Inactivation of the srtA gene in Streptococcus gordonii inhibits cell wall anchoring of surface proteins and decreases in vitro and in vivo adhesion

Therapeutic potential of kaempferol on Streptococcus pneumoniae infection

Co-infections with pathogens and secondary bacterial infections play significant roles during the pandemic coronavirus disease 2019 (COVID-19) pathogenetic process, caused by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). Notably, co-infections with Streptococcus pneumoniae (S. pneumoniae), as a major Gram-positive pathogen causing pneumonia or meningitis, severely threaten the diagnosis, therapy, and prognosis of COVID-19 worldwide. Accumulating evidences have emerged indicating that S. pneumoniae evolves multiple virulence factors, including pneumolysin (PLY) and sortase A (SrtA), which have been extensively explored as alternative anti-infection targets. In our study, natural flavonoid kaempferol was identified as a potential candidate drug for infection therapeutics via anti-virulence mechanisms. We found that kaempferol could interfere with the pore-forming activity of PLY by engaging with catalytic active sites and consequently inhibit PLY-mediated cytotoxicity. Additionally, exposed to kaempferol significantly reduced the SrtA peptidase activity by occupying the active sites of SrtA. Further, the biofilms formation and bacterial adhesion to the host cells could be significantly thwarted by kaempferol incubation. In vivo infection model by S. pneumoniae highlighted that kaempferol oral administration exhibited notable treatment benefits, as evidenced by decreased bacterial burden, suggesting that kaempferol has tremendous potential to attenuate S. pneumoniae pathogenicity. Scientifically, our study implies that kaempferol is a promising therapeutic option by targeting bacterial virulence factors.

Streptococcus oralis Employs Multiple Mechanisms of Salivary Mucin Binding That Differ Between Strains

Streptococcus oralis is an oral commensal and opportunistic pathogen that can enter the bloodstream and cause bacteremia and infective endocarditis. Here, we investigated the mechanisms of S. oralis binding to oral mucins using clinical isolates, isogenic mutants and glycoconjugates. S. oralis bound to both MUC5B and MUC7, with a higher level of binding to MUC7. Mass spectrometry identified 128 glycans on MUC5B, MUC7 and the salivary agglutinin (SAG). MUC7/SAG contained a higher relative abundance of Lewis type structures, including Lewis b/y, sialyl-Lewis a/x and α2,3-linked sialic acid, compared to MUC5B. S. oralis subsp. oralis binding to MUC5B and MUC7/SAG was inhibited by Lewis b and Lacto-N-tetraose glycoconjugates. In addition, S. oralis binding to MUC7/SAG was inhibited by sialyl Lewis x. Binding was not inhibited by Lacto-N-fucopentaose, H type 2 and Lewis x conjugates. These data suggest that three distinct carbohydrate binding specificities are involved in S. oralis subsp. oralis binding to oral mucins and that the mechanisms of binding MUC5B and MUC7 differ. Efficient binding of S. oralis subsp. oralis to MUC5B and MUC7 required the gene encoding sortase A, suggesting that the adhesin(s) are LPXTG-containing surface protein(s). Further investigation demonstrated that one of these adhesins is the sialic acid binding protein AsaA.

Covalent sortase A inhibitor ML346 prevents Staphylococcus aureus infection of Galleria mellonella

The housekeeping sortase A (SrtA), a membrane-associated cysteine transpeptidase, is responsible for anchoring surface proteins to the cell wall peptidoglycan in Gram-positive bacteria. This process is essential for the regulation of bacterial virulence and pathogenicity. Therefore, SrtA is considered to be an ideal target for antivirulence therapy. In this study, we report that ML346, a compound with a barbituric acid and cinnamaldehyde scaffold, functions as an irreversible inhibitor of Staphylococcus aureus SrtA (SaSrtA) and Streptococcus pyogenes SrtA (SpSrtA) in vitro at low micromolar concentrations. According to our X-ray crystal structure of the SpSrtAΔN81/ML346 complex (Protein Data Bank ID: 7V6K), ML346 covalently modifies the thiol group of Cys208 in the active site of SpSrtA. Importantly, ML346 significantly attenuated the virulence phenotypes of S. aureus and exhibited inhibitory effects on Galleria mellonella larva infection caused by S. aureus. Collectively, our results indicate that ML346 has potential for development as a covalent antivirulence agent for treating S. aureus infections, including methicillin-resistant S. aureus.

Oxyresveratrol promotes biofilm formation, cell attachment and aggregation of Streptococcus gordonii in the presence of sucrose

Streptococcus gordonii is a commensal colonizer of oral cavity that initiates the formation of dental plaque. Oxyresveratrol is a natural purification from plants with antibacterial effects on various oral bacteria including Streptococcus mutans. The aim of this study was to investigate the effects of oxyresveratrol on S. gordonii. The basic viability, biofilm formation and cell aggregation of S. gordonii treated with oxyresveratrol were investigated. Oxyresveratrol dose-dependently inhibited the growth of S. gordonii in the absence of sucrose. However, in the presence of sucrose, it promoted biofilm formation under MIC. Both the biofilm formation and extracellular polysaccharides synthesis reached the maximum level at ½ MIC (250 μg/mL) oxyresveratrol. The gene expressions of abpA, abpB, scaA, gtfG, hsa, cshA, cshB, ccpA, srtA and sspB were upregulated when treated with 62.5 and 125 μg/mL oxyresveratrol. A total eight of the ten genes were significantly upregulated at 250 μg/mL oxyresveratrol except abpB and sspB, which were downregulated at 250 μg/mL without significance. In conclusion, oxyresveratrol has dual-effects on S. gordonii. Considering its specific biofilm suppressive effect on S. mutans, it might be a candidate for bacterial interspecies modulator applied in caries prevention.

State-of-the-Art Vaccine Research for Aquaculture Use: The Case of Three Economically Relevant Fish Species

In the last three decades, the aquaculture sector has experienced a 527% growth, producing 82 million tons for a first sale value estimated at 250 billion USD. Infectious diseases caused by bacteria, viruses, or parasites are the major causes of mortality and economic losses in commercial aquaculture. Some pathologies, especially those of bacterial origin, can be treated with commercially available drugs, while others are poorly managed. In fact, despite having been recognized as a useful preventive measure, no effective vaccination against many economically relevant diseases exist yet, such as for viral and parasitic infections. The objective of the present review is to provide the reader with an updated perspective on the most significant and innovative vaccine research on three key aquaculture commodities. European sea bass (Dicentrarchus labrax), Nile tilapia (Oreochromis niloticus), and Atlantic salmon (Salmo salar) were chosen because of their economic relevance, geographical distinctiveness, and representativeness of different culture systems. Scientific papers about vaccines against bacterial, viral, and parasitic diseases will be objectively presented; their results critically discussed and compared; and suggestions for future directions given.

