Structural and functional studies of histidine-rich glycoprotein in relation to its roles in angiogenesis and coagulation

Abstract

Histidine-rich glycoprotein (HRG) is a plasma protein that regulates key cardiovascular processes such as coagulation, angiogenesis and immune response. The protein consists of six distinct functional domains: two N-terminal domains (N1 and N2), two proline-rich regions (PRR1 and PRR2), a central histidine-rich region (HRR) and a C-terminal domain. The HRR binds Zn²⁺, which alters the affinity of HRG towards various ligands including the anticoagulant, heparin. A key aim of this study was to structurally characterise HRG. The 1.93 Å crystal structure of the HRG N2 domain presented here represents the first crystallographic snapshot of the molecule. The N2 domain is cystatin-like and N-glycosylated at Asn184. An S-glutathionyl adduct was observed at Cys185, providing in vivo evidence that release of an anti-angiogenic HRR/PRR fragment is controlled in part by a redox mechanism, representing a novel further role for GSH in regulation of angiogenesis. Since Zn²⁺ regulates some of the functions of HRG, the dynamics of Zn²⁺ in plasma were investigated using a combination of ITC, ELISA and thrombin assay systems. Zn²⁺ is normally associated with albumin in circulation, but its ability to bind Zn²⁺ is allosterically inhibited upon fatty acids binding to albumin. Elevated plasma fatty acid levels are associated with some disease states. It is proposed that this may alter the proportion of Zn²⁺ bound to HRG, which could in turn activate thrombin to promote coagulation. These studies provide evidence to suggest that Zn²⁺-dependent activation of HRG (following fatty acid binding to albumin) may play a role in the development of haemostatic complications in susceptible individuals. Finally, the Zn²⁺ binding ability of albumin was probed in order to locate unidentified sites using recombinant albumin mutants. H9A, H67A, E252A, D256A and H288A mutants all exhibited diminished Zn²⁺ binding ability, indicating that these residues are involved directly or indirectly in Zn²⁺ binding.