Identification of novel human cellular substrates of Staphylococcus aureus serine protease SplB

Staphylococcus aureus colonizes up to one third of the human population yet retains the capacity to cause invasive, life-threatening infections. The growing prevalence of antimicrobial resistance further complicates treatment. A major contributor to the versatility of S. aureus is its broad repertoire of virulence factors, among which secreted proteases facilitate dissemination from colonization sites into deeper tissues. Twelve extracellular proteases are secreted, with the serine protease-like protein (Spl) family (SplA-SplF), encoded within a single operon, accounting for half of them. Despite this prominence, the pathophysiological roles and substrate specificities of the Spl proteases remain poorly understood. Here, we employed a direct protein-protein interaction approach to identify novel SplB substrates in human serum. We demonstrate that SplB cleaves three intermediate filament proteins, namely desmin, vimentin, and nestin, as well as heat shock protein {beta}1 and -enolase, which have not previously been recognized as targets of S. aureus proteases. Moreover, SplB was found to cleave native IgG, a feature otherwise described only for the glutamyl endopeptidase V8. These findings expand the host protein repertoire targeted by SplB and suggest broader roles for Spl proteases in immune evasion and tissue invasion.