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

Mass spectrometric methods were used to investigate the proteolytic process ing and glycopeptide structures of three seed defensive proteins from Phase olus vulgaris, The proteins were the a-amylase inhibitors alpha AI-1 and al pha AI-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 A sn residue in alpha AI-1, and in all three proteins, seven or more residues were clipped from the C-termini, in the manner of the seed lectins, In mos t instances, individual glycoforms could be assigned at each Asn site, due to the unique masses of the plant glycopeptides, It was found that alpha AI -1 and alpha AI-2 differed significantly in their glycosylation patterns, d espite their high sequence homology. These data complement the previous X-r ay studies of the alpha(1)-amylase inhibitor and arcelin, where many of the C-terminal residues and glycopeptide residues could not be observed. (C) 1 999 Federation of European Biochemical Societies.