| Protein A (SpA) is a major surface virulence factor of Staphylococcus aureus and a member of the immunoglobulin-binding protein family. It is expressed on the bacterial cell wall with multiple tandem IgG-binding domains. SpA has high affinity for the Fc region of IgG from humans, rabbits, and other mammals, but a low affinity for mouse IgG1. This interaction allows SpA to interfere with host immune defenses by binding the Fcγ domain of IgG, blocking the Fab arms from opsonizing the bacterial surface, and preventing phagocytosis by neutrophils and macrophages. As a result, S. aureus can evade clearance in bloodstream infections. SpA also acts as a B cell superantigen by cross-linking B cell receptors that use VH3-family Fabs, leading to extensive polyclonal B cell expansion and subsequent apoptosis. This cripples adaptive immunity by deleting B-1 and marginal zone B cells, resulting in long-term defects in humoral immunity and promoting recurrent infections. The protein activates tyrosine kinase signaling pathways through phospholipase C, PKC, and MAP kinases, causing aberrant B cell proliferation before deletion. SpA is widely used for antibody purification by affinity chromatography and for detection in Western blots, radioimmunoassays (RIAs), and immunoprecipitations due to its conserved Fc binding. Monoclonal anti-SpA IgG1 antibodies are used to detect and quantify SpA-tagged fusion proteins and S. aureus bacteria. Dysregulated SpA expression contributes to chronic and methicillin-resistant S. aureus (MRSA) infections. |
Lane 1: Recombinant Alkalitolerant Protein A
