M1-PROTEIN AND PROTEIN-H - IGGFC-BINDING AND ALBUMIN-BINDING STREPTOCOCCAL SURFACE-PROTEINS ENCODED BY ADJACENT GENES

M1 protein and Protein H are surface proteins simultaneously present a t the surface of certain strains of Streptococcus pyogenes, important pathogenic bacteria in humans. The present study concerns the structur e, protein-binding properties and relationship between these two molec ules. The gene encoding M1 protein (emm1) was found immediately upstre am of the Protein H gene (sph). Both genes were preceded by a promoter region. Comparison of the sequences revealed a high degree of similar ity in the signal peptides, the C repeats located in the central parts of the molecules and in the C-terminal cell-wall-attached regions, wh ereas the N-terminal sequences showed no significant similarity. Prote in H has affinity for the Pc region of IgG antibodies. Also M1 protein , isolated from streptococcal culture supernatants or from Escherichia coli expressing emm1, was found to bind human IgGFc. When tested agai nst polyclonal IgG from eight other mammalian species, M1 protein and Protein H both showed affinity for baboon, rabbit and pig IgG. M1 prot ein also reacted with guinea-pig IgG, whereas both streptococcal prote ins were negative in binding experiments with rat, mouse, bovine and h orse IgG. The two proteins were also tested against other members of t he immunoglobulin super family: human IgM, IgA, IgD, IgE, beta(2)-micr oglobulin, and major histocompatibility complex (MHC) class-I and clas s-II antigens. M1 protein showed no affinity for any of these molecule s whereas Protein H reacted with MHC class-II antigens. M1 protein is known to bind albumin and fibrinogen also. The binding sites for these two plasma proteins and for IgGFc were mapped to different sites on M 1 protein. Thus albumin bound to the C repeats and IgGFc to a region ( S) immediately N-terminal of the C repeats. Finally, fibrinogen bound further towards the N-terminus but close to the IgGFc-binding site. On the fibrinogen molecule, fragment D was found to mediate binding to M 1 protein. The IgGFc-binding region of M1 protein showed no similarity to that of Protein H. Still, competitive binding experiments demonstr ated that the two streptococcal proteins bound to overlapping sites on IgGFc.