Transcriptional activity of ecdysone receptor isoforms is regulated by modulation of receptor stability and interaction with Ab‐ and C‐domains of the heterodimerization partner ultraspiracle

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.