Bigler J, Hokanson W, Eisenman R N
Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98104.
Mol Cell Biol. 1992 May;12(5):2406-17. doi: 10.1128/mcb.12.5.2406-2417.1992.
ErbA/thyroid hormone receptor is a nuclear receptor that can affect transcription from promoters containing a thyroid hormone response element (TRE) in a thyroid hormone (T3)-dependent manner. We reported earlier that the thyroid hormone receptor is expressed in embryonic avian erythroid cells as a nested set of four proteins with a common C terminus. The full-length receptor is capable of both high-affinity binding to thyroid hormone and specific binding to DNA. We now report that the two smallest ErbA forms, which contain the hormone-binding domain but lack the N-terminal DNA-binding domain, have the same affinity for T3 as does full-length ErbA but are incapable of specific DNA binding. In transactivation assays, these N-terminally truncated proteins are able to specifically suppress both transcriptional repression and hormone-dependent transcriptional activation by the full-length ErbA. We also find that retinoic acid-dependent transactivation by retinoic acid receptors is inhibited by the truncated ErbA proteins. Furthermore, the smaller ErbA forms inhibit binding to TREs by full-length ErbA in vitro. Results from experiments involving site-specific mutagenesis of a conserved region within the hormone-binding domain of the smaller ErbA proteins indicate that the suppressive effect of the smaller receptor forms is independent of hormone binding and that this region is important in mediating protein-hormone as well as protein-protein interactions. We have also found that full-length ErbA homodimers can be detected only in the presence of a specific DNA-binding site. However, no association between full-length and the N-terminally truncated non-DNA-binding ErbA proteins could be detected, indicating that the complex either is unstable or does not form. Our results suggest that inhibition of receptor function occurs through transient formation of heterodimers which lack DNA-binding activity or by competition for factors which positively affect DNA binding by the full-length protein. This finding raises the possibility that thyroid hormone receptor transcriptional activity is autoregulated by means of alternative receptor translation products acting in a dominant negative manner.
ErbA/甲状腺激素受体是一种核受体，它能够以甲状腺激素（T3）依赖的方式影响含有甲状腺激素反应元件（TRE）的启动子的转录。我们之前报道过，甲状腺激素受体在胚胎期禽类红细胞中表达为一组具有共同C末端的四种嵌套蛋白。全长受体既能与甲状腺激素进行高亲和力结合，又能与DNA特异性结合。我们现在报道，两种最小的ErbA形式，它们含有激素结合结构域但缺乏N末端DNA结合结构域，对T3的亲和力与全长ErbA相同，但不能进行特异性DNA结合。在反式激活分析中，这些N末端截短的蛋白能够特异性抑制全长ErbA的转录抑制和激素依赖性转录激活。我们还发现，截短的ErbA蛋白会抑制视黄酸受体介导的视黄酸依赖性反式激活。此外，较小的ErbA形式在体外会抑制全长ErbA与TREs的结合。对较小ErbA蛋白激素结合结构域内保守区域进行位点特异性诱变的实验结果表明，较小受体形式的抑制作用与激素结合无关，并且该区域在介导蛋白-激素以及蛋白-蛋白相互作用中很重要。我们还发现，只有在存在特定DNA结合位点的情况下才能检测到全长ErbA同二聚体。然而，未检测到全长与N末端截短的非DNA结合ErbA蛋白之间存在关联，这表明该复合物要么不稳定，要么不形成。我们的结果表明，受体功能的抑制是通过缺乏DNA结合活性的异二聚体的瞬时形成，或者是通过竞争对全长蛋白的DNA结合有正向影响的因子来实现的。这一发现增加了甲状腺激素受体转录活性通过以显性负性方式起作用的替代受体翻译产物进行自动调节的可能性。
