Abstract
Vigna radiata plant defensin 1 (VrD1) is the first reported plant defensin exhibiting insecticidal activity. We report herein the nuclear magnetic resonance solution structure of VrD1 and the implication on its insecticidal activity. The root-mean-square deviation values are 0.51 ± 0.35 and 1.23 ± 0.29 Å for backbone and all heavy atoms, respectively. The VrD1 structure comprises a triple-stranded antiparallel β-sheet, an α-helix, and a 3 10 helix stabilized by four disulfide bonds, forming a typical cysteine-stabilized αβ motif. Among plant defensins of known structure, VrD1 is the first to contain a 3 10 helix. Glu 26 is highly conserved among defensins; VrD1 contains an arginine at this position, which may induce a shift in the orientation of Trp 10 , thereby promoting the formation of this 3 10 helix. Moreover, VrD1 inhibits Tenebrio molitor α-amylase. α-Amylase has an essential role in the digestion of plant starch in the insect gut, and expression of the common bean α-amylase inhibitor 1 in transgenic pea imparts complete resistance against bruchids. These results imply that VrD1 insecticidal activity has its basis in the inhibition of a polysaccharide hydrolase. Sequence and structural comparisons between two groups of plant defensins having different specificity toward insect α-amylase reveal that the loop between β2 and β3 is the probable binding site for the α-amylase. Computational docking experiments were used to study VrD1-α-amylase interactions, and these results provide information that may be used to improve the insecticidal activity of VrD1. © 2006 Wiley-Liss, Inc.