Wansa K D Senali Abayratna, Harris Jonathan M, Muscat George E O
University of Queensland Centre for Molecular and Cellular Biology Institute for Molecular Bioscience, St. Lucia, Queensland 4072, Australia.
J Biol Chem. 2002 Sep 6;277(36):33001-11. doi: 10.1074/jbc.M203572200. Epub 2002 Jun 24.
Nur77/NR4A1 is an "orphan member" of the nuclear hormone receptor superfamily. Nur77 and its close relatives Nurr1 and NOR-1 bind as monomers to a consensus binding site, the nerve growth factor induced protein I-B (NGFI-B)-binding response element (NBRE). The Nur77/NURR1/NOR1 nuclear receptors are classified as immediate early response genes which are induced through multiple signal transduction pathways. They have been implicated in cell proliferation, differentiation, and apoptosis. However, the mechanism of coactivation and ligand independent trans-activation remains unclear. Hence we examined the molecular basis of Nur77-mediated cofactor recruitment and activation. We observed that Nur77 trans-activates gene expression in a cell-specific manner, and operates in an activation function-1 (AF-1)-dependent manner. The AB region encodes an uncommonly potent N-terminal AF-1 domain delimited to between amino acids 50 and 160 and is essential for the ligand-independent activation of gene expression. Steroid receptor coactivator-2 (SRC-2) modulates the activity of the N-terminal AF-1 domain. Moreover, SRC-2 dramatically potentiates the retinoid induced RXR-dependent activation of the Nur77 ligand binding domain (LBD). Interestingly, the N-terminal AB region (not the LBD) facilitates coactivator recruitment and directly interacts with SRC, p300, PCAF, and DRIP-205. Consistent with this, homology modeling indicated that the Nur77 LBD coactivator binding cleft was substantially different from that of retinoic acid receptor gamma, a closely related AF-2-dependent receptor. In particular, the hydrophobic cleft characteristic of nuclear receptors was replaced with a much more hydrophilic surface with a distinct topology. This observation accounts for the inability of this nuclear receptor LBD to directly mediate cofactor recruitment. Furthermore, the AF-1 domain physically associates with the Nur77 C-terminal LBD and synergizes with the retinoid X receptor LBD. Thus, the AF-1 domain plays a major role in Nur77-mediated transcriptional activation, cofactor recruitment, and intra- and intermolecular interactions.
Nur77/NR4A1是核激素受体超家族的一个“孤儿成员”。Nur77及其近亲Nurr1和NOR-1以单体形式结合到一个共有结合位点，即神经生长因子诱导蛋白I-B（NGFI-B）结合反应元件（NBRE）上。Nur77/NURR1/NOR1核受体被归类为即时早期反应基因，可通过多种信号转导途径诱导产生。它们与细胞增殖、分化和凋亡有关。然而，共激活和非配体依赖性反式激活的机制仍不清楚。因此，我们研究了Nur77介导的辅因子募集和激活的分子基础。我们观察到，Nur77以细胞特异性方式反式激活基因表达，并以激活功能-1（AF-1）依赖性方式发挥作用。AB区域编码一个非常有效的N端AF-1结构域，其范围限定在氨基酸50至160之间，对于基因表达的非配体依赖性激活至关重要。类固醇受体共激活因子-2（SRC-2）调节N端AF-1结构域的活性。此外，SRC-2显著增强视黄酸诱导的Nur77配体结合结构域（LBD）的RXR依赖性激活。有趣的是，N端AB区域（而非LBD）促进辅因子募集，并直接与SRC、p300、PCAF和DRIP-205相互作用。与此一致的是，同源建模表明，Nur77 LBD辅激活因子结合裂隙与视黄酸受体γ（一种密切相关的AF-2依赖性受体）的结合裂隙有很大不同。特别是，核受体特有的疏水裂隙被一个拓扑结构独特、亲水性强得多的表面所取代。这一观察结果解释了该核受体LBD无法直接介导辅因子募集的原因。此外，AF-1结构域与Nur77 C端LBD发生物理缔合，并与视黄酸X受体LBD协同作用。因此，AF-1结构域在Nur77介导的转录激活、辅因子募集以及分子内和分子间相互作用中起主要作用。
