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

The molecular structure and carbohydrate-binding activity of the lectin from 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 reasonable sequence similarity to the monocot mannose-binding lectins. Post-translational cleavage of the precursor yields two equally sized polypeptides. Mature CVA consists of two pairs of polypeptides and hence is a heterotetrameric protein. Surface plasmon resonance studies of the interaction of the crocus 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 three-dimensional structure of the mannose-binding sites of the crocus lectin and other monocot mannose-binding lectins. However, docking experiments indicate that only one of the six putative mannose-binding sites of the CVA protomer is active. These results can explain the weak carbohydrate-binding activity and low specific agglutination activity of the lectin. As the cloning and characterization of the spring crocus lectin demonstrate that the monocot mannose-binding lectins occur also within the family Iridaceae a refined model of the molecular evolution of this lectin family is proposed.