Lipid intermediates in the biosynthesis of the linkage unit between teichoic acids and peptidoglycan

Staphylococcal Protein Secretion and Envelope Assembly

The highly cross-linked peptidoglycan represents the rigid layer of the bacterial envelope and protects bacteria from osmotic lysis. In Gram-positive bacteria, peptidoglycan also functions as a scaffold for the immobilization of capsular polysaccharide, wall teichoic acid (WTA), and surface proteins. This chapter captures recent development on the assembly of the envelope of Staphylococcus aureus including mechanisms accounting for immobilization of molecules to peptidoglycan as well as hydrolysis of peptidoglycan for the specific release of bound molecules, facilitation of protein secretion across the envelope and cell division. Peptidoglycan, WTA and capsular polysaccharide are directly synthesized onto undecaprenol. Surface proteins are anchored by Sortase A, a membrane-embedded transpeptidase that scans secreted polypeptides for the C-terminal LPXTG motif of sorting signals. The resulting acyl enzyme intermediate is resolved by lipid II, the undecaprenol-bound peptidoglycan precursor. While these pathways share membrane diffusible undecaprenol, assembly of these molecules occurs either at the cross-walls or the cell poles. In S. aureus, the cross-wall represents the site of de novo peptidoglycan synthesis which is eventually split to complete the cell cycle yielding newly divided daughter cells. Peptidoglycan synthesized at the cross-wall is initially devoid of WTA. Conversely, lipoteichoic acid (LTA) synthesis which does not require bactoprenol is seemingly restricted to septal membranes. Similarly, S. aureus distinguishes two types of surface protein precursors. Polypeptides with canonical signal peptides are deposited at the cell poles, whereas precursors with conserved YSIRK-GXXS motif signal peptides traffic to the cross-wall. A model for protein trafficking in the envelope and uneven distribution of teichoic acids is discussed.

Determinants of murein hydrolase targeting to cross-wall of Staphylococcus aureus peptidoglycan

Cells of eukaryotic or prokaryotic origin express proteins with LysM domains that associate with the cell wall envelope of bacteria. The molecular properties that enable LysM domains to interact with microbial cell walls are not yet established. Staphylococcus aureus, a spherical microbe, secretes two murein hydrolases with LysM domains, Sle1 and LytN. We show here that the LysM domains of Sle1 and LytN direct murein hydrolases to the staphylococcal envelope in the vicinity of the cross-wall, the mid-cell compartment for peptidoglycan synthesis. LysM domains associate with the repeating disaccharide β-N-acetylmuramic acid, (1→4)-β-N-acetylglucosamine of staphylococcal peptidoglycan. Modification of N-acetylmuramic acid with wall teichoic acid, a ribitol-phosphate polymer tethered to murein linkage units, prevents the LysM domain from binding to peptidoglycan. The localization of LytN and Sle1 to the cross-wall is abolished in staphylococcal tagO mutants, which are defective for wall teichoic acid synthesis. We propose a model whereby the LysM domain ensures septal localization of LytN and Sle1 followed by processive cleavage of peptidoglycan, thereby exposing new LysM binding sites in the cross-wall and separating bacterial cells.

Use of CDP-glycerol as an alternate acceptor for the teichoic acid polymerase reveals that membrane association regulates polymer length

The study of bacterial extracellular polysaccharide biosynthesis is hampered by the fact that these molecules are synthesized on membrane-resident carrier lipids. To get around this problem, a practical solution has been to synthesize soluble lipid analogs and study the biosynthetic enzymes using a soluble system. This has been done for the Bacillus subtilis teichoic acid polymerase, TagF, although several aspects of catalysis were inconsistent with the results obtained with reconstituted membrane systems or physiological observations. In this work we explored the acceptor substrate promiscuity and polymer length disregulation that appear to be characteristic of TagF activity away from biological membranes. Using isotope labeling, steady-state kinetics, and chemical lability studies, we demonstrated that the enzyme can synthesize poly(glycerol phosphate) teichoic acid using the elongation substrate CDP-glycerol as an acceptor. This suggests that substrate specificity is relaxed in the region distal to the glycerol phosphate moiety in the acceptor molecule under these conditions. Polymer synthesis proceeded at a rate (27 min(-1)) comparable to that in the reconstituted membrane system after a distinct lag period which likely represented slower initiation on the unnatural CDP-glycerol acceptor. We confirmed that polymer length became disregulated in the soluble system as the polymers synthesized on CDP-glycerol acceptors were much larger than the polymers synthesized on the membrane or previously found attached to bacterial cell walls. Finally, polymer synthesis on protease-treated membranes suggested that proper length regulation is retained in the absence of accessory proteins and provided evidence that such regulation is conferred through proper association of the polymerase with the membrane.

