Factor H (FH) is an abundant regulator of complement activation and protects host cells from self-attack by complement. Here we provide insights into the regulatory activity of FH by solving the crystal structure of the first four domains of FH in complex with its target C3b. FH interacts with multiple domains of C3b, covering a large, extended surface area. The structure indicated that FH destabilizes the C3 convertase by competition and electrostatic repulsion and that FH enables proteolytic degradation of C3b by providing a binding platform for the protease factor I, while stabilizing the overall domain arrangement of C3b. The results offer general models for complement regulation and provide structural explanations for disease-related mutations in both FH and C3b.
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