Structural and functional studies on BEL beta-trefoil, a novel lectin from king bolete (Boletus edulis) mushrooms

This thesis deals with the purification and the structural characterization of a novel lectin, completely different from the formerly described members of the saline soluble family of mushroom specific lectins. The lectin was called BEL (Boletus edulis lectin) beta-trefoil and it was purified from the fruiting bodies of king bolete mushrooms, commonly known as porcini. BEL beta-trefoil was structurally characterized: its amino acid sequence and three dimensional structure were determined. The new protein is a homodimer and each protomer folds as a beta-trefoil domain as indicated by the name itself. The lectin has potent anti-proliferative effects on human cancer cells which confers to it an interesting therapeutic potential as an antineoplastic agent. Several crystal forms of the apoprotein and of complexes with different carbohydrates were studied by X-ray diffraction. The structure of the apoprotein was solved at 1.12 Å resolution. The interaction of the lectin with lactose, galactose, N-acetylgalactosamine, the T-antigen disaccharide (Gal beta1-3GalNAc) and the T-Antigen (Ser-Gal beta1-3GalNAc) was examined in detail. All the three potential binding sites present in the beta-trefoil fold are occupied in at least one crystal form. No important conformational changes are observed in the lectin when comparing the structure of the complexes with carbohydrates and those of the ligand free protein.