Functional analysis of ecdysone receptor

Abstract

Ecdysteroids are important regulators of insect development. They perform their actions through intracellular receptors belonging to the superfamily of nuclear receptors (NRs). In the present study, the relative performance of the three Drosophila ecdysone receptor (EcR)-isoforms in terms of their affinity to the ligand Ponasterone A in mammalian cell line CHO-K1 was determined. In the absence of a heterodimerization partner hormone binding of EcR is rather weak. The presence of ecdysone response elements (EcREs) stimulates the ligand binding to different degrees depending on the EcR-isoform involved. When Ultraspiracle (Usp), the invertebrate orthologue of mammalian RXR, is used as dimerization partner, all EcR isoforms of Drosophila bind the ligand Ponasterone A with the same high affinity already in the absence of EcREs. Depending on the EcR isoform, Usp variant and EcREs, addition of DNA results in augmented ligand binding to the heterodimer EcR/Usp. In the absence of EcREs, hormone binding is not affected if the A/B domain of wild type Usp is replaced by the activation domain (AD) of herpex simplex virus (VP16) to circumvent the inhibition of transcriptional activity of the wild type Usp. By contrast, substantial differences were observed with VP16-Usp fusion proteins in the presence of EcREs. In contrast to Usp, the mammalian ortholog-RXR confers ligand affinity to the receptor complex only in the presence of an EcRE. Finally, specific mutations in the EcR ligand binding domain (LBD) showed an abolishing effect on the ligand binding function of this receptor.