Transcription regulation by Mediator kinases CDK8 and CDK19

Immune reactions are controlled by small secreted proteins, such as cytokines and interferons, such that a robust but not destructively strong immune response against infection is launched. The effects of interferons are caused by their ability to precisely stimulate transcription (i.e., the reading of the gene sequence and mRNA synthesis) of dozens of genes that perform important tasks in the immune response. However, different genes are activated at different strength and for different period of time. How this important gene-specific activation comes about is essentially unexplained. Transcriptional gene activation requires the interaction of specific transcription factors with RNA polymerase II (RNAPII), an enzyme that synthesizes mRNA. The communication between transcription factors and RNAPII is mediated by the so called Mediator, a complex of many different proteins, which together form a hub of transcriptional control. An important part of the mediator are the Mediator kinases and control switches CDK8 and CDK19. These kinases are very similar in sequence and have long been believed to perform similar functions as well. Accordingly, these functions should be predominantly based on the phosphorylation (i.e., modification) of other proteins by CDK8 or CDK19. However, the results of our ongoing study clearly indicate that CDK8 and CDK19 regulate different genes and that they also employ different regulatory mechanisms. These results imply that CDK8 and CDK19 play a central role in gene-specific transcriptional activation. In the proposed study, we plan to mechanistically explain the different functions of CDK8 and CDK19. We will investigate interferon-stimulated transcription and test the findings in other immune responses. The results of the proposed study will significantly improve our understanding of the regulation of transcription in immune responses. The mediator kinases CDK8 and CDK19 can be pharmacologically regulated; however, a therapeutic use of the CDK8 or CDK19 inhibition is not possible without knowledge of the exact function of these enzymes. The proposed study will therefore also help to identify possible applications for therapeutic inhibition of CDK8 and CDK19 in immune disorders.

CDK8 & CDK19 - Twin enzymes with non-twin roles in antiviral defense The goal of the project was to better understand how genes driving the response against viral infections are activated. Precise regulation of gene expression, or gene activity, is essential for life. For example, activation of genes involved in protection of cells against viruses is critical for successful defense against viral infections. The antiviral gene expression is induced by interferons which are small proteins secreted by virus-infected cells. Interferons dock on cell surface receptors and initiate a cascade of signals that ultimately cause the cell to transcribe, that is to activate, a set of antiviral genes. It is critical that each antiviral gene is induced to the right extent since an inadequate (both too low or too high) expression may be harmful to the host organism. Mechanisms that regulate the quantity of gene transcription, and consequently the level of gene expression, remain poorly understood. To address this important question, we focused on the enzymes CDK8 and CDK19 which are components of the Mediator kinase, an indispensable regulator of gene transcription. The Mediator kinase controls the quantity of transcription, but the underlying molecular principles are not known. CDK8 and CDK19 are highly similar proteins that were thought to be functionally redundant. The project results demonstrated that CDK8 and CDK19 are functionally distinct proteins. They activate different sets of genes and employ distinct mechanisms in the interferon-induced antiviral response. The results show that CDK8 and CDK19 precisely adjust the transcriptional output to meet the needs. The project discoveries revise the concept of anti-viral immunity and of gene regulation in general. Future work will investigate in detail the molecular mechanisms of how CDK8 and CDK19 determine the quantity of transcription. The challenge of this aim is to develop experimental systems that mirrors the infection condition and at the same time allows the inactivation of either of the two proteins at a given time. First parts of this task were successfully established in the course of the completed project.