The pupal cuticle of Drosophila: biphasic synthesis of pupal cuticle proteins in vivo and in vitro in response to 20-hydroxyecdysone.

We investigated the synthesis and localization of Drosophila pupal cuticle proteins by immunochemical techniques using both a complex antiserum and monoclonal antibodies. A set of low molecular weight (15,000-25,000) pupal cuticle proteins are synthesized by the imaginal disk epithelium before pupation. After pupation, synthesis of the low molecular weight proteins ceases and a set of unrelated high molecular weight proteins (40,000-82,000) are synthesized and incorporated into the pupal cuticle. Ultrastructural changes in the cuticle deposited before and after pupation correlate with the switch in cuticle protein synthesis. A similar biphasic accumulation of low and high molecular weight pupal cuticle proteins is also seen in imaginal discs cultured in vitro. The low molecular weight pupal cuticle proteins accumulate in response to a pulse of the insect steroid hormone 20-hydroxyecdysone and begin to appear 6 h after the withdrawal of the hormone from the culture medium. The high molecular weight pupal cuticle proteins accumulate later in culture; a second pulse of hormone appears to be necessary for the accumulation of two of these proteins.