Purification and characterization of sortase, the transpeptidase that cleaves surface proteins of Staphylococcus aureus at the LPXTG motif

Anchoring foreign substances on live cell surfaces using Sortase A specific binding peptide

A simple, rapid, and efficient live cell surface labeling method has been developed that uses a direct conjugation between Sortase A expressed transiently at the cell surface and Sortase A specific binding peptide.

Synthetic LPETG-containing peptide incorporation in the Staphylococcus aureus cell-wall in a sortase A- and growth phase-dependent manner

The majority of Staphylococcus aureus virulence- and colonization-associated surface proteins contain a pentapeptide recognition motif (LPXTG). This motif can be recognized and cleaved by sortase A (SrtA) which is a membrane-bound transpeptidase. After cleavage these proteins are covalently incorporated into the peptidoglycan. Therefore, SrtA plays a key role in S. aureus virulence. We aimed to generate a substrate mimicking this SrtA recognition motif for several purposes: to incorporate this substrate into the S. aureus cell-wall in a SrtA-dependent manner, to characterize this incorporation and to determine the effect of substrate incorporation on the incorporation of native SrtA-dependent cell-surface-associated proteins. We synthesized substrate containing the specific LPXTG motif, LPETG. As a negative control we used a scrambled version of this substrate, EGTLP and a S. aureus srtA knockout strain. Both substrates contained a fluorescence label for detection by FACScan and fluorescence microscope. A spreading assay and a competitive Luminex assay were used to determine the effect of substrate treatment on native LPXTG containing proteins deposition in the bacterial cell-wall. We demonstrate a SrtA-dependent covalent incorporation of the LPETG-containing substrate in wild type S. aureus strains and several other Gram-positive bacterial species. LPETG-containing substrate incorporation in S. aureus was growth phase-dependent and peaked at the stationary phase. This incorporation negatively correlated with srtA mRNA expression. Exogenous addition of the artificial substrate did not result in a decreased expression of native SrtA substrates (e.g. clumping factor A/B and protein A) nor induced a srtA knockout phenotype.

Semienzymatic cyclization of disulfide-rich peptides using Sortase A

Disulfide-rich cyclic peptides have generated great interest in the development of peptide-based therapeutics due to their exceptional stability toward chemical, enzymatic, or thermal attack. In particular, they have been used as scaffolds onto which bioactive epitopes can be grafted to take advantage of the favorable biophysical properties of disulfide-rich cyclic peptides. To date, the most commonly used method for the head-to-tail cyclization of peptides has been native chemical ligation. In recent years, however, enzyme-mediated cyclization has become a promising new technology due to its efficiency, safety, and cost-effectiveness. Sortase A (SrtA) is a bacterial enzyme with transpeptidase activity. It recognizes a C-terminal penta-amino acid motif, LPXTG, and cleaves the amide bond between Thr and Gly to form a thioacyl-linked intermediate. This intermediate undergoes nucleophilic attack by an N-terminal poly-Gly sequence to form an amide bond between the Thr and N-terminal Gly. Here, we demonstrate that sortase A can successfully be used to cyclize a variety of small disulfide-rich peptides, including the cyclotide kalata B1, α-conotoxin Vc1.1, and sunflower trypsin inhibitor 1. These peptides range in size from 14 to 29 amino acids and contain three, two, or one disulfide bond, respectively, within their head-to-tail cyclic backbones. Our findings provide proof of concept for the potential broad applicability of enzymatic cyclization of disulfide-rich peptides with therapeutic potential.

Release of protein A from the cell wall of Staphylococcus aureus

Staphylococcal protein A (SpA) is anchored to the cell wall envelope of Staphylococcus aureus by sortase A, which links the threonyl (T) of its C-terminal LPXTG motif to peptidoglycan cross-bridges (i.e., Gly5). SpA binds the Fcγ domains of IgG and protects staphylococci from opsonophagocytic clearance. Moreover, SpA cross-links B-cell receptors to modify host adaptive immune responses. The mechanisms whereby SpA is released from the bacterial surface to access the host's immune system are not known. Here we demonstrate that SpA is released with murein tetrapeptide-tetraglycyl [L-Ala-D-iGln-(SpA-Gly5)L-Lys-D-Ala-Gly4] linked to its C-terminal threonyl. LytN, a cross-wall murein hydrolase, contributes to the release of SpA by removing amino sugars [i.e., N-acetylmuramic acid-N-acetylglucosamine (MurNAc-GlcNAc)] from attached peptidoglycan, whereas LytM, a pentaglycyl-endopeptidase, triggers polypeptide release from the bacterial envelope. A model is proposed whereby murein hydrolases cleave the anchor structure of released SpA to modify host immune responses.

Depsipeptide substrates for sortase-mediated N-terminal protein ligation

Technologies that allow the efficient chemical modification of proteins under mild conditions are widely sought after. Sortase-mediated peptide ligation provides a strategy for modifying the N or C terminus of proteins. This protocol describes the use of depsipeptide substrates (containing an ester linkage) with sortase A (SrtA) to completely modify proteins carrying a single N-terminal glycine residue under mild conditions in 4-6 h. The SrtA-mediated ligation reaction is reversible, so most labeling protocols that use this enzyme require a large excess of both substrate and sortase to produce high yields of ligation product. In contrast, switching to depsipeptide substrates effectively renders the reaction irreversible, allowing complete labeling of proteins with a small excess of substrate and catalytic quantities of sortase. Herein we describe the synthesis of depsipeptide substrates that contain an ester linkage between a threonine and glycolic acid residue and an N-terminal FITC fluorophore appended via a thiourea linkage. The synthesis of the depsipeptide substrate typically takes 2-3 d.

