Hong H, Kohli K, Garabedian M J, Stallcup M R
Department of Pathology, University of Southern California, Los Angeles 90033, USA.
Mol Cell Biol. 1997 May;17(5):2735-44. doi: 10.1128/MCB.17.5.2735.
After binding to enhancer elements, transcription factors require transcriptional coactivator proteins to mediate their stimulation of transcription initiation. A search for possible coactivators for steroid hormone receptors resulted in identification of glucocorticoid receptor interacting protein 1 (GRIP1). The complete coding sequence for GRIP1, isolated from a mouse brain cDNA library, contains an open reading frame of 1,462 codons. GRIP1 is the probable ortholog of the subsequently identified human protein transcription intermediary factor 2 (TIF2) and is also partially homologous to steroid receptor coactivator 1 (SRC-1). The full-length GRIP1 interacted with the hormone binding domains (HBDs) of all five steroid receptors in a hormone-dependent manner and also with HBDs of class II nuclear receptors, including thyroid receptor alpha, vitamin D receptor, retinoic acid receptor alpha, and retinoid X receptor alpha. In contrast to agonists, glucocorticoid antagonists did not promote interaction between the glucocorticoid receptor and GRIP1. In yeast cells, GRIP1 dramatically enhanced the transcriptional activation function of proteins containing the HBDs of any of the above-named receptors fused to the GAL4 DNA binding domain and thus served as a transcriptional coactivator for them. This finding contrasts with previous reports of TIF2 and SRC-1, which in mammalian cells enhanced the transactivation activities of only a subset of the steroid and nuclear receptors that they physically interacted with. GRIP1 also enhanced the hormone-dependent transactivation activity of intact glucocorticoid receptor, estrogen receptor, and mineralocorticoid receptor. Experiments with glucocorticoid receptor truncation and point mutants indicated that GRIP1 interacted with and enhanced the activity of the C-terminal AF-2 but not the N-terminal AF-1 transactivation domain of the glucocorticoid receptor. These results demonstrate directly that AF-1 and AF-2 domains accomplish their transactivation activities through different mechanisms: AF-2 requires GRIP1 as a coactivator, but AF-1 does not.
转录因子与增强子元件结合后，需要转录共激活蛋白来介导其对转录起始的刺激作用。对类固醇激素受体可能的共激活因子进行搜索，结果鉴定出糖皮质激素受体相互作用蛋白1（GRIP1）。从鼠脑cDNA文库中分离出的GRIP1完整编码序列包含一个1462个密码子的开放阅读框。GRIP1可能是随后鉴定出的人类蛋白转录中介因子2（TIF2）的直系同源物，并且与类固醇受体共激活因子1（SRC-1）也有部分同源性。全长GRIP1以激素依赖的方式与所有五种类固醇受体的激素结合结构域（HBD）相互作用，还与II类核受体的HBD相互作用，包括甲状腺激素受体α、维生素D受体、视黄酸受体α和维甲酸X受体α。与激动剂不同，糖皮质激素拮抗剂不会促进糖皮质激素受体与GRIP1之间的相互作用。在酵母细胞中，GRIP1显著增强了与GAL4 DNA结合结构域融合的上述任何一种受体的含HBD蛋白的转录激活功能，因此作为它们的转录共激活因子。这一发现与之前关于TIF2和SRC-1的报道形成对比，在哺乳动物细胞中，TIF2和SRC-1仅增强了它们与之物理相互作用的一部分类固醇和核受体 的反式激活活性。GRIP1还增强了完整的糖皮质激素受体、雌激素受体和盐皮质激素受体的激素依赖性反式激活活性。对糖皮质激素受体截短突变体和点突变体的实验表明，GRIP1与糖皮质激素受体的C端AF-2反式激活结构域相互作用并增强其活性，但不与N端AF-1反式激活结构域相互作用。这些结果直接证明，AF-1和AF-2结构域通过不同机制完成其反式激活活性：AF-2需要GRIP1作为共激活因子，而AF-1则不需要。
