PRELIMINARY X-RAY STUDY OF TETRACARPIDIUM CONOPHORUM AGGLUTININ II, AN ISOLECTIN FROM THE NIGERIAN WALNUT. Tracey Barrett, Kim Henrick, Guy Dodson, Theresa Animashaun, Colin Hughes, National Institute For Medical Research, Mill Hill, London, UK

Lectins form a group of structurally diverse proteins that bind to specific oligosaccharide sequences. They occur in almost all living organisms and despite having their roles well characterised in mammals are of largely unknown function in plants.

Two isolectins (TCAI and TCAII) have been isolated from seed extracts of the Nigerian walnut (Tetracarpidium conophorum). Both TCAI and TCAII are glycosylated and have the respective molecular weights of 70 and 30kda. TCAI exists as a disulphide linked dimer and TCAII as a monomer. Both isolectins have specificity for oligosaccharides with terminal galactose residues consistent with this lectin family which includes Ricin and Ricinus communis agglutinin.

Orthorhombic crystals of TCAII were obtained (space group P2_l2_l2₁ with cel dimensions a=65.7Å, b=86.3Å, c=118.3Å and two molecules in the assymetric unit) which diffracted beyond 2.4Å. It was possible to locate the position of both molecules in the unit cell using molecular replacement with a search model consisting of the Ricin B-chain. The overall fold of TCAII is very similar to that of Ricin where the molecule can be divided into two globular domains which are formed from a series of disulphide linked gamma loops (there is little significant secondary structure). TCAII was crystallised in the presence of lactose and it is possible to identify electron density for at least galactose in both sugar binding sites. The structure is currently undergoing refinement.

References:Animashaun, T., Togun, R.A. and Hughes, R.C., (1994) Glycoconjugate Journal, 11, 299-303.