Characterization of EcR and RXR homologues in the ixodid tick, Amblyomma americanum (L.)

Ecdysteroid hormones have been shown to regulate growth and development in insects, chelicerates and crustaceans, White they presumably mediate analog ous functions in all arthropods, their action outside Insecta is poorly und erstood. Ecdysteroid receptors are heterodimeric proteins composed of two n uclear receptor superfamily members, the ecdysone receptor (EcR) and Ultras piracle (USP), the invertebrate homologue of retinoid x receptors (RXRs), W hen paired, EcR/USP diners function as ligand-activated transcription facto rs, binding to DNA response elements in target genes and activating transcr iption. 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 receptor 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 isolate d chelicerate EcR and RXR homologues from the ixodid tick, Amblyomma americ anum, Like insects, ticks possess a single EcR homologue that encodes multi ple protein isoforms, However, ticks possess at least two RXR genes that en code proteins with greater overall similarity to vertebrate RXRs. While tic k EcR and RXR proteins can partner to form functional: ecdysteroid receptor s, the DNA and ligand binding domains of tick EcR and RXR proteins are quit e divergent, and suggest that there may be important functional differences in both DNA and ligand binding of tick ecdysteroid receptors.