Irreversible sortase A-mediated ligation driven by diketopiperazine formation

Sortase A (SrtA)-mediated ligation has emerged as an attractive tool in bioorganic chemistry attributing to the remarkable specificity of the ligation reaction and the physiological reaction conditions. However, the reversible nature of this reaction limits the efficiency of the ligation reaction and has become a significant constraint to its more widespread use. We report herein a novel set of SrtA substrates (LPETGG-isoacyl-Ser and LPETGG-isoacyl-Hse) that can be irreversibly ligated to N-terminal Gly-containing moieties via the deactivation of the SrtA-excised peptide fragment through diketopiperazine (DKP) formation. The convenience of the synthetic procedure and the stability of the substrates in the ligation buffer suggest that both LPETGG-isoacyl-Ser and LPETGG-isoacyl-Hse are valuable alternatives to existing irreversible SrtA substrate sequences.

Immobilization of actively thromboresistant assemblies on sterile blood-contacting surfaces

Rapid one-step modification of thrombomodulin with alkylamine derivatives such as azide, biotin, and PEG is achieved using an evolved sortase (eSrtA) mutant. The feasibility of a point-of-care scheme is demonstrated herein to site-specifically immobilize azido-thrombomodulin on sterilized commercial ePTFE vascular grafts, which exhibit superior thromboresistance compared with commercial heparin-coated grafts in a primate model of acute graft thrombosis.

Sortase A catalyzed reaction pathways: a comparative study with six SrtA variants

Different enzyme variants of sortase A fromStaphylococcus aureuswere found to have distinct catalytic properties with regard to site-directed protein fusion and competing intermolecular crosslinking reactions.

Sortase-catalyzed in vitro functionalization of a HER2-specific recombinant Fab for tumor targeting of the plant cytotoxin gelonin

We report on the preparation of a new type of immunotoxin via in vitro ligation of the αHer2 antigen binding fragment (Fab) of the clinically-validated antibody trastuzumab to the plant toxin gelonin, employing catalysis by the bacterial enzyme sortase A (SrtA). The αHer2 Fab was fused with the extended SrtA recognition motif LPET↓GLEH 6 at the C-terminus of its heavy chain, thereby preventing interference with antigen binding, while the toxin was equipped with a Gly 2 sequence at its N-terminus, distant to the catalytically active site in the C-terminal region. Site-specific in vitro transpeptidation led to a novel antibody-toxin conjugate wherein gelonin had effectively replaced the Fc region of a conventional (monomerized) immunoglobulin. After optimization of reaction conditions and incubation time, the resulting Fab-Gelonin ligation product was purified to homogeneity in a two-step procedure by means of Strep-Tactin affinity chromatography--utilizing the Strep-tag II appended to gelonin--and size exclusion chromatography. Binding activity of the immunotoxin for the Her2 ectodomain was indistinguishable from the unligated Fab as measured by real-time surface plasmon resonance spectroscopy. Specific cytotoxic potency of Fab-Gelonin was demonstrated against two Her2-positive cell lines, resulting in EC 50 values of ~1 nM or lower, indicating a 1000-fold enhanced cell-killing activity compared with gelonin itself. Thus, our strategy provides a convenient route to the modular construction of functional immunotoxins from Fabs of established tumor-specific antibodies with gelonin or related proteotoxins, also avoiding the elevated biosafety levels that would be mandatory for the direct biotechnological preparation of corresponding fusion proteins.

Sortase A inhibitory metabolites from the roots of Pulsatilla koreana

Three new lignans (3, 4, and 6) along with eight known lignans and phenyl propanoids were isolated from the dried roots of Pulsatilla koreana, an oriental folk medicine. Based upon the results of combined spectroscopic and chemical methods, the structures of new compounds were determined to be lignan glycosides. Included among the known compounds are three compounds (5, 7, and 8) isolated first time from this plant as well as two compounds (2 and 11) previously reported only as synthetic derivatives. These compounds significantly inhibited the action of Sortase A from Streptococcus mutans OMZ65, an isolate from human oral cavity.

Chemoenzymatic bio-orthogonal chemistry for site-specific double modification of recombinant thrombomodulin

Best of both worlds: A one-pot strategy for site-specific PEGylation through strain-promoted alkyne-azide cycloaddition (SPAAC) and fluorescent labeling through sortase A-mediated ligation (SML) of recombinant thrombomodulin without prior chemical modification and without diminishing the protein activity has been developed.

Role of sortase in Streptococcus mutans under the effect of nicotine

Streptococcus mutans is a common Gram-positive bacterium and plays a significant role in dental caries. Tobacco and/or nicotine have documented effects on S. mutans growth and colonization. Sortase A is used by many Gram-positive bacteria, including S. mutans, to facilitate the insertion of certain cell surface proteins, containing an LPXTGX motif such as antigen I/II. This study examined the effect of nicotine on the function of sortase A to control the physiology and growth of S. mutans using wild-type S. mutans NG8, and its isogenic sortase-defective and -complemented strains. Briefly, the strains were treated with increasing amounts of nicotine in planktonic growth, biofilm metabolism, and sucrose-induced and saliva-induced antigen I/II-dependent biofilm formation assays. The strains exhibited no significant differences with different concentrations of nicotine in planktonic growth assays. However, they had significantly increased (P≤0.05) biofilm metabolic activity (2- to 3-fold increase) as the concentration of nicotine increased. Furthermore, the sortase-defective strain was more sensitive metabolically to nicotine than the wild-type or sortase-complemented strains. All strains had significantly increased sucrose-induced biofilm formation (2- to 3-fold increase) as a result of increasing concentrations of nicotine. However, the sortase-defective strain was not able to make as much sucrose- and saliva-induced biofilm as the wild-type NG8 did with increasing nicotine concentrations. These results indicated that nicotine increased metabolic activity and sucrose-induced biofilm formation. The saliva-induced biofilm formation assay and qPCR data suggested that antigen I/II was upregulated with nicotine but biofilm was not able to be formed as much as wild-type NG8 without functional sortase A.

Site-specific N-terminal labeling of proteins using sortase-mediated reactions

This protocol describes the use of sortase-mediated reactions to label the N terminus of any given protein of interest. The sortase recognition sequence, LPXTG (for Streptococcus aureus sortase A) or LPXTA (for Staphylococcus pyogenes sortase A), can be appended to a variety of probes such as fluorophores, biotin or even to other proteins. The protein to be labeled acts as a nucleophile by attacking the intermediate formed between the probe containing the LPXTG/A motif and the sortase enzyme. If sortase, the protein of interest and a suitably functionalized label are available, the reactions usually require less than 3 h.

