Histidine-rich glycoprotein: A novel adaptor protein in plasma that modulates the immune, vascular and coagulation systems

Histidine-rich glycoprotein (HRG) is an abundant plasma glycoprotein that has a multidomain structure, interacts with many ligands, and has been shown to regulate a number of important biological processes. HRG ligands include Zn ²⁺ and haem, tropomyosin, heparin and heparan sulphate, plasminogen, plasmin, fibrinogen, thrombospondin, IgG, FcγR and complement. In many cases, the histidine-rich region of the molecule enhances ligand binding following interaction with Zn²⁺ or exposure to low pH, conditions associated with sites of tissue injury or tumour growth. The multidomain nature of HRG indicates that it can act as an extracellular adaptor protein, bringing together disparate ligands, particularly on cell surfaces. HRG binds to most cells primarily via heparan sulphate proteoglycans, binding which is also potentiated by elevated free Zn²⁺ levels and low pH. Recent reports have shown that HRG can modulate angiogenesis and additional studies have shown that it may regulate other physiological processes such as cell adhesion and migration, fibrinolysis and coagulation, complement activation, immune complex clearance and phagocytosis of apoptotic cells. This review outlines the molecular, structural, biological and clinical properties of HRG as well as describing the role of HRG in various physiological processes.