Effects of srtA variation on phagocytosis resistance and immune response of Streptococcus equi

Strangles, which is caused by Streptococcus equi subspecies equi (S. equi), is one of the most prevalent equine infectious diseases with worldwide distribution and leads to serious economic loss in the horse industry. Sortase A (srtA) is a transpeptidase that anchors multiple virulence-associated surface proteins to the cell surface of S. equi. srtA plays a major role in S. equi infection and colonization of the host cell. In this study, we aimed to investigate the effects of srtA mutation on the phagocytic activity and immunogenicity of S. equi. The point-mutated recombinant sortases, including srtA-HT1112 (I88V), srtA-5012 (R147G), and srtA-ZZM17 (control), were expressed, purified, and used to immunize the mouse models. Phagocytic activity was assessed using equine polymorphonuclear cells, whereas opsonophagocytic function and adherence inhibition were measured using the antiserum of these mutants. Mouse serum antibody, bacterial load, and weight gain were also measured. The srtA-HT1112 (I88V) mutant showed significantly enhanced antiphagocytic capability, and its antiserum exhibited increased adherence inhibition activity. In addition, the srtA-HT1112 (I88V) mutant presented the highest lung bacterial load and lowest protection rate (50%) after the challenge with S. equi ZZM17. The srtA-5012 (R147G) mutant exhibited a high IgG2a level and protection rate (62.5%-75%) and the lowest lung bacterial load. These results indicate that the I88V mutation is associated with a high antiphagocytic activity, whereas R147G mutation is associated with the decreased lung bacterial load. Our findings may be useful for the evaluation and development of vaccines.

Uncovering Roles of Streptococcus gordonii SrtA-Processed Proteins in the Biofilm Lifestyle

Streptococcus gordonii is a commensal oral organism. Harmless in the oral cavity, S. gordonii is an opportunistic pathogen. S. gordonii adheres to body surfaces using surface adhesive proteins (adhesins), which are critical to subsequent formation of biofilm communities. As in most Gram-positive bacteria, S. gordonii surface proteins containing the C-terminal LPXTG motif cleavage sequence are processed by sortase A (SrtA) to become covalently attached to the cell wall. To characterize the functional diversity and redundancy in the family of SrtA-processed proteins, an S. gordonii DL1 markerless deletion mutant library was constructed of each of the 26 putative SrtA-processed proteins. Each library member was evaluated for growth in rich medium, biofilm formation on plastic, saliva and salivary fractions, cell surface hydrophobicity (CSH), hemagglutination, and integration into an ex vivo plaque biofilm community. Library members were compared to the non-SrtA-processed adhesins AbpA and AbpB. While no major growth differences in rich medium were observed, many S. gordonii LPXTG/A proteins impacted biofilm formation on one or more of the substrates. Several mutants showed significant differences in hemagglutination, hydrophobicity, or fitness in the ex vivo plaque model. From the identification of redundant and unique functions in these in vitro and ex vivo systems, functional stratification among the LPXTG/A proteins is apparent.IMPORTANCE S. gordonii interactions with its environment depend on the complement of cell wall proteins. A subset of these cell wall proteins requires processing by the enzyme sortase A (SrtA). The identification of SrtA-processed proteins and their functional characterization will help the community to better understand how S. gordonii engages with its surroundings, including other microbes, integrates into the plaque community, adheres to the tooth surface, and hematogenously disseminates to cause blood-borne infections. This study identified 26 putative SrtA-processed proteins through creation of a markerless deletion mutant library. The library was subject to functional screens that were chosen to better understand key aspects of S. gordonii physiology and pathogenesis.

Astilbin Inhibits the Activity of Sortase A from Streptococcus mutans

Streptococcus mutans (S. mutans) is the primary etiological agent of dental caries. The S. mutans enzyme sortase A (SrtA) is responsible for anchoring bacterial cell wall surface proteins involved in host cell attachment and biofilm formation. Thus, SrtA is an attractive target for inhibiting dental caries caused by S. mutans-associated acid fermentation. In this study, we observed that astilbin, a flavanone compound extracted from Rhizoma Smilacis Glabrae, has potent inhibitory activity against the S. mutans SrtA, with an IC₅₀ of 7.5 μg/mL. In addition, astilbin was proven to reduce the formation of biofilm while without affecting the growth of S. mutans. The results of a molecular dynamics simulation and a mutation analysis revealed that the Arg213, Leu111, and Leu116 of SrtA are important for the interaction between SrtA and astilbin. The results of this study demonstrate the potential of using astilbin as a nonbactericidal agent to modulate pathogenicity of S. mutans by inhibiting the activity of SrtA.

Ca2+ binding induced sequential allosteric activation of sortase A: An example for ion-triggered conformational selection

The allosteric activation of the intrinsically disordered enzyme Staphylococcus aureus sortase A is initiated via binding of a Ca²⁺ ion. Although Ca²⁺ binding was shown to initiate structural changes inducing disorder-to-order transitions, the details of the allosteric activation mechanism remain elusive. We performed long-term molecular dynamics simulations of sortase A without (3 simulations of 1.6 μs) and with bound Ca²⁺ (simulations of 1.6 μs, 1.8 μs, and 2.5 μs). Our results show that Ca²⁺ binding causes not only ordering of the disordered β6/β7 loop of the protein, but also modulates hinge motions in the dynamic β7/β8 loop, which is important for the catalytic activity of the enzyme. Cation binding triggers signal transmission from the Ca²⁺ binding site to the dynamic β7/β8 loop via the repetitive folding/unfolding of short helical stretches of the disordered β6/β7 loop. These correlated structural rearrangements lead to several distinct conformational states of the binding groove, which show optimal binding features for the sorting signal motif and feature binding energies up to 20 kcal/mol more favorable than observed for the sortase A without Ca²⁺. The presented results indicate a highly correlated, conformational selection-based activation mechanism of the enzyme triggered by cation binding. They also demonstrate the importance of the dynamics of intrinsically disordered regions for allosteric regulation.

Role of Neuraminidase-Producing Bacteria in Exposing Cryptic Carbohydrate Receptors for Streptococcus gordonii Adherence

Streptococcus gordonii is an early colonizer of the oral cavity. Although a variety of S. gordonii adherence mechanisms have been described, current dogma is that the major receptor for S. gordonii is sialic acid. However, as many bacterial species in the oral cavity produce neuraminidase that can cleave terminal sialic acid, it is unclear whether S. gordonii relies on sialic acid for adherence to oral surfaces or if this species has developed alternative binding strategies. Previous studies have examined adherence to immobilized glycoconjugates and identified binding to additional glycans, but no prior studies have defined the contribution of these different glycan structures in adherence to oral epithelial cells. We determined that the majority of S. gordonii strains tested did not rely on sialic acid for efficient adherence. In fact, adherence of some strains was significantly increased following neuraminidase treatment. Further investigation of representative strains that do not rely on sialic acid for adherence revealed binding not only to sialic acid via the serine-rich repeat protein GspB but also to β-1,4-linked galactose. Adherence to this carbohydrate occurs via an unknown adhesin distinct from those utilized by Streptococcus oralis and Streptococcus pneumoniae Demonstrating the potential biological relevance of binding to this cryptic receptor, we established that S. oralis increases S. gordonii adherence in a neuraminidase-dependent manner. These data suggest that S. gordonii has evolved to simultaneously utilize both terminal and cryptic receptors in response to the production of neuraminidase by other species in the oral environment.

Directed evolution provides insight into conformational substrate sampling by SrtA

The Sortase family of transpeptidases are found in numerous gram-positive bacteria and involved in divergent physiological processes including anchoring of surface proteins to the cell wall as well as pili assembly. As essential proteins, sortase enzymes have been the focus of considerable interest for the development of novel anti-microbials, however, more recently their function as unique transpeptidases has been exploited for the synthesis of novel bio-conjugates. Yet, for synthetic purposes, SrtA-mediated conjugation suffers from the enzyme's inherently poor catalytic efficiency. Therefore, to identify SrtA variants with improved catalytic efficiency, we used directed evolution to select a catalytically enhanced SrtA enzyme. An analysis of improved SrtA variants in the context of sequence conservation, NMR and x-ray crystal structures, and kinetic data suggests a novel mechanism for catalysis involving large conformational changes that delivers substrate to the active site pocket. Indeed, using DEER-EPR spectroscopy, we reveal that upon substrate binding, SrtA undergoes a large scissors-like conformational change that simultaneously translates the sort-tag substrate to the active site in addition to repositioning key catalytic residues for esterification. A better understanding of Sortase dynamics will significantly enhance future engineering and drug discovery efforts.

