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Gram-positive bacteria are some of the earliest known life forms, diverging from gram-negative bacteria 2 billion years ago. These organisms utilize sortase enzymes to attach proteins to their peptidoglycan cell wall, a structural feature that distinguishes the two types of bacteria. The transpeptidase activity of sortases make them an important tool in protein engineering applications, e.g., in sortase-mediated ligations or sortagging. However, due to relatively low catalytic efficiency, there are ongoing efforts to create better sortase variants for these uses. Here, we use bioinformatics tools, principal component analysis and ancestral sequence reconstruction, in combination with protein biochemistry, to analyze natural sequence variation in these enzymes. Principal component analysis on the sortase superfamily distinguishes previously described classes and identifies regions of relatively high sequence variation in structurally-conserved loops within each sortase family, including those near the active site. Using ancestral sequence reconstruction, we determined sequences of ancestral Staphylococcus and Streptococcus Class A sortase proteins. Enzyme assays revealed that the ancestral Streptococcus enzyme is relatively active and shares similar sequence variation with other Class A Streptococcus sortases. Taken together, we highlight how natural sequence variation can be utilized to investigate this important protein family, arguing that these and similar techniques may be used to discover or design sortases with increased catalytic efficiency and/or selectivity for sortase-mediated ligation experiments.
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This content will become publicly available on December 1, 2025
Amino acid variability at W194 of Staphylococcus aureus sortase A alters nucleophile specificity
Abstract Bacterial sortases are a family of cysteine transpeptidases in Gram‐positive bacteria of which sortase A (SrtA) enzymes are responsible for ligating proteins to the peptidoglycan layer of the cell surface. Engineered versions of sortases are also used in sortase‐mediated ligation (SML) strategies for a variety of protein engineering applications. Although a versatile tool, substrate recognition byStaphylococcus aureusSrtA (saSrtA), the most commonly utilized enzyme in SML, is stringent and relies on an LPXTG pentapeptide motif. Previous structural studies revealed that the requirement of a glycine in the binding motif may be due to potential steric hindrance of amino acids possessing a β‐carbon by W194, a tryptophan located in the β7‐β8 loop of the enzyme. Here, we measured the effect of seven single point mutants of W194 (A, D, F, G, N, S, Y) saSrtA using a FRET‐based activity assay. We found that while the LPXTG motif remains a requirement for initial proteolytic cleavage, the nucleophile specificity of our variants is altered. In particular, W194A and W194S saSrtA recognize a D‐Ala nucleophile and are able to perform ligation reactions. Notably, an LPXT(D‐Ala) peptide was not cleaved by either mutant enzyme. We hypothesize that these variants may potentially be utilized to develop an irreversible sortase‐mediated reaction. Taken together, this experiment reveals new insight into sortase specificity and possible future SML strategies.
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- Award ID(s):
- 2044958
- PAR ID:
- 10629462
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Protein Science
- Volume:
- 33
- Issue:
- 12
- ISSN:
- 0961-8368
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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