Structural variation in the glycan strands of bacterial peptidoglycan

The normal, unmodified glycan strands of bacterial peptidoglycan consist of alternating residues of beta-1,4-linked N-acetylmuramic acid and N-acetylglucosamine. In many species the glycan strands become modified after their insertion into the cell wall. This review describes the structure of secondary modifications and of attachment sites of surface polymers in the glycan strands of peptidoglycan. It also provides an overview of the occurrence of these modifications in various bacterial species. Recently, enzymes responsible for the N-deacetylation, N-glycolylation and O-acetylation of the glycan strands were identified. The presence of these modifications affects the hydrolysis of peptidoglycan and its enlargement during cell growth. Glycan strands are frequently deacetylated and/or O-acetylated in pathogenic species. These alterations affect the recognition of bacteria by host factors, and contribute to the resistance of bacteria to host defence factors such as lysozyme.

Mechanism of action of the mannopeptimycins, a novel class of glycopeptide antibiotics active against vancomycin-resistant gram-positive bacteria

The naturally occurring mannopeptimycins (formerly AC98-1 through AC98-5) are a novel class of glycopeptide antibiotics that are active against a wide variety of gram-positive bacteria. The structures of the mannopeptimycins suggested that they might act by targeting cell wall biosynthesis, similar to other known glycopeptide antibiotics; but the fact that the mannopeptimycins retain activity against vancomycin-resistant organisms suggested that they might have a unique mode of action. By using a radioactive mannopeptimycin derivative bearing a photoactivation ligand, it was shown that mannopeptimycins interact with the membrane-bound cell wall precursor lipid II [C(55)-MurNAc-(peptide)-GlcNAc] and that this interaction is different from the binding of other lipid II-binding antibiotics such as vancomycin and mersacidin. The antimicrobial activities of several mannopeptimycin derivatives correlated with their affinities toward lipid II, suggesting that the inhibition of cell wall biosynthesis was primarily through lipid II binding. In addition, it was shown that mannopeptimycins bind to lipoteichoic acid in a rather nonspecific interaction, which might facilitate the accumulation of antibiotic on the bacterial cell surface.

Purified, recombinant TagF protein from Bacillus subtilis 168 catalyzes the polymerization of glycerol phosphate onto a membrane acceptor in vitro

We report the first characterization of a recombinant protein involved in the polymerization of wall teichoic acid. Previously, a study of the teichoic acid polymerase activity associated with membranes from Bacillus subtilis 168 strains bearing thermosensitive mutations in tagB, tagD, and tagF implicated TagF as the poly(glycerol phosphate) polymerase (Pooley, H. M., Abellan, F. X., and Karamata, D. (1992) J. Bacteriol. 174, 646-649). In the work reported here, we have demonstrated an unequivocal role for tagF in the thermosensitivity of one such mutant (tagF1) by conditional complementation at the restrictive temperature with tagF under control of the xylose promoter at the amyE locus. We have overexpressed and purified recombinant B. subtilis TagF protein, and we provide direct biochemical evidence that this enzyme is responsible for polymerization of poly(glycerol phosphate) teichoic acid in B. subtilis 168. Recombinant hexahistidine-tagged TagF protein was purified from Escherichia coli and was used to develop a novel membrane pelleting assay to monitor poly(glycerol phosphate) polymerase activity. Purified TagF was shown to incorporate radioactivity from its substrate CDP-[(14)C]glycerol into a membrane fraction in vitro. This activity showed a saturable dependence on the concentration of CDP-glycerol (K(m) of 340 microm) and the membrane acceptor (half-maximal activity at 650 microg of protein/ml of purified B. subtilis membranes). High pressure liquid chromatography analysis confirmed the polymeric nature of the reaction product, approximately 35 glycerol phosphate units in length.

