STRUCTURAL STUDIES ON JACALIN Ð CARBOHYDRATE COMPLEXES 

 

K. Sekar,a  A. A. Jeyaprakash,b  P. G. Rani,b  S. Katiyar,b A. Surolia,b and M. Vijayanb

 

aBioinformatics Centre and Supercomputer Education and Research Centre;  bMolecular Biophysics Unit, Indian Institute of Science, Bangalore 560 012, India (sekar@physics.iisc.ernet.in)

 

 

Jacalin is a 66kDa tetrameric lectin from the seeds of jackfruit (Artocarpus integrifolia). Each subunit consists of two chains, a long a chain and a short b chain, produced by post-translational proteolysis. The structure of the lectin-bound to Me-a-galactose was earlier solved in this laboratory. The structure revealed a novel lectin fold and the use of post-translational modification as a strategy for generating specificity for galactose at the primary binding site. At the disaccharide level, it is specific to tumour-related Galb1-3GalNAc, known as T-antigen. We have now prepared and solved the structures of the complexes of the lectin with Gal, Me-a-GalNAc, T-antigen, Me-a-T-antigen and GalNAcb1-3Gal-O-Me.  In the complexes with T-antigen and  Me-a-T-antigen, the primary site is occupied  by the GalNac residue.  The interactions of the Gal residue with the protein are confined to water bridges.  The methyl group in Me-a-T-antigen has favourable interaction with an aromatic residue, accounting for its increased affinity to the lectin.  In the complex with GalNAcb1-3Gal-O-Me, however, the primary site is occupied by the Gal residue presumably on account of the favourable interaction of the methyl group with the residue.  The structures of the complexes provide a reasonable explanation for the available thermodynamic data of jacalin-carbohydrate interactions.  The details will be presented.