Site-specific C-terminal and internal loop labeling of proteins using sortase-mediated reactions

Methods for site-specific modification of proteins should be quantitative and versatile with respect to the nature and size of the biological or chemical targets involved. They should require minimal modification of the target, and the underlying reactions should be completed in a reasonable amount of time under physiological conditions. Sortase-mediated transpeptidation reactions meet these criteria and are compatible with other labeling methods. Here we describe the expression and purification conditions for two sortase A enzymes that have different recognition sequences. We also provide a protocol that allows the functionalization of any given protein at its C terminus, or, for select proteins, at an internal site. The target protein is engineered with a sortase-recognition motif (LPXTG) at the place where modification is desired. Upon recognition, sortase cleaves the protein between the threonine and glycine residues, facilitating the attachment of an exogenously added oligoglycine peptide modified with the functional group of choice (e.g., fluorophore, biotin, protein or lipid). Expression and purification of sortase takes ∼3 d, and sortase-mediated reactions take only a few minutes, but reaction times can be extended to increase yields.

Role of protein A in the evasion of host adaptive immune responses by Staphylococcus aureus

Heritable defects in human B cell/antibody development are not associated with increased susceptibility to Staphylococcus aureus infection. Protein A (SpA), a surface molecule of S. aureus, binds the Fcγ domain of immunoglobulin (Ig) and cross-links the Fab domain of V_H3-type B cell receptors (IgM). Here we generated S. aureus spa variants harboring amino acid substitutions at four key residues in each of the five Ig-binding domains of SpA. Wild-type S. aureus required SpA binding to Ig to resist phagocytosis and SpA-mediated B cell receptor cross-linking to block antibody development in mice. The spa_KKAA mutant, which cannot bind Ig or IgM, was phagocytosed and elicited B cell responses to key virulence antigens that protected animals against lethal S. aureus challenge. The immune evasive attributes of S. aureus SpA were abolished in µMT mice lacking mature B cells and antibodies. Thus, while wild-type S. aureus escapes host immune surveillance, the spa_KKAA variant elicits adaptive responses that protect against recurrent infection.

Staphylococcus aureus causes recurrent skin and bloodstream infections without eliciting immunity. Heritable defects in neutrophil and T cell function, but not B cell or antibody development, are associated with increased incidence of S. aureus infection, and efforts to develop antibody-based S. aureus vaccines have thus far been unsuccessful. We show here that the Fcγ and V_H3-type Fab binding activities of staphylococcal protein A (SpA) are essential for S. aureus escape from host immune surveillance in mice. The virulence attributes of SpA in mice required mature B cells and immunoglobulin. These results suggest that antibodies and B cells play a key role in the pathogenesis of staphylococcal infections and provide insights into the development of a vaccine against S. aureus.

Receptor-directed chimeric toxins created by sortase-mediated protein fusion

Chimeric protein toxins that act selectively on cells expressing a designated receptor may serve as investigational probes and/or antitumor agents. Here, we report use of the enzyme sortase A (SrtA) to create four chimeric toxins designed to selectively kill cells bearing the tumor marker HER2. We first expressed and purified: (i) a receptor recognition-deficient form of diphtheria toxin that lacks its receptor-binding domain and (ii) a mutated, receptor-binding-deficient form of anthrax-protective antigen. Both proteins carried at the C terminus the sortase recognition sequence LPETGG and a H₆ affinity tag. Each toxin protein was mixed with SrtA plus either of two HER2-recognition proteins--a single-chain antibody fragment or an Affibody--both carrying an N-terminal G₅ tag. With wild-type SrtA, the fusion reaction between the toxin and receptor-recognition proteins approached completion only after several hours, whereas with an evolved form of the enzyme, SrtA*, the reaction was virtually complete within 5 minutes. The four fusion toxins were purified and shown to kill HER2-positive cells in culture with high specificity. Sortase-mediated ligation of binary combinations of diverse natively folded proteins offers a facile way to produce large sets of chimeric proteins for research and medicine.

Staphylococcus aureus mutants lacking the LytR-CpsA-Psr family of enzymes release cell wall teichoic acids into the extracellular medium

The LytR-CpsA-Psr (LCP) proteins are thought to transfer bactoprenol-linked biosynthetic intermediates of wall teichoic acid (WTA) to the peptidoglycan of Gram-positive bacteria. In Bacillus subtilis, mutants lacking all three LCP enzymes do not deposit WTA in the envelope, while Staphylococcus aureus Δlcp mutants display impaired growth and reduced levels of envelope phosphate. We show here that the S. aureus Δlcp mutant synthesized WTA yet released ribitol phosphate polymers into the extracellular medium. Further, Δlcp mutant staphylococci no longer restricted the deposition of LysM-type murein hydrolases to cell division sites, which was associated with defects in cell shape and increased autolysis. Mutations in S. aureus WTA synthesis genes (tagB, tarF, or tarJ2) inhibit growth, which is attributed to the depletion of bactoprenol, an essential component of peptidoglycan synthesis (lipid II). The growth defect of S. aureus tagB and tarFJ mutants was alleviated by inhibition of WTA synthesis with tunicamycin, whereas the growth defect of the Δlcp mutant was not relieved by tunicamycin treatment or by mutation of tagO, whose product catalyzes the first committed step of WTA synthesis. Further, sortase A-mediated anchoring of proteins to peptidoglycan, which also involves bactoprenol and lipid II, was not impaired in the Δlcp mutant. We propose a model whereby the S. aureus Δlcp mutant, defective in tethering WTA to the cell wall, cleaves WTA synthesis intermediates, releasing ribitol phosphate into the medium and recycling bactoprenol for peptidoglycan synthesis.

