Structural mechanisms for the recruitment of factor H by Streptococcus pyogenes

The major bacterial pathogen Streptococcus pyogenes (Group A Streptococcus, or Strep A) recruits the negative regulator of the alternative complement pathway factor H (FH) to its surface. Antigenically sequence variable regions of several Strep A M proteins, including M5 and M6, bind FH but have no obvious sequence homology. A second Strep A surface-localized protein, FbaA, binds FH through a purported coiled-coil region, suggesting mimicry of the well-known coiled coil of M proteins. We determined the structures of fragments of M5 protein, M6 protein, and FbaA complexed with FH domains 6 and 7 (FH(6-7)). M5 and M6 proteins formed dimeric α-helical coiled coils, as expected, while FbaA instead consisted of a monomeric three-helix bundle preceded by a loop. FH(6-7) accommodated different binding modes in these three proteins, with very few common interacting amino acids. Based on contributions to binding, distinct FH-binding sequence patterns were constructed for M5 and M6 proteins, enabling identification of FH-binding sequences in M or M-like Enn proteins in 32 strains of differing M types. While FbaA was allelically sequence variable, its critical FH-binding amino acids were absolutely conserved in 95 strains of differing M types. Together, FH-binding sequences were identified in about half of the known 250 Strep A strains, with the majority due to FbaA. Our structural and functional elucidation of the mechanism of FH recruitment is applicable to precise investigation of its role in Strep A virulence.