Functional properties of complement factor H-related proteins FHR-3 and FHR-4: binding to the C3d region of C3b and differential regulation by heparin

Abstract

The human factor H-related proteins FHR-3 and FHR-4 are members of a family of proteins related to the complement factor H. Here, we report that the two proteins bind to the C3d region of complement C3b. The apparent K A values for the interactions of FHR-3 and FHR-4 with C3b are 7.5×10 6 M −1 and 2.9×10 6 M −1, respectively. Binding studies performed with C3b-coated pneumococci confirmed the results obtained with the biosensor system. A C-terminal construct of factor H showed similar binding characteristics. The interaction of FHR-3, but not of FHR-4, with opsonised pneumococci was inhibited by heparin.