CRISPR/dCas9-mediated inhibition of gene expression in Staphylococcus aureus

The understanding of the genetic mechanism of Staphylococcus aureus requires efficient tools, however, genetic manipulation in S. aureus is always laborious and time-consuming. Here we proposed a novel CRISPR/dCas9 interference method for the rapid knockdown of target genes. Furthermore, multiple genes can be repressed simultaneously by using this method.

Epigallocatechin gallate inhibits Streptococcus pneumoniae virulence by simultaneously targeting pneumolysin and sortase A

Streptococcus pneumoniae (pneumococcus), the causative agent of several human diseases, possesses numerous virulence factors associated with pneumococcal infection and pathogenesis. Pneumolysin (PLY), an important virulence factor, is a member of the cholesterol-dependent cytolysin family and has cytolytic activity. Sortase A (SrtA), another crucial pneumococcal virulence determinate, contributes greatly to the anchoring of many virulence-associated surface proteins to the cell wall. In this study, epigallocatechin gallate (EGCG), a natural compound with little known antipneumococcal activity, was shown to directly inhibit PLY-mediated haemolysis and cytolysis by blocking the oligomerization of PLY and simultaneously reduce the peptidase activity of SrtA. The biofilm formation, production of neuraminidase A (NanA, the pneumococcal surface protein anchored by SrtA), and bacterial adhesion to human epithelial cells (Hep2) were inhibited effectively when S. pneumoniae D39 was cocultured with EGCG. The results from molecular dynamics simulations and mutational analysis confirmed the interaction of EGCG with PLY and SrtA, and EGCG binds to Glu277, Tyr358, and Arg359 in PLY and Thr169, Lys171, and Phe239 in SrtA. In vivo studies further demonstrated that EGCG protected mice against S. pneumoniae pneumonia. Our results imply that EGCG is an effective inhibitor of both PLY and SrtA and that an antivirulence strategy that directly targets PLY and SrtA using EGCG is a promising therapeutic option for S. pneumoniae pneumonia.

Staphylococcus epidermidis ΔSortase A strain elicits protective immunity against Staphylococcus aureus infection

Staphylococcus aureus and Staphylococcus epidermidis are two of the most significant opportunistic human pathogens, causing medical implant and nosocomial infections worldwide. These bacteria contain surface proteins that play crucial roles in multiple biological processes. It has become apparent that they have evolved a number of unique mechanisms by which they can immobilise proteins on their surface. Notably, a conserved cell membrane-anchored enzyme, sortase A (SrtA), can catalyse the covalent attachment of precursor bacterial cell wall-attached proteins to peptidoglycan. Considering its indispensable role in anchoring substrates to the cell wall and its effects on virulence, SrtA has attracted great attention. In this study, a 549-bp gene was cloned from a pathogenic S. epidermidis strain, YC-1, which shared high identity with srtA from other Staphylococcus spp. A mutant strain, YC-1ΔsrtA, was then constructed by allelic exchange mutagenesis. The direct survival rate assay suggested that YC-1ΔsrtA had a lower survival capacity in healthy mice blood compare with the wild-type strain, indicating that the deletion of srtA affects the virulence and infectious capacity of S. epidermidis YC-1. YC-1ΔsrtA was then administered via intraperitoneal injection and it provided a relative percent survival value of 72.7 % in mice against S. aureus TC-1 challenge. These findings demonstrate the possbility that YC-1ΔsrtA might be used as a live attenuated vaccine to produce cross-protection against S. aureus.

Streptococcus pyogenes Sortase Mutants Are Highly Susceptible to Killing by Host Factors Due to Aberrant Envelope Physiology

Cell wall anchored virulence factors are critical for infection and colonization of the host by Gram-positive bacteria. Such proteins have an N-terminal leader sequence and a C-terminal sorting signal, composed of an LPXTG motif, a hydrophobic stretch, and a few positively charged amino acids. The sorting signal halts translocation across the membrane, allowing sortase to cleave the LPXTG motif, leading to surface anchoring. Deletion of sortase prevents the anchoring of virulence factors to the wall; the effects on bacterial physiology however, have not been thoroughly characterized. Here we show that deletion of Streptococcus pyogenes sortase A leads to accumulation of sorting intermediates, particularly at the septum, altering cellular morphology and physiology, and compromising membrane integrity. Such cells are highly sensitive to cathelicidin, and are rapidly killed in blood and plasma. These phenomena are not a loss-of-function effect caused by the absence of anchored surface proteins, but specifically result from the accumulation of sorting intermediates. Reduction in the level of sorting intermediates leads to a return of the sortase mutant to normal morphology, while expression of M protein with an altered LPXTG motif in wild type cells leads to toxicity in the host environment, similar to that observed in the sortase mutant. These unanticipated effects suggest that inhibition of sortase by small-molecule inhibitors could similarly lead to the rapid elimination of pathogens from an infected host, making such inhibitors much better anti-bacterial agents than previously believed.

Equilibrium of sortase A dimerization on Staphylococcus aureus cell surface mediates its cell wall sorting activity

Staphylococcus aureus sortase A (SrtA) transpeptidase is a therapeutically important membrane-bound enzyme in Gram-positive bacteria, which organizes the covalently attached cell surface proteins on the peptidoglycan cell wall of the organism. Here, we report the direct observation of the highly selective homo-dimerization of SrtA on the cell membrane. To address the biological significance of the dimerization towards enzyme function, site-directed mutagenesis was performed to generate a SrtA mutant, which exists as monomer on the cell membrane. We observed that the cell surface display of adhesive proteins in S. aureus cells expressing monomeric SrtA mutant is more prominent than the cells expressing the wild-type enzyme. A cell-based invasion assay was also performed to evaluate the activities of wild-type SrtA and its monomeric mutant as well. Our data demonstrated that S. aureus cells expressing SrtA in monomeric form invade host mammalian cells more efficiently than those expressing wild-type SrtA in dimer-monomer equilibrium. The results suggested that the monomeric form of SrtA is more active than the dimeric form of the enzyme in terms of cell surface display of virulence factors for infection. This is the first study to present the oligomerization of SrtA and its related biological function on the cell membrane. Study of SrtA dimerization has implications for understanding its catalytic mechanism at the cellular level as well as the development of novel anti-infective agents.

