Biochemical and biological properties of the binding of human fibrinogen to M protein in group A streptococci

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Abstract
Fibrinogen is known to bind to group A streptococci and precipitate with extracts containing streptococcal M protein. We have previously shown that the binding of fibrinogen to M-positive streptococci prevents opsonization by complement and protects the organisms from phagocytosis in nonimmune blood. In the present study, we used 3H-labeled fibrinogen, a highly purified peptide fragment of type 24 M protein (pep M24), and anti-pep M sera to show that (i) fibrinogen binds to M-positive streptococci with high affinity (dissociation constants, 1 to 5 nM); (ii) occupation of the high-affinity binding sites suffices to protect the organisms from phagocytosis; (iii) proteolytic treatments that remove M protein from streptococcal cells abolish binding; (iv) binding is competitively inhibited by anti-pep M sera; (v) pep M24 precipitates fibrinogen; and (vi) bindng to type 24 cells is inhibited by pep M24. We conclude that M protein is the cell surface structure principally responsible for binding fibrinogen on the surface on M-positive streptococci and that this binding contributes to the known antiopsonic prooperty of M proteins.