RZRs, a New Family of Retinoid-related Orphan Receptors That Function As Both Monomers and Homodimers

Members of the superfamily of nuclear receptors share the greatest homology in their DNA-binding domains. We have used reverse transcription-polymerase chain reaction and highly degenerate primers based on the amino acid sequence of the zinc finger motif of known nuclear receptors to identify novel members of the family. Starting with rat brain RNA, we have isolated an orphan receptor that we call RZR beta. The sequence of its nearly full-length complementary DNA shows great similarity to RZR alpha, a receptor we recently identified from human umbilical vein endothelial cells. These RZR subtypes represent members of a new family of orphan nuclear receptors that most likely regulate specific gene expression. Sequence comparison with other known nuclear receptors reveals great similarity for both RZR subtypes to retinoic acid and retinoid-X receptors. By Northern blot analyses, we found RZR beta messenger RNA only in brain, whereas RZR alpha is expressed in many tissues. We show here that the RZRs bind as monomers to natural retinoid response elements formed by (A/G)GGTCA half-sites. However, a T-residue in the -1 position of this motif greatly enhances the DNA binding affinity of RZRs, whereas the -2 position has no influence. We show that RZRs can bind as homodimers on response elements formed by palindromes, inverted palindromes, or direct repeats of two TAGGTCA half-sites. Interestingly, these response elements display dramatically reduced affinity for retinoic acid receptor-retinoid-X receptor heterodimers. Thus, the 5'-flanking sequence of hexameric half-sites appears to be crucial to direct the activity of several nuclear receptors. On monomeric as well as dimeric binding sites, RZRs show constitutive transactivational activity that can be enhanced by unidentified components of fetal calf serum.