Architecture of the central region of factor H and its interaction with PspC of S. pneumoniae
The complement system is a major component of innate immunity and an effector of antibody-mediated immune responses. Unlike the other two activation pathways of the complement system, the alternative pathway is permanently switched on. Discrimination by complement between self and foreign is therefore achieved by selective protection of healthy host tissue and cells. This study investigated the alternative pathway regulator factor H (FH), which is crucial for protection of self surfaces from complement. FH engages via its N- and C- terminal ends with activation-specific fragments of C3, C3b and C3d. The middle region of FH has no binding sites for complement components. It presumably ensures that the binding sites at either end of the extended and flexible FH molecule cooperate in recognizing C3b in fluid phase or on self surfaces, but not on foreign targets. This study was aimed at achieving an atomic level understanding of the structure of the middle portion of FH, thereby testing hypotheses as to how it promotes the overall biological efficacy of the intact protein. High-resolution NMR-derived structures of two module pairs FH-10-11 and FH-11-12 were solved and combined with SAXS data to produce a model of FH-10-12. This was combined, in silico, with the previously solved FH-12-13 structure, then the model of FH-10-13 was used to revisit SAXS data for FH-10-15 and FH-8-15. A unique structure emerged, unlike any other encountered previously in the family of complement regulators, in which CCPs 13, 14 and 15 have a highly compacted organization that has repercussions for function. While devoid of binding affinity for host ligands, this central region is a binding site for PspC, a virulence factor of S. pneumoniae. It has been speculated that the bacteria use this interaction to sequester FH in a conformation that resembles the one adopted by FH on self cells and makes it particularly good at regulating complement. Structural and functional investigations of this interaction were performed to establish the molecular basis of the use of FH by this pathogen in order to avoid complement-mediated elimination. It was found that PspC and FH form a near-irreversible complex, while FH-8-15 binds PspC almost as tightly as intact protein. When bound to PspC, FH has a higher affinity for some of its targets, supporting the theory that this bacterial protein stabiles a particularly active conformation of the regulator.