Crystal Structure of Streptococcus pyogenes Sortase A

Abstract

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.Cell wall-anchored proteins play critical roles in the virulence of most Gram-positive bacterial pathogens by acting as adhesins or invasins and/or interfering with various arms of the host innate or specific immune defenses. The vast majority of these virulence proteins are retained at the bacterial surface after secretion by a mechanism that involves the covalent linkage of target proteins to the peptidoglycan layer of the cell wall. This linkage is catalyzed by membrane-associated transpeptidases called sortases (1, 2). Proteins destined for cell-surface attachment contain a sorting signal recognized by these enzymes. As this mechanism is unique to Gram-positive pathogens, inhibiting the reaction is an attractive target for the development of novel antibacterials (3, 4). The sortase-mediated transpeptidation reaction is also being increasingly used in a variety of biotechnology applications (5-8).The sorting signal that targets proteins for cell surface attachment is located at the C terminus of substrates and comprises a pentapeptide motif, typically LPXTG (where X is any amino acid), followed by a hydrophobic region and a tail of positively charged residues that locates the substrate to the cell surfacefollowingsecretion(2,9).Inonecurrentmodelofsortasedependent transpeptidation, the LPXTG motif is specifically recognized by the enzyme (10), and the thiolate group of an essential active sit...

Antiinfective therapy with a small molecule inhibitor of Staphylococcus aureus sortase

“…The LPXTG motif is conserved in a wide variety of surface proteins (35); however, the chemical structure of the peptidoglycan crossbridge varies between different bacterial species (36). Although structurally similar (37)(38)(39)(40), the amino acid identity between sortases from different Gram-positive bacteria is limited to a few key residues at or near the active site (34). We therefore asked whether compound 6e can inhibit sortase from three different microbes: Bacillus anthracis, Streptococcus pneumoniae, and Streptococcus pyogenes.…”

The Sortase A Enzyme That Attaches Proteins to the Cell Wall of Bacillus anthracis Contains an Unusual Active Site Architecture

“…SrtA-type enzymes are believed to play a housekeeping role in the cell by anchoring a large number of distinct proteins that contain a LPXTG sorting signal (10). The structures of two other SrtA-type enzymes have been determined: S. pyogenes SrtA (SpSrtA) and S. aureus SrtA (Sa-SrtA) (12,16,17,19). These proteins share only limited sequence homology with Ba-SrtA; Sa-SrtA and Sp-SrtA share 29 and 32% sequence identity with Ba-SrtA, respectively.…”

“…The molecular basis of Ba-SrtA function is not well understood because it shares only limited sequence identity with previously characterized SrtA-type enzymes; the structures of the Sa-SrtA and Sp-SrtA enzymes have been determined and share 29 and 32% sequence identity with Ba-SrtA, respectively (12,16,17,19). Here we report studies of the structure, dynamics, and function of Ba-SrtA.…”

“…NMR and crystal structures of several sortase enzymes have revealed that they adopt a common eight-stranded ␤-barrel fold that contains conserved active site residues (12)(13)(14)(15)(16)(17)(18)(19)(20). The mechanism of the sortase-catalyzed transpeptidation reaction is best understood for the Sa-SrtA enzyme.…”

Sortase-catalyzed transformations that improve the properties of cytokines

“…This sortase (SrtA strep ) cleaves the LPXTA motif between theronine and alanine and allows installation of modified alanine-based nucleophiles. SrtA strep also recognizes and cleaves LPXTG motifs, albeit with reduced efficiency, however the LPXTA motif is refractory to cleavage by SrtA Staph (13,14).…”