Preferentially cleaves peptide bonds on the carboxyl-terminal side of aspartate and glutamate. Along with other extracellular proteases it is involved in colonization and infection of human tissues. Required for proteolytic maturation of thiol protease SspB and inactivation of SspC, an inhibitor of SspB. It is the most important protease for degradation of fibronectin-binding protein (FnBP) and surface protein A, which are involved in adherence to host cells. May also protect bacteria against host defense mechanism by cleaving the immunoglobulin classes IgG, IgA and IgM. May be involved in the stability of secreted lipases. Preferential cleavage: Glu-|-Xaa, Asp-|-Xaa. Expression occurs in a growth-phase-dependent manner with optimal expression at post-exponential phase. Environmental conditions such as degree of aeration and salt concentration are also important in control of transcription and processing of SspA. Up-regulated by Agr (accessory gene regulator) and repressed by sigmaB factor and SarA. Proteolytically cleaved by aureolysin (aur). This cleavage leads to the activation of SspA. The cascade of activation of extracellular proteases proceeds from the metalloprotease aureolysin (aur), through SspA to SspB. Belongs to the peptidase S1B family.