Immunologically active and structurally similar fragments of protein A from Staphylococcus aureus

To study the active site(s) in protein A, partial tryptic digestions of the protein and of intact Staphylococcus aureus were performed. Fragments which bind to the Fc-part of human IgG were isolated by affinity chromatography on IgG-Sepharose 4B and purified by ion-exchange chromatography on phosphocellulose. From a partial tryptic digest of pure protein A at 30 degrees C, pH 8.2 for 30 min we have isolated and characterized six active fragments with molecular weights ranging from 6000 to 8000. Two active fragments, obtained in high yields by digestion at pH 7.2 of intact protein-A-containing bacteria, were shown to be similar to two of the six characterized fragments from the digest of pure protein A. All fragments appeared to have similar amino acid sequences, judged by peptide mapping, specific staining and amino acid analysis; some are very possibly overlapping peptides. Each fragment probably contains only one active site region since all are monovalent in the Fc-reaction when studied with a hemagglutination technique. The maximal molar yield of active fragments obtained from the digestion of pure protein A accounts for about 210% of the amount of protein A used. Thus protein A, suggested to consist of repeating units, should exhibit at least three similar if not identical active regions.