Lethality of sortase depletion in Actinomyces oris caused by excessive membrane accumulation of a surface glycoprotein

Sortase, a cysteine-transpeptidase conserved in Gram-positive bacteria, anchors on the cell wall many surface proteins that facilitate bacterial pathogenesis and fitness. Genetic disruption of the housekeeping sortase in several Gram-positive pathogens reported thus far attenuates virulence, but not bacterial growth. Paradoxically, we discovered that depletion of the housekeeping sortase SrtA was lethal for Actinomyces oris; yet, all of its predicted cell wall-anchored protein substrates (AcaA-N) were individually dispensable for cell viability. Using Tn5-transposon mutagenesis to identify factors that upend lethality of srtA deletion, we uncovered a set of genetic suppressors harbouring transposon insertions within genes of a locus encoding AcaC and a LytR-CpsA-Psr (LCP)-like protein. AcaC was shown to be highly glycosylated and dependent on LCP for its glycosylation. Upon SrtA depletion, the glycosylated form of AcaC, hereby renamed GspA, was accumulated in the membrane. Overexpression of GspA in a mutant lacking gspA and srtA was lethal; conversely, cells overexpressing a GspA mutant missing a membrane-localization domain were viable. The results reveal a unique glycosylation pathway in A. oris that is coupled to cell wall anchoring catalysed by sortase SrtA. Significantly, this novel phenomenon of glyco-stress provides convenient cell-based assays for developing a new class of inhibitors against Gram-positive pathogens.

Clostridium difficile has a single sortase, SrtB, that can be inhibited by small-molecule inhibitors

Background

Bacterial sortases are transpeptidases that covalently anchor surface proteins to the peptidoglycan of the Gram-positive cell wall. Sortase protein anchoring is mediated by a conserved cell wall sorting signal on the anchored protein, comprising of a C-terminal recognition sequence containing an “LPXTG-like” motif, followed by a hydrophobic domain and a positively charged tail.

Results

We report that Clostridium difficile strain 630 encodes a single sortase (SrtB). A FRET-based assay was used to confirm that recombinant SrtB catalyzes the cleavage of fluorescently labelled peptides containing (S/P)PXTG motifs. Strain 630 encodes seven predicted cell wall proteins with the (S/P)PXTG sorting motif, four of which are conserved across all five C. difficile lineages and include potential adhesins and cell wall hydrolases. Replacement of the predicted catalytic cysteine residue at position 209 with alanine abolishes SrtB activity, as does addition of the cysteine protease inhibitor MTSET to the reaction. Mass spectrometry reveals the cleavage site to be between the threonine and glycine residues of the (S/P)PXTG peptide. Small-molecule inhibitors identified through an in silico screen inhibit SrtB enzymatic activity to a greater degree than MTSET.

Conclusions

These results demonstrate for the first time that C. difficile encodes a single sortase enzyme, which cleaves motifs containing (S/P)PXTG in-vitro. The activity of the sortase can be inhibited by mutation of a cysteine residue in the predicted active site and by small-molecule inhibitors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0219-1) contains supplementary material, which is available to authorized users.

Sorting of LPXTG peptides by archetypal sortase A: role of invariant substrate residues in modulating the enzyme dynamics and conformational signature of a productive substrate

Transpeptidase sortase catalyzes the covalent anchoring of surface proteins to the cell wall in Gram-positive bacteria. Sortase A (SrtA) of Staphylococcus aureus is a prototype enzyme and considered a bona fide drug target because several substrate proteins are virulence-related and implicated in pathogenesis. Besides, SrtA also works as a versatile tool in protein engineering. Surface proteins destined for cell wall anchoring contain a LPXTG sequence located in their C-terminus which serves as a substrate recognition motif for SrtA. Recent studies have implicated substrate-induced conformational dynamics in SrtA. In the present work, we have explored the roles of invariant Leu and Pro residues of the substrate in modulating the enzyme dynamics with a view to understand the selection process of a catalytically competent substrate. Overall results of molecular dynamics simulations and experiments carried out with noncanonical substrates and site-directed mutagenesis reveal that the kinked conformation due to Pro in LPXTG is obligatory for productive binding but does not per se control the enzyme dynamics. The Leu residue of the substrate appears to play the crucial role of an anchor to the beta6-beta7 loop directing the conformational transition of the enzyme from an "open" to a "closed" state subsequent to which the Pro residue facilitates the consummation of binding through predominant engagement of the loop and catalytic motif residues in hydrophobic interactions. Collectively, our study provides insights about specificity, tolerance, and conformational sorting of substrate by SrtA. These results have important implications in designing newer substrates and inhibitors for this multifaceted enzyme.

Impact of Lactobacillus plantarum sortase on target protein sorting, gastrointestinal persistence, and host immune response modulation

Sortases are transpeptidases that couple surface proteins to the peptidoglycan of Gram-positive bacteria, and several sortase-dependent proteins (SDPs) have been demonstrated to be crucial for the interactions of pathogenic and nonpathogenic bacteria with their hosts. Here, we studied the role of sortase A (SrtA) in Lactobacillus plantarum WCFS1, a model Lactobacillus for probiotic organisms. An isogenic srtA deletion derivative was constructed which did not show residual SrtA activity. DNA microarray-based transcriptome analysis revealed that the srtA deletion had only minor impact on the full-genome transcriptome of L. plantarum, while the expression of SDP-encoding genes remained completely unaffected. Mass spectrometry analysis of the bacterial cell surface proteome, which was assessed by trypsinization of intact bacterial cells and by LiCl protein extraction, revealed that SrtA is required for the appropriate subcellular location of specific SDPs and for their covalent coupling to the cell envelope, respectively. We further found that SrtA deficiency did not affect the persistence and/or survival of L. plantarum in the gastrointestinal tract of mice. In addition, an in vitro immature dendritic cell (iDC) assay revealed that the removal of surface proteins by LiCl strongly affected the proinflammatory signaling properties of the SrtA-deficient strain but not of the wild type, which suggests a role of SDPs in host immune response modulation.

Human pathogenic streptococcal proteomics and vaccine development

Gram-positive streptococci are non-motile, chain-forming bacteria commonly found in the normal oral and bowel flora of warm-blooded animals. Over the past decade, a proteomic approach combining 2-DE and MS has been used to systematically map the cellular, surface-associated and secreted proteins of human pathogenic streptococcal species. The public availability of complete streptococcal genomic sequences and the amalgamation of proteomic, genomic and bioinformatic technologies have recently facilitated the identification of novel streptococcal vaccine candidate antigens and therapeutic agents. The objective of this review is to examine the constituents of the streptococcal cell wall and secreted proteome, the mechanisms of transport of surface and secreted proteins, and describe the current methodologies employed for the identification of novel surface-displayed proteins and potential vaccine antigens.

Synthesis and structure activity relationship studies of novel Staphylococcus aureus Sortase A inhibitors

Synthetic methods have been developed for lead Sortase A inhibitors identified from previous studies. Several derivatives of the lead inhibitor were synthesized to derive preliminary structure activity relationships (SAR). Different regions of the lead inhibitor that are critical for the enzyme activity have been determined by systematic SAR studies. The E stereochemistry of the lead compound was found to be critical for its activity. Replacement of the E double bond with Z double bond or a rigid triple bond reduced the enzyme inhibitory activity in most cases. Reduction of the double bond to a C-C single bond resulted in complete loss of activity. Amide carbonyl and NH groups were also found to be crucial for the activity of this class of inhibitors, as well. The morpholine ring oxygen atom was also found to be an important factor for the activity of the lead inhibitor. Preliminary SAR studies led to the identification of compounds with improved enzyme inhibition. The most active compound was found to have an IC(50) value of 58 microM against the enzyme.