Curcumin inhibits the Sortase A activity of the Streptococcus mutans UA159

Streptococcus mutans (S. mutans) forms part of the commensal microflora and is deemed to be the major pathogen responsible for the generation of dental caries. The enzyme, sortase A enzyme, modulates the surface properties and cariogenicity of S. mutans. Curcumin has been reported to be an inhibitor of Staphylococcus aureus sortase A. In this study, inhibition of a purified S. mutans UA159 sortase A by curcumin was evaluated. Curcumin exerted strong inhibitory activity with a half maximal inhibitory concentration (IC50) of 10.2 ± 0.7 μM which was lower than the minimum inhibitory concentration of 175 μM and the minimum bactericidal concentration of 350 μM. These results indicated that curcumin is a S. mutans UA159 sortase A inhibitor and therefore represents as a promising anticaries agent.

Curcumin reduces Streptococcus mutans biofilm formation by inhibiting sortase A activity

BACKGROUND

Sortase A is an enzyme responsible for the covalent attachment of Pac proteins to the cell wall in Streptococcus mutans. It has been shown to play a role in modulating the surface properties and the biofilm formation and influence the cariogenicity of S. mutans. Curcumin, an active ingredient of turmeric, was reported to be an inhibitor for Staphylococcus aureus sortase A. The aim of this study was to investigate the inhibitory ability of curcumin against S. mutans sortase A and the effect of curcumin for biofilm formation.

METHODS

The antimicrobial activity of the curcumin to the S. mutans and inhibitory ability of the curcumin against the purified sortase A in vitro were detected. Western-blot and real-time PCR were used to analysis the sortase A mediated Pac protein changes when the S. mutans was cultured with curcumin. The curcumin on the S. mutans biofilm formation was determined by biofilm formation analysis.

RESULTS

Curcumin can inhibit purified S. mutans sortase A with a half-maximal inhibitory concentration (IC50) of (10.2±0.7)μmol/l, which is lower than minimum inhibitory concentration (MIC) of 175μmol/l. Curcumin (15μmol/l) was found to release the Pac protein to the supernatant and reduce S. mutans biofilm formation.

CONCLUSIONS

These results indicated that curcumin is an S. mutans sortase A inhibitor and has promising anti-caries characteristics through an anti-adhesion-mediated mechanism.

Haloferax volcanii archaeosortase is required for motility, mating, and C-terminal processing of the S-layer glycoprotein

Cell surfaces are decorated by a variety of proteins that facilitate interactions with their environments and support cell stability. These secreted proteins are anchored to the cell by mechanisms that are diverse, and, in archaea, poorly understood. Recently published in silico data suggest that in some species a subset of secreted euryarchaeal proteins, which includes the S-layer glycoprotein, is processed and covalently linked to the cell membrane by enzymes referred to as archaeosortases. In silico work led to the proposal that an independent, sortase-like system for proteolysis-coupled, carboxy-terminal lipid modification exists in bacteria (exosortase) and archaea (archaeosortase). Here, we provide the first in vivo characterization of an archaeosortase in the haloarchaeal model organism Haloferax volcanii. Deletion of the artA gene (HVO_0915) resulted in multiple biological phenotypes: (a) poor growth, especially under low-salt conditions, (b) alterations in cell shape and the S-layer, (c) impaired motility, suppressors of which still exhibit poor growth, and (d) impaired conjugation. We studied one of the ArtA substrates, the S-layer glycoprotein, using detailed proteomic analysis. While the carboxy-terminal region of S-layer glycoproteins, consisting of a putative threonine-rich O-glycosylated region followed by a hydrophobic transmembrane helix, has been notoriously resistant to any proteomic peptide identification, we were able to identify two overlapping peptides from the transmembrane domain present in the ΔartA strain but not in the wild-type strain. This clearly shows that ArtA is involved in carboxy-terminal post-translational processing of the S-layer glycoprotein. As it is known from previous studies that a lipid is covalently attached to the carboxy-terminal region of the S-layer glycoprotein, our data strongly support the conclusion that archaeosortase functions analogously to sortase, mediating proteolysis-coupled, covalent cell surface attachment.

Synthetic Studies of Glycosylphosphatidylinositol (GPI) Anchors and GPI-Anchored Peptides, Glycopeptides, and Proteins

Glycosylphosphatidylinositol (GPI) anchorage of proteins and glycoproteins onto the cell surface is ubiquitous in eukaryotes, and GPI-anchored proteins and glycoproteins play an important role in many biological processes. To study GPI anchorage and explore the functions of GPIs and GPI-anchored proteins and glycoproteins, it is essential to have access to these molecules in homogeneous and structurally defined forms. This review is focused on the progress that our laboratory has made towards the chemical and chemoenzymatic synthesis of structurally defined GPI anchors and GPI-anchored peptides, glycopeptides, and proteins. Briefly, highly convergent strategies were developed for GPI synthesis and were employed to successfully synthesize a number of GPIs, including those carrying unsaturated lipids and other useful functionalities such as the azido and alkynyl groups. The latter enabled further site-specific modification of GPIs by click chemistry. GPI-linked peptides, glycopeptides, and proteins were prepared by regioselective chemical coupling of properly protected GPIs and peptides/glycopeptides or through site-specific ligation of synthetic GPIs and peptides/glycopeptides/proteins under the influence of sortase A. The investigation of interactions between GPI anchors and pore-forming bacterial toxins by means of synthetic GPI anchors and GPI analogs is also discussed.

