β‐Microglobulin is Bound to Streptococcal M Protein

The presence of receptors for three human proteins (fibrinogen, IgG, and beta 2-microglobulin (beta 2m)) was studied in group A streptococcal strains. M type 1, 12 and 14, and in M protein-negative variants of these strains. Fibrinogen binding was detected in all six strains. IgG did not bind to the two type-12 strains and uptake was low in both type-14 strains. The type-1, M protein-positive strain was negative in binding experiments with radiolabelled IgG, whereas the M protein-negative variant was strongly positive. Aggregated beta 2m showed a high degree of reactivity with the M protein-positive strains but not with the corresponding M protein-negative variants. The results indicate that the receptor for aggregated beta 2m is located on M protein in group A streptococci.