Department of Biochemistry and Molecular Biology, McGovern Medical School, University of Texas Health Science Center at Houston, Houston, TX, USA.
Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, USA.
Mol Cell. 2024 Oct 17;84(20):3932-3949.e10. doi: 10.1016/j.molcel.2024.09.001. Epub 2024 Sep 24.
The eukaryotic transcriptional Mediator comprises a large core (cMED) and a dissociable CDK8 kinase module (CKM). cMED recruits RNA polymerase II (RNA Pol II) and promotes pre-initiation complex formation in a manner repressed by the CKM through mechanisms presently unknown. Herein, we report cryoelectron microscopy structures of the complete human Mediator and its CKM. The CKM binds to multiple regions on cMED through both MED12 and MED13, including a large intrinsically disordered region (IDR) in the latter. MED12 and MED13 together anchor the CKM to the cMED hook, positioning CDK8 downstream and proximal to the transcription start site. Notably, the MED13 IDR obstructs the recruitment of RNA Pol II/MED26 onto cMED by direct occlusion of their respective binding sites, leading to functional repression of cMED-dependent transcription. Combined with biochemical and functional analyses, these structures provide a conserved mechanistic framework to explain the basis for CKM-mediated repression of cMED function.
真核转录中介体由一个大核心（cMED）和一个可分离的 CDK8 激酶模块（CKM）组成。cMED 招募 RNA 聚合酶 II（RNA Pol II），并以一种通过目前未知的机制被 CKM 抑制的方式促进起始前复合物的形成。在此，我们报告了完整的人类中介体及其 CKM 的冷冻电子显微镜结构。CKM 通过 MED12 和 MED13 结合到 cMED 的多个区域，包括后者中的一个大的固有无序区域（IDR）。MED12 和 MED13 一起将 CKM 锚定在 cMED 的钩上，将 CDK8 定位在转录起始位点的下游和近端。值得注意的是，MED13 IDR 通过直接阻断其各自的结合位点，阻止 RNA Pol II/MED26 招募到 cMED 上，从而导致 cMED 依赖性转录的功能抑制。这些结构与生化和功能分析相结合，为 CKM 介导的 cMED 功能抑制提供了一个保守的机制框架。
