A glucocorticoid/retinoic acid receptor chimera that displays cytoplasmic/nuclear translocation in response to retinoic acid - A real time sensing assay for nuclear receptor ligands

Members of the nuclear receptor superfamily play key roles in a host of phy siologic and pathologic processes from embryogenesis to cancer. Some member s, including the retinoic acid receptor (RAR), are activated by ligand bind ing but are unaffected in their subcellular distribution, which is predomin antly nuclear. In contrast, several members of the steroid receptor family, including the glucocorticoid receptor, are cytoplasmic and only translocat e to the nucleus after ligand binding. We have constructed chimeras between RAR and glucocorticoid receptor that selectively respond to RAR agonists b ut display cytoplasmic localization in the absence of ligand. These chimeri c receptors manifest both nuclear translocation and gene activation functio ns in response to physiological concentrations of PLAR ligands. The ability to achieve regulated subcellular trafficking with a heterologous ligand bi nding domain has implications both for current models of receptor transloca tion and for structural-functional conservation of ligand binding domains b roadly across the receptor superfamily. When coupled to the green fluoresce nt protein, chimeric receptors offer a powerful new tool to 1) study mechan isms of steroid receptor translocation, 2) detect dynamic and graded distri butions of ligands in complex microenvironments such as embryos, and 3) scr een for novel ligands of "orphan" receptors in vivo.