Synthesis and Posttranslational Activation of Sulfhydryl-Endopeptidase in Cotyledons of Germinating Vigna mungo Seeds

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Abstract
A sulfhydryl-endopeptidase was purified as a 33 kilodalton (kD) mass polypeptide from cotyledons of Vigna mungo seedlings. Immunoblot analysis with antiserum made against the purified enzyme showed that the sulfhydryl-endopeptidase was synthesized only in the cotyledons during germination and that the amount of the enzyme increased until 4 days after imbibition and decreased thereafter. Next, an RNA fraction was prepared from cotyledons of 3 day old seedlings and translated in a wheat germ system. The synthesis of a 45 kD polypeptide was shown by the analysis of its translation products by immunoprecipitation with the antiserum to the endopeptidase and gel electrophoresis. When the RNA fraction was translated in the presence of canine microsomal membranes, a smaller polypeptide, having a 43 kD molecular mass, was detected as the translation product. When membrane-bound polysomes, but not free polysomes, prepared from cotyledons were used for translation in the wheat germ system, both the 43 and 45 kD polypeptides were synthesized. By incubation of a crude enzyme extract from cotyledons at 5 ± 1°C at neutral pH, the 43 kD polypeptide was sequentially cleaved to the 33 kD polypeptide via 39 and 36 kD intermediate polypeptides. The endopeptidase was activated simultaneously with the processing. Two-dimensional polyacrylamide gel electrophoresis showed that the 33 kD polypeptide was the fully activated form of the enzyme, whereas little or no activity was detected in other forms. From the present results, we postulate that the sulfhydryl-endopeptidase is first synthesized as the 45 kD precursor with a 2 kD signal peptide being cleaved, and that the 43 kD polypeptide is further cleaved to give the 33kD mature enzyme.