Isolation and characterization of a lectin from the cortical granules of Xenopus laevis eggs

A cortical granule lectin was isolated from eggs of the South African clawed toad Xenopus laevis. The lectin was released from the cortical granules by activation of dejellied eggs with the Ca2+ ionophore A23187. The lectin was purified by affinity chromatography with its natural ligand, the egg jelly coat, chemically coupled to a Sepharose matrix. The purified lectin was homogeneous by the criteria of isoelectric focusing (pI = 4.6), immunodiffusion, and immunoelectrophoresis but existed in two different molecular weight isomers as determined by sedimentation velocity ultracentrifugation and disc gel electrophoresis. Molecular weights of the isomers were determined by ultracentrifugation, disc gel electrophoresis, and gel filtration and found to be 539,000 and 655,000. Chemically, the lectin was a metalloglycoprotein, composed of 84.0% protein, 15.8% carbohydrate, and 0.19% calcium. No unusual types or amounts of amino acids were present. The carbohydrate moiety was composed of fucose, mannose, galactose, glucosamine, galactosamine, and sialic acid. The monosaccharide specificity of the lectin was investigated with the sugar inhibition of the precipitin reaction in gels. The lectin was specific for D-galactosyl sugars with the configuration at carbon atoms 2-4 of primary importance.