DNA-INDEPENDENT AND DNA-DEPENDENT MECHANISMS REGULATE THE DIFFERENTIAL HETERODIMERIZATION OF THE ISOFORMS OF THE THYROID-HORMONE RECEPTOR WITH RETINOID-X RECEPTOR

Thyroid hormone receptors (TRs) require heterodimerization with retino id X receptor (RXR) for maximum DNA binding affinity. Interaction with RXR occurs via two dimerization interfaces, one in the DNA-binding do main and one in the C-terminal ''ninth heptad'' of the receptors. We s tudied the relative importance of these two dimerization domains in na turally occurring C-terminal TR variants. TR alpha 1 has a conserved n inth heptad and formed stable heterodimers with RXR in solution. TR al pha 1 . RXR heterodimers bound similarly to direct repeat 4 (DR4) site s with different 5'-flanking and spacer sequences. In contrast, TR alp ha 2, which contains a highly divergent ninth heptad, did not interact with RXR in solution and bound as a heterodimer with RXR only to spec ific DR4 sequences in which the downstream half-site was the preferred octameric binding site of TR (TNAGGTCA). Although the ninth heptad of TR alpha 2 was insufficient for interaction with RXR off DNA, this re gion was required for DNA-dependent heterodimerization with RXR. TR al pha 3, another naturally occurring TR alpha isoform whose ninth heptad differs from those of both TR alpha 1 and TR alpha 2, displayed inter mediate behavior in heterodimerization with RXR. Thus, in the absence of a strong ninth heptad interaction an octameric downstream half-site allosterically promotes RXR heterodimerization with TR alpha 2. Diffe rential dependence upon DNA-binding for heterodimerization with RXR ma y influence transcriptional regulation by TR alpha isoforms.