Structural stability and heat-induced conformational change of two complement inhibitors: C4b-binding protein and factor H.

The complement inhibitors C4b-binding protein (C4BP) and factor H (FH) both consist of complement control protein (CCP) domains. Here we examined the secondary structure of both proteins by circular dichroism and Fourier-transform infrared technique at temperatures ranging from 30 degrees C-90 degrees C. We found that predominantly beta-sheet structure of both proteins was stable up to 70 degrees C, and that a reversible conformational change toward alpha-helix was apparent at temperatures ranging from 70 degrees C to 90 degrees C. The ability of both proteins to inhibit complement was not impaired after incubation at 95 degrees C, exposure to extreme pH conditions, and storage at room temperature for several months. Similar remarkable stability was previously observed for vaccinia virus control protein (VCP), which is also composed of CCP domains; it therefore seems to be a general property of CCP-containing proteins. A typical CCP domain has a hydrophobic core, which is wrapped in beta-sheets and stabilized by two disulphide bridges. How the CCP domains tolerate harsh conditions is unclear, but it could be due to a combination of high content of prolines, hydrophobic residues, and the presence of two disulphide bridges within each domain. These findings are of interest because CCP-containing complement inhibitors have been proposed as clinical agents to be used to control unwanted complement activation that contributes to many diseases.