Further evidence that a cell wall precursor [C(55)-MurNAc-(peptide)-GlcNAc] serves as an acceptor in a sorting reaction

Previous studies suggested that a Gly-containing branch of cell wall precursor [C(55)-MurNAc-(peptide)-GlcNAc], which is often referred to as lipid II, might serve as a nucleophilic acceptor in sortase-catalyzed anchoring of surface proteins in Staphylococcus aureus. To test this hypothesis, we first simplified the procedure for in vitro biosynthesis of Gly-containing lipid II by using branched UDP-MurNAc-hexapeptide isolated from the cytoplasm of Streptomyces spp. Second, we designed a thin-layer chromatography-based assay in which the mobility of branched but not linear lipid II is shifted in the presence of both sortase and LPSTG-containing peptide. These results and those of additional experiments presented in this study further suggest that lipid II indeed serves as a natural substrate in a sorting reaction.

Lack of inhibition by colicin M suggests bactoprenol independence of MDO biosynthesis

Biosynthesis of membrane-derived oligosaccharides (MDO), located in the periplasmic space of Escherichia coli, was not inhibited by colicin M, an inhibitor of bactoprenyl phosphate regeneration. This result suggests that bactoprenol does not serve as a lipid carrier of MDO oligosaccharides across the cytoplasmic membrane.

Biosynthesis of linkage units for teichoic acids in gram-positive bacteria: distribution of related enzymes and their specificities for UDP-sugars and lipid-linked intermediates

The distribution and substrate specificities of enzymes involved in the formation of linkage units which contain N-acetylglucosamine (GlcNAc) and N-acetylmannosamine (ManNAc) or glucose and join teichoic acid chains to peptidoglycan were studied among membrane systems obtained from the following two groups of gram-positive bacteria: group A, including Bacillus subtilis, Bacillus licheniformis, Bacillus pumilus, Staphylococcus aureus, and Lactobacillus plantarum; group B, Bacillus coagulans. All the membrane preparations tested catalyzed the synthesis of N-acetylglucosaminyl pyrophosphorylpolyprenol (GlcNAc-PP-polyprenol). The enzymes transferring glycosyl residues to GlcNAc-PP-polyprenol were specific to either UDP-ManNAc (group A strains) or UDP-glucose (group B strains). In the synthesis of the disaccharide-bound lipids, GlcNAc-PP-dolichol could substitute for GlcNAc-PP-undecaprenol. ManNAc-GlcNAc-PP-undecaprenol, ManNAc-GlcNAc-PP-dolichol, Glc-GlcNAc-PP-undecaprenol, Glc-GlcNAc-PP-dolichol, and GlcNAc-GlcNAc-PP-undecaprenol were more or less efficiently converted to glycerol phosphate-containing lipid intermediates and polymers in the membrane systems of B. subtilis W23 and B. coagulans AHU 1366. However, GlcNAc-GlcNAc-PP-dolichol could not serve as an intermediate in either of these membrane systems. Further studies on the exchangeability of ManNAc-GlcNAc-PP-undecaprenol and Glc-GlcNAc-PP-undecaprenol revealed that in the membrane systems of S. aureus strains and other B. coagulans strains both disaccharide-inked lipids served almost equally as intermediates in the synthesis of polymers. In the membrane systems of other B. subtilis strains as well as B. licheniformis and B. pumilus strains, however, the replacement of ManNAc-GlcNAc-PP-undecaprenol by Glc-GlcNAc-PP-undecaprenol led to a great accumulation of (glycerol phosphate)-Glc-GlcNAc-PP-undecaprenol accompanied by a decrease in the formation of polymers.