Sortase anchored proteins of Streptococcus uberis play major roles in the pathogenesis of bovine mastitis in dairy cattle

Streptococcus uberis, strain 0140J, contains a single copy sortase A (srtA), encoding a transamidase capable of covalently anchoring specific proteins to peptidoglycan. Unlike the wild-type, an isogenic mutant carrying an inactivating ISS1 insertion within srtA was only able to infect the bovine mammary gland in a transient fashion. For the first 24 h post challenge, the srtA mutant colonised at a similar rate and number to the wild type strain, but unlike the wild type did not subsequently colonise in higher numbers. Similar levels of host cell infiltration were detected in response to infection with both strains, but only in those mammary quarters infected with the wild type strain were clinical signs of disease evident. Mutants that failed to express individual sortase substrate proteins (sub0135, sub0145, sub0207, sub0241, sub0826, sub0888, sub1095, sub1154, sub1370, and sub1730) were isolated and their virulence determined in the same challenge model. This revealed that mutants lacking sub0145, sub1095 and sub1154 were attenuated in cattle. These data demonstrate that a number of sortase anchored proteins each play a distinct, non-redundant and important role in pathogenesis of S. uberis infection within the lactating bovine mammary gland.

Discovery and structure-activity relationship analysis of Staphylococcus aureus sortase A inhibitors

Methicillin resistant Staphylococcus aureus (MRSA) is a major health problem that has created a pressing need for new antibiotics. Compounds that inhibit the S. aureus SrtA sortase may function as potent anti-infective agents as this enzyme attaches virulence factors to the cell wall. Using high-throughput screening, we have identified several compounds that inhibit the enzymatic activity of the SrtA. A structure-activity relationship (SAR) analysis led to the identification of several pyridazinone and pyrazolethione analogs that inhibit SrtA with IC(50) values in the sub-micromolar range. Many of these molecules also inhibit the sortase enzyme from Bacillus anthracis suggesting that they may be generalized sortase inhibitors.

Sortase A confers protection against Streptococcus pneumoniae in mice

Streptococcus pneumoniae sortase A (SrtA) is a transpeptidase that is highly conserved among pneumococcal strains, whose involvement in adhesion/colonization has been reported. We found that intraperitoneal immunization with recombinant SrtA conferred to mice protection against S. pneumoniae intraperitoneal challenge and that the passive transfer of immune serum before intraperitoneal challenge was also protective. Moreover, by using the intranasal challenge model, we observed a significant reduction of bacteremia when mice were intraperitoneally immunized with SrtA, while a moderate decrease of lung infection was achieved by intranasal immunization, even though no influence on nasopharynx colonization was seen. Taken together, our results suggest that SrtA is a good candidate for inclusion in a multicomponent, protein-based, pneumococcal vaccine.

Identification of novel LPXTG-linked surface proteins from Streptococcus gordonii

Surface adhesion plays an essential part in the survival of the commensal organism Streptococcus gordonii in the oral cavity as well as during opportunistic infections such as endocarditis. At least two types of cell surface protein involved in adhesion are found on the surface of Gram-positive bacteria: those anchored via an LPXTG motif by the enzyme sortase A (SrtA) and those associated with the cell surface by, as yet, unknown mechanisms. In srtA(-) mutants, LPXTG-containing proteins have been shown to be released rather than cross-linked to the cell wall. We have therefore used 2D gel electrophoresis of released proteins from an srtA(-) mutant as well as the wild-type strain, followed by peptide identification by MS, to identify a set of novel proteins predicted to be present on the surface of S. gordonii DL1. This includes two large LPXTG-linked proteins (SGO_0707 and SGO_1487), which both contain tandemly repeated sequences similar to those present in known fibrillar adhesins. A 5'-nucleotidase and a protein with a putative collagen-binding domain, both containing LPXTG motifs, were also identified. Anchorless proteins with known chaperone, stress response and elongation factor functions, apparently responsible for bacterial binding to keratinocytes and saliva-coated surfaces in the absence of the LPXTG-linked adhesins, were also associated with the cell surface. These data reveal a range of proteins to be present on the S. gordonii DL1 cell surface, the expression of which plays an important role in adhesion to epithelia and which represent likely candidates for novel virulence factors in S. gordonii.

Crystal structure of Streptococcus pyogenes sortase A: implications for sortase mechanism

Sortases are a family of Gram-positive bacterial transpeptidases that anchor secreted proteins to bacterial cell surfaces. These include many proteins that play critical roles in the virulence of Gram-positive bacterial pathogens such that sortases are attractive targets for development of novel antimicrobial agents. All Gram-positive pathogens express a "housekeeping" sortase that recognizes the majority of secreted proteins containing an LPXTG wall-sorting motif and covalently attaches these to bacterial cell wall peptidoglycan. Many Gram-positive pathogens also express additional sortases that link a small number of proteins, often with variant wall-sorting motifs, to either other surface proteins or peptidoglycan. To better understand the mechanisms of catalysis and substrate recognition by the housekeeping sortase produced by the important human pathogen Streptococcus pyogenes, the crystal structure of this protein has been solved and its transpeptidase activity established in vitro. The structure reveals a novel arrangement of key catalytic residues in the active site of a sortase, the first that is consistent with kinetic analysis. The structure also provides a complete description of residue positions surrounding the active site, overcoming the limitation of localized disorder in previous structures of sortase A-type proteins. Modification of the active site Cys through oxidation to its sulfenic acid form or by an alkylating reagent supports a role for a reactive thiol/thiolate in the catalytic mechanism. These new insights into sortase structure and function could have important consequences for inhibitor design.

The orphan response regulator CovR: a globally negative modulator of virulence in Streptococcus suis serotype 2

Streptococcus suis serotype 2 is an emerging zoonotic pathogen responsible for a wide range of life-threatening diseases in pigs and humans. However, the pathogenesis of S. suis serotype 2 infection is not well understood. In this study, we report that an orphan response regulator, CovR, globally regulates gene expression and negatively controls the virulence of S. suis 05ZYH33, a streptococcal toxic shock syndrome (STSS)-causing strain. A covR-defective (DeltacovR) mutant of 05ZYH33 displayed dramatic phenotypic changes, such as formation of longer chains, production of thicker capsules, and increased hemolytic activity. Adherence of the DeltacovR mutant to epithelial cells was greatly increased, and its resistance to phagocytosis and killing by neutrophils and monocytes was also significantly enhanced. More importantly, inactivation of covR increased the lethality of S. suis serotype 2 in experimental infection of piglets, and this phenotype was restored by covR complementation. Colonization experiments also showed that the DeltacovR mutant exhibited an increased ability to colonize susceptible tissues of piglets. The pleiotropic phenotype of the DeltacovR mutant is in full agreement with the large number of genes controlled by CovR as revealed by transcription profile analysis: 2 genes are positively regulated, and 193 are repressed, including many that encode known or putative virulence factors. These findings suggested that CovR is a global repressor in virulence regulation of STSS-causing S. suis serotype 2.

Mutagenesis studies of substrate recognition and catalysis in the sortase A transpeptidase from Staphylococcus aureus

The Staphylococcus aureus transpeptidase sortase A (SrtA) is responsible for anchoring a range of virulence- and colonization-associated proteins to the cell wall. SrtA recognizes substrates that contain a C-terminal LPXTG motif. This sequence is cleaved following the threonine, and an amide bond is formed between the threonine and the pentaglycine cross-bridge of branched lipid II. Previous studies have implicated the beta6/beta7 loop region of SrtA in LPXTG recognition but have not systematically characterized this domain. To better understand the individual roles of the residues within this loop, we performed alanine-scanning mutagenesis. Val-168 and Leu-169 were found to be important for substrate recognition, and Glu-171 was also found to be important, consistent with its hypothesized role as a Ca(2+)-binding residue. Gly-167 and Asp-170 were dispensable for catalysis, as was Gln-172. The role of Arg-197 in SrtA has been the subject of much debate. To explore its role in catalysis, we used native chemical ligation to generate semi-synthetic SrtA in which we replaced Arg-197 with citrulline, a non-ionizable analog. This change resulted in a decrease of <3-fold in k(cat)/K(m), indicating that Arg-197 utilizes a hydrogen bond, rather than an electrostatic interaction. Our results are consistent with a model for LPXTG recognition wherein the Leu-Pro sequence is recognized primarily by hydrophobic contacts with SrtA Val-168 and Leu-169, as well as a hydrogen bond from Arg-197. This model contradicts the previously proposed mechanism of binding predicted by the x-ray crystal structure of SrtA.

