Abstract
The stability of ecdysone receptor (EcR) expressed in a heterologous system is regulated in an isoform-specific manner and modified by ligand and heterodimerization partner. Transcriptional activities of various receptor complexes with Usp and ligand as determined by reporter assays are the result of two effects: change in receptor concentration and altered transcriptional capability. Transcriptional activity of EcR-A is low when compared to EcR-B1 independent of the absence or presence of Ultraspiracle (Usp). Ligand increased the concentration of EcR-A, but had no effect on the transcriptional capability, in contrast to EcR-B1, which is not stabilized by hormone or Usp, but the transcriptional capability is enhanced by heterodimerization and ligand. Exchange of the AB-domain of Usp by the activation domain (AD) of Vp16 revealed that the N-terminus of Usp inhibited transcriptional activity only with EcR-B isoforms, whereas the hexapeptide in the AB-domain of wild type Usp adjacent to the C-domain of Usp harbours an activating function. Deletion of the C-domain of Usp did not affect the stability of the receptor complex, but reduced the transcriptional capability of heterodimers with all EcR-isoforms, indicating that the stability of the receptor, which is important for termination of the hormone signal transduction, is regulated in a cooperative manner by the AB-domains of EcR and Usp, and ligand. We show the active role of Usp in modulation of the transcriptional activity of the heterodimer in an isoform-specific manner by the inhibitory N-terminus, the activating hexapeptide in the AB-domain, and the C-domain of Usp.
