Streptococcal M protein: an antiphagocytic molecule assembled on the cell wall.
After extraction with nonionic detergent, type 6 streptococcal M protein was found to be composed of multiple proteins ranging in molecular weight from 35,000 to 6,000 daltons. The antiphagocytic proteins, however, were found to be limited to three species having molecular weights of 28,000, 31,000, and 35,000 daltons. These molecules which removed opsonic antibodies from immune serum could be separated from those proteins that had only type specificity. Pulse chase experiments supported by chemical and immunological data suggest that the smaller, type-specific molecules are used to assemble the larger, antiphagocytic proteins. Type 6 M protein was radialabeled and used in a binding assay for the measurement of opsonic antibodies in human serum. Good correlation was observed between binding and the presence of opsonic antibodies in both systems. However, certain sera did exhibit binding but lacked type-specific opsonic activity. Results of competitive inhibition experiments demonstrated that the nonopsonic serum was deficient in certain antibodies that were present in opsonics serum and that the anitphagocytic molecules contained the sites necessary to bind these antibodies.