Heterodimerization and deoxyribonucleic acid-binding properties of a retinoid X receptor-related factor

The extent thyroid hormone receptors (TRs) bind to AGGTCA-related motifs as monomers and/or homodimers, and as heterodimers with retinoid X receptors (RXRs) depends on the number, spacing, and orientation of these half-sites. Here we show that recombinant RXR alpha affects TR binding to DNA in diverse ways; it enhances recombinant TR beta 1 binding to four-nucleotide-spaced direct repeat and palindromes but not to inverted palindromes. We also used an endogenous factor termed RXR alpha-RF that cross-reacted with antibodies to RXR alpha and copurified and formed heterodimers on DNA with rat liver TRs (mostly TR beta 1 isoform), supporting the fact that endogenous TRs are commonly heterodimers. RXR alpha-RF formed, like recombinant RXR alpha, heterodimers on DNA with vitamin D and retinoic acid but not estrogen receptors. RXR alpha-RF differed from recombinant RXR alpha in that it provoked enhancement of TR beta 1 binding to DNA irrespective of half-site architecture, was resistant to heating to 50 C, and did not form heterodimers with recombinant TR alpha 2 on four-nucleotide-spaced direct repeat. The overall enhancement of TR-DNA recognition by endogenous RXR alpha-RF, not found in studies with recombinant RXR alpha, might exemplify properties acquired in vivo by endogenous RXRs; this could promote wider DNA recognition by TRs and expand the thyroid hormone transcriptional influence in the cell.