Class A sortases are membrane-bound cysteine transpeptidases distributed in Gram-positive bacteria (mainly present in Firmicutes). They perform a housekeeping role in the cell as members of this group are capable of anchoring a large number of functionally distinct surface proteins containing a cell wall sorting signal to an amino group located on the bacterial cell wall. They do so by catalyzing a transpeptidation reaction in which the surface protein substrate is cleaved at a conserved cell wall-sorting signal (Class A sortases recognize a canonical LPXTG motif, X can be any amino acid), and covalently linked to peptidoglycan for display on the bacterial surface. The prototypical sortase A protein from Staphylococcus aureus (named Sa-SrtA) cleaves the amide bond between threonine and glycine residues of the canonical LPXTG motif in a wide range of protein substrates with diverse functions that can promote bacterial adhesion, nutrient acquisition, host cell invasion, and immune evasion. Next, it catalyzes a transpeptidation reaction by which the proteins are covalently linked to the peptidoglycan precursor lipid II. SrtA is therefore affects the ability of a pathogen to establish successful infection. SrtA contains an N-terminal hydrophobic segment, a linker region and an extra-cellular C-terminal catalytic domain. The hydrophobic segment functions as both a signal peptide for secretion and a stop-transfer signal for membrane anchoring. The catalytic domain contains the catalytic TLXTC signature sequence where X is usually a valine, isoleucine or a threonine. The gene encoding SrtA is generally not located in the same gene cluster as its substrates while the gene encoding SrtB is usually clustered in the same locus as its substrate.
Comment:The SrtA enzymes recognize a conserved five amino acid sequence (LPXTG) at the C-terminus of their protein substrates and cleave the amide bond between the threonine and glycine residues using a highly-conserved cysteine residue in the active site. Two other active site residues have been identified as essential for optimal enzyme activity, a His side-chain which serves as a general base and an Arg side-chain which stabilises the oxoanion intermediate formed during the reaction.
Comment:The catalytic triad is formed by His, Cys, and Arg. The sulfhydryl of Cys is ionized by His and the resultant thiolate attacks the LPXTG peptide.
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