The asialoglycoprotein receptor of mammalian hepatocytes is a well- studied model system for carbohydrate-protein interactions, receptor mediated endocytosis, and glycoprotein trafficking. Although it binds many types of galactose containing molecules, it now appears that the optimal oligosaccharide ligand for the receptor is a single-type of N- linked triantennary oligosaccharide. This is surmised from binding studies and photoaffinity labeling experiments which demonstrate that the three galactose residues of triantennary bind stereospecifically to the major and minor subunits of the rat receptor. Based on these and other results, the active receptor is proposed to be a hetero-oligomer of subunits. Refinement of this model requires detailed information of the number and location of galactose binding sites on the subunits. The polypeptide sequence of each subunit from both rat and human are known, but the region(s) of the polypeptide which form active sites for galactose have not been elucidated. The solution conformation of the three antenna of oligosaccharide ligand are related to the spacial arrangement of galactose binding site on the receptor. Triantennary contains two semi-flexible and one completely rigid antenna which leads to the question of how antenna flexibility functions to position the galactose residues for stereospecific binding by the receptor? To address these questions we propose to utilize modified triantennary ligands to probe both the rat and human asialoglycoprotein receptor. The stoichiometry of subunits which comprise a triantennary binding site will be determined by photoaffinity labeling cross-linking experiments. Peptides will be derived from the active sites of the major and minor subunits and sequenced to identify the number and location of individual galactose binding sites within each subunit. The solution conformation of the triantennary oligosaccharide while bound to the receptor will be determined using fluorescence energy transfer. These experiments will lead to a further understanding of the molecular structure of the asialoglycoprotein receptor and may explain the role of oligosaccharide flexibility involved in carbohydrate-protein interactions.
