Staphylococcus aureus, strain Cowan I, contains 1.7 protein A and its isolated cell walls contain 6.7O/,.Removal of contaminating cytoplasmic membrane fragments from the cell walls does not decrease the content of protein A. Extraction of cell walls with trichloroacetic acid releases teichoic acid but not protein A. Extractions with high concentrations of LiCl do not remove protein A. Only bacteriolytic enzymes can release the protein.Protein A isolated from bacteria after lysozyme digestion contains peptidoglycan constituents. More effective bacteriolytic enzymes such as lysostaphin release peptidoglycan fragments from this protein A preparation.We conclude from these results that protein A is a cell wall component of X. aureas and is covalently linked to the peptidoglycan structure.

Staphylococcus aureus contains cell wall protein A as well as extracellular protein A. The two types of protein A have very similar amino acid compositions, electrophoretic mobilities and sizes. The release of extracellular protein A from exponentially growing bacteria is dependent on protein synthesis do novo and protein A is released directly after being synthesized on the ribosomes. Bacteria in the stationary phase, however, release protein A as a result of cell lysis. Protoplasts have been isolated which produce protein A as extensively as the intact bacteria but because of the absence of cell wall all the protein A is of the extracellular type. In the presence of puromycin, an enhancement of the formation of extracellular protein A is observed from cells also producing cell wall protein A.

Mutant strains of Staphylococcus aureus have been isolated using a simple cosedimentation technique.

Protein A has been isolated from a methicillin‐resistant strain of Staphylococcus aureus, A676, which produces only extracellular protein A. The material is homogeneous after a one‐step separation and has a molecular weight of 41000. Its physicochemical and immunochemical properties have been studied.

Protein A is a cell wall constituent of Staphylococcus aureus. After being synthesized on the ribosomes, the protein A molecules are linked to the peptidoglycan. This is a very rapid reaction which is completed within approximately 1 min; it is suggested that the molecules leave the ribosomes and are then directly incorporated into the peptidoglycan without passing a soluble cytoplasmic stage. The synthesis of protein A de novo and peptidoglycan take place independently of each other. Also, the incorporation of protein A into the cell wall proceeds when the synthesis of peptidoglycan has been completely inhibited by vancomycin, i.e. the synthesis of peptidoglycan is not essential for the incorporation of protein A. It is therefore concluded that protein A can be linked to pre‐existing peptidoglycan chains.