Isolation, characterization, molecular cloning and molecular modelling of two lectins of different specificities from bluebell (Scilla campanulata) bulbs

Two lectins have been isolated from bluebell (Scilla campanulata) bulbs. From their isolation by affinity chromatography, they are characterized as a mannose-binding lectin (SCAman) and a fetuin-binding lectin (SCAfet). SCAman preferentially binds oligosaccharides with α(1,3)- and α(1,6)-linked mannopyranosides. It is a tetramer of four identical protomers of approx. 13 kDa containing 119 amino acid residues; it is not glycosylated. The fetuin-binding lectin (SCAfet), which is not inhibited by any simple sugars, is also unglycosylated. It is a tetramer of four identical subunits of approx. 28 kDa containing 244 residues. Each 28 kDa subunit is composed of two 14 kDa domains. Both lectins have been cloned from a cDNA library and sequenced. X-ray crystallographic analysis and molecular modelling studies have demonstrated close relationships in sequence and structure between these lectins and other monocot mannose-binding lectins. A refined model of the molecular evolution of the monocot mannose-binding lectins is proposed.