the AcPIM5 synthesis. Mutation of an Asp residue in PimE that is conserved in and required for the activity of human PIG-M resulted in loss of PIM-biosynthetic activity, indicating that PimE is the catalytic component. Finally, PimE was localized to a distinct membrane fraction enriched in AcPIM4-6 biosynthesis. Taken together, PimE represents the first PPM-dependent mannosyl-transferase shown to be involved in PIM biosynthesis, where it mediates the fifth mannose transfer.

Identification of the required acyltransferase step in the biosynthesis of the phosphatidylinositol mannosides of mycobacterium species

Fatty acyl functions of the glycosylated phosphatidylinositol (GPI) anchors of the phosphatidylinositol mannosides (PIM), lipomannan (LM), and lipoarabinomannan (LAM) of mycobacteria play a critical role in both the physical properties and biological activities of these molecules. In a search for the acyltransferases that acylate the GPI anchors of PIM, LM, and LAM, we examined the function of the mycobacterial Rv2611c gene that encodes a putative acyltransferase involved in the early steps of phosphatidylinositol mannoside synthesis. A Rv2611c mutant of Mycobacterium smegmatis was constructed which exhibited severe growth defects and contained an increased amount of phosphatidylinositol mono- and di-mannosides and a decreased amount of acylated phosphatidylinositol di-mannosides compared with the wild-type parental strain. In cell-free assays, extracts from M. smegmatis overexpressing the M. tuberculosis Rv2611c gene incorporated [14C]palmitate into acylated phosphatidylinositol mono- and di-mannosides, and transferred cold endogenous fatty acids onto 14C-labeled phosphatidylinositol mono- and di-mannosides more efficiently than extracts from the wild-type strain. Cell-free extracts from the Rv2611c mutant of M. smegmatis were greatly impaired in these respects. This work provides evidence that Rv2611c is the acyltransferase that catalyzes the acylation of the 6-position of the mannose residue linked to position 2 of myo-inositol in phosphatidylinositol mono- and di-mannosides, with the mono-mannosylated lipid acceptor being the primary substrate of the enzyme. We also provide the first evidence that two distinct pathways lead to the formation of acylated PIM2 from PIM1 in mycobacteria.

Acylation state of the phosphatidylinositol hexamannosides from Mycobacterium bovis bacillus Calmette Guerin and mycobacterium tuberculosis H37Rv and its implication in Toll-like receptor response

The dimannoside (PIM2) and hexamannoside (PIM6) phosphatidyl-myo-inositol mannosides are the two most abundant classes of PIM found in Mycobacterium bovis bacillus Calmette Guérin, Mycobacterium tuberculosis H37Rv, and Mycobacterium smegmatis 607. Recently, these long known molecules received a renewed interest due to the fact that PIM2 constitute the anchor motif of an important constituent of the mycobacterial cell wall, the lipoarabinomannans (LAM), and that both LAM (phosphoinositol-capped LAM) and PIM are agonists of Toll-like receptor 2 (TLR2), a pattern recognition receptor involved in innate immunity. Due to the biological importance of these molecules, the chemical structure of PIM was revisited. The structure of PIM2 was recently published (Gilleron, M., Ronet, C., Mempel, M., Monsarrat, B., Gachelin, G., and Puzo, G. (2001) J. Biol. Chem. 276, 34896-34904). Here we report the purification and molecular characterization of PIM6 in their native form. For the first time, four acyl forms of this molecule have been purified, using hydrophobic interaction chromatography. Mono- to tetra-acylated molecules were identified in M. bovis bacillus Calmette Guérin, M. tuberculosis H37Rv, and M. smegmatis 607 using a sophisticated combination of analytical tools, including matrix-assisted laser desorption/ionization-time of flight-mass spectrometry and two-dimensional homo- and heteronuclear NMR spectroscopy. These experiments revealed that the major acyl forms are similar to the ones described for PIM2. Finally, we show that PIM6, like PIM2, activate primary macrophages to secrete TNF-alpha through TLR2, irrespective of their acylation pattern, and that they signal through the adaptor MyD88.

