Ecdysteroid receptors in imaginal discs of Drosophila melanogaster.

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Abstract
[3H]Ponasterone A (PNA) of high specific activity was used to identify and begin characterization of ecdysteroid (formerly called ecdysone) receptors in cytoplasmic and nuclear fractions of imaginal discs of D. melanogaster. The equilibrium Kd of the observed macromolecular binding, 3-4 .times. 100-9 M PNA, is in good agreement with the minimal concentration required for induction of complete morphogenesis in vitro, 4.2 .times. 10-9 M PNA. Binding is analog-specific and has kinetics consistent with a role in hormone response. On gentle homogenization, < 5% of the binding capacity of the cell is released as soluble receptor; the other 95% remains with the nuclear fraction. This nuclear fraction specifically binds [3H]PNA in vitro. More than 95% of nuclear PNA receptors are released by extraction with 0.3 M KCl. The binding properties of the nuclear receptors are indistinguishable from those of the cytosol fraction or the whole cell.