Abstract
At the end of the third larval instar of Drosophila melanogaster, larval serum 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 translation of Fbp-1 is stage-specifically restricted to the end of the third larval instar, starting around 99 h after egg laying. Expression of Fbp-1 is induced by a low level of 20-hydroxy-ecdysone (>/= 10-7 m). After translation, the FBP-1 protein is thought to be proteolytically cleaved in three subsequent steps. The final cleavage step is delayed by 6 h and relies on a higher concentration of ecdysone (>/= 10-5 m). Therefore, 20-hydroxy-ecdysone regulates Fbp-1 expression and function at two different levels. To the best of our knowledge, this study is the first to date to demonstrate two distinct functions for different concentrations of a steroid hormone on a single biochemical process.
