The isolation and characterization of purified heterocomplexes of recombinant retinoic acid receptor and retinoid X receptor ligand binding domains.

Retinoic acid exerts many of its biological effects by interaction with heterocomplexes of nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). To further examine this interaction, a glutathione S-transferase (GST) fusion protein containing the ligand binding domain of human RXR alpha has been used to copurify the ligand binding domain of human RAR gamma by affinity chromatography over glutathione-agarose. Complexes of recombinant RAR-RXR ligand binding domains retaining full ligand binding capacity were purified, and their interactions with various retinoids were characterized by fluorometric titration and photoaffinity labeling. Analyses of the distribution of limiting amounts of [3H]-all-trans-retinoic acid between cytoplasmic retinoic acid binding proteins, CRABP-I and CRABP-II, and the purified heterocomplexes indicate that all-trans-retinoic acid binds with comparable affinity to CRABP-I and the heterocomplexes, but with approximately 10-fold less affinity to CRABP-II. The aromatic retinoid acitretin, which is used in the treatment of psoriasis, binds relatively poorly to the purified heterocomplexes, although it binds with high affinity to the CRABPs. Acitretin displaces [3H]-all-trans-retinoic acid from the CRABPs and increases retinoic acid occupancy of the heterocomplexes. These results suggest that certain retinoids could potentially perturb the distribution of endogenous retinoic acid between the CRABPs and the nuclear receptors and thus affect retinoid signaling. The purified recombinant complexes should provide a useful model system for further structural analysis of the dimerization interface between the RAR and RXR ligand binding domains.