Expression of the ligand-binding domain-containing region of retinoic acid receptors alpha, beta and gamma in Escherichia coli and evaluation of ligand-binding selectivity.

The complete molecule or the ligand-binding domain-containing region of each of the three subtypes of human retinoic acid receptors (hRAR alpha, hRAR beta and hRAR gamma) was expressed in Escherichia coli. The expressed recombinant RARs (rRARs: rRAR alpha/E, rRAR beta/E and rRAR gamma) showed nearly the same magnitude of binding affinity toward [3H]retinoic acid (RA) as hRARs extracted from human cells (Ka values: 6.0 x 10(9) M-1 for rRAR alpha/E and 2.7 x 10(10) M-1 for both rRAR beta/E and rRAR gamma). Therefore, the ligand-binding selectivity of the rRARs toward RA and synthetic retinoids (4-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenylcarbamoyl )benzoic acid (Am80), (E)-4-[3-(3,5-di-tert-butylphenyl)-3-oxo-1-propenyl]benzoic acid (Ch55)) was examined. Am80 bound rRAR alpha/E preferentially and showed no binding activity toward rRAR gamma, which is consistent with the case of hRAR gamma. Ch55 bound all three subtypes of rRARs, preferentially rRAR beta/E. These results suggest that the intrinsic nature of the binding of each retinoid can be investigated by usage of the rRARs. However, rRARs show quantitatively different ligand-selectivity from that of hRARs: RA showed higher binding affinity toward rRARs than both Am80 and Ch55, but Ch55 binds all three subtypes of hRARs stronger than RA and Am80, which binds hRAR beta stronger than RA.