Purification and characterization of an extracellular lectin (Lectin I) from Agrobacterium radiobacter NCIM 2443

A lectin from culture filtrate of Agrobacterium radiobacter NCIM 2443 is purified to homogeneity by ion exchange chromatography on a DEAE cellulose column followed by hydrophobic chromatography on phenyl sepharose and hydroxyapatite column chromatography. The protein (Lectin I) is a monomer of relative molecular mass 37,000, as determined by denaturing gel electrophoresis as well as size exclusion chromatography. Lectin I is stable at pH 5.0 and its isoelectric point is pH 4.0. Amino acid analysis reveals that acidic amino acids and glycine are predominant amino acids and cysteine is absent in the lectin. Chemical modification of tryptophan residues causes more than 80% loss of haemagglutination activity of the lectin and 60% loss of activity is caused by modification of carboxyl groups. Lectin I agglutinates rabbit erythrocytes but does not agglutinate human A, B and O types of erythrocytes. It is specific for N-acetyl D-glucosamine, chitobiose, pNP-beta-mannoside as well as high mannose type glycopeptides. The relative inhibition by disaccharides, oligosaccharides and glycoproteins indicates that Lectin I recognizes Man3-GlcNAc-GlcNAc core carbohydrate structure of asparagine linked glycopeptides. Tobacco tissue extracts also inhibit the haemagglutination activity of Lectin I.