Consequences of a sortase A mutation in Streptococcus gordonii

Sortase A (SrtA) is required for cell-wall anchoring of LPXTG-containing Gram-positive surface proteins. It was hypothesized, therefore, that disruption of the srtA gene would alter surface anchoring and functions of target LPXTG motif-bearing SspA and SspB proteins of Streptococcus gordonii. Mutant strains in srtA (V288srtA(-), DL1srtA(-)) were constructed in S. gordonii V288 (wtV288) and DL1 (wtDL1). When compared to wtV288, the V288srtA(-) mutant showed decreased biofilm formation on polystyrene, and reduced binding to immobilized purified salivary agglutinin (BIAcore analysis). The wtV288 and V288srtA(-) strains were similar in ultrastructure, but immunogold-labelled SspA/SspB surface expression was reduced on the V288srtA(-) mutant. DL1srtA(-) was also complemented to obtain DL1srtA(+). From the wild-type strains (wtV288, wtDL1), srtA(-) mutants (V288srtA(-), DL1srtA(-)), and the complemented mutant (DL1srtA(+)), cytoplasmic, cell-wall and released extracellular protein fractions were isolated. Each fraction was analysed by SDS-PAGE and immunoblotting with anti-P1. Spent medium from srtA(-) mutant cells contained over-represented proteins, including SspA/SspB (P1 antigen). Mutants showed less P1 on the cell surface than wild-types, as estimated using whole-cell ELISA, and no P1 appeared in the cytoplasmic fractions. Expression of several adhesin genes (sspA/B, cshA/B, fbpA) was generally upregulated in the mutants (V288srtA(-), DL1srtA(-)), but restored to wild-type levels in DL1srtA(+). These data therefore imply that in addition to its role in processing LPXTG-containing adhesins, sortase A has the novel function of contributing to transcriptional regulation of adhesin gene expression.

Single mutation on the surface of Staphylococcus aureus Sortase A can disrupt its dimerization

Staphylococcus aureus Sortase A (SrtA) is an important Gram-positive membrane enzyme which catalyzes the anchoring of many cell surface proteins conserved with the LPXTG sequence. Recently SrtA has been demonstrated to be a dimer with a Kd of 55 microM in vitro. Herein, we show that a single point mutation of amino acid residue on the surface of SrtA can completely disrupt the dimerization. Native polyacrylamide gel electrophoresis and analytical gel filtration chromatography were used to detect the dimer-monomer equilibrium of SrtA mutants. Circular dichroism spectrum experiments were performed to study the conformational change of each SrtA mutant. An enzyme activity assay confirmed that all the SrtA mutants were active in vitro. Our results not only are important for understanding the SrtA protein self-associating mechanism but also provided the necessary starting materials for the study of sortase A pathway in vivo, which may have significant implications for discovering microbial physiology and give a potential target for novel Gram-positive antibiotics.

Staphylococcus aureus sortase A exists as a dimeric protein in vitro

We report the first direct observation of the self-association behavior of the Staphylococcus aureus sortase A (SrtA) transpeptidase. Formation of a SrtA dimer was observed under native conditions by polyacrylamide gel electrophoresis and fast protein liquid chromatography (FPLC). Subsequent peptide mass fingerprinting and protein sequencing experiments confirmed the dimeric form of the SrtA protein. Furthermore, SrtA can be selectively cross-linked both in vitro and in Escherichia coli. Multiple samples of enzyme were subjected to analytical sedimentation equilibrium ultracentrifugation to obtain an apparent Kd for dimer formation of about 55 microM. Finally, enzyme kinetic studies suggested that the dimeric form of SrtA is more active than the monomeric enzyme. Discovery of SrtA dimerization may have significant implications for understanding microbial physiology and developing new antibiotics.

Lactobacillus plantarum for oral peptide delivery

AIMS

To evaluate strains of lactobacilli for their ability to persist and secrete heterologous protein in the oral cavity.

METHODS AND RESULTS

Four different strains of common oral lactobacilli, Lactobacillus brevis, Lactobacillus johnsonii, Lactobacillus murinus and Lactobacillus plantarum, were transformed with the plasmid pKTH2121, which contains a secretion cassette for beta-lactamase. Lactobacilli isolated from the mouth of host mice were also transformed with pKTH2121 for later feeding. Lactococcus lactis, transformed with pKTH2121, was also fed to mice as a negative control. All transformed isolates were fed to C57Black mice in varying schedules. The number of transformed bacteria persisting in the mouth was reported as a percentage of total oral bacteria recovered by swabbing.

CONCLUSIONS

The transformed L. lactis, L. brevis, L. johnsonii, L. murinus, and the endogenous murine lactobacillus strain failed to persist in the mouth. Transformed L. plantarum, however, persisted in the mouth and comprised up to 25% of the total lactobacilli at 18 h and 10% at 24 h after feeding. L. plantarum recovered after feeding retained its ability to secrete beta-lactamase into culture medium efficiently. Beta-lactamase activity could be detected in oral secretions at 8 h after feedings. After repeated feedings, however, the L. plantarum containing pKTH2121 gradually lost its ability to persist after feedings. This experiment demonstrates that L. plantarum can transiently colonize the oral mucosa in large numbers, while continuously secreting foreign proteins, raising the possibility of using lactobacilli as a vector for delivery of oral mucosal peptides.

Engineering the substrate specificity of Staphylococcus aureus Sortase A. The beta6/beta7 loop from SrtB confers NPQTN recognition to SrtA

The Staphylococcus aureus transpeptidase Sortase A (SrtA) anchors virulence and colonization-associated surface proteins to the cell wall. SrtA selectively recognizes a C-terminal LPXTG motif, whereas the related transpeptidase Sortase B (SrtB) recognizes a C-terminal NPQTN motif. In both enzymes, cleavage occurs after the conserved threonine, followed by amide bond formation between threonine and the pentaglycine cross-bridge of cell wall peptidoglycan. Genetic and biochemical studies strongly suggest that SrtA and SrtB exhibit exquisite specificity for their recognition motifs. To better understand the origins of substrate specificity within these two isoforms, we used sequence and structural analysis to predict residues and domains likely to be involved in conferring substrate specificity. Mutational analyses and domain swapping experiments were conducted to test their function in substrate recognition and specificity. Marked changes in the specificity profile of SrtA were obtained by replacing the beta6/beta7 loop in SrtA with the corresponding domain from SrtB. The chimeric beta6/beta7 loop swap enzyme (SrtLS) conferred the ability to acylate NPQTN-containing substrates, with a k(cat)/K(m)(app) of 0.0062 +/- 0.003 m(-1) s(-1). This enzyme was unable to perform the transpeptidation stage of the reaction, suggesting that additional domains are required for transpeptidation to occur. The overall catalytic specificity profile (k(cat)/K(m)(app)(NPQTN)/k(cat)/K(m)(app)(LPETG)) of SrtLS was altered 700,000-fold from SrtA. These results indicate that the beta6/beta7 loop is an important site for substrate recognition in sortases.

