Exploitation of the Mediator complex by viruses

Just as biophysicists were identifying the component proteins of functional RNA polymerase II (RNAPII) complexes, virologists were identifying viral proteins that control it with peculiar efficiency [1]. Purified transcription components phosphorylate the RNAPII carboxyl-terminal domain (CTD) to initiate transcription, and this is enhanced by viral transactivators. In this way, viral proteins with their potent transcription activation domains (TADs) served as powerful tools for the characterization of DNA binding domains and TAD structures, e.g., the “acidic activator” (an acidic amphipathic alpha helix) and in the identification of host proteins that control transcription. The herpes simplex virus (HSV) transactivator, virion protein 16 (VP16, α-TIF, and Vmw65), was particularly valuable in transcription assays [1–3].

In the early 90s, prior to discovery of Mediator, a cyclin of unknown function, cyclin C, identified in screens for the rescue of yeast deficient in CLN genes (G1 cyclins) [4–6], was found to activate cyclin-dependent kinase 8 (CDK8) to phosphorylate the RNAPII CTD, placing it firmly at transcription control, not cell cycle [7–9]. At the same time, virologists discovered that VP16 and the adenovirus E1A proteins were associated with a large complex with cyclin C/CDK8 kinase activity [10].

This same period saw the sequence of an acutely transforming oncogenic retrovirus, walleye dermal sarcoma virus (WDSV) [11]. The WDSV genome encodes a retroviral cyclin (RV-cyclin) with only distant homology to any eukaryotic cyclin. The RV-cyclin rescued CLN-deficient yeast and induced hyperplastic lesions in mice carrying its transgene [12,13].

At this point, many components of Mediator had been identified as parts of a large, thyroid-hormone receptor complex (TRAP, THRAP, activator-recruited cofactor (ARC), and cofactor required for Sp1 activation (CRSP)) [14,15], and in 2002, the whole ARC/CRSP complex (Mediator) was purified by its VP16 affinity and its structure visualized by cryo-EM [16]. We showed that the RV-cyclin colocalized with transcription and splicing complexes in nuclei of mammalian cells and, in 2002, demonstrated its specific binding to and activation of human CDK8 (walleye and human CDK8 proteins are 98% identical) [17,18]. This was the first association of CDK8 with oncogenesis. In 2008, CDK8 was identified as an oncogene in a majority of human colon carcinomas and has since been implicated in a variety of human metastatic cancers [19,20].