Isolation of two functional retinoid X receptor subtypes from the Ixodid tick, Amblyomma americanum (L.).

Retinoid X receptors (RXR) play a central role in a variety of nuclear signaling pathways in both vertebrates and invertebrates. Vertebrate RXRs are encoded by a multigene family whereas the insect RXR homologue, ultraspiracle (USP), is encoded by a single gene. To determine if acarines possess an RXR homologue similar to insect USPs, we isolated cDNAs encoding two distinct RXR genes, AamRXR1 and AamRXR2, from the ixodid tick, Amblyomma americanum (L.). The DNA binding domains share 95 and 87% identity, respectively, with DNA binding domains from insect USP and vertebrate RXR proteins. However, the ligand binding domains of the AamRXRs are more similar to vertebrate RXRs than to insect USP ligand binding domains (approximately 71 vs approximately 52%). Northern blot and RT-PCR analysis reveal both unique and overlapping patterns of AamRXR1 and AamRXR2 expression. Transactivation analysis show that both AamRXRs encode proteins which can form functional ecdysteroid receptors but are unlikely to bind retinoic acids.