Plasminogen Is Tethered with High Affinity to the Cell Surface by the Plasma Protein, Histidine-rich Glycoprotein

Abstract

Plasminogen has been implicated in extracellular matrix degradation by invading cells, but few high affinity cell surface receptors for the molecule have been identified. Previous studies have reported that the plasma protein, histidine-rich glycoprotein (HRG), interacts with plasminogen and cell surfaces, raising the possibility that HRG may immobilize plasminogen/plasmin to cell surfaces. Here we show, based on optical biosensor analyses, that immobilized HRG interacts with soluble plasminogen with high affinity and with an extremely slow dissociation rate. Furthermore, the HRG-plasminogen interaction is lysine-dissociable and involves predominately the amino-terminal domain of HRG, and the fifth kringle domain of plasminogen, but not the carboxyl-terminal lysine of HRG. HRG was also shown to tether plasminogen to cell surfaces, with this interaction being potentiated by elevated Zn 2؉ levels and low pH, conditions that prevail at sites of tissue injury, tumor growth, and angiogenesis. Based on these data we propose that HRG acts as a soluble adaptor molecule that binds to cells at sites of tissue injury, tumor growth, and angiogenesis, providing a high affinity receptor for tethering plasminogen to the cell surface and thereby enhancing the migratory potential of cells.

The Anti-angiogenic His/Pro-rich Fragment of Histidine-rich Glycoprotein Binds to Endothelial Cell Heparan Sulfate in a Zn2+-dependent Manner

“…The N-terminal domain of the protein, as well as peptides derived from sequences downstream of HRGP330 in the His/Pro-rich region, showed much weaker heparin binding. These findings suggest that the N-terminal domain is not the main heparin-binding domain as earlier proposed (36). One possible explanation for this discrepancy may be that significant parts of the His/Pro-rich region were not included in the latter study.…”

“…Zn 2ϩ is a known ligand for HRGP (42) and has recently been proposed to induce a conformational change in HRGP to expose the N-terminal domain for binding to cell surface HS (36). Our results show a direct Zn 2ϩ -dependent binding between the 26-amino-acid residue peptide HRGP335 and HS/heparin.…”

“…However, unlike heparin, HS is present on the cell surface and is therefore the natural ligand for most heparin-binding proteins. HRGP does not bind to cells lacking HS (36), indicating that HRGP can make use of cellular HS as a natural cell surface ligand. Indeed, binding of biotinylated HRGP330 was competed by the removal of cell surface HS, heparin lyase treatment, heparin pre-incubation of BT-HRGP330, and by HRGP, demonstrating HS structures as receptor molecules for HRGP and HRGP330 on endothelial cells.…”

“…HRGP has been shown to interact with heparin and HS (35,36,39,40), a feature shared by many pro-and anti-angiogenic factors (for a review, see Ref. 9).…”

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

“…Therefore, Zn 2ϩ may act as a cofactor that simultaneously binds HRG and fibrin(ogen) in a coordinated fashion (39). Alternatively, Zn 2ϩ binding to HRG may induce conformational changes that facilitate its interaction with fibrin(ogen), a concept supported by the observation that Zn 2ϩ alters the conformation of a synthetic His-Pro-rich peptide (16 (15,40). These data point to a mechanism whereby Zn 2ϩ modulates the structure and function of HRG.…”