Interaction of histidine-proline-rich glycoprotein with plasminogen: effect of ligands, pH, ionic strength, and chemical modification.

The association of plasma histidine-proline-rich glycoprotein (HPRG) with plasminogen (PLG) was examined using a sucrose density gradient assay in order to evaluate the effects of several relevant conditions on complex formation. Addition of PLG shifts the S-value of 125I-labeled HPRG from 4.8S to 6.8S, providing the first direct evidence that HPRG associates with the zymogen form of plasmin in solution. Complex formation is not sensitive to pH in the range of pH 6.5-8.5, but is abolished at high ionic strength (1 M NaCl). No species differences were found, as either rabbit or human HPRG bound readily to rabbit or human PLG. Of the ligands of HPRG tested, mesoheme (20 microM) but not heparin (M(r) 10,000, 10 microM) inhibits the formation of the HPRG-PLG complex. Modification of lysine residues of HPRG or competitive binding by lysine and anti-fibrinolytic agents containing primary amino groups also inhibits association. Conversely, modification of arginine or histidine residues of HPRG has no effect on complex formation. These results indicate that HPRG has independent binding sites for heparin and PLG and confirm that one or more lysine residues of HPRG are involved in its recognition by PLG. The protein-protein interaction was also quantitatively characterized at thermodynamic equilibrium by analytical ultracentrifugation. The stoichiometry and dissociation constant (KD) of the complex were determined from the equilibrium distribution of fluorescein-isothiocyanate-labeled PLG in the presence of HPRG. The experimental data were analyzed by nonlinear least-squares curve fitting and indicated that a heterodimer is formed.(ABSTRACT TRUNCATED AT 250 WORDS)