The complete primary structure of rabbit plasma histidine-proline-rich glycoprotein (HPRG), also known as histidine-rich glycoprotein, was determined by a combination of cDNA and peptide sequencing. Limited proteolysis with plasmin yielded three disulfide-linked fragments that were further purified. Reduction of the disulfide bonds with dithiothreitol under nondenaturing conditions releases the central, histidine-proline-rich domain, which contains 15 tandem repeats of the pentapeptide [H/P]-[H/P]PHG. The N-terminal fragment (295 amino acids), consisting of two cystatin-like modules, is bound to the proline-rich C-terminal fragment (105 amino acids) via a buried disulfide bond whose reduction requires prior denaturation. Far-UV circular dichroism spectra revealed beta-sheet with some alpha-helix, polyproline-II helix, and random coil in the secondary structure of the N-terminal, central, and C-terminal domains, respectively. The modular architecture of HPRG suggests that it may have several independent binding sites and that its biological role may be to bring two or more ligands together. The histidine-proline-rich domain, which contains 34 of the 53 histidine residues of HPRG, binds heparin and has an isoelectric point of 7.15 and a relatively high apparent pKa (7.0) of its histidine residues, and thus it probably mediates the interaction between HPRG and heparin, which is strikingly sensitive to pH in the range 7.0-7.4 [Peterson et al. (1987) J. Biol. Chem. 262, 7567-7574]. Solvent perturbation and second-derivative UV spectroscopy of HPRG revealed changes in the environment of tryptophan residues upon lowering the pH. This transition had a midpoint at pH 6.0 and required the disulfide bond bridging the histidine-proline-rich domain to the N/C fragment. The data are consistent with the mutual repulsion of protonated histidine residues in the histidine-proline-rich region causing a conformational change transmitted to the rest of the molecule via the disulfide bond.