Activity and Selectivity of Class B Sortase Enzymes

Gram-positive bacteria attach many proteins to their cell walls via sortase enzymes. Sortases are cysteine transpeptidases and are grouped into 6 classes, A-F. Sortase enzymes, particularly sortase A from Staphylococcus aureus, have been used extensively for in vitro protein ligations. Here, we investigate substrate-binding in sortase A from Streptococcus pyogenes. In addition, class B sortases are typically overlooked for research and development due to low in vitro activity and incomplete knowledge of substrate specificity. Here, we investigate the activity of class B sortases from Bacillus anthracis (baSrtB), Clostridioides difficile (cdSrtB), Listeria monocytogenes (lmSrtB), and Staphylococcus aureus (saSrtB). Of these, baSrtB was the most active in our hands and was selected for further study. Mutant enzymes were created to study the impact of a class B N-terminal α-helix and a structurally conserved, but sequentially variable, loop on baSrtB activity. Mutations to the structurally conserved loop were impactful on enzyme activity, with some mutations decreasing activity while others greatly increased it. A substrate-bound enzyme model generated using Alphafold2 (Galaxy) allowed us to explore enzyme-substrate interactions in greater detail. This model was validated through molecular dynamics simulations and mutagenesis. This work shows that baSrtB is a viable tool for protein engineering studies and lends greater insight into the structural features that underpin sortase activity and selectivity.