Protein A from Staphylococcus aureus

Protein A, a cell wall protein from Staphylococcus aureus, has been chemically modified by acetylation with acetylimidazole. All the four tyrosines present in the molecule as well as 40 out of 52 lysines were acetylated. The conformation of protein A was not changed by the acetylation. The ability of protein A to react with the Fc fragment of immunoglobulins was decreased to 40%, but was restored to 60%, when acetylated protein A was O‐deacetylated with hydroxylamine. The reactivity of protein A against specific rabbit antiserum was not decreased by acetylation. The significance of unmodified tyrosines and lysines for the protein A reactivity is discussed.