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The surface M protein of group A streptococci binds factor H, a regulatory protein of the alternative complement pathway, which may contribute to the antiphagocytic activity of the M molecules. To locate the factor H binding domain in the alpha-helical coiled-coil structure of the M molecule, the M protein was cleaved with pepsin at pH 5.8, which separates the molecule approximately in half. Western blot (immunoblot), amino acid sequence, and mass spectrometric analyses revealed that factor H bound to a 14.6-kDa C-terminal fragment of the M molecule. Competitive inhibition of factor H binding to the 14.6-kDa fragment with M 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 conserved among heterologous M and M-like molecules. These studies also identified a second pepsin-susceptible site with the sequence ELAK located within the cell wall-associated region of the M molecule.

M protein, the major virulence factor of group A streptococci, has antiopsonic activity in that it inhibits activation of the alternative complement pathway on the streptococcal surface. Two properties of M protein have been claimed to account for the inhibitory activity, namely, (i) its binding affinity for complement factor H, which is an inhibitor of alternative pathway activation, and (ii) its high binding affinity for fibrinogen. We have recently shown that fibrinogen, like M protein, inhibits alternative pathway activation by possessing binding affinity for factor H. Here we report that fibrinogen effectively competes with factor H for binding to M protein but retains its own binding affinity for factor H. The presence of fibrinogen did not significantly affect alternative pathway inhibition on the streptococcal surface.

Pathogenic forms of the protozoan parasite Entamoeba histolytica were reported previously to resist the cytolytic effect of the alternative complement pathway (AP) only temporarily during exposure to complement. In contrast, nonpathogenic forms of E. histolytica had been found to show AP resistance as a stable property. We studied the mechanisms of AP resistance of the two forms. Upon exposure to AP activity, resistant pathogenic or nonpathogenic forms bound significantly less C3 products than complement-sensitive pathogenic amebae, indicating that the two resistant forms both inhibited AP amplification. Various enzymatic treatments and inhibition of membrane mobility by cytochalasin B and glutaraldehyde fixation showed that the mechanisms of AP inhibition differed between pathogenic and nonpathogenic forms; in contrast to nonpathogenic forms, pathogenic amebae required intact membrane mobility and a trypsin-sensitive surface component(s) to inhibit AP activation.