In vitro translation of the protease catalyzing Bombyx mori egg-specific protein and identification of a nascent peptide with biological activity.

Abstract A yolk protein, egg-specific protein (ESP), of Bombyx mori is sequentially degraded by the ESP-specific protease which appears at the later stages of embryogenesis. In order to find the biological origin of this protease, an in vitro translation was done on RNAs prepared throughout embryogenesis using a rabbit reticulocyte lysate. Among several peptides translated, a 26-kDa peptide was selectively precipitated by the ESP protease antiserum. The mRNA activity increased slowly and then abruptly, reaching the maximum level on Day 8 of embryogenesis. By cotranslation with dog pancreatic microsomal membranes, the 26-kDa peptide was converted to a 24.5-kDa peptide, suggesting the cleavage of a signal peptide of 1.5 kDa. The direct incubation of the translation mixture with ESP failed to hydrolyze ESP, whereas the immunoprecipitate of the primary translation products clearly hydrolyzed ESP into the same peptides as were given by the authentic ESP protease. These results indicate that the protease becomes biologically active before chemical maturation.