Structure of FXII, gC1qR, and HK. (A) Domain organization of FXII, gC1qR, and HK. The gC1qR anionic loops are red and residue numbers for domain boundaries are labeled. HK domain D4 represents the bradykinin (BK) peptide. (B) Gel filtration elution profiles for FXIIFnII (blue) and gC1qR (black) in isolation and a gC1qR-FXIIFnII mixture (red). (C) Coomassie-stained SDS-PAGE gel of the fractions collected from gel filtration of the FXIIFnII-gC1qR mixture. Lanes labeled 15 to 18 correspond to elution volumes 15 and 18 mL from panel B showing both proteins coeluting as a complex with excess FXIIFnII observed at 20 mL. The first lane (labeled M) is the protein marker, and the second lane is gC1qR in isolation. (D) Diagram of the FXIIFnII-gC1qR complex crystal structure showing the FXIIFnII domain (cyan) in complex with gC1qR (wheat, purple, and green), with Zn²⁺ in blue, the anionic G1-loop in red, and key interacting side chains as sticks. (E) Space-filling representation of the gC1qR complex with the FXIIFnII domain as spheres (cyan), with residues Arg36 and Arg65 in orange.