ALTHOUGH the biological activity of the insect moulting hormone ecdyso
ne, is manifested through a hormonally regulated transcriptional casca
de associated with chromosomal puffing1-3, a direct association of the
receptor with the puff has yet to be established. The cloned ecdysone
receptor4 (EcR) is by itself incapable of high-affinity DNA binding o
r transcriptional activation. Rather, these activities are dependent o
n heterodimer formation with Ultraspiracle5 (USP) the insect homologue
of vertebrate retinoid X receptor6. Here we report that native EcR an
d USP are co-localized on ecdysone-responsive loci of polytene chromos
omes. Moreover, we show that natural ecdysones selectively promote phy
sical association between EcR and USP, and conversely, that high-affin
ity hormone binding requires both EcR and USP. Replacement of USP with
retinoid X receptor produces heterodimers with distinct pharmacologic
al and functional properties. These results redefine the ecdysone rece
ptor as a dynamic complex whose activity may be altered by combinatori
al interactions among subunits and ligand.