Ontario Cancer Institute, University Health Network, Toronto, ON, Canada M5G 1L7.
Proc Natl Acad Sci U S A. 2012 Apr 17;109(16):6078-83. doi: 10.1073/pnas.1119073109. Epub 2012 Apr 2.
Forkhead box class O 3a (FOXO3a) is a transcription factor and tumor suppressor linked to longevity that determines cell fate through activating transcription of cell differentiation, survival, and apoptotic genes. Recruitment of the coactivator CBP/p300 is a crucial step in transcription, and we revealed that in addition to conserved region 3 (CR3) of FOXO3a, the C-terminal segment of CR2 (CR2C) binds CBP/p300 and contributes to transcriptional activity. CR2C and CR3 of FOXO3a interact with the KIX domain of CBP/p300 at both "MLL" and "c-Myb" binding sites simultaneously. A FOXO3a CR2C-CR3 peptide in complex with KIX exists in equilibrium between two equally populated conformational states, one of which has CR2C bound to the MLL site and CR3 bound to the c-Myb site, whereas in the other, CR2C and CR3 bind the c-Myb and MLL sites, respectively. This promiscuous interaction between FOXO3a and CBP/p300 is further supported by additional binding sites on CBP/p300, namely, the TAZ1 and TAZ2 domains. In functional studies, our structure-guided mutagenesis showed that both CR2C and CR3 are involved in the activation of certain endogenous FOXO3a target genes. Further, phosphorylation of S626, a known AMP-dependent protein kinase target in CR3, increased affinity for CBP/p300 and the phosphomimetic mutation enhanced transactivation of luciferase. These findings underscore the significance of promiscuous multivalent interactions and posttranslational modification in the recruitment of transcriptional coactivators, which may allow transcription factors to adapt to various gene-specific genomic and chromatin structures and respond to cell signals.
叉头框蛋白 O 3a（FOXO3a）是一种转录因子和肿瘤抑制因子，与长寿有关，通过激活细胞分化、存活和凋亡基因的转录来决定细胞命运。共激活因子 CBP/p300 的募集是转录的关键步骤，我们揭示除了 FOXO3a 的保守区 3（CR3）之外，CR2 的 C 端片段（CR2C）与 CBP/p300 结合并有助于转录活性。FOXO3a 的 CR2C 和 CR3 与 CBP/p300 的 KIX 结构域同时在“MLL”和“c-Myb”结合位点相互作用。与 KIX 结合的 FOXO3a CR2C-CR3 肽在两种同等占据的构象状态之间处于平衡状态，其中一种状态是 CR2C 结合到 MLL 位点，CR3 结合到 c-Myb 位点，而在另一种状态下，CR2C 和 CR3 分别结合到 c-Myb 和 MLL 位点。这种 FOXO3a 和 CBP/p300 之间的混杂相互作用进一步得到了 CBP/p300 上其他结合位点的支持，即 TAZ1 和 TAZ2 结构域。在功能研究中，我们的结构导向突变显示 CR2C 和 CR3 都参与了某些内源性 FOXO3a 靶基因的激活。此外，CR3 中已知的 AMP 依赖性蛋白激酶靶标 S626 的磷酸化增加了与 CBP/p300 的亲和力，磷酸模拟突变增强了荧光素酶的转录激活。这些发现强调了混杂的多价相互作用和翻译后修饰在募集转录共激活因子中的重要性，这可能使转录因子能够适应各种基因特异性基因组和染色质结构，并响应细胞信号。
