The middle domain of plasma histidine-proline-rich glycoprotein (HPRG) contains unusual tandem pentapeptide repeats (consensus G(H/P)(H/P)PH) and binds heparin and transition metals. Unlike other proteins that interact with heparin via lysine or arginine residues, HPRG relies exclusively on histidine residues for this interaction. To assess the consequences of this unusual requirement, we have studied the interaction between human plasma HPRG and immobilized glycosaminoglycans (GAGs) using resonant mirror biosensor techniques. HPRG binding to immobilized heparin was strikingly pH-sensitive, producing a titration curve with a midpoint at pH 6.8. There was little binding of HPRG to heparin at physiological pH in the absence of metals, but the interaction was promoted by nanomolar concentrations of free zinc and copper, and its pH dependence was shifted toward alkaline pH by zinc. The affinity of HPRG for various GAGs measured in a competition assay decreased in the following order: heparin > dermatan sulfate > heparan sulfate > chondroitin sulfate A. Binding of HPRG to immobilized dermatan sulfate had a midpoint at pH 6.5, was less influenced by zinc, and exhibited cooperativity. Importantly, plasminogen interacted specifically with GAG-bound HPRG. We propose that HPRG is a physiological pH sensor, interacting with negatively charged GAGs on cell surfaces only when it acquires a net positive charge by protonation and/or metal binding. This provides a mechanism to regulate the function of HPRG (the local pH) and rationalizes the role of its unique, conserved histidine-prolinerich domain. Thus, under conditions of local acidosis (e.g. ischemia or hypoxia), HPRG can co-immobilize plasminogen at the cell surface as well as compete for heparin with other proteins such as antithrombin.
HPRG1 is a relatively abundant plasma glycoprotein (125 g/ml or about 1.5 M) (1). Its two N-terminal cystatin modules identify it as a member of the cystatin superfamily, along with kininogen and fetuin (2). Based on its interactions with various ligands, HPRG has been proposed to be a regulator of coagulation, fibrinolysis, and the immune response and to play a role in zinc transport. In vitro HPRG binds components of the coagulation and fibrinolytic systems, including heparin (3), plasminogen (4), and fibrinogen (5); cells, viz. T-cells (6), macrophages (7), and platelets (8); and small ligands, such as transition metal ions and heme (9). No hypothesis of the function of HPRG has been able to rationalize these numerous interactions or explain how the activity of HPRG may be regulated in vivo.The most noteworthy feature of HPRG is a very high content of histidine, 13 mol %, concentrated in the central His-Pro-rich domain which contains a pentapeptide consensus sequence (GHHPH) repeated in tandem 12 times in human HPRG (10) and the sequences GHPPH and GPPPH repeated 18 times in the rabbit protein (11). Thus, the ionic charge of HPRG is exquisitely sensitive to pH in the range of pH 6 -7, and an electrophoretic titration curve of rabbit HP...