Histidine-rich glycoprotein binds to cell-surface heparan sulfate via its N-terminal domain following Zn²⁺ chelation

Histidine-rich glycoprotein (HRG) is an α₂-glycoprotein found in mammalian plasma at high concentrations (∼150 μg/ml) and is distinguished by its high content of histidine and proline. Structurally, HRG is a modular protein consisting of an N-terminal cystatin-like domain (N1N2), a central histidine-rich region (HRR) flanked by proline-rich sequences, and a C-terminal domain. HRG binds to cell surfaces and numerous ligands such as plasminogen, fibrinogen, thrombospondin, C1q, heparin, and IgG, suggesting that it may act as an adaptor protein either by targeting ligands to cell surfaces or by cross-linking soluble ligands. Despite the suggested functional importance of HRG, the cell-binding characteristics of the molecule are poorly defined. In this study, HRG was shown to bind to most cell lines in a Zn²⁺- dependent manner, but failed to interact with the Chinese hamster ovary cell line pgsA-745, which lacks cell-surface glycosaminoglycans (GAGs). Subsequent treatment of GAG-positive Chinese hamster ovary cells with mammalian heparanase or bacterial heparinase III, but not chondroitinase ABC, abolished HRG binding. Furthermore, blocking studies with various GAG species indicated that only heparin was a potent inhibitor of HRG binding. These data suggest that heparan sulfate is the predominate cell-surface ligand for HRG and that mammalian heparanase is a potential regulator of HRG binding. Using recombinant forms of full-length HRG and the N-terminal N1N2 domain, it was shown that the N1N2 domain bound specifically to immobilized heparin and cell-surface heparan sulfate. In contrast, synthetic peptides corresponding to the Zn ²⁺-binding HRR of HRG did not interact with cells. Furthermore, the bind. ing of full-length HRG, but not the N1N2 domain, was greatly potentiated by physiological concentrations of Zn²⁺. Based on these data, we propose that the N1N2 domain binds to cell-surface heparan sulfate and that the interaction of Zn²⁺ with the HRR can indirectly enhance cell-surface binding.