Surface Proteins of Gram-Positive Bacteria and How They Get There

Surface proteins are critical in determining the identifying characteristics of individual bacteria and their interaction with the environment. Because the structure of the cell surface is the major characteristic that distinguishes gram-positive from gram-negative bacteria, the processes used to transport and attach these proteins show significant differences between these bacterial classes. This review is intended to highlight these differences and to focus attention on areas that are ripe for further investigation.

Comparative and functional analysis of sortase-dependent proteins in the predicted secretome of Lactobacillus salivarius UCC118

Surface proteins are important factors in the interaction of probiotic and pathogenic bacteria with their environment or host. We performed a comparative bioinformatic analysis of four publicly available Lactobacillus genomes and the genome of Lactobacillus salivarius subsp. salivarius strain UCC118 to identify secreted proteins and those linked to the cell wall. Proteins were identified which were predicted to be anchored by WXL-binding domains, N- or C-terminal anchors, GW repeats, lipoprotein anchors, or LysM-binding domains. We identified 10 sortase-dependent surface proteins in L. salivarius UCC118, including three which are homologous to mucus-binding proteins (LSL_0152, LSL_0311, and LSL_1335), a collagen-binding protein homologue (LSL_2020b), two hypothetical proteins (LSL_1838 and LSL_1902b), an enterococcal surface protein homologue (LSL_1085), a salivary agglutinin-binding homologue (LSL_1832b), an epithelial binding protein homologue (LSL_1319), and a proteinase homologue (LSL_1774b). However, two of the genes are gene fragments and four are pseudogenes, suggesting a lack of selection for their function. Two of the 10 genes were not transcribed in vitro, and 1 gene showed a 10-fold increase in transcript level in stationary phase compared to logarithmic phase. The sortase gene was deleted, and three genes encoding sortase-dependent proteins were disrupted. The sortase mutant and one sortase-dependent protein (mucus-binding homologue) mutant showed a significant reduction in adherence to human epithelial cell lines. The genome-wide investigation of surface proteins can thus help our understanding of their roles in host interaction.

Sortases and the art of anchoring proteins to the envelopes of gram-positive bacteria

The cell wall envelopes of gram-positive bacteria represent a surface organelle that not only functions as a cytoskeletal element but also promotes interactions between bacteria and their environment. Cell wall peptidoglycan is covalently and noncovalently decorated with teichoic acids, polysaccharides, and proteins. The sum of these molecular decorations provides bacterial envelopes with species- and strain-specific properties that are ultimately responsible for bacterial virulence, interactions with host immune systems, and the development of disease symptoms or successful outcomes of infections. Surface proteins typically carry two topogenic sequences, i.e., N-terminal signal peptides and C-terminal sorting signals. Sortases catalyze a transpeptidation reaction by first cleaving a surface protein substrate at the cell wall sorting signal. The resulting acyl enzyme intermediates between sortases and their substrates are then resolved by the nucleophilic attack of amino groups, typically provided by the cell wall cross bridges of peptidoglycan precursors. The surface protein linked to peptidoglycan is then incorporated into the envelope and displayed on the microbial surface. This review focuses on the mechanisms of surface protein anchoring to the cell wall envelope by sortases and the role that these enzymes play in bacterial physiology and pathogenesis.

The role of Streptococcus pneumoniae sortase A in colonisation and pathogenesis

Sortase enzymes are found throughout Gram-positive bacteria and are responsible for the covalent attachment of specific proteins to the cell wall. Through the anchoring of these cell wall proteins, sortase enzymes are important in the ability of several Gram-positive pathogens to cause disease. Previously, deletion of srtA from Streptococcus pneumoniae (the pneumococcus) was shown to disturb the localisation of surface proteins, and decrease bacterial adherence to human pharyngeal cells in vitro. Here we present data demonstrating, for the first time, a role for srtA as a pneumococcal fitness factor in experimental models of pneumonia and bacteraemia. In addition, srtA contributed to nasopharyngeal colonisation in vivo. Furthermore, we find that the contribution of srtA varied between two pneumococcal strains. We show that the known role of srtA in adherence in vitro is dependent on capsule expression, the role of SrtA in adherence to human cells only being apparent in the absence of the pneumococcal capsule. The srtA gene was detected by PCR in all 82 clinical isolates examined and sequencing of the gene from 20 strains showed srtA to be highly conserved. The ubiquitous distribution of srtA was in contrast to the other known pneumococcal sortase genes, srtB, C and D, which were found in only 14 of the 82 tested strains (17%).

Synthesis of (2R,3S) 3-amino-4-mercapto-2-butanol, a threonine analogue for covalent inhibition of sortases

L-Threonine 2 was converted in seven steps into the protected aminomercaptoalcohol 8, a threonine mimic. This compound 8 was coupled to various oligopeptides to produce two different tetrapeptide analogues, for example, 11 and 17, which were shown to inhibit the Sortase enzymes (SrtA and SrtB) via covalent attachment of the thiol groups of 11 and 17 to the catalytically active cysteine residue of the Sortase enzymes.

Identification of substrates of theListeria monocytogenes sortases A and B by a non-gel proteomic analysis

Sortases are enzymes that anchor surface proteins to the cell wall of Gram-positive bacteria by cleaving a sorting motif located in the C-terminus of the protein substrate. The best-characterized motif is LPXTG, which is cleaved between the T and G residues. In this study, a non-gel proteomic approach was used to identify surface proteins recognized by the two sortases of Listeria monocytogenes, SrtA and SrtB. Material containing peptidoglycan and strongly associated proteins was purified from sortase-defective mutants, digested with trypsin, and the resulting peptide mixture analysed by two-dimensional nano-liquid chromatography coupled to ion-trap mass spectrometry. Unlike enzymes involved in peptidoglycan metabolism, other surface proteins displayed uneven distribution in the mutants. A total of 13 LPXTG-containing proteins were identified exclusively in strains having a functional SrtA. In contrast, two surface proteins, Lmo2185 and Lmo2186, were identified only when SrtB was active. The analysis of the peptides identified in these proteins suggests that SrtB of L. monocytogenes may recognize two different sorting motifs, NXZTN and NPKXZ. Taken together, these data demonstrate that non-gel proteomics is a powerful technique to rapidly identify sortase substrates and to gain insights on potential sorting motifs.

Staphylococcus aureus Sortase A transpeptidase. Calcium promotes sorting signal binding by altering the mobility and structure of an active site loop

Many virulence factors in gram-positive bacteria are covalently anchored to the cell-wall peptidoglycan by sortase enzymes, a group of widely distributed cysteine transpeptidases. The Staphylococcus aureus Sortase A protein (SrtA) is the archetypal member of the Sortase family and is activated by Ca2+, an adaptation that may facilitate host colonization as elevated concentrations of this ion are encountered in human tissue. Here we show that a single Ca2+ ion bound to an ordered pocket on SrtA allosterically activates catalysis by modulating both the structure and dynamics of a large active site loop. Detailed nitrogen-15 relaxation measurements indicate that Ca2+ may facilitate the adaptive recognition of the substrate by inducing slow micro- to millisecond time-scale dynamics in the active site. Interestingly, relaxation compensated Carr-Purcell-Meiboom-Gill experiments suggest that the time scale of these motions is directly correlated with ion binding. The results of site-directed mutagenesis indicate that this motional coupling is mediated by the side chain of Glu-171, which is positioned within the beta6/beta7 loop and shown to contribute to Ca2+ binding. The available structural and dynamics data are compatible with a loop closure model of Ca2+ activation, in which the beta6/beta7 loop fluctuates between a binding competent closed form that is stabilized by Ca2+, and an open, highly flexible state that removes key substrate contacting residues from the active site.