Structure and functions of linkage unit intermediates in the biosynthesis of ribitol teichoic acids in Staphylococcus aureus H and Bacillus subtilis W23

The stepwise formation and characterization of linkage unit intermediates and their functions in ribitol teichoic acid biosynthesis were studied with membranes obtained from Staphylococcus aureus H and Bacillus subtilis W23. The formation of labeled polymer from CDP-[14C]ribitol and CDP-glycerol in each membrane system was markedly stimulated by the addition of N-acetylmannosaminyl(beta 1----4)N-acetylglucosamine (ManNAc-GlcNAc) linked to pyrophosphorylyisoprenol. Whereas incubation of S. aureus membranes with CDP-glycerol and ManNAc-[14C]GlcNAc-PP-prenol led to synthesis of (glycerol phosphate) 1-3-ManNAc-[14C]GlcNAc-PP-prenol, incubation of B. subtilis membranes with the same substrates yielded (glycerol phosphate)1-2-ManNAc-[14C]GlcNAc-PP-prenol. In S. aureus membranes, (glycerol phosphate)2-ManNAc-[14C]GlcNAc-PP-prenol as well as (glycerol phosphate)3-ManNAc-[14C]GlcNAc-PP-prenol served as an acceptor for ribitol phosphate units, but (glycerol phosphate)-ManNAc-[14C]GlcNAc-PP-prenol did not. In B. subtilis W23 membranes, (glycerol phosphate)-ManNAc-[14C]GlcNAc-PP-prenol served as a better acceptor for ribitol phosphate units than (glycerol phosphate)2-ManNAc-[14C]GlcNAc-PP-prenol. In this membrane system (ribitol phosphate)-(glycerol phosphate)-ManNAc-[14C]GlcNAc-PP-prenol was formed from ManNAc-[14C]GlcNAc-PP-prenol, CDP-glycerol and CDP-ribitol. The results indicate that (glycerol phosphate)1-3-ManNAc-GlcNAc-PP-prenol and (glycerol phosphate)1-2-ManNac-GlcNAc-PP-prenol are involved in the pathway for the synthesis of wall ribitol teichoic acids in S. aureus H and B. subtilis W23 respectively.

Biosynthesis of wall teichoic acids in Staphylococcus aureus H, Micrococcus varians and Bacillus subtilis W23. Involvement of lipid intermediates containing the disaccharide N-acetylmannosaminyl N-acetylglucosamine

The precursors for linkage unit (LU) synthesis in Staphylococcus aureus H were UDP-GlcNAc, UDP-N-acetylmannosamine (ManNAc) and CDP-glycerol and synthesis was stimulated by ATP. Moraprenol-PP-GlcNAc-ManNAc-(glycerol phosphate)1-3 was formed from chemically synthesised moraprenol-PP-GlcNAc, UDP-ManNAc and CDP-glycerol in the presence of Triton X-100. LU intermediates formed under both conditions served as acceptors for ribitol phosphate residues, from CDP-ribitol, which comprise the main chain. The initial transfer of GlcNAc-1-phosphate from UDP-GlcNAc was very sensitive to tunicamycin whereas the subsequent transfer of ManNAc from UDP-ManNAc was not. Poly(GlcNAc-1-phosphate) and LU synthesis in Micrococcus varians, with endogenous lipid acceptor, UDP-GlcNAc and CDP-glycerol, was stimulated by UDP-ManNAc. Synthesis of LU on exogenous moraprenol-PP-GlcNAc, with Triton X-100, was dependent on UDP-ManNAc and CDP-glycerol and the intermediates formed served as substrates for polymer synthesis. Membranes from Bacillus subtilis W23 had much lower levels of LU synthesis, but UDP-ManNAc was again required for optimal synthesis in the presence of UDP-GlcNAc and CDP-glycerol. Conditions for LU synthesis on exogenous moraprenol-PP-GlcNAc were not found in this organism. LU synthesis on endogenous acceptor in the absence of UDP-ManNAc was explained by contamination of membranes with UDP-GlcNAc 2-epimerase. Under appropriate conditions, low levels of this enzyme were sufficient to convert UDP-GlcNAc into a mixture of UDP-Glc-NAc and UDP-ManNAc and account for LU synthesis. The results indicate the formation of prenol-PP-GlcNAc-ManNAc-(glycerol phosphate)1-3 which is involved in the synthesis of wall teichoic acids in S. aureus H, M. varians and B. subtilis W23 and their attachment to peptidoglycan.

