Glue secretion in the Drosophila salivary gland: A model for steroid-regulated exocytosis

Small hydrophobic hormones like steroids control many tissue-specific physi ological responses in higher organisms. Hormone response is characterized b y changes in gene expression, but the molecular details connecting target-g ene transcription to the physiology of responding cells remain elusive. The salivary glands of Drosophila provide an ideal model system to investigate gaps in our knowledge, because exposure to the steroid 20-hydroxyecdysone (20E) leads to a robust regulated secretion of glue granules after a stereo typical pattern of puffs (activated 20E-regulated genes) forms on the polyt ene chromosomes. Here, we describe a convenient bioassay for glue secretion and use it to analyze mutants in components of the puffing hierarchy. We s how that 20E mediates secretion through the EcR/USP receptor, and two early -gene products, the rbp(+) function of BR-C and the Ca2+-binding protein E6 3-1, are involved. Furthermore, we demonstrate that 20E treatment of saliva ry glands leads to Ca2+ elevations by a genomic mechanism and that elevated Ca2+ levels are required for ectopically produced E63-1 to drive secretion . The results presented establish a connection between 20E exposure and cha nges in Ca2+ levels that are mediated by Ca2+-effector proteins, and thus e stablish a mechanistic framework for future studies. (C) 2001 Academic Pres s.