Cell-free translation of silkmoth chorion mRNAs: Identification of protein precursors and characterization of cloned DNAs by hybrid-selected translation

The secretory silkmoth chorion proteins are synthesized as precursors bearing signal peptides. Precursors are detected upon cell-free translation of chorion mRNAs in the wheat germ system; they are processed into products identical in size to authentic chorion proteins when translation is performed in the presence of microsomal membranes from dog pancreas. Precursors corresponding to specific protein size classes and subclasses are identified by three approaches: comparison of precursors and products encoded by stage-specific mRNAs, comparison of precursors and products encoded by mRNAs specifically hybridizing to individual chorion cDNA clones, and comparison of relative amino acid compositions of precursors and authentic chorion proteins. Translation of stage-specific mRNA preparations indicates that, in general, the developmental changes of in vivo chorion protein synthesis are based on changes in concentrations of the corresponding mRNAs. Characterization of the precursors makes it possible to identify, for any chorion DNA clone, the protein subclass, a member of which is encoded by the clone sequence.