Thyroid hormone receptor (TR) binding to thyroid hormone response elements is enhanced by heterodimerization with T3 receptor auxiliary proteins (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 are RXRs. In this study, we used the electrophoretic mobility shift assay to demonstrate that at least two different TR-TRAP heterodimer complexes can be formed from nuclear extracts of various rat tissues and cell lines. Additionally, the TRAPs appear to be differentially expressed in rat tissues. In antibody super-shift experiments, most of the faster migrating TR-TRAP heterodimer bands and some of the slower migrating TR-TRAP heterodimer bands were recognized in several tissues and cell lines by the anti-RXRα-specific antibody. Immunodepletion assays showed that the slower migrating TR-TRAP heterodimer bands in most tissues and cell lines were recognized by the anti-RXRβ-specific antibody. Therefore, we have demonstrated that RXRα and RXRβ are endogenous TRAPs in a variety of tissues and cell lines, and are differentially expressed. We speculate that this heterogeneity of TRAP distribution may contribute to tissue and cell-specific expression of T3-regulated genes.
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