Characterization of the Major Protease Involved in the Soybean beta-Conglycinin Storage Protein Mobilization

Protease C1, the protease responsible for the initial degradation of the alpha' and alpha subunits of the soybean beta-conglycinin storage protein (Glycine max [L.] Merrill), has been purified. The enzyme was found by sodium dodecyl sulfate-polyacrylamide gel electrophoresis to have a molecular weight of 70,000 and a pH optimum of 3.5 to 4.5. Susceptibility to protease inhibitors indicates that protease C1 is a serine protease. Study of the proteolytic intermediates generated suggests that the cleavage of the alpha' and alpha subunits of beta-conglycinin by protease C1 results in intermediates that are 1 or 2 kilodaltons smaller than the native alpha' and alpha subunits. Following that, a succession of intermediates exhibiting molecular masses of 70.0 and 58.0 kilodaltons, then 63.0, 61.0, 55.0, and 53.5 kilodaltons, are observed. A 50.0- and a 48.0- kilodalton intermediate are the final products of protease C1 action. Comparison of these intermediates with the prominent anti-beta-conglycinin cross-reacting bands that increase during the first few days of germination and early growth show that protease C1 plays an important physiological role, but not an exclusive one, in the living plant.