Purification of an endopeptidase involved with storage-protein degradation in Phaseolus vulgaris L. cotyledons

Abstract

Phaseolin, the major seed storage protein of Phaseolus vulgaris L., is degraded in the cotyledons in the first 7–10 d following seed germination. We assayed cotyledon extracts for protease activity by using [³H]phaseolin as a substrate and then fractionated the digestion mixtures by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in order to identify the cleavage products. The cotyledons of 4-d-old seedlings contain an endopeptidase which cleaves the polypeptides of [³H]phaseolin (apparent molecular weights=51 000, 48 000, 46 000 and 43 000) into three discrete clusters of proteolytic fragments (M_rs=27 000, 25 000 and 23 000). Endopeptidase activity is not detected in the cotyledons until the protein content of these organs starts to decline, shortly after the first day of seedling growth. Endopeptidase activity increases to a maximum level in the cotyledons of 5-d-old seedlings and then declines to a minimum value by day 10. The enzyme was purified 335-fold by ammonium-sulfate precipitation, organomercurial-agarose chromatography, gel filtration and ion-exchange chromatography. The endopeptidase constitutes 0.3% of the protein content in the cotyledons of 4-d-old seedlings. It is a cysteine protease with a single polypeptide chain (M_r=30 000). Optimum hydrolysis of [³H]phaseolin occurs at pH 5. The enzyme is irreversibly inactivated at pH values above 7 and at temperatures above 45° C. The endopeptidase attacks only a limited number of peptide bonds in [³H]phaseolin, without causing any appreciable change in the native molecular weight of the storage protein. The endopeptidase is also able to hydrolyze the bean-seed lectin, phytohemagglutinin. Thus, this enzyme may play a general role in degrading cotyledon proteins of P. vulgaris following seed germination.