Post-translational processing of two α-amylase inhibitors and an arcelin from the common bean, Phaseolus vulgaris

Abstract

Mass spectrometric methods were used to investigate the proteolytic processing and glycopeptide structures of three seed defensive proteins from Phaseolus vulgaris. The proteins were the α-amylase inhibitors αAI-1 and αAI-2 and arcelin-5, all of which are related to the seed lectins, PHA-E and PHA-L. The mass data showed that the proteolytic cleavage required for activation of the amylase inhibitors is followed by loss of the terminal Asn residue in αAI-1, and in all three proteins, seven or more residues were clipped from the C-termini, in the manner of the seed lectins. In most instances, individual glycoforms could be assigned at each Asn site, due to the unique masses of the plant glycopeptides. It was found that αAI-1 and αAI-2 differed significantly in their glycosylation patterns, despite their high sequence homology. These data complement the previous X-ray studies of the α 1-amylase inhibitor and arcelin, where many of the C-terminal residues and glycopeptide residues could not be observed.