title: Characterization of the receptor for protease nexin-I:protease complexes on human fibroblasts.
abstract: Fibroblasts as well as several other cell types, secrete a number of protease inhibitors into their culture media. Among these inhibitors are the protease nexins, a class of proteins which covalently bind serine proteases, thereby inactivating their specific targets. Protease nexin-I, first discovered in human foreskin fibroblasts, binds thrombin, plasmin, and urokinase with high affinity, forming covalently linked complexes. Human fibroblasts bind complexes of protease nexin-I and its target protease via a cell-surface, high-affinity receptor. We have analyzed a number of characteristics of this receptor, and found them to be typical of class II receptors in general. At 4 degrees C binding of PN-I:protease complexes was competed by heparin. In addition, binding was independent of the particular protease bound to the PN-I; purified complexes of PN-I with thrombin or urokinase competed equipotently for [125]I-thrombin:PN-I binding. As the pH of the binding buffer was lowered, binding to cells increased. A twofold increase in binding was attained by lowering the pH from 7.5 to 4.5. This phenomenon was not due to irreversible, pH-induced changes to either the cell surface or the labeled complexes. At 37 degrees C, the removal of labeled complexes from culture medium was rapid; approximately 80% was removed by 4 hours under given conditions. The internalization of complexes was also very rapid, with an estimated ke (endocytic rate constant) of 1.0 min-1. At neutral pH, fibroblasts bind complexes in a saturable manner. Scatchard analysis yields a receptor number of 250,000 per cell and a Kd of 1 nM.
mesh_terms: D016564:Amyloid beta-Protein Precursor; D001667:Binding, Competitive; D002352:Carrier Proteins / Q000378:metabolism*; D002413:Cations, Divalent / Q000494:pharmacology; D004705:Endocytosis; D005347:Fibroblasts / Q000378:metabolism*; D006493:Heparin / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010447:Peptide Hydrolases / Q000378:metabolism; D058255:Protease Nexins; D011956:Receptors, Cell Surface / Q000378:metabolism*; D013997:Time Factors
chemical_list: D016564:Amyloid beta-Protein Precursor; D002352:Carrier Proteins; D002413:Cations, Divalent; D058255:Protease Nexins; D011956:Receptors, Cell Surface; C052677:protease nexin-protease complex receptor; D006493:Heparin; D010447:Peptide Hydrolases