The binding mechanism, multiple binding modes, and allosteric regulation of Staphylococcus aureus Sortase A probed by molecular dynamics simulations

Sortase enzymes are vitally important for the virulence of gram-positive bacteria as they play a key role in the attachment of surface proteins to the cell wall. These enzymes recognize a specific sorting sequence in proteins destined to be displayed on the surface of the bacteria and catalyze the transpeptidation reaction that links it to a cell wall precursor molecule. Because of their role in establishing pathogenicity, and in light of the recent rise of antibiotic-resistant bacterial strains, sortase enzymes are novel drug targets. Here, we present a study of the prototypical sortase protein Staphylococcus aureus Sortase A (SrtA). Both conventional and accelerated molecular dynamics simulations of S. aureus SrtA in its apo state and when bound to an LPATG sorting signal (SS) were performed. Results support a binding mechanism that may be characterized as conformational selection followed by induced fit. Additionally, the SS was found to adopt multiple metastable states, thus resolving discrepancies between binding conformations in previously reported experimental structures. Finally, correlation analysis reveals that the SS actively affects allosteric pathways throughout the protein that connect the first and the second substrate binding sites, which are proposed to be located on opposing faces of the protein. Overall, these calculations shed new light on the role of dynamics in the binding mechanism and function of sortase enzymes.

Pasteurella multocida toxin as a transporter of non-cell-permeating proteins

The protein toxin Pasteurella multocida toxin (PMT) is the causative agent of atrophic rhinitis in pigs, leading to atrophy of the nasal turbinate bones by affecting osteoblasts and osteoclasts. The mechanism of PMT-induced intoxication is a deamidation of α-subunits of heterotrimeric G proteins, including Gαq, Gα13, and Gαi, thereby causing persistent activation of the G proteins. Here we utilized PMT as a transporter of the non-cell-permeating A domain of diphtheria toxin (DTa). Fusion proteins of PMT and DTa ADP-ribosylated elongation factor 2, the natural target of diphtheria toxin, leading to cell toxicity. PMT-DTa effects were competed by PMT, indicating binding to the same cell surface receptor. Fluorescently labeled PMT-DTa and PMT colocalized with specific markers of early and late endosomes. Bafilomycin A, which inhibits vacuolar H(+)-ATPase, blocked PMT-DTa-induced intoxication of HEK-293 cells. By constructing various PMT-DTa chimeras, we identified a minimal region of PMT necessary for uptake of DTa. The data suggest that PMT is able to transport cargo proteins into eukaryotic cells by utilizing the PMT-specific uptake route.

Sortase A as a tool to functionalize surfaces

A widely accepted approach to combat surface fouling is based on the prevention of biofoulants to attach to a surface by the functionalization with poly(ethylene glycol) (PEG). The goal of this study was to generate a proof of concept for the enzymatic coupling of PEG to a peptide precoated surface by using the enzyme Sortase A (SrtA). A hydrophobic polystyrene surface was primed with anchoring peptide P3 equipped with a pentaglycine acceptor motif for SrtA, to enable subsequent transpeptidation with either biotin or a PEG-tail containing the sortase recognition motif LPETG. High levels of surface-bound biotin were detected only in cases with biotin-LPETG and SrtA. Little if any reactivity was detected in wells treated with the SrtA scrambled motif EGLTP, or in the absence of SrtA. Conjugation of PEG resulted in a significant decrease of bacterial adherence to the surface.

Large scale modification of biomolecules using immobilized sortase A from Staphylococcus aureus

Recently, sortase A (SrtA) from Staphyloccus aureus moved into the focus of bioscience because of its ability to incorporate site specific modifications into proteins. The enzyme was mostly used to modify target proteins in an analytical scale, to study biomolecules in their cellular context. In this study, we show the applicability of SrtA mediated ligation for site specific modification of proteins in a large scale. Therefore, the reaction was first optimized using peptides and subsequently new reaction conditions were applied for the large scale biotinylation of interleukin-8. Furthermore, we established C-terminal immobilization of the SrtA on a PEG based resin and could demonstrate maintaining enzymatic activity. Immobilized SrtA significantly facilitates previous ligation protocols as the enzyme can be easily recycled. Also, the removal of excess reaction solution and the whole washing process is significantly accelerated, as centrifugation or filtration techniques can be applied instead of time-consuming chromatography steps.

The SHOCT domain: a widespread domain under-represented in model organisms

We have identified a new protein domain, which we have named the SHOCT domain (Short C-terminal domain). This domain is widespread in bacteria with over a thousand examples. But we found it is missing from the most commonly studied model organisms, despite being present in closely related species. It's predominantly C-terminal location, co-occurrence with numerous other domains and short size is reminiscent of the Gram-positive anchor motif, however it is present in a much wider range of species. We suggest several hypotheses about the function of SHOCT, including oligomerisation and nucleic acid binding. Our initial experiments do not support its role as an oligomerisation domain.

Small molecule inhibitor of lipoteichoic acid synthesis is an antibiotic for Gram-positive bacteria

The current epidemic of infections caused by antibiotic-resistant gram-positive bacteria requires the discovery of new drug targets and the development of new therapeutics. Lipoteichoic acid (LTA), a cell wall polymer of gram-positive bacteria, consists of 1,3-polyglycerol-phosphate linked to glycolipid. LTA synthase (LtaS) polymerizes polyglycerol-phosphate from phosphatidylglycerol, a reaction that is essential for the growth of gram-positive bacteria. We screened small molecule libraries for compounds inhibiting growth of Staphylococcus aureus but not of gram-negative bacteria. Compound 1771 [2-oxo-2-(5-phenyl-1,3,4-oxadiazol-2-ylamino)ethyl 2-naphtho[2,1-b]furan-1-ylacetate] blocked phosphatidylglycerol binding to LtaS and inhibited LTA synthesis in S. aureus and in Escherichia coli expressing ltaS. Compound 1771 inhibited the growth of antibiotic-resistant gram-positive bacteria and prolonged the survival of mice with lethal S. aureus challenge, validating LtaS as a target for the development of antibiotics.

Designing disorder: Tales of the unexpected tails

Protein tags of various sizes and shapes catalyze progress in biosciences. Well-folded tags can serve to solubilize proteins. Small, unfolded, peptide-like tags have become invaluable tools for protein purification as well as protein-protein interaction studies. Intrinsically Disordered Proteins (IDPs), which lack unique 3D structures, received exponentially increasing attention during the last decade. Recently, large ID tags have been developed to solubilize proteins and to engineer the pharmacological properties of protein and peptide pharmaceuticals. Here, we contrast the complementary benefits and applications of both folded and ID tags based on predictions of ID. Less structure often means more function in a shorter tag.