Bacillus anthracis sortase A (SrtA) anchors LPXTG motif-containing surface proteins to the cell wall envelope

Cell wall-anchored surface proteins of gram-positive pathogens play important roles during the establishment of many infectious diseases, but the contributions of surface proteins to the pathogenesis of anthrax have not yet been revealed. Cell wall anchoring in Staphylococcus aureus occurs by a transpeptidation mechanism requiring surface proteins with C-terminal sorting signals as well as sortase enzymes. The genome sequence of Bacillus anthracis encodes three sortase genes and eleven surface proteins with different types of cell wall sorting signals. Purified B. anthracis sortase A cleaved peptides encompassing LPXTG motif-type sorting signals between the threonine (T) and the glycine (G) residues in vitro. Sortase A activity could be inhibited by thiol-reactive reagents, similar to staphylococcal sortases. B. anthracis parent strain Sterne 34F(2), but not variants lacking the srtA gene, anchored the collagen-binding MSCRAMM (microbial surface components recognizing adhesive matrix molecules) BasC (BA5258/BAS4884) to the bacterial cell wall. These results suggest that B. anthracis SrtA anchors surface proteins bearing LPXTG motif sorting signals to the cell wall envelope of vegetative bacilli.

Staphylococcus aureusSortase Transpeptidase SrtA: Insight into the Kinetic Mechanism and Evidence for a Reverse Protonation Catalytic Mechanism†

The Staphylococcus aureus transpeptidase SrtA catalyzes the covalent attachment of LPXTG-containing virulence and colonization-associated proteins to cell-wall peptidoglycan in Gram-positive bacteria. Recent structural characterizations of staphylococcal SrtA, and related transpeptidases SrtB from S. aureus and Bacillus anthracis, provide many details regarding the active site environment, yet raise questions with regard to the nature of catalysis and active site cysteine thiol activation. Here we re-evaluate the kinetic mechanism of SrtA and shed light on aspects of its catalytic mechanism. Using steady-state, pre-steady-state, bisubstrate kinetic studies, and high-resolution electrospray mass spectrometry, revised steady-state kinetic parameters and a ping-pong hydrolytic shunt kinetic mechanism were determined for recombinant SrtA. The pH dependencies of kinetic parameters k(cat)/K(m) and k(cat) for the substrate Abz-LPETG-Dap(Dnp)-NH(2) were bell-shaped with pK(a) values of 6.3 +/- 0.2 and 9.4 +/- 0.2 for k(cat) and 6.2 +/- 0.2 and 9.4 +/- 0.2 for k(cat)/K(m). Solvent isotope effect (SIE) measurements revealed inverse behavior, with a (D)2(O)k(cat) of 0.89 +/- 0.01 and a (D)2(O)(k(cat)/K(m)) of 0.57 +/- 0.03 reflecting an equilibrium SIE. In addition, SIE measurements strongly implicated Cys184 participation in the isotope-sensitive rate-determining chemical step when considered in conjunction with an inverse linear proton inventory for k(cat). Last, the pH dependence of SrtA inactivation by iodoacetamide revealed a single ionization for inactivation. These studies collectively provide compelling evidence for a reverse protonation mechanism where a small fraction (ca. 0.06%) of SrtA is competent for catalysis at physiological pH, yet is highly active with an estimated k(cat)/K(m) of >10(5) M(-)(1) s(-)(1).

The SrtA Sortase of Streptococcus agalactiae is required for cell wall anchoring of proteins containing the LPXTG motif, for adhesion to epithelial cells, and for colonization of the mouse intestine

Streptococcus agalactiae (group B streptococcus [GBS]) is the leading cause of neonatal pneumonia, sepsis, and meningitis. An in silico genome analysis indicated that GBS strain NEM316 encodes 35 proteins containing an LPXTG motif which are thought to be covalently linked to the peptidoglycan by an enzyme called sortase. The role of these cell wall-anchored proteins in GBS pathogenesis was evaluated on a global level by inactivating the srtA gene. This gene encodes the major sortase SrtA that anchors most of the LPXTG-containing proteins. We chose the C5a peptidase (ScpB) and Alp2, an abundant immunogenic protein, as prototypical LPXTG-containing proteins. As expected, the SrtA knockout mutant was unable to anchor the C5a peptidase (ScpB) and Alp2 to the cell wall. Complementation with plasmid-borne srtA inserted into the chromosome restored the correct surface localization of both ScpB and Alp2. Interestingly, the SrtA mutant was impaired for binding to the major extracellular matrix components fibronectin and fibrinogen and displayed a significant reduction in adherence to human (A549, HeLa, and Caco-2) and murine (L2) epithelial cells compared to the parental wild-type strain. Surprisingly, the inactivation of srtA had no effect on the virulence of the type III strain of GBS in a neonatal rat model (measured by the 50% lethal dose and lung colonization) but strongly impaired the capacity of the strain to colonize the intestines of gnotobiotic mice in a competition assay. These results demonstrate that LPXTG-containing proteins are involved in cell adhesion and GBS persistence in vivo.

Structures of sortase B from Staphylococcus aureus and Bacillus anthracis reveal catalytic amino acid triad in the active site

Surface proteins attached by sortases to the cell wall envelope of bacterial pathogens play important roles during infection. Sorting and attachment of these proteins is directed by C-terminal signals. Sortase B of S. aureus recognizes a motif NPQTN, cleaves the polypeptide after the Thr residue, and attaches the protein to pentaglycine cross-bridges. Sortase B of B. anthracis is thought to recognize the NPKTG motif, and attaches surface proteins to m-diaminopimelic acid cross-bridges. We have determined crystal structure of sortase B from B. anthracis and S. aureus at 1.6 and 2.0 A resolutions, respectively. These structures show a beta-barrel fold with alpha-helical elements on its outside, a structure thus far exclusive to the sortase family. A putative active site located on the edge of the beta-barrel is comprised of a Cys-His-Asp catalytic triad and presumably faces the bacterial cell surface. A putative binding site for the sorting signal is located nearby.

Sorting sortases: a nomenclature proposal for the various sortases of Gram-positive bacteria

Bacterial surface proteins constitute a diverse group of molecules with important functions, such as adherence, invasion, signaling and interaction with the host immune system or the environment. In Gram-positive bacteria, many surface proteins are anchored to the cell wall envelope by an enzyme named sortase, which recognizes a conserved carboxylic sorting motif. The sequence of the prototype staphylococcal SrtA has been widely used to identify homologs in bacterial genomes, revealing a profusion of sortases in almost all Gram-positive bacteria, often with more than one sortase-like protein per genome [M.J. Pallen, A.C. Lam, M. Antonio, K. Dunbar, Trends Microbiol. 9 (2001) 97-102]. In light of increasing reports on the identification and/or characterization of paralogous sortase genes, a classification of sortases now appears necessary. This report provides an analysis of sixty-one sortases from complete Gram-positive genomes, and suggests the existence of four structural groups of sortases. We propose the classification of sortases into 4 classes designated A, B, C and D. This classification should help to discriminate between sortases in the future.