ARCELIN AND ALPHA-AMYLASE INHIBITOR FROM THE SEEDS OF COMMON BEAN (PHASEOLUS-VULGARIS L) ARE TRUNCATED LECTINS

Lectin (PHA) and lectin-like proteins (arcelin, alpha-amylase inhibitor) are insecticidal proteins present in common bean (Phaseolus vulgaris) seeds. Due to their high degree of homology with other Leguminosae lectins, they could be modeled from the coordinates of other well known Leguminosae lectins. All the three-dimensional models are very similar but show that arcelin and alpha-amylase inhibitor correspond to truncated lectins. Deletions of one (arcelin) or two (alpha-amylase inhibitor) loops located in the region of the monosaccharide-binding site are responsible for this alteration. As a result, arcelin exhibits a weak agglutinating activity while alpha-amylase inhibitor cannot agglutinate erythrocytes. These lectin-like proteins are thus suspected to act against insect predators by other mechanisms than those implying the monosaccharide-binding site of lectins. These models also confirm that lectins and lectin-like proteins from common bean are closely evolutionary related.