Purification and characterization of LPXTGase from Staphylococcus aureus: the amino acid composition mirrors that found in the peptidoglycan

Evolution of Streptococcus pyogenes has maximized the efficiency of the Sortase A cleavage motif for cell wall transpeptidation

Trafficking of M-protein (Mprt) from the cytosol of Group A Streptococcus pyogenes (GAS) occurs via Sec translocase membrane channels that associate with Sortase A (SrtA), an enzyme that catalyzes cleavage of Mprt at the proximal C-terminal [-LPST355∗GEAA-] motif and subsequent transpeptidation of the Mprt-containing product to the cell wall (CW). These steps facilitate stable exposure of the N-terminus of Mprt to the extracellular milieu where it interacts with ligands. Previously, we found that inactivation of SrtA in GAS cells eliminated Mprt CW transpeptidation but effected little reduction in its cell surface exposure, indicating that the C-terminus of Mprt retained in the cytoplasmic membrane (CM) extends its N-terminus to the cell surface. Herein, we assessed the effects of mutating the Thr355 residue in the WT SrtA consensus sequence (LPST355∗GEAA-) in a specific Mprt, PAM. In vitro, we found that synthetic peptides with mutations (LPSX355GEAA) in the SrtA cleavage site displayed slower cleavage activities with rSrtA than the WT peptide. Aromatic residues at X had the lowest activities. Nonetheless, PAM/[Y355G] still transpeptidated the CW in vivo. However, when using isolated CMs from srtA-inactivated GAS cells, rapid cleavage of PAM/[LPSY355GEAA] occurred at E357∗ but transpeptidation did not take place. These results show that another CM-resident enzyme nonproductively cleaved PAM/[LPSYGE357∗AA]. However, SrtA associated with the translocon channel in vivo cleaved and transpeptidated PAM/[LPSX355∗GEAA] variants. These CM features allow diverse cleavage site variants to covalently attach to the CW despite the presence of other potent nonproductive CM proteases.

Surface Proteins on Gram-Positive Bacteria

Surface proteins are critical for the survival of gram-positive bacteria both in the environment and to establish an infection. Depending on the organism, their surface proteins are evolutionarily tailored to interact with specific ligands on their target surface, be it inanimate or animate. Most surface molecules on these organisms are covalently anchored to the peptidoglycan through an LPxTG motif found at the C-terminus. These surface molecules are generally modular with multiple binding or enzymatic domains designed for a specific survival function. For example, some molecules will bind serum proteins like fibronectin or fibrinogen in one domain and have a separate function in another domain. In addition, enzymes such as those responsible for the production of ATP may be generally found on some bacterial surfaces, but when or how they are used in the life of these bacteria is currently unknown. While surface proteins are required for pathogenicity but not viability, targeting the expression of these molecules on the bacterial surface would prevent infection but not death of the organism. Given that the number of different surface proteins could be in the range of two to three dozen, each with two or three separate functional domains (with hundreds to thousands of each protein on a given organism), exemplifies the complexity that exists on the bacterial surface. Because of their number, we could not adequately describe the characteristics of all surface proteins in this chapter. However, since the streptococcal M protein was one of the first gram-positive surface protein to be completely sequenced, and perhaps one of the best studied, we will use M protein as a model for surface proteins in general, pointing out differences with other surface molecules when necessary.

SecA localization and SecA-dependent secretion occurs at new division septa in group B Streptococcus

Exported proteins of Streptococcus agalactiae (GBS), which include proteins localized to the bacterial surface or secreted into the extracellular environment, are key players for commensal and pathogenic interactions in the mammalian host. These proteins are transported across the cytoplasmic membrane via the general SecA secretory pathway and those containing the so-called LPXTG sorting motif are covalently attached to the peptidoglycan by sortase A. How SecA, sortase A, and LPXTG proteins are spatially distributed in GBS is not known. In the close relative Streptococcus pyogenes, it was shown that presence of the YSIRKG/S motif (literally YSIRKX3Gx2S) in the signal peptide (SP) constitutes the targeting information for secretion at the septum. Here, using conventional and deconvolution immunofluorescence analyses, we have studied in GBS strain NEM316 the localization of SecA, SrtA, and the secreted protein Bsp whose signal peptide contains a canonical YSIRKG/S motif (YSLRKykfGlaS). Replacing the SP of Bsp with four other SPs containing or not the YSIRKG/S motif did not alter the localized secretion of Bsp at the equatorial ring. Our results indicate that secretion and cell wall-anchoring machineries are localized at the division septum. Cell wall- anchored proteins displayed polar (PilB, Gbs0791), punctuate (CspA) or uniform distribution (Alp2) on the bacterial surface. De novo secretion of Gbs0791 following trypsin treatment indicates that it is secreted at the septum, then redistributed along the lateral sides, and finally accumulated to the poles. We conclude that the ±YSIRK SP rule driving compartimentalized secretion is not true in S. agalactiae.

