Protective mechanism of the Mexican bean weevil against high levels of alpha-amylase inhibitor in the common bean

alpha-Amylase inhibitor (alphaAI) protects seeds of the common bean (Phaseolus vulgaris) against predation by certain species of bruchids such as the cowpea weevil (Callosobruchus maculatus) and the azuki bean weevil (Callosobruchus chinensis), but not against predation by the bean weevil (Acanthoscelides obtectus) or the Mexican bean weevil (Zabrotes subfasciatus), insects that are common in the Americas. We characterized the interaction of alphaAI-1 present in seeds of the common bean, of a different isoform, alphaAI-2, present in seeds of wild common bean accessions, and of two homologs, alphaAI-Pa present in seeds of the tepary bean (Phaseolus acutifolius) and alphaAI-Pc in seeds of the scarlet runner bean (Phaseolus coccineus), with the midgut extracts of several bruchids. The extract of the Z. subfasciatus larvae rapidly digests and inactivates alphaAI-1 and alphaAI-Pc, but not alphaAI-2 or alphaAI-Pa. The digestion is caused by a serine protease. A single proteolytic cleavage in the beta subunit of alphaAI-1 occurs at the active site of the protein. When degradation is prevented, alphaAI-1 and alphaAI-Pc do not inhibit the alpha-amylase of Z. subfasciatus, although they are effective against the alpha-amylase of C. chinensis. alphaAI-2 and alphaAI-Pa, on the other hand, do inhibit the alpha-amylase of Z. subfasciatus, suggesting that they are good candidates for genetic engineering to achieve resistance to Z. subfasciatus