Molecular Chaperones Activate the Drosophila Ecdysone Receptor, an RXR Heterodimer

The steroid hormone 20-hydroxyecdysone coordinates the stages of Drosophila development by activating a nuclear receptor heterodimer consisting of the ecdysone receptor, EcR, and the Drosophila RXR receptor, USP. We show that EcR/USP DNA binding activity requires activation by a chaperone heterocomplex like that required for activation of the vertebrate steroid receptors, but not previously shown to be required for activation of RXR heterodimers. Six proteins normally present in the chaperone complex were individually purified and shown to be sufficient for this activation. We also show that two of the six (Hsp90 and Hsc70) are required in vivo for ecdysone receptor activity, and that EcR is the primary target of the chaperone complex.