Protein A is a cell wall protein from Staphylococcus aureus with a molecular weight between 35-50 kDa and shows high specificity for the Fc region of immunoglobulin molecules of many mammalian species. It is encoded by the spa gene, and its regulation is carried out at the level of the DNA topology, as a function of cellular osmolarity, by a two-component system called ArlS-ArlR. The quality of the resin (Protein A Agarose or equivalent) and stability of the immobilisation is important to avoid leakage of Protein A during the elution procedure.
Protein A affinity chromatography is a rapid one-step purification, which removes most nonIgG contaminants and can achieve purities close to homogeneity. It is particularly useful for purifications of tissue culture supernatant, where 10-100 fold concentrations can be achieved.