Ternary complexes of FXII, HK and gC1qR analyzed by gel filtration. (A) Analytical gel filtration elution profiles showing FXIIFnII, D5 and gC1qR combined with increasing concentrations of HKD5 in the presence of Zn²⁺. On the right coomassie stained SDS-PAGE gels showing gC1qR, HKD5 and FXIIFnII in the fractions collected. As the concentration of HKD5 is increased, FXIIFnII shifts from the high-molecular-weight peak to the low-molecular-weight peak, suggesting D5 is outcompeting FXIIFnII for gC1qR binding. (B) Analytical gel filtration (Superose 6 10/300) of full-length proteins HK (green), FXII (blue), gC1qR (black), and the gC1qR-HK-FXII ternary complex (red). Coomassie-stained SDS-PAGE gel showing the peak fraction of the gC1qR-HK-FXII ternary complex. (C) Schematic diagram of a hypothetical FXII-HK-gC1qR-PK complex with a 1:2:6:2 stoichiometry. In this model, gC1qR is capable of stimulating reciprocal FXII-PK activation by aligning the activation loops and active sites of the FXII and PK proteases.