Definition of the first mannosylation step in phosphatidylinositol mannoside synthesis. PimA is essential for growth of mycobacteria

We examined the function of the pimA (Rv2610c) gene, located in the vicinity of the phosphatidylinositol synthase gene in the genomes of Mycobacterium tuberculosis and Mycobacterium smegmatis, which encodes a putative mannosyltransferase involved in the early steps of phosphatidylinositol mannoside synthesis. A cell-free assay was developed in which membranes from M. smegmatis overexpressing the pimA gene incorporate mannose from GDP-[(14)C]Man into di- and tri-acylated phosphatidylinositol mono-mannosides. Moreover, crude extracts from Escherichia coli producing a recombinant PimA protein synthesized diacylated phosphatidylinositol mono-mannoside from GDP-[(14)C]Man and bovine phosphatidylinositol. To determine whether PimA is an essential enzyme of mycobacteria, we constructed a pimA conditional mutant of M. smegmatis. The ability of this mutant to synthesize the PimA mannosyltransferase was dependent on the presence of a functional copy of the pimA gene carried on a temperature-sensitive rescue plasmid. We demonstrate here that the pimA mutant is unable to grow at the higher temperature at which the rescue plasmid is lost. Thus, the synthesis of phosphatidylinositol mono-mannosides and derived higher phosphatidylinositol mannosides in M. smegmatis appears to be dependent on PimA and essential for growth. This work provides the first direct evidence of the essentiality of phosphatidylinositol mannosides for the growth of mycobacteria.

Biosynthesis of mycobacterial lipoarabinomannan

The mycobacterial lipoglycans, lipomannan (LM) and lipoarabinomannan (LAM), are potent immunomodulators in tuberculosis and leprosy. Little is known of their biosynthesis, other than being based on phosphatidylinositol (PI), and they probably originate in the phosphatidylinositol mannosides (PIMs; PIMans). A novel form of cell-free incubation involving in vitro and in situ labeling with GDP-[14C]Man of the polyprenyl-P-mannoses (C35/C50-P-Man) and the simpler PIMs of mycobacterial membranes, reisolation of the [14C]Man-labeled membranes, and in situ chase demonstrated the synthesis of a novel alpha(1-->6)-linked linear form of LM at the expense of the C35/C50-P-Man. There was little or no synthesis under these conditions of PIMan5 with its terminal alpha(1-->2)Man unit or the mature LM or LAM with copious alpha(1-->2)Man branching. Synthesis of the linear LM, but not of the simpler PIMan2, was susceptible to amphomycin, a lipopeptide antibiotic that specifically inhibits polyprenyl-P-requiring translocases. A mixture of P[3H]I and P[3H]IMan2 was incorporated into the linear LM, supporting other evidence that, like the PIMs, LM and LAM, it is a lipid-linked mannooligosaccharide and a new member of the mycobacterial glycosylphosphatidylinositol lipoglycan/glycolipid class. Hence, the simpler PIMs originate in PI and GDP-Man, but further growth of the linear backbone emanates from C35-/C50-P-Man and is amphomycin-sensitive. The origin of the alpha(1-->2)Man branches of mature PIMan5, LM, and LAM is not known at this time but is probably GDP-Man.

Biosynthesis of hexamannophosphoinositides in Mycobacterium smegmatis ATCC 607

The biosynthesis of hexamannophosphoinositides in Mycobacterium smegmatis ATCC 607 was examined using labelled tri- and tetraacylated dimannophosphoinositides (PIM2(-3)F and PIM2(-4)F) as precursors, by in vivo and in vitro incorporation. Tetraacylated dimannoside was metabolically more active as compared to triacylated dimannoside and seems to be the precursor for the synthesis of hexamannophosphoinositides.