中国科大Cell Res纠正转录研究错误理论

　　

　近日，中国科学技术大学生命科学学院教授蔡刚研究组首次破解了“转录中央控制器”——中介体的模块化结构，颠覆了影响转录研究领域长达十余年的错误认识。相关研究成果于5月9日在线发表在《细胞研究》上。

　　中介体在转录中扮演关键角色，被称为“转录中央控制器”。它是由几十个不同蛋白质组成的庞大的分子机器，由头部、中部、尾部3个稳定模块构成；其模块化结构和亚基组成从低等的酵母到高等哺乳动物高度保守。冷冻电镜是当前能够解析完整中介体三维结构的唯一方法。

   但受限于结构上的复杂性，已报道的电镜结构分辨率很低，无法定位各个模块，甚至头部模块的晶体结构对接到中介体电镜结构的位置都不确定。原先对于中介体的模块化结构认识模糊，且与大量结构和功能实验数据相抵触，大大限制了对中介体精细结构和作用分子机制的认识。

　　蔡刚研究组长期专注于采用冷冻电镜解析中介体的结构和功能，相继解析了迄今唯一的中介体冷冻电镜三维结构（Structure 2009）、首个中介体头部模块三维结构（Nat Struct Mol Biol 2010）、首个中介体头部模块与RNA聚合酶II的复合物的冷冻电镜三维结构（Structure 2012）和中介体可解离Cdk8激酶模块的三维结构（Protein Cell 2013）；此外，参与解析头部模块的首个晶体结构（Nature 2011）。

   在此基础上，该研究组采取“庖丁解牛”的研究策略，将完整中介体肢解成各个模块和模块的组合，通过细致比较中介体及其各个功能模块组合的精细三维结构，首次清晰划分了各个模块，重新定义了中介体的模块化的组织架构，颠覆了影响长达十余年的错误模块划分。

　　审稿人认为，“这项工作极大推进了对于中介体模块化结构的认识，为阐明中介体调控转录的分子机制打下了坚实的基础”。

原文摘要：

Redefining the modular organization of the core Mediator complex

The Mediator complex plays an essential role in the regulation of eukaryotic transcription. The Saccharomyces cerevisiae core Mediator comprises 21 subunits, which are organized into Head, Middle and Tail modules. Previously, the Head module was assigned to a distinct dense domain at the base, and the Middle and Tail modules were identified to form a tight structure above the Head module, which apparently contradicted findings from many biochemical and functional studies. Here, we compared the structures of the core Mediator and its subcomplexes, especially the first 3D structure of the Head + Middle modules, which permitted an unambiguous assignment of the three modules. Furthermore, nanogold labeling pinpointing four Mediator subunits from different modules conclusively validated the modular assignment, in which the Head and Middle modules fold back on one another and form the upper portion of the core Mediator, while the Tail module forms a distinct dense domain at the base. The new modular model of the core Mediator has reconciled the previous inconsistencies between the structurally and functionally defined Mediator modules. Collectively, these analyses completely redefine the modular organization of the core Mediator, which allow us to integrate the structural and functional information into a coherent mechanism for the Mediator's modularity and regulation in transcription initiation