Inhibition of Staphylococcus aureus cysteine proteases by human serpin potentially limits staphylococcal virulence.

Bacterial proteases are considered virulence factors and it is presumed that by abrogating their activity, host endogenous protease inhibitors play a role in host defense against invading pathogens. Here we present data showing that Staphylococcus aureus cysteine proteases (staphopains) are efficiently inhibited by Squamous Cell Carcinoma Antigen 1 (SCCA1), an epithelial-derived serpin. The high association rate constant (k(ass)) for inhibitory complex formation (1.9×10(4) m/s and 5.8×10(4) m/s for staphopain A and staphopain B interaction with SCCA1, respectively), strongly suggests that SCCA1 can regulate staphopain activity in vivo at epithelial surfaces infected/colonized by S. aureus. The mechanism of staphopain inhibition by SCCA1 is apparently the same for serpin interaction with target serine proteases whereby the formation of a covalent complex result in cleavage of the inhibitory reactive site peptide bond and associated release of the C-terminal serpin fragment. Interestingly, the SCCA1 reactive site closely resembles a motif in the reactive site loop of native S. aureus-derived inhibitors of the staphopains (staphostatins). Given that S. aureus is a major pathogen of epithelial surfaces, we suggest that SCCA1 functions to temper the virulence of this bacterium by inhibiting the staphopains.

Identification of an intracellular M17 family leucine aminopeptidase that is required for virulence in Staphylococcus aureus.

Staphylococcus aureus is a highly virulent bacterial pathogen capable of causing a variety of ailments throughout the human body. It is a major public health concern due to the continued emergence of highly pathogenic methicillin resistant strains (MRSA) both within hospitals and in the community. Virulence in S. aureus is mediated by an array of secreted and cell wall associated virulence factors, including toxins, hemolysins and proteases. In this work we identify a leucine aminopeptidase (LAP, pepZ) that strongly impacts the pathogenic abilities of S. aureus. Disruption of the pepZ gene in either Newman or USA300 resulted in a dramatic attenuation of virulence in both localized and systemic models of infection. LAP is required for survival inside human macrophages and gene expression analysis shows that pepZ expression is highest in the intracellular environment. We examine the cellular location of LAP and demonstrate that it is localized to the bacterial cytosol. These results identify for the first time an intracellular leucine aminopeptidase that influences disease causation in a Gram-positive bacterium.