Cloning and characterization of a monocot mannose-binding lectin from Crocus vernus (family Iridaceae)

The molecular structure and carbohydrate-binding activity of the lectin fro m bulbs of spring crocus (Crocus vernus) has been determined unambiguously using a combination of protein analysis and cDNA cloning. Molecular cloning revealed that the lectin called C. vernus agglutinin (CVA) is encoded by a precursor consisting of two tandemly arrayed lectin domains with a reasona ble sequence similarity to the monocot mannose-binding lectins. Post-transl ational cleavage of the precursor yields two equally sized polypeptides. Ma ture CVA consists of two pairs of polypeptides and hence is a heterotetrame ric protein. Surface plasmon resonance studies of the interaction of the cr ocus lectin with high mannose-type glycans showed that the lectin interacts specifically with exposed alpha-1,3-dimannosyl motifs. Molecular modelling studies confirmed further the close relationships in overall fold and thre e-dimensional structure of the mannose-binding sites of the crocus lectin a nd other monocot mannose-binding lectins. However, docking experiments indi cate that only one of the six putative mannose-binding sites of the CVA pro tomer is active. These results can explain the weak carbohydrate-binding ac tivity and low specific agglutination activity of the lectin. As the clonin g and characterization of the spring crocus lectin demonstrate that the mon ocot mannose-binding lectins occur also within the family Iridaceae a refin ed model of the molecular evolution of this lectin family is proposed.