Interactions of the nuclear thyroid hormone receptor with core histones

These studies concern the interactions of the rat liver thyroid hormone nuclear receptor with histones and factors influencing the receptor's assay and stability. Heating certain crude receptor preparations at 50°C produces a selective loss of triiodothyronine (T₃) but not thyroxine (T₄) binding activity, whereas, with more purified preparations, such heating decreases both T₃ and T₄ binding. The selective T₃-binding loss in crude preparations was found to be due to the simultaneous denaturation of the receptor's high-affinity hormone-binding activity for both T₃ and T₄ and generation of new low-affinity T₄-binding sites. The fraction in which T₄ binding can be activated could be separated from the receptors by Sephadex G-100 chromatography. Core histones stimulated both T₃- and T₄-binding activity of 6-fold-purified receptor preparations, and data from several different experimental approaches suggest that this stimulation is due to the capability of the core histones to prevent the receptor from binding to or being denatured by Sephadex G-25 assay columns. The core histones were also found to stabilize 500-fold-purified but not 6-fold-purified or crude receptor preparations. A number of other acidic or basic proteins had little or none of these stimulatory effects, whereas a few proteins (such as the insulin B chain and histone H1) did have activity, although it was less than that of the core histones. There were no significant differences between the purified core histone subfractions (H2A, H2B, H3, and H4). That core histones can interact with the thyroid hormone receptors was demonstrated more directly by the finding that the receptors bind to histone-Sepharose but not Sepharose or insulin- or ovalbumin-Sepharose columns and that this binding was blocked by core histones at concentrations suggestive of an affinity for the receptor-core histone interaction of around 3 μM at 0.15 M salt concentration. The results demonstrate the utility of the histones in the assay and stabilization of purified thyroid hormone receptors, but they fail to support our previous hypothesis of a receptor subunit where T₃- but not T₄-binding activity is regulated selectively by histones. However, the results indicate that histones may interact with the receptors with some degree of specificity, and they raise the possibility that the histones participate in the nuclear localization of the receptors.