We reported that the 3'-untranslated region (3'-UTR) of cathepsin B mRNA of Sarcophaga peregrina (flesh fly) is necessary for the repression of its translation, and detected the 3'-UTR-binding protein in lysates of larval hemocytes in which cathepsin B mRNA translation was repressed [Yano, T., Kurata, S. and Natori, S. (1995) Eur. J. Biochem. 234, 39-43]. In this study, we purified the 3'-UTR-binding protein from an embryonic cell line of Sarcophaga. The purified protein (CBBP) was found to repress cathepsin B mRNA translation in a rabbit reticulocyte lysate. We found that the CBBP contents of the hemocytes did not change during metamorphosis, although the cathepsin B mRNA became translatable only at the pupal stage. Moreover, we found that pupal, but not larval hemocytes, contained a factor that inhibited the binding of CBBP to the 3'-UTR. A regulatory mechanism of cathepsin B expression in Sarcophaga hemocytes is discussed.
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