Histidine-rich glycoprotein (HRGP) is a multifunctional protein that is found in plasma and platelets. It specifically interacts with plasminogen, heparin, and thrombospondin, suggesting that it may be important in modulating fibrinolysis and regulating the anticoagulant activity of heparin. The overall objective of this project is to study the cell biology, biochemistry, and molecular biology of HRGP. Preliminary data have shown a new specific interaction of HRGP with fibrinogen and demonstrated specific incorporation of HRGP into fibrin clots. The functional role(s) of the HRGP-fibrinogen/fibrin interactions on fibrin polymerization as well as on the structure of the fibrin gel will be studied. Whether the HRGP-fibrinogen/fibrin interactions play a role in mediating HRGP binding to thrombin-activated platelets and mouse peritoneal macrophages will be investigated. The cell biology of these new HRGP interactions relating to heparin binding to cell surfaces and platelet-macrophage interactions will be studied. A detailed analysis of the structure-function relationships of HRGP is crucial for the further elucidation of the biology of this molecule and will be carried out by two approaches. The separate functional ligand-binding domains of HRGP will be characterized by isolation of HRGP fragments generated by enzymatic digestion and chemical fragmentation. A detailed map of the HRGP molecule will be established by generating anti-HRGP monoclonals with reactivity directed against specific ligand-binding domains of HRGP. HRGP cDNA will be isolated by screening a human liver cDNA library both immunochemically and by synthetic oligonucleotides to establish the primary sequence of HRGP and to examine the structure and regulation of expression of the HRGP gene(s). The cellular HRGP receptors and the HRGP-binding domains on the various ligands will be studied using monoclonal anti-idiotypic anti-receptor antibodies. These studies will provide important insights into the biology of HRGP and the molecular mechanisms of hemostasis and thrombosis.