Metal-free bioconjugation reactions

The recent strategy to apply chemical reactions to address fundamental biological questions has led to the emergence of entirely new conjugation reactions that are fast and irreversible, yet so mild and selective that they can be performed even in living cells or organisms. These so-called bioorthogonal reactions open novel avenues, not only in chemical biology research, but also in many other life sciences applications, including the modulation of biopharmaceuticals by site-specific modification approaches.

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.

Identification of an atypical zinc metalloproteinase, ZmpC, from an epidemic conjunctivitis-causing strain of Streptococcus pneumoniae

Streptococcus pneumoniae is a pathogen associated with a range of invasive and noninvasive infections. Despite the identification of the majority of virulence factors expressed by S. pneumoniae, knowledge of the strategies used by this bacterium to trigger infections, especially those originating at wet-surfaced epithelia, remains limited. In this regard, we recently reported a mechanism used by a nonencapsulated, epidemic conjunctivitis-causing strain of S. pneumoniae (strain SP168) to gain access into ocular surface epithelial cells. Mechanistically, strain SP168 secretes a zinc metalloproteinase, encoded by a truncated zmpC gene, to cleave off the ectodomain of a vital defense component - the membrane mucin MUC16 - from the apical glycocalyx barrier of ocular surface epithelial cells and, thereby invades underlying epithelial cells. Here, we compare the truncated SP168 ZmpC to its highly conserved archetype from S. pneumoniae serotype 4 (TIGR4), which has been linked to pneumococcal virulence in previous studies. Comparative nucleotide sequence analyses revealed that the zmpC gene corresponding to strain SP168 has two stretches of DNA deleted near its 5' end. A third 3 bp in-frame deletion, resulting in the elimination of an alanine residue, was found towards the middle segment of the SP168 zmpC. Closer examination of the primary structure revealed that the SP168 ZmpC lacks the canonical LPXTG motif - a signature typical of several surface proteins of gram-positive bacteria and of other pneumococcal zinc metalloproteinases. Surprisingly, in vitro assays performed using recombinant forms of ZmpC indicated that the truncated SP168 ZmpC induces more cleavage of the MUC16 ectodomain than its TIGR4 counterpart. This feature may help explain, in part, why S. pneumoniae strain SP168 is better equipped at abrogating the MUC16 glycocalyx barrier en route to causing epidemic conjunctivitis.

Functional analysis of the Lactobacillus casei BL23 sortases

Sortases are a class of enzymes that anchor surface proteins to the cell wall of Gram-positive bacteria. Lactobacillus casei BL23 harbors four sortase genes, two belonging to class A (srtA1 and srtA2) and two belonging to class C (srtC1 and srtC2). Class C sortases were clustered with genes encoding their putative substrates that were homologous to the SpaEFG and SpaCBA proteins that encode mucus adhesive pili in Lactobacillus rhamnosus GG. Twenty-three genes encoding putative sortase substrates were identified in the L. casei BL23 genome with unknown (35%), enzymatic (30%), or adhesion-related (35%) functions. Strains disrupted in srtA1, srtA2, srtC1, and srtC2 and an srtA1 srtA2 double mutant were constructed. The transcription of all four sortase encoding genes was detected, but only the mutation of srtA1 resulted in a decrease in bacterial surface hydrophobicity. The β-N-acetyl-glucosaminidase and cell wall proteinase activities of whole cells diminished in the srtA1 mutant and, to a greater extent, in the srtA1 srtA2 double mutant. Cell wall anchoring of the staphylococcal NucA reporter protein fused to a cell wall sorting sequence was also affected in the srtA mutants, and the percentages of adhesion to Caco-2 and HT-29 intestinal epithelial cells were reduced for the srtA1 srtA2 strain. Mutations in srtC1 or srtC2 result in an undetectable phenotype. Together, these results suggest that SrtA1 is the housekeeping sortase in L. casei BL23 and SrtA2 would carry out redundant or complementary functions that become evident when SrtA1 activity is absent.

Solution structure of the sortase required for efficient production of infectious Bacillus anthracis spores

Bacillus anthracis forms metabolically dormant endospores that upon germination can cause lethal anthrax disease in humans. Efficient sporulation requires the activity of the SrtC sortase (BaSrtC), a cysteine transpeptidase that covalently attaches the BasH and BasI proteins to the peptidoglycan of the forespore and predivisional cell, respectively. To gain insight into the molecular basis of protein display, we used nuclear magnetic resonance to determine the structure and backbone dynamics of the catalytic domain of BaSrtC (residues Ser(56)-Lys(198)). The backbone and heavy atom coordinates of structurally ordered amino acids have coordinate precision of 0.42 ± 0.07 and 0.82 ± 0.05 Å, respectively. BaSrtC(Δ55) adopts an eight-stranded β-barrel fold that contains two short helices positioned on opposite sides of the protein. Surprisingly, the protein dimerizes and contains an extensive, structurally disordered surface that is positioned adjacent to the active site. The surface is formed by two loops (β2-β3 and β4-H1 loops) that surround the active site histidine, suggesting that they may play a key role in associating BaSrtC with its lipid II substrate. BaSrtC anchors proteins bearing a noncanonical LPNTA sorting signal. Modeling studies suggest that the enzyme recognizes this substrate using a rigid binding pocket and reveals the presence of a conserved subsite for the signal. This first structure of a class D member of the sortase superfamily unveils class-specific features that may facilitate ongoing efforts to discover sortase inhibitors for the treatment of bacterial infections.

