The effect of arcelin-1 on the structure of the midgut of bruchid larvae and immunolocalization of the arcelin protein.

Some wild accessions of the common bean (Phaseolus vulgaris) contain a family of proteins called arcelins, that are toxic to the larvae of certain bruchid species. Among the six allelic variants of arcelin tested so far, arcelin-5 and arcelin-1 confer the highest level of resistance against the Mexican bean weevil, Zabrotes subfasciatus. The same proteins are not toxic to the bean weevil, Acanthoscelides obtectus, which is also a serious pest of cultivated beans. Arcelins belong to the bean lectin family that includes phytohemaggutinins and alpha-amylase inhibitors. Although homologous to lectins, arcelins are themselves only very weak lectins, and their binding properties have not been clearly established. The toxic properties of arcelins may be related to their recognition of and interaction with the glycoproteins and other constituents of the membranes along the digestive tract of insects. Since arcelin-1 was shown to have growth inhibitory effects for the larvae of Z. subfasciatus but not of A. obtectus, we examined the effect of an arcelin-1 containing diet on the structure of the cells that line the intestinal tract of the larvae of these two bruchid species, and used antibodies against arcelin to examine the distribution of arcelin within the cells and tissues. Here we show that dietary arcelin-1 caused an alteration of the gut structure and the penetration of arcelin into the haemolymph in Z. subfasciatus but not in A. obtectus. These results lead us to suggest that arcelins exert their toxic effect by severely damaging the epithelial cells.