Characterization of alpha-Amylase-Inhibitor, a Lectin-Like Protein in the Seeds of Phaseolus vulgaris.

The common bean, Phaseolus vulgaris, contains a glycoprotein that inhibits the activity of mammalian and insect alpha-amylases, but not of plant alpha-amylases. It is therefore classified as an antifeedant or seed defense protein. In P. vulgaris cv Greensleeves, alpha-amylase inhibitor (alphaAl) is present in embryonic axes and cotyledons, but not in other organs of the plant. The protein is synthesized during the same time period that phaseolin and phytohemagglutinin are made and also accumulates in the protein storage vacuoles (protein bodies). Purified alphaAl can be resolved by SDS-PAGE into five bands (M(r) 15,000-19,000), four of which have covalently attached glycans. These bands represent glycoforms of two different polypeptides. All the glycoforms have complex glycans that are resistant to removal by endoglycosidase H, indicating transport of the protein through the Golgi apparatus. The two different polypeptides correspond to the N-terminal and C-terminal halves of a lectin-like protein encoded by an already identified gene or a gene closely related to it (LM Hoffman [1984] J Mol Appl Genet 2: 447-453; J Moreno, MJ Chrispeels [1989] Proc Natl Acad Sci USA 86:7885-7889). The primary translation product of alphaAl is a polypeptide of M(r) 28,000. Immunologically cross-reacting glycopolypeptides of M(r) 30,000 to 35,000 are present in the endoplasmic reticulum, while the smaller polypeptides (M(r) 15,000-19,000) accumulate in protein storage vacuoles (protein bodies). Together these data indicate that alphaAl is a typical bean lectin-type protein that is synthesized on the rough endoplasmlc reticulum, modified in the Golgi, and transported to the protein storage vacuoles.