A NOVEL MODE OF CARBOHYDRATE-RECOGNITION IN JACALIN, A MORACEAE PLANTLECTIN WITH A BETA-PRISM FOLD

Jacalin, a tetrameric two-chain lectin (66,000 M(r)) from jackfruit se eds, is highly specific for the tumour associated T-antigenic disaccha ride. The crystal structure of jacalin with methyl-alpha-D-galactose r eveals that each subunit has a three-fold symmetric beta-prism fold ma de up of three four-stranded beta-sheets. The lectin exhibits a novel carbohydrate-binding site involving the N terminus of the alpha-chain which is generated through a post-translational modification involving proteolysis, the first known instance where such a modification has b een used to confer carbohydrate specificity. This new lectin fold may be characteristic of the Moraceae plant family. The structure provides an explanation for the relative affinities of the lectin for galactos e derivatives and provides insights into the structural basis of its T -antigen specificity.