Characterization of EcR and RXR Homologues in the Ixodid Tick,Amblyomma americanum(L.)

Abstract

Synopsis. Ecdysteroid hormones have been shown to regulate growth and devel? opment in insects, chelicerates and crustaceans. While they presumably mediate analogous functions in all arthropods, their action outside Insecta is poorly understood. Ecdysteroid receptors are heterodimeric proteins composed of two nuclear receptor superfamily members, the ecdysone receptor (EcR) and Ultraspiracle (USP), the invertebrate homologue of retinoid x receptors (RXRs). When paired, EcR/USP dimers function as ligand-activated transcription factors, binding to DNA response elements in target genes and activating transcription. A curious feature of the insect EcR and USP homologues isolated to date is the striking degree of heterogeneity in both EcR and USP proteins, relative to vertebrate nuclear recep? tor homologues. This feature has raised a number of questions regarding their evolution and functional equivalence. To examine the question of ecdysteroid action in ixodid ticks, we isolated chelicerate EcR and RXR homologues from the ixodid tick, Amblyomma americanum. Like insects, ticks possess a single EcR homologue that encodes multiple protein isoforms. However, ticks possess at least two RXR genes that encode proteins with greater overall similarity to vertebrate RXRs. While tick EcR and RXR proteins can partner to form functional ecdysteroid receptors, the DNA and ligand binding domains of tick EcR and RXR proteins are quite divergent, and suggest that there may be important functional differences in both DNA and ligand binding of tick ecdysteroid receptors.