Role of lipid intermediate(s) in the synthesis of serogroup B Neisseria meningitidis capsular polysaccharide

Neisseria meningitidis serogroup B strain M986 was examined for the involvement of lipid intermediate(s) participating in the biosynthesis of the sialic acid capsular polysaccharide. The addition of exogenous undecaprenyl phosphate, phosphatidylethanolamine, or phosphatidylglycerol to particulate membranes, in the presence of cytidine 5'-monophosphosialic acid, resulted in the stimulation of sialyltransferase activity specifically by undecaprenyl phosphate. Sialyltransferase activity, after delipidation of particulate membrane proteins, was specifically reconstituted by undecaprenyl phosphate. After the addition of 14C-labeled cytidine 5'-monophosphosialic acid to particulate membranes, the level of labeled lipid intermediate(s), extracted by chloroform-methanol (2:1), increased up to a maximum level between 3.75 and 5.0 min, which subsequently decreased to a lower steady-state level. Pulse-chase experiments revealed a transient, solvent-extractable, lipid-linked component. The extracted N-acetylneuraminic acid was in polymeric form. Sequential oxidation and reduction of the extracted radioactivity followed by neuraminidase treatment revealed an average degree of polymerization of four or five N-acetylneuraminic acid residues. Bacitracin-sensitive peptidoglycan was synthesized in vitro by particulate membranes. Cross-competition experiments between peptidoglycan and capsular polysaccharide synthesis by preincubation of precursors of one pathway during synthesis of the other revealed a competitive effect for a common component. This component was believed to be a common pool of undecaprenyl phosphate. A model for the production and regulation of the capsular polysaccharide is proposed.

Formation of competent Bacillus subtilis cells

The process of competent cell formation for transformation has been studied with early-stationary-phase (T1) cells of Bacillus subtilis which had been grown in an enriched Spizizen minimal medium and transferred to a second synthetic medium. Rifampin, chloramphenicol, and tunicamycin were strong inhibitors of competent cell formation, as well as vegetative growth. After formation, competent cells were no longer sensitive to the above agents. Methicillin and an inhibitor of chromosomal replication, hydroxyphenylazouracil, did not inhibit the development of competence. A D-alanine-requiring mutant strain developed competence even in the absence of D-alanine in the second medium. A T1-stage culture showed the activity of extracellular serine protease which is necessary for sporulation. Competent cell formation was completely blocked by 0.7 M ethanol, which is a specific inhibitor of early events during sporulation, including forespore septum formation. Competent cells were formed even in media which supported sporulation. The development of competence was also studied with spo0 mutants at 10 different loci. Most spo0 mutations repressed the development of competence except for spo0C, spo0G, and spo0J. These results suggest that competent cells are formed from early sporulating cells with the synthesis of cell wall materials and by factors whose genes are activated by the supply of nutrients. It is suggested that common steps are involved both in forespore septation and in competent cell formation.

Synthesis of teichoic acid by Bacillus subtilis protoplasts

Protoplasts of Bacillus subtilis W23 readily synthesized ribitol teichoic acid from nucleotide precursors in the surrounding medium. With cytidine diphosphate-ribitol they made poly(ribitol phosphate), presumably attached to lipoteichoic acid carrier; when cytidine diphosphate-glycerol and uridine diphosphate-N-acetylglucosamine were also present a 10-fold increase in the rate of polymer synthesis occurred, and the product contained both the main chain and the linkage unit. Synthesis was inhibited by trypsin or p-chloromercuribenzenesulfonate in the medium, and we concluded that it occurred at the outer surface of the membrane. During synthesis, which was also achieved readily by whole cells after a brief period of wall lysis, the cytidine phosphate portion of the nucleotide precursors did not pass through the membrane. No evidence could be obtained for a transphosphorylation mechanism for the translocation process. It is suggested that reaction with exogenous substrates was due to temporary exposure of a protein component of the enzyme complex at the outer surface of the membrane during the normal biosynthetic cycle.

