Ecdysone-regulation of synthesis and processing of Fat Body Protein 1, thelarval serum protein receptor of Drosophila melanogaster

At the end of the third larval instar of Drosophila melanogaster, larval se rum proteins 1 and 2 (LSP-1 and -2) are taken up by cells of the fat body. Here, we show that the product of the ecdysteroid-inducible gene Fbp-1 (Fat Body Protein 1) is the receptor that binds LSP-1. Transcription and transl ation of Fbp-1 is stage-specifically restricted to the end of the third lar val instar, starting around 99 h after egg laying. Expression of Fbp-1 is i nduced by a low level of 20-hydroxy-ecdysone (greater than or equal to 10(- 7) M). After translation, the FBP-1 protein is thought to be proteolyticall y cleaved in three subsequent steps. The final cleavage step is delayed by 6 h and relies on a higher concentration of ecdysone (greater than or equal to 10(-5) M). Therefore, 20-hydroxy-ecdysone regulates Fbp-1 expression an d function at two different levels. To the best of our knowledge, this stud y is the first to date to demonstrate two distinct functions for different concentrations of a steroid hormone on a single biochemical process.