The sortase A substrates FnbpA, FnbpB, ClfA and ClfB antagonize colony spreading of Staphylococcus aureus

Staphylococcus aureus is an important human pathogen that is renowned both for its rapid transmission within hospitals and the community, and for the formation of antibiotic resistant biofilms on medical implants. Recently, it was shown that S. aureus is able to spread over wet surfaces. This motility phenomenon is promoted by the surfactant properties of secreted phenol-soluble modulins (PSMs), which are also known to inhibit biofilm formation. The aim of the present studies was to determine whether any cell surface-associated S. aureus proteins have an impact on colony spreading. To this end, we analyzed the spreading capabilities of strains lacking non-essential components of the protein export and sorting machinery. Interestingly, our analyses reveal that the absence of sortase A (SrtA) causes a hyper-spreading phenotype. SrtA is responsible for covalent anchoring of various proteins to the staphylococcal cell wall. Accordingly, we show that the hyper-spreading phenotype of srtA mutant cells is an indirect effect that relates to the sortase substrates FnbpA, FnbpB, ClfA and ClfB. These surface-exposed staphylococcal proteins are known to promote biofilm formation, and cell-cell interactions. The hyper-spreading phenotype of srtA mutant staphylococcal cells was subsequently validated in Staphylococcus epidermidis. We conclude that cell wall-associated factors that promote a sessile lifestyle of S. aureus and S. epidermidis antagonize the colony spreading motility of these bacteria.

M13 bacteriophage display framework that allows sortase-mediated modification of surface-accessible phage proteins

We exploit bacterial sortases to attach a variety of moieties to the capsid proteins of M13 bacteriophage. We show that pIII, pIX, and pVIII can be functionalized with entities ranging from small molecules (e.g., fluorophores, biotin) to correctly folded proteins (e.g., GFP, antibodies, streptavidin) in a site-specific manner, and with yields that surpass those of any reported using phage display technology. A case in point is modification of pVIII. While a phage vector limits the size of the insert into pVIII to a few amino acids, a phagemid system limits the number of copies actually displayed at the surface of M13. Using sortase-based reactions, a 100-fold increase in the efficiency of display of GFP onto pVIII is achieved. Taking advantage of orthogonal sortases, we can simultaneously target two distinct capsid proteins in the same phage particle and maintain excellent specificity of labeling. As demonstrated in this work, this is a simple and effective method for creating a variety of structures, thus expanding the use of M13 for materials science applications and as a biological tool.

Isopeptide bonds of the major pilin protein BcpA influence pilus structure and bundle formation on the surface of Bacillus cereus

Bacillus cereus strains elaborate pili on their surface using a mechanism of sortase-mediated cross-linking of major and minor pilus components. Here we used a combination of electron microscopy and atomic force microscopy to visualize these structures. Pili occur as single, double or higher order assemblies of filaments formed from monomers of the major pilin, BcpA, capped by the minor pilin, BcpB. Previous studies demonstrated that within assembled pili, four domains of BcpA - CNA(1), CNA(2), XNA and CNA(3) - each acquire intramolecular lysine-asparagine isopeptide bonds formed via catalytic glutamic acid or aspartic acid residues. Here we showed that mutants unable to form the intramolecular isopeptide bonds in the CNA(2) or CNA(3) domains retain the ability to form pilus bundles. A mutant lacking the CNA(1) isopeptide bond assembled deformed pilin subunits that failed to associate as bundles. X-ray crystallography revealed that the BcpA variant Asp(312) Ala, lacking an aspartyl catalyst, did not generate the isopeptide bond within the jelly-roll structure of XNA. The Asp(312) Ala mutant was also unable to form bundles and promoted the assembly of deformed pili. Thus, structural integrity of the CNA(1) and XNA domains are determinants for the association of pili into higher order bundle structures and determine native pilus structure.

Development of an influenza virus protein array using Sortagging technology

Protein array technology is an emerging tool that enables high-throughput screening of protein-protein or protein-lipid interactions and identification of immunodominant antigens during the course of a bacterial or viral infection. In this work, we developed an Influenza virus protein array using the sortase-mediated transpeptidation reaction known as "Sortagging". LPETG-tagged Influenza virus proteins from bacterial and eukaryotic cellular extracts were immobilized at their carboxyl-termini onto a preactivated amine-glass slide coated with a Gly3 linker. Immobilized proteins were revealed by specific antibodies, and the newly generated Sortag-protein chip can be used as a device for antigen and/or antibody screening. The specificity of the Sortase A (SrtA) reaction avoids purification steps in array building and allows immobilization of proteins in an oriented fashion. Previously, this versatile technology has been successfully employed for protein labeling and protein conjugation. Here, the tool is implemented to covalently link proteins of a viral genome onto a solid support. The system could readily be scaled up to proteins of larger genomes in order to develop protein arrays for high-throughput screening.

Protein secretion and surface display in Gram-positive bacteria

The cell wall peptidoglycan of Gram-positive bacteria functions as a surface organelle for the transport and assembly of proteins that interact with the environment, in particular, the tissues of an infected host. Signal peptide-bearing precursor proteins are secreted across the plasma membrane of Gram-positive bacteria. Some precursors carry C-terminal sorting signals with unique sequence motifs that are cleaved by sortase enzymes and linked to the cell wall peptidoglycan of vegetative forms or spores. The sorting signals of pilin precursors are cleaved by pilus-specific sortases, which generate covalent bonds between proteins leading to the assembly of fimbrial structures. Other precursors harbour surface (S)-layer homology domains (SLH), which fold into a three-pronged spindle structure and bind secondary cell wall polysaccharides, thereby associating with the surface of specific Gram-positive microbes. Type VII secretion is a non-canonical secretion pathway for WXG100 family proteins in mycobacteria. Gram-positive bacteria also secrete WXG100 proteins and carry unique genes that either contribute to discrete steps in secretion or represent distinctive substrates for protein transport reactions.

