SELECTIVE SUGAR BINDING TO THE CARBOHYDRATE-RECOGNITION DOMAINS OF THE RAT HEPATIC AND MACROPHAGE ASIALOGLYCOPROTEIN RECEPTORS

Asialoglycoprotein receptors on the surfaces of both hepatocytes and p eritoneal macrophages bind terminal galactose residues of desialylated glycoproteins and mediate endocytosis and eventual degradation of the se ligands. The hepatic receptor binds oligosaccharides with terminal N-acetylgalactosamine residues more tightly than ligands with terminal galactose residues, but the macrophage receptor shows no such differe ntial binding affinity. Carbohydrate recognition domains from the macr ophage receptor and the major subunit of the hepatic receptor have bee n expressed in a bacterial system and have been shown to retain the di stinct binding selectivities of the receptors from which they derive. Binding of a series of N-acyl derivatives of galactosamine suggests th at the a-substituent of these sugars interacts with the surface of the hepatic receptor with highest affinity binding observed for the N-pro pionyl derivative. Chimeric sugar-binding domains have been used to id entify three regions of the hepatic receptor that are essential for es tablishing selectivity for N-acetylgalactosamine over galactose, Based on these results and the orientation of N-acetylgalactosamine when bo und to an homologous galactose-binding mutant of rat serum mannose-bin ding protein, a fourth region likely to interact with N-acetylgalactos amine has been identified and probed by site-directed mutagenesis. The results of these studies define a binding pocket for the 2-substituen t of N-acetylgalactosamine in the hepatic asialoglycoprotein receptor.