SELECTIVE BINDING OF N-ACETYLGLUCOSAMINE TO THE CHICKEN HEPATIC LECTIN

Among Ca2+-dependent (C-type) animal lectins, the chicken hepatic lect in (CHL) is unique in displaying almost complete selectivity for N-ace tylglucosamine over other monosaccharide ligands. The crystal structur es of the carbohydrate-recognition domain (CRD) from serum mannose-bin ding protein (MBP) and of a complex between the CRD from liver MBP and the methyl glycoside of N-acetylglucosamine were used to model the bi nding site in CHL. Substitution of portions of CHL into the MBP framew ork did not substantially increase selectivity. A bacterial expression system for the CRD of CHL was developed so that specific residues pre dicted to be near the 2-acetamido substituent of N-acetylglucosamine c ould be altered by site-directed mutagenesis. The results indicate tha t the ligand is bound to CHL in the same orientation as it binds to li ver MBP. A tyrosine and a valine residue that probably contact the the N-acetyl group have been identified. These results, together with stu dies of ligand-binding selectivity, suggest that these residues form p art of a binding pocket for the N-acetyl group, which confers selectiv e binding of N-acetylglucosamine.