Staphylococcal SplA and SplB serine proteases target ubiquitin(-like) specific proteases

Staphylococcus aureus is a Gram-positive opportunistic pathogen that has colonized nearly 30% of the human population and can cause life-threatening infections. S. aureus exports a variety of virulence factors, such as a novel 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 the infection are largely unknown. Here we characterized the substrate and cleavage specificity of recombinantly expressed SplA and SplB proteins. We identified a group of ubiquitin or ubiquitin-like modifying enzymes including deubiquitinating enzymes from human as well as from bacterial sources to be so far unknown SplA and SplB substrates. Distinct cleavage sites within these substrates for SplA (YLY^↓T, FMY^↓N) and SplB (VCD^↓S) were identified by mass spectrometry and confirmed by site-directed mutagenesis of the target proteins. Since many cellular immune signaling pathways are tightly regulated by ubiquitination, the specific cleavage of ubiquitin modifying enzymes strongly suggests a specific role of Spls in manipulating immune signaling and in competing with other bacteria.