Li H, Chen J D
Department of Pharmacology and Molecular Toxicology, University of Massachusetts Medical School, Worcester, Massachusetts 01655-0126, USA.
J Biol Chem. 1998 Mar 6;273(10):5948-54. doi: 10.1074/jbc.273.10.5948.
Transcriptional coactivators are involved in gene activation by nuclear hormone receptors. The receptor-associated coactivator 3 (RAC3) was recently identified to be highly related to the steroid receptor coactivator-1 and transcriptional intermediate factor 2, thereby establishing a novel family of nuclear receptor coactivators. In this study, we identified a RAC3 fragment containing three LXXLL motifs conserved among this family, which is sufficient to mediate nuclear receptor interaction in vivo and in vitro. Point mutations that disrupt ligand-dependent activation function of the receptor inhibited the interaction. We found that a 162-amino acid fragment of RAC3 conferred transcriptional activation and recruited the CREB-binding protein and that three distinct LXXLL motifs mediated the transcriptional activation. A trimeric far Western analysis demonstrated the formation of a ternary complex containing CREB-binding protein, RAC3, and the receptor. In addition, we showed that RAC3, transcriptional intermediate factor 2, and steroid receptor coactivator-1 are expressed in specific tissues and cancer cells and that RAC3 transcript is directly up-regulated by retinoid treatment. These results suggest that RAC3 may contribute to amplified transcriptional responses through both recruitment of additional coactivators and autoregulation by the receptor-coactivator complex.
转录共激活因子参与核激素受体介导的基因激活过程。受体相关共激活因子3（RAC3）最近被鉴定与类固醇受体共激活因子-1和转录中间因子2高度相关，从而确立了一个新的核受体共激活因子家族。在本研究中，我们鉴定出一个RAC3片段，其含有该家族中保守的三个LXXLL基序，该片段足以在体内和体外介导核受体相互作用。破坏受体配体依赖性激活功能的点突变抑制了这种相互作用。我们发现RAC3的一个162个氨基酸的片段赋予转录激活能力，并募集CREB结合蛋白，且三个不同的LXXLL基序介导了转录激活。三聚体远缘Western分析证明形成了包含CREB结合蛋白、RAC3和受体的三元复合物。此外，我们表明RAC3、转录中间因子2和类固醇受体共激活因子-1在特定组织和癌细胞中表达，且视黄酸处理可直接上调RAC3转录本。这些结果表明，RAC3可能通过募集额外的共激活因子和受体-共激活因子复合物的自动调节作用，促进转录反应的放大。
