Translation and processing of egg-specific protein of the silkworm, Bombyx mori

Abstract Synthesis of two egg-specific protein (ESP) subunits of the silkworm, Bombyx mori , ESP72 and ESP64 with approximate molecular masses of 72 kDa and 64 kDa, respectively, was analyzed by means of in vivo labeling and in vitro translation systems. In vitro translation in a rabbit reticulocyte lysate showed that among more than 30 polypeptides translated through mRNA from developing ovaries, a polypeptide with a molecular mass of 69 kDa (p69) changed strikingly during the ovarian development. The p69 was immunoprecipitated with the anti-ESP serum, indicating the p69 as being related to ESP. In vitro translation of the ovarian RNA in the presence of dog pancreas microsomal membranes yielded two additional polypeptides of 67 kDa (p67) and 72 kDa (p72) with a concomitant decrease in p69. The p67 and p72 interacted with the anti-ESP serum, indicating that these polypeptides were processed from p69. In vivo labeling experiments showed that during short (less than 24 h) labeling periods, ESP72 was preferentially labelled with [ 35 S]methionine, and that radioactivity in ESP64 increased with increasing labeling times after a lag period of about 24 h. A pulse-chase experiment suggested that ESP64 was derived from ESP72. Moreover, peptide mapping by limited proteolysis demonstrated that there was an amino-acid sequence homology between ESP72 and ESP64. These results indicate that p69 is the common primary gene product for two ESP subunits, which is being processed to ESP72 and ESP64 via p67. It has also been suggested that the mechanism for the conversion from p69 to p67 is a cleavage of signal peptide and that from p67 to ESP72 involves glycosylation.