HPRG

Human histidine-rich glycoprotein (HPRG) is a multidomain, monomeric, secreted, 67-75 kDa member of the cystatin superfamily of molecules. Its name derives from the fact that 26% of its amino acids (aa) are histidine and proline. In human, it is synthesized as a 525 aa precursor that contains an 18 aa signal sequence and a 507 aa mature region. Five distinct domains are recognized in the mature molecule. There are two N-terminal cystatin-like modules (aa 19-254) and one His-Pro-rich region (aa 350-497) that is flanked by two Pro-rich segments (aa 276-321 and 498-525) . The His-Pro-rich region contains 10 tandem repeats with an HHPHG motif, and the N-and C-termini are linked by a disulfide bond. Human HPRG is only 60% aa identical to mouse HPRG. There are multiple ligands for HPRG. These include small molecular weight molecules (metal ions; heme), hemostatic molecules (heparan sulfate; TSP; plasminogen), and immune system components (T cells; macrophages) . About 50% of plasma plasminogen circulates bound to HPRG. Upon immobilization to cell surface tropomyosin in a Zn ++dependent manner, it is converted to plasmin by tPA. HPRG also shows anti-angiogenic activity on vascular endothelial cells.