MECHANISM OF N-ACETYLGALACTOSAMINE BINDING TO A C-TYPE ANIMAL LECTIN CARBOHYDRATE-RECOGNITION DOMAIN

The mammalian hepatic asialoglycoprotein receptor, a member of the C-t ype animal lectin family, displays preferential binding to N-acetylgal actosamine compared with galactose. The structural basis for selective binding to N-acetylgalactosamine has been investigated. Regions of th e carbohydrate-recognition domain of the receptor believed to be impor tant in preferential binding to N-acetylgalactosamine have been insert ed into the homologous carbohydrate-recognition domain of a mannose-bi nding protein mutant that was previously altered to bind galactose, In troduction of a single histidine residue corresponding to residue 256 of the hepatic asialoglycoprotein receptor was found to cause a 14-fol d increase in the relative affinity for N-acetylgalactosamine compared with galactose, The relative ability of various acyl derivatives of g alactosamine 60 compete for binding to this modified carbohydrate-reco gnition domain suggest that it is a good model for the natural N-acety lgalactosamine binding site of the asialoglycoprotein receptor. Crysta llographic analysis of this mutant carbohydrate-recognition domain in complex with N-acetylgalactosamine reveals a direct interaction betwee n the inserted histidine residue and the methyl group of the N-acetyl substituent of the sugar. Evidence for the role of the side chain at p osition 208 of the receptor in positioning this key histidine residue was obtained from structural analysis and mutagenesis experiments. The corresponding serine residue in the modified carbohydrate-recognition domain of mannose-binding protein forms a hydrogen bond to the imidaz ole side chain. When this serine residue is changed to valine, loss in selectivity for N-acetylgalactosamine is observed, The structure of t his mutant reveals that the beta-branched valine side chain interacts directly with the histidine side chain, resulting in an altered imidaz ole ring orientation.