Structure and noncanonical Cdk8 activation mechanism within an Argonaute-containing Mediator kinase module

• Published in: Science advances 2021-01, Vol.7 (3)
• Main Authors: Li, Yi-Chuan, Chao, Ti-Chun, Kim, Hee Jong, Cholko, Timothy, Chen, Shin-Fu, Li, Guojie, Snyder, Laura, Nakanishi, Kotaro, Chang, Chia-En, Murakami, Kenji, Garcia, Benjamin A, Boyer, Thomas G, Tsai, Kuang-Lei
• Format: Article
• Language: English
• Subjects: Biochemistry; SciAdv r-articles; Structural Biology
• ISSN: 2375-2548; 2375-2548
• DOI: 10.1126/sciadv.abd4484

The Cdk8 kinase module (CKM) in Mediator, comprising Med13, Med12, CycC, and Cdk8, regulates RNA polymerase II transcription through kinase-dependent and -independent functions. Numerous pathogenic mutations causative for neurodevelopmental disorders and cancer congregate in CKM subunits. However, the structure of the intact CKM and the mechanism by which Cdk8 is non-canonically activated and functionally affected by oncogenic CKM alterations are poorly understood. Here, we report a cryo-electron microscopy structure of CKM that redefines prior CKM structural models and explains the mechanism of Med12-dependent Cdk8 activation. Med12 interacts extensively with CycC and activates Cdk8 by stabilizing its activation (T-)loop through conserved Med12 residues recurrently mutated in human tumors. Unexpectedly, Med13 has a characteristic Argonaute-like bi-lobal architecture. These findings not only provide a structural basis for understanding CKM function and pathological dysfunction, but also further impute a previously unknown regulatory mechanism of Mediator in transcriptional modulation through its Med13 Argonaute-like features.