NEW OBSERVATIONS ON STRUCTURE AND ANTIGENICITY OF M PROTEINS OF GROUP A STREPTOCOCCUS

Purified type 12 M protein extracted from streptococcal cell walls with alkaline buffers at 37°C was found to have a molecular weight over 100,000. This high molecular weight M protein (Hi-M) was compared with purified M protein extracted by the more conventional procedure employing dilute HCl at pH 2 at 95°C. The acid-extracted M protein (Lo-M) had a minimum molecular weight of about 20,000. Amino acid analyses and peptide mapping of these proteins indicated that Hi-M was composed of at least 4 units of Lo-M. The usual procedures of reduction and alkylation followed by electrophoresis in 8 M urea or guanidine were ineffective in cleaving Hi-M into smaller units, although heating to 95°C at pH partially degraded Hi-M into smaller serologically active fragments with electrophoretic mobilities similar to Lo-M. Neither succinylation nor periodate oxidation had any effect in fragmenting Hi-M to Lo-M. The nature of binding of these units of form Hi-M is still unknown. By antigenic analyses employing immunodiffusion, quantitative precipitation and passive hemagglutination it was shown that Hi-M contained only antigenic determinants found in Lo-M. However, Hi-M exhibited greater affinity and was able to bind more antibody per unit weight than Lo-M and Hi-M appeared to be a better immunogen in rabbits when administered in low doses. © 1969.