The biosynthesis of wall teichoic acid by toluenised cells of Bacillus subtilis W23

Toluenised cells of Bacillus subtilis W23 synthesized the teichoic acid, poly(ribitol phosphate), from exogenous precursors. The synthesis was dependent on concomitant synthesis of the linkage unit that joins teichoic acid to peptidoglycan. Under conditions that reduced cell autolytic activity, a large proportion of the teichoic acid became linked to the cell wall, independently of peptidoglycan synthesis. The specific activity of the system was more than 30 times that of isolated membranes, so that activity could be measured readily in the cells from 2 ml of an exponential culture of bacteria.

Fractionation studies of the enzyme complex involved in teichoic acid synthesis

The membrane-bound enzymes participating in the syntheses of the teichoic acid main chain and linkage unit have been solubilized with Triton X-100 and fractionated by sucrose density gradient centrifugation. Two main fractions were obtained: a heavy fraction, containing enzymes effecting synthesis of the main chain attached to the linkage unit, which was associated with only a small amount of lipid, and a light fraction which was rich in prenyl phosphate and catalyzed only linkage-unit synthesis. The separation by density was not based entirely on polypeptide chain length, as some of the shortest chains appeared in the denser fractions and some relatively high-molecular-weight peptides occurred in the lightest fraction. High activity for linkage-unit synthesis was observed in a fraction containing only a few peptides. Addition of ficaprenyl phosphate to the enzyme preparations had no stimulatory effect. It is concluded that the enzymes for main-chain and linkage unit syntheses frm one or more fairly tightly associated complexes and that polyprenyl phosphate is an integral firmly bound component of the complex in which the linkage unit is synthesized.

Teichoic and teichuronic acids: biosynthesis, assembly, and location

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Attachment of the main chain to the linkage unit in biosynthesis of teichoic acids

The main chain of teichoic acids can be assembled in cell-free membrane preparations by the transfer of residues from the appropriate nucleotide precursors to an incompletely characterized amphiphilic molecule, lipoteichoic acid carrier (LTC). However, in the cell wall, the main chain is attached to peptidoglycan through a linkage unit which is synthesized independently. It is believed that, in these cell-free systems, lipid intermediates carrying linkage units are also able to accept residues directly from nucleotide precursors to build up the main chain. In this paper, we have shown that the main chain attached to LTC was transferred from LTC to lipids containing the linkage unit. Thus, in these systems, there appear to be two routes to the biosynthesis of teichoic acid-linkage unit complexes, one by direct assembly of the main chain on linkage unit lipids and the other by transfer of the preassembled main chain from LTC to the linkage unit. It was also shown that linkage unit lipids from different organisms were interchangeable and that these were used for polymer synthesis by Bacillus subtilis 3610, in which the teichoic acid is a poly(glycerol phosphate).

Structure of the Escherichia coli K2 capsular antigen, a teichoic acid-like polymer

The primary structure of the K2 antigen of Escherichia coli was elucidated by composition, alkaline fragmentation, dephosphorylation with hydrofluoric acid, periodate oxidation, and methylation analysis. The polymer contains galactose in the pyranosidic and furanosidic ring form. It consists of phosphogaolactopyranosyl glycerol and phosphagalactofuranosyl glycerol units in a molar ratio of 2:1. The sequence of these units is not known. The structure of the K2 antigen is reminiscent of that of certain teichoic acids of gram-positive bacteria. Using microprecipitation, it was shown that in the polymer galactoside is immunodominant.

Tunicamycin-resistant mutants and chromosomal locations of mutational sites in Bacillus subtilis

The types of tunicamycin-resistant mutants of Bacillus subtilis were analyzed, and their mutational sites on the chromosome were mapped. A type 1 mutation that simultaneously expressed hyperproductivity of extracellular alpha-amylase was located close to amy E. Type 2 mutations were near aroI.