CHARACTERIZATION AND TISSUE EXPRESSION OF MULTIPLE TRIIODOTHYRONINE RECEPTOR-AUXILIARY PROTEINS AND THEIR RELATIONSHIP TO THE RETINOID X-RECEPTORS

Thyroid hormone receptor (TR) binding to thyroid hormone response elem ents is enhanced by heterodimerization with T3 receptor auxiliary prot eins (TRAPs). Although retinoid X-receptors (RXRs) behave similarly to TRAP by heterodimerizing with TRs and enhancing TR binding to thyroid hormone response elements, it is not known whether endogenous TRAPs a re RXRs. In this study, we used the electrophoretic mobility shift ass ay to demonstrate that at least two different TR-TRAP heterodimer comp lexes can be formed from nuclear extracts of various rat tissues and c ell lines. Additionally, the TRAPs appear to be differentially express ed in rat tissues. In antibody super-shift experiments, most of the fa ster migrating TR-TRAP heterodimer bands and some of the slower migrat ing TR-TRAP heterodimer bands were recognized in several tissue and ce ll lines by the anti-RXRalpha-specific antibody. Immunodepletion assay s showed that the slower migrating TR-TRAP heterodimer bands in most t issues and cell lines were recognized by the anti-RXRbeta-specific ant ibody. Therefore, we have demonstrated that RXRalpha and RXRbeta are e ndogenous TRAPs in a variety of tissues and cell lines, and are differ entially expressed. We speculate that this heterogeneity of TRAP distr ibution may contribute to tissue and cell-specific expression of T3-re gulated genes.