title: Sites within the 39-kDa protein important for regulating ligand binding to the low-density lipoprotein receptor-related protein.
abstract: A 39-kDa protein copurifies with the low-density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) and inhibits the binding and/or cellular uptake of ligands by this receptor. We recently utilized glutathione S-transferase (GST)-39-kDa fusion protein constructs to demonstrate that constructs encoding amino-terminal residues 1-114 and carboxy-terminal residues 115-319 of the 39-kDa protein independently bind to purified LRP and to LRP on hepatoma cells with similar affinities as the full-length GST-39-kDa protein (Kd approximately 8-10 nM). These regions, however, inhibit ligand binding to LRP differently: GST/1-114 inhibits both tissue-type plasminogen activator (t-PA) and alpha 2-macroglobulin-methylamine (alpha 2M*) binding whereas GST/115-319 only potently inhibits t-PA binding. Four domains, containing residues 18-24 and 100-107 within amino-terminal constructs and residues 200-225 and 311-319 within carboxy-terminal constructs, are required for inhibition of ligand binding. In the present study, we generated additional 39-kDa protein constructs to precisely define residues within each domain required for inhibition of t-PA and alpha 2M* binding to LRP. The potential importance of these residues in mediating direct binding both to purified LRP and to LRP on hepatoma cells was examined. Within amino-terminal residues 1-114, alanine 103 and leucine 104 are required for inhibition of t-PA and alpha 2M* binding. These residues, however, are not required for binding either to purified LRP or to LRP on hepatoma cells. Within domain 18-24, arginine 21 is required for inhibition of t-PA and alpha 2M* binding as well as for the direct binding of amino-terminal constructs to LRP. Within carboxy-terminal domains 200-225 and 311-319, leucine 222 and leucine 319 are both required for inhibition of t-PA binding. Deletion of leucine 319 changes the ligand specificity from inhibition of t-PA binding to inhibition of alpha 2M* binding. Thus, leucine 319 is not required for direct binding to LRP whereas leucine 222 is required for high-affinity binding to LRP.
authors.affiliation: Edward Mallinckrodt Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: I
authors.initials: I
authors.lastname: Warshawsky
authors.forename: G
authors.initials: G
authors.lastname: Bu
authors.forename: A L
authors.initials: AL
authors.lastname: Schwartz
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D001665:Binding Sites; D003001:Cloning, Molecular; D017931:DNA Primers / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008024:Ligands; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D008969:Molecular Sequence Data; D008970:Molecular Weight; D011485:Protein Binding; D011971:Receptors, Immunologic / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011973:Receptors, LDL / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017384:Sequence Deletion; D010959:Tissue Plasminogen Activator / Q000378:metabolism; D014407:Tumor Cells, Cultured / Q000378:metabolism; D000511:alpha-Macroglobulins / Q000378:metabolism
chemical_list: D017931:DNA Primers; D008024:Ligands; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D011971:Receptors, Immunologic; D011973:Receptors, LDL; D011993:Recombinant Fusion Proteins; D000511:alpha-Macroglobulins; D010959:Tissue Plasminogen Activator