Characterization of a soybean beta-conglycinin-degrading protease cleavage site

Protease C1, an enzyme from soybean (Glycine max [L.] Merrill cv Amsoy 71) seedling cotyledons, was previously determined to be the enzyme responsible for the initial degradation of the alpha' and alpha subunits, but not the beta subunit, of beta-conglycinin storage protein. The sizes of the proteolytic products generated by the action of protease C1 suggest that the cleavage sites on the alpha' and alpha subunits of beta-conglycinin may be located in their N-terminal domain, which is not found in the beta subunit of beta-conglycinin. To check this hypothesis, storage proteins from other plant species that are homologous to either the alpha'/alpha or the beta subunit of beta-conglycinin were tested as substrates. As expected, the convicilin from pea (Pisum sativum), a protein homologous to the alpha' and alpha subunits of beta-conglycinin, was digested by protease C1. The vicilins from pea as well as vicilins from adzuki bean (Vigna angularis), garden bean (Phaseolus vulgaris), black-eyed pea (Vigna unguiculata), and mung bean (Vigna radiata), storage proteins that are homologous to the beta subunit of soybean beta-conglycinin, were not degraded by protease C1. Degradation of soybean beta-conglycinin involves a sequential attack of the alpha subunit at multiple sites, culminating in the formation of a stable intermediate of 53.5 kD and a final product of 48.0 kD. The cleavage sites resulting in this formation of the intermediates and final product were determined by N-terminal analysis. These were compared to the known amino acid sequences of the three beta-conglycinin subunits. Results showed these two polypeptides to be generated by proteolysis of the alpha subunit at regions bearing long strings of acidic amino acid residues.