Functional ecdysone receptor is the product of EcR and Ultraspiracle genes

Abstract

Although the biological activity of the insect moulting hormone ecdysone, is manifested through a hormonally regulated transcriptional cascade associated with chromosomal puffing, a direct association of the receptor with the puff has yet to be established. The cloned ecdysone receptor (EcR) is by itself incapable of high-affinity DNA binding or transcriptional activation. Rather, these activities are dependent on heterodimer formation with Ultraspiracle (USP) the insect homologue of vertebrate retinoid X receptor. Here we report that native EcR and USP are co-localized on ecdysone-responsive loci of polytene chromosomes. Moreover, we show that natural ecdysones selectively promote physical association between EcR and USP, and conversely, that high-affinity hormone binding requires both EcR and USP. Replacement of USP with retinoid X receptor produces heterodimers with distinct pharmacological and functional properties. These results redefine the ecdysone receptor as a dynamic complex whose activity may be altered by combinatorial interactions among subunits and ligand.

Discrete pulses of molting hormone, 20‐hydroxyecdysone, during late larval development of Drosophila melanogaster: Correlations with changes in gene activity

“…This has been especially true for the critical third (final) larval instar when the insect must be reprogrammed so that it can undergo metamorphosis to the pupa and then the adult in response to a large surge in 20E, a phenomenon studied mainly in the Lepidoptera (Bollenbacher et al, 1975;Riddiford, 1976Riddiford, , 1995. The requisite involvement of ecdysteroid signaling in these complex processes is central to present theories of arthropod development (Ashburner et al, 1974;Yao et al, 1993;Buszczak and Segraves, 1998;Bialeccki et al, 2002;Lafont et al, 2005;Henrich, 2005). However, the RIA, particularly when applied directly to whole body extracts, yields only a rough estimate of the molting hormone titer, as each ecdysteroid component present gives a variable response (cross-reactivity) in the assay depending on affinity characteristics of the antibody used.…”

Thyroid Hormone-mediated Enhancement of Heterodimer Formation between Thyroid Hormone Receptor β and Retinoid X Receptor

“…The addition of the ligand 1,25-dihydroxyvitamin D 3 enhanced the affinity of the vitamin D 3 receptor for RXR by 7-fold as a result of an increase in the association rate constant, and we hypothesize that the T 3 -mediated enhancement of the TR-RXR interaction might involve a similar mechanism. Another example in which ligand promotes dimerization is provided by the ecdysone receptor-ultraspiracle interaction, where muristerone has been shown to promote formation of ecdysone receptor-ultraspiracle complexes as well as formation of ecdysone receptor-ultraspiracle or ecdysone receptor-RXR heterodimers on DNA (48). Last, a genomic footprinting study has shown that occupancy of the retinoid response element in the RAR␤2 gene occurs only after treatment of cells with retinoic acid (49).…”

A comparison of invivo and invitro DNA-binding specificities suggests a new model for homeoprotein DNA binding in Drosophila embryos

“…In contrast, Zeste crosslinks in vivo to short regions of the Ubx promoter at at least 100-fold higher levels than it does to other genes (Walter et al, 1994;Laney and Biggin, 1997). Studies of proteins bound to polytene chromosomes also suggest that some transcription factors bind selectively to only a small number of genes (Urness and Thummel, 1990;Yao et al, 1993). We suggest that metazoan transcription factors will show a spectrum of DNA binding, from factors that bind very selectively to those that bind as broadly as Bicoid, Paired, Eve and Ftz.…”