Identification of a Domain in Human Factor H and Factor H-Like Protein-1 Required for the Interaction with Streptococcal M Proteins

The plasma protein factor H (FH) inhibits the alternative pathway of complement activation. Previous work has shown that FH binds to group A streptococci and that the interaction does not interfere with the complement-inhibitory capacity of FH. In this work, we report a molecular analysis of this interaction. In absorption experiments with human plasma, M protein-expressing group A streptococci bound both FH and FH-like protein-1 (FHL-1), an active 42-kDa splice product of the FH-gene transcript comprising the first 7 of its 20 short consensus repeat (SCR) domains. rFHL-1 also bound to M protein-expressing streptococci, but rFH fragments containing SCR 1-5 or SCR 1-6 did not. rFHL-1 bound to purified M5 protein with an affinity that was higher than the value calculated for the interaction between FH and M5 protein. The binding of radiolabeled rFHL-1 to immobilized M5 was blocked completely by unlabeled rFHL-1, but was inhibited only partially by SCR 1-6, emphasizing the importance of SCR 7 for the interaction. In experiments with the FH-related proteins FHR-3 and FHR-4, only the former bound to M protein-expressing streptococci, again pointing to an involvement of SCR 7, since FHR-3, but not FHR-4, contains a domain that is similar to SCR 7. Finally, the interaction between rFHL-1 and purified M5 protein was inhibited by heparin, which binds FH via SCR 7. Together, these data indicate that the interaction between streptococcal M proteins and FH or FHL-1 requires SCR 7.