High‐Affinity Ligand Binding to Subunit HI of the Asialoglycoprotein Receptor in the Absence of Subunit H2

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 K d 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 HI‐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 HI 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.