Staphylococcal SplA and SplB Serine Protease Allelic Variants Exhibit Different Substrate Specificities

Staphylococcus aureus is an opportunistic pathogen that is persistently colonizing nearly 30% of the human population and can cause life-threatening infections. S. aureus secretes a variety of virulence factors, such as a set of extracellular serine protease-like proteins (Spls). Spls are expressed by most clinical isolates of S. aureus, but their pathophysiological substrates and role during infection are largely unknown. Pathogens use allelic variation of virulence factors to allow an adaption to different host cells and their defense mechanisms. The differences of these variants are marginally characterized so far. Here, we performed a biochemical characterization of selected allelic variants of the S. aureus SplA and SplB. Our data suggests different variants show differences in their stability, enzymatic activity, and substrate specificity. For the recently identified Spl target proteins RickULP and SseL, different cleavage patterns were observed, upon treatment with different Spl allelic variants. One SplB variant strongly differed in its substrate recognition at the P3/P4 position, more closely resembling SplE than SplB wildtype in its substrate selectivity. Our data provide valuable insights into the evolution of bacterial virulence factors and highlight the importance of including allelic variation of virulence factors to fully understand their role in host-pathogen interaction.