The Interactive Fly

Evolutionarily conserved developmental pathways

Molting and the response to small hydrophobic hormones - the steroid receptor superfamily

Steroid receptor superfamily transcription factors consists of zinc finger proteins that possess ligand binding domains, protein interaction domains, and DNA binding domains (the so-called zinc fingers). The function of these proteins is to relay extracellular signals (provided by diverse hydrophobic hormones that are related to steroids) directly to nuclear targets (the promoters of genes responsive to steroid receptors). Organic ligands activate these receptors; in turn the receptors directly regulate the transcription of specific genes. The actions of steroid hormones directly convey information from outside the cell to the nucleus, without an intervening cytoplasmic signal transduction pathway

Three evolutionarily conserved types of nuclear receptors are the RAR (retinoic acid receptor), RXR (a dimerization partner of RAR), and ROR (an orphan nuclear receptor with unknown or nonexistent ligands). The Drosophila versions of these proteins are Ecdysone receptor, Ultraspiracle and Hormone receptor-like (DHR3 or Hr46). Ecdysone receptor is the master regulator of the molt cycle in Drosophila; Ultraspiracle is its dimerization partner, and Hormone receptor-like (an early-late gene) bridges the gap between the earliest response to ecdysone (the molting hormone) and subsequent responses, such as metamorphosis into the adult.

Of the many hormone responses studied in vertebrates, two serve as useful analogs for the functioning of steroid receptors in Drosophila. The metamorphosis of tadpoles into adult frogs involves the thyroid hormone receptor (an RAR type receptor) and its RXR dimerization partner. Information about Xenopus molting can be found in the evolutionary homologs sections of Ecdysone receptor and Ultraspiracle. The responses of HOX protein transcription factors to retinoids during segmentation and limb outgrowth are documented in the evolutionary homologs sections of Labial and Ultrabithorax.

date revised: 10 July 97

Developmental Pathways conserved in Evolution

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