LOCATION OF THE COMPLEMENT FACTOR-H BINDING-SITE ON STREPTOCOCCAL M6 PROTEIN

The surface M protein of group A streptococci binds factor H, a regula tory protein of the alternative complement pathway, which may contribu te to the antiphagocytic activity of the M molecules. To locate the fa ctor H binding domain in the alpha-helical coiled-coil structure of th e M molecule, the M protein was cleaved with pepsin at pH 5.8, which s eparates the molecule approximately in half. Western blot (immunoblot) , amino acid sequence, and mass spectrometric analyses revealed that f actor H bound to a 14.6-kDa C-terminal fragment of the M molecule, Com petitive inhibition of factor H binding to the 14.6-kDa fragment with M I protein peptides Localized the binding site to amino acids 256 to 292, This segment is located within the surface-exposed region of the M6 protein, identified as the C-repeat region, whose sequence is conse rved among heterologous RI and M-like molecules. These studies also id entified a second pepsin-susceptible site with the sequence ELAK locat ed within the cell wall-associated region of the M molecule.