Disorder-to-order transition of an intrinsically disordered region of sortase revealed by multiscale enhanced sampling

Molecular functions of intrinsically disordered proteins (IDPs) or intrinsically disordered regions (IDRs), such as molecular recognition and cellular signaling, are ascribed to dynamic changes in the conformational space in response to binding of target molecules. Sortase, a transpeptitase in Gram-positive bacteria, has an IDR in a loop which undergoes a disordered-to-ordered transition (called "disordered loop"), accompanying a tilt of another loop ("dynamic loop"), upon binding of a signal peptide and a calcium ion. In this study, all-atom conformational ensembles of sortase were calculated for the four different binding states (with/without the peptide and with/without a calcium ion) by the multiscale enhanced sampling (MSES) simulation to examine how the binding of the peptide and/or calcium influences the conformational ensemble. The MSES is a multiscale and multicopy simulation method that allows an enhanced sampling of the all-atom model of large proteins including explicit solvent. A 100 ns MSES simulation of the ligand-free sortase using 20 replicas (in total 2 μs) demonstrated large flexibility in both the disordered and dynamic loops; however, their distributions were not random but had a clear preference which populates the N-terminal part of the disordered loop near the bound form. The MSES simulations of the three binding states clarified the allosteric mechanism of sortase: the N- and C-terminal parts of the disordered loop undergo a disorder-to-order transition independently of each other upon binding of the peptide and a calcium ion, respectively; however, upon binding of both ligands, the two parts work cooperatively to stabilize the bound peptide.

Sortase-conjugation generates a capsule vaccine that protects guinea pigs against Bacillus anthracis

Capsules protect bacteria against phagocytic clearance. Capsular polysaccharides or polyglutamates have evolved also to resist antigen presentation by immune cells, thereby interfering with the production of opsonophagocytic antibodies. Linking capsular material to a carrier protein stimulates its presentation to the immune system. For many conjugate vaccines this is achieved by a process of random chemical cross-linking. Here we describe a new technology, designated sortase-conjugation, which generates a single amide bond between the C-terminal end of a carrier protein and the capsular material. Sortase-conjugation was used to link the poly-D-γ-glutamic acid (PDGA) capsule of Bacillus anthracis to the receptor binding domain (D4) of protective antigen (PagA). When used as a vaccine, PDGA-D4 conjugate elicited robust antibody responses against both capsule and D4. Immunization with PDGA-D4 afforded guinea pigs complete protection against anthrax challenge with wild-type or pagA mutant B. anthracis Ames.

Chemoenzymatic site-specific labeling of influenza glycoproteins as a tool to observe virus budding in real time

The influenza virus uses the hemagglutinin (HA) and neuraminidase (NA) glycoproteins to interact with and infect host cells. While biochemical and microscopic methods allow examination of the early steps in flu infection, the genesis of progeny virions has been more difficult to follow, mainly because of difficulties inherent in fluorescent labeling of flu proteins in a manner compatible with live cell imaging. We here apply sortagging as a chemoenzymatic approach to label genetically modified but infectious flu and track the flu glycoproteins during the course of infection. This method cleanly distinguishes influenza glycoproteins from host glycoproteins and so can be used to assess the behavior of HA or NA biochemically and to observe the flu glycoproteins directly by live cell imaging.

Enveloped viruses such as the influenza virus cause significant disease in humans. In order to cause a productive infection, the virus particle must interact with the host cell using the viral proteins encoded within its genome. For many such viruses, it is possible to directly observe the early steps in infection, yet for technical reasons it has been extremely difficult to study the genesis of daughter virions as they bud off of infected host cells. Here we devised a chemoenzymatic labeling strategy to site-specifically append probes to the influenza hemagglutinin (HA) and neuraminidase (NA) proteins using the bacterial sortase A enzyme. Because labeling is confined to surface exposed HA and NA in the context of live, infected cells, it is possible to study budding biochemically and microscopically in real-time. Using this system, we can observe budding of flu virions from discrete sites at the cell surface. Our work will enable detailed investigation into the birth of viruses from infected host cells and can likely be applied to viruses other than influenza that have been similarly resistant to real-time microscopic observation during budding.

End-point immobilization of recombinant thrombomodulin via sortase-mediated ligation

We report an enzymatic end-point modification and immobilization of recombinant human thrombomodulin (TM), a cofactor for activation of anticoagulant protein C pathway via thrombin. First, a truncated TM mutant consisting of epidermal growth factor-like domains 4-6 (TM(456)) with a conserved pentapeptide LPETG motif at its C-terminal was expressed and purified in E. coli. Next, the truncated TM(456) derivative was site-specifically modified with N-terminal diglycine containing molecules such as biotin and the fluorescent probe dansyl via sortase A (SrtA) mediated ligation (SML). The successful ligations were confirmed by SDS-PAGE and fluorescence imaging. Finally, the truncated TM(456) was immobilized onto an N-terminal diglycine-functionalized glass slide surface via SML directly. Alternatively, the truncated TM(456) was biotinylated via SML and then immobilized onto a streptavidin-functionalized glass slide surface indirectly. The successful immobilizations were confirmed by fluorescence imaging. The bioactivity of the immobilized truncated TM(456) was further confirmed by protein C activation assay, in which enhanced activation of protein C by immobilized recombinant TM was observed. The sortase A-catalyzed surface ligation took place under mild conditions and occurs rapidly in a single step without prior chemical modification of the target protein. This site-specific covalent modification leads to molecules being arranged in a definitively ordered fashion and facilitating the preservation of the protein's biological activity.

Identification of host cell factors required for intoxication through use of modified cholera toxin

We describe a novel labeling strategy to site-specifically attach fluorophores, biotin, and proteins to the C terminus of the A1 subunit (CTA1) of cholera toxin (CTx) in an otherwise correctly assembled and active CTx complex. Using a biotinylated N-linked glycosylation reporter peptide attached to CTA1, we provide direct evidence that ~12% of the internalized CTA1 pool reaches the ER. We also explored the sortase labeling method to attach the catalytic subunit of diphtheria toxin as a toxic warhead to CTA1, thus converting CTx into a cytolethal toxin. This new toxin conjugate enabled us to conduct a genetic screen in human cells, which identified ST3GAL5, SLC35A2, B3GALT4, UGCG, and ELF4 as genes essential for CTx intoxication. The first four encode proteins involved in the synthesis of gangliosides, which are known receptors for CTx. Identification and isolation of the ST3GAL5 and SLC35A2 mutant clonal cells uncover a previously unappreciated differential contribution of gangliosides to intoxication by CTx.