Cysteine protease that plays an important role in the inhibition of host innate immune response. Cleaves host elastins found in connective tissues, pulmonary surfactant protein A in the lungs, and the chemokine receptor CXCR2 on leukocytes. Proteolytic cleavage of surfactant protein A impairs bacterial phagocytosis by neutrophils while CXCR2 degradation blocks neutrophil activation and chemotaxis. Additionally, promotes vascular leakage by activating the plasma kallikerin/kinin system, resulting in hypotension. Broad endopeptidase action on proteins including elastin, but rather limited hydrolysis of small-molecule substrates. Assays are conveniently made with hemoglobin, casein or Z-Phe-Arg-NHMec as substrate. Prematurely activated/folded staphopain A is inhibited by staphostatin A (ScpB), which is probably required to protect staphylococcal cytoplasmic proteins from degradation by ScpA. In the cytoplasm, prematurely activated/folded ScpA forms a stable non-covalent complex with ScpB. Expression occurs in a growth phase-dependent manner with optimal expression at post-exponential phase. Up-regulated by Agr (accessory gene regulator) and repressed by SarA (staphylococcal accessory regulator) and sigmaB factor. Cleavage leads to the activation of ScpA probably by an auto-catalytic manner. The catalytic maturation of ScpA appears to reside outside the cascade of activation started by the metalloprotease aureolysin (aur). Belongs to the peptidase C47 family.