Histidine-rich Glycoprotein Binds Fibrin(ogen) with High Affinity and Competes with Thrombin for Binding to the γ′-Chain

Histidine-rich glycoprotein (HRG) is an abundant protein that binds fibrinogen and other plasma proteins in a Zn2(+)-dependent fashion but whose function is unclear. HRG has antimicrobial activity, and its incorporation into fibrin clots facilitates bacterial entrapment and killing and promotes inflammation. Although these findings suggest that HRG contributes to innate immunity and inflammation, little is known about the HRG-fibrin(ogen) interaction. By immunoassay, HRG-fibrinogen complexes were detected in Zn2(+)-supplemented human plasma, a finding consistent with a high affinity interaction. Surface plasmon resonance determinations support this concept and show that in the presence of Zn2(+), HRG binds the predominant gamma(A)/gamma'-fibrinogen and the gamma-chain elongated isoform, gamma(A)/gamma'-fibrinogen, with K-d values of 9 nM. Likewise, I-125-labeled HRG binds gamma(A)/gamma(A)-or gamma(A)/gamma'-fibrin clots with similar K-d values when Zn2(+) is present. There are multiple HRG binding sites on fibrin(ogen) because HRG binds immobilized fibrinogen fragment D or E and gamma'-peptide, an analog of the COOH terminus of the gamma'-chain that mediates the high affinity interaction of thrombin with gamma(A)/gamma'-fibrin. Thrombin competes with HRG for gamma'-peptide binding and displaces I-125-HRG from gamma(A)/gamma'-fibrin clots and vice versa. Taken together, these data suggest that (a) HRG circulates in complex with fibrinogen and that the complex persists upon fibrin formation, and (b) by competing with thrombin for gamma(A)/gamma'-fibrin binding, HRG may modulate coagulation. Therefore, the HRG-fibrin interaction may provide a novel link between coagulation, innate immunity, and inflammation.