Characterization of the sortase repertoire in Bacillus anthracis

LPXTG proteins, present in most if not all Gram-positive bacteria, are known to be anchored by sortases to the bacterial peptidoglycan. More than one sortase gene is often encoded in a bacterial species, and each sortase is supposed to specifically anchor given LPXTG proteins, depending of the sequence of the C-terminal cell wall sorting signal (cwss), bearing an LPXTG motif or another recognition sequence. B. anthracis possesses three sortase genes. B. anthracis sortase deleted mutant strains are not affected in their virulence. To determine the sortase repertoires, we developed a genetic screen using the property of the gamma phage to lyse bacteria only when its receptor, GamR, an LPXTG protein, is exposed at the surface. We identified 10 proteins that contain a cell wall sorting signal and are covalently anchored to the peptidoglycan. Some chimeric proteins yielded phage lysis in all sortase mutant strains, suggesting that cwss proteins remained surface accessible in absence of their anchoring sortase, probably as a consequence of membrane localization of yet uncleaved precursor proteins. For definite assignment of the sortase repertoires, we consequently relied on a complementary test, using a biochemical approach, namely immunoblot experiments. The sortase anchoring nine of these proteins has thus been determined. The absence of virulence defect of the sortase mutants could be a consequence of the membrane localization of the cwss proteins.

Functional characterization and localization of a Bacillus subtilis sortase and its substrate and use of this sortase system to covalently anchor a heterologous protein to the B. subtilis cell wall for surface display

Sortases catalyze the covalent anchoring of proteins to the cell surface on Gram-positive bacteria. Bioinformatic analysis suggests the presence of structural genes encoding sortases and their substrates in the Bacillus subtilis genome. In this study, a β-lactamase reporter was fused to the cell wall anchoring domain from a putative sortase substrate, YhcR. Covalent anchoring of this fusion protein to the cell wall was confirmed by using the eight-protease-deficient B. subtilis strain WB800 as the host. Inactivation of yhcS abolished the cell wall anchoring reaction. The amounts of fusion protein anchored to the cell wall were proportional to the levels of YhcS. These data demonstrate that YhcS and YhcR are the sortase and sortase substrate, respectively, in B. subtilis. Furthermore, yhcS is not essential for the survival of B. subtilis under the cultivation condition tested. YhcR fusions were distributed helically in the lateral cell wall. Interestingly, when viewed with an epifluorescence microscope, YhcS also appeared to form short helical arcs. This is the first report to illustrate such distribution of sortases in a rod-shaped bacterium. Models for the spatial distribution of both the sortase and its substrate are discussed. The amount of the reporters displayed on the surface was unambiguously quantified via a unique strategy. Under optimal conditions with the overproduction of YhcS, 47,300 YhcR fusions could be displayed per cell. Displayed reporters were biologically functional and surface accessible. Characterization of the sortase-substrate system allowed the successful development of a YhcR-based covalent surface display system. This system may have various biotechnological applications.

A biosynthetic strategy for re-engineering the Staphylococcus aureus cell wall with non-native small molecules

Staphylococcus aureus (S. aureus) is a Gram-positive bacterial pathogen that has emerged as a major public health threat. Here we report that the cell wall of S. aureus can be covalently re-engineered to contain non-native small molecules. This process makes use of endogenous levels of the bacterial enzyme sortase A (SrtA), which ordinarily functions to incorporate proteins into the bacterial cell wall. Thus, incubation of wild-type bacteria with rationally designed SrtA substrates results in covalent incorporation of functional molecular handles (fluorescein, biotin, and azide) into cell wall peptidoglycan. These conclusions are supported by data obtained through a variety of experimental techniques (epifluorescence and electron microscopy, biochemical extraction, and mass spectrometry), and cell-wall-incorporated azide was exploited as a chemical handle to perform an azide-alkyne cycloaddition reaction on the bacterial cell surface. This report represents the first example of cell wall engineering of S. aureus or any other pathogenic Gram-positive bacteria and has the potential for widespread utility.