title: High-affinity ligand binding to subunit H1 of the asialoglycoprotein receptor in the absence of subunit H2.
abstract: The hepatic asialoglycoprotein receptor is a hetero-oligomer composed of two homologous subunits. The specificity and affinity of ligand binding depends on the number and spatial arrangement of several galactose-binding sites within the receptor complex. Previous studies indicated that both subunits are required for high-affinity ligand binding, i.e. for the simultaneous interaction with three galactose residues within an N-linked glycan. However, we found that asialoorosomucoid (ASOR) and asialofetuin (ASF) bind to transfected COS-7 cells expressing subunit H1 in the absence of the second subunit H2. ASOR binding occurred with a dissociation constant of approximately 40 nM, approximately four-times higher than the Kd of ASOR binding to the hetero-oligomeric receptor. Normalized to the amount of H1 expressed, approximately 10-times fewer binding sites were produced by H1 alone. A glycopeptide with a single tri-antennary N-linked glycan purified from ASF bound to the hetero-oligomeric receptor, but did not bind detectably to H1-expressing COS-7 cells. H1 is thus unable to simultaneously recognize all three galactose residues in a glycan. From this, we conclude that, at a sufficiently high density of H1 on the cell surface, high-affinity binding of ASOR and ASF is the result of two or more glycans interacting with H1 oligomers with low affinity in a bivalent manner.
authors.affiliation: Department of Biochemistry, Biozentrum, University of Basel, Switzerland.
authors.forename: M D
authors.initials: MD
authors.lastname: Bider
authors.forename: R
authors.initials: R
authors.lastname: Cescato
authors.forename: P
authors.initials: P
authors.lastname: Jenö
authors.forename: M
authors.initials: M
authors.lastname: Spiess
mesh_terms: D016475:3T3 Cells; D000595:Amino Acid Sequence; D000818:Animals; D037263:Asialoglycoprotein Receptor; D001212:Asialoglycoproteins / Q000032:analysis; D002478:Cells, Cultured; D060748:Fetuins; D008024:Ligands; D051379:Mice; D008969:Molecular Sequence Data; D011956:Receptors, Cell Surface / Q000378:metabolism*; D014162:Transfection; D000509:alpha-Fetoproteins / Q000032:analysis
chemical_list: D037263:Asialoglycoprotein Receptor; D001212:Asialoglycoproteins; D060748:Fetuins; D008024:Ligands; D011956:Receptors, Cell Surface; D000509:alpha-Fetoproteins; C017029:asialofetuin