RZRS, A NEW FAMILY OF RETINOID-RELATED ORPHAN RECEPTORS THAT FUNCTIONAS BOTH MONOMERS HOMODIMERS

Members of the superfamily of nuclear receptors share the greatest hom ology 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 receptor s to identify novel members of the family. Starting with rat brain RNA , we have isolated an orphan receptor that we call RZR beta. The seque nce 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 receptor s reveals great similarity for both RZR subtypes to retinoic acid and retinoid-X receptors. By Northern blot analyses, we found RZR beta mes senger RNA only in brain, whereas RZR alpha is expressed in many tissu es. We show here that the RZRs bind as monomers to natural retinoid re sponse elements formed by (A/G)GGTCA half-sites. However, a T-residue in the -1 position of this motif greatly enhances the DNA binding affi nity of RZRs, whereas the -2 position has no influence. We show that R ZRs can bind as homodimers on response elements formed by palindromes, inverted palindromes, or direct repeats of two TAGGTCA half-sites. In terestingly, these response elements display dramatically reduced affi nity for retinoic acid receptor-retinoid-X receptor heterodimers. Thus , the 5'-flanking sequence of hexameric half-sites appears to be cruci al to direct the activity of several nuclear receptors. On monomeric a s well as dimeric binding sites, RZRs show constitutive transactivatio nal activity that can be enhanced by unidentified components of fetal calf serum.