Disease relevance of CDK8

• To examine the involvement of CTD phosphorylation in modulation of RNAP II function, we have analyzed the ability of a known CTD kinase, human Cdk8, to modulate HIV-1 LTR-driven gene expression upon directed targeting to a promoter-proximal nascent RNA element [1].
• Surprisingly, the HIV promoter in unactivated Jurkat T cells is partially occupied and carries Mediator containing the CDK8 repressive module, TFIID and RNAP II that is hypophosphorylated and confined to the promoter region [2].
• First, deleting its corepressor CDK8 does not suppress the slt2 hypersensitivity phenotype [3].
• The expression levels of cyclin-dependent kinase 8, phosphoinositide-3-kinase, interferon regulatory factor 3 and tubulin were significantly higher in the tumors of surviving patients with stage I lung cancer (p<0.01) [4].

High impact information on CDK8

• The cdk8/cyclin C protein complex is also found in a number of mammalian Mediator-like protein complexes, which repress activated transcription independently of the CTD in vitro [5].
• In addition, mimicking cdk8 phosphorylation of cyclin H in vivo has a dominant-negative effect on cell growth [5].
• This goes beyond Cdk8-repressed genes, indicating that Mediator can mark some regulatory regions ahead of additional signals [6].
• Upon stimulation of the cells by TNF-alpha, NF-kappaB and TFIIH are rapidly recruited to the promoter together with additional Mediator and RNAP II, but CDK8 is lost [2].
• The cyclin-dependent kinase 8 module sterically blocks Mediator interactions with RNA polymerase II [7].

Biological context of CDK8

• CDK8 and CDK9 complexes, bound to viral activators E1A and Tat, respectively, target only serine 5 for phosphorylation in the CTD peptides, and binding to the viral activators does not change the substrate preference of these kinases [8].
• Targeting of CDK8 to a promoter-proximal RNA element demonstrates catalysis-dependent activation of gene expression [1].
• Furthermore, regulation of cell adhesion appears to be a feature unique to cyclin C, but not other G1 cyclins, E and D3, and its regulatory function is independent of CDK8 kinase activity [9].
• A kinase subunit of the human mediator complex, CDK8, positively regulates transcriptional activation [10].
• In a final experiment, knockdown of CDK8 using RNA interference prevented transcriptional activation by Gal4-VP16 in a luciferase-assay [10].

Anatomical context of CDK8

• To further understand the structural and functional diversity of these complexes we established three HeLa cell lines each expressing one of three epitope-tagged human TRAP/Mediator subunits, MED6, MED7, and CDK8 and isolated the complexes in which these subunits were contained by affinity and HPLC-gel filtration chromatography [10].
• Colocalization of cyclin C and its preferred binding partner, Cdk8, was only observed in astrocytes but not in neurons [11].

Associations of CDK8 with chemical compounds

• Immunoprecipitation of the approximately 2-MDa fraction with anti-Cdk8 antibody indicated that at least two classes of Mediator complexes occur, one containing CDK8 and cyclin C and one lacking this CDK-cyclin pair [12].
• The composition of these complexes was analyzed by centrifugation over sucrose gradients, by gel filtration, by RIA with either IgG Sepharose or K35 Sepharose and by double-labeling studies [13].
• In the absence of the Zn(2+) ion, K6 and K35 on the top of the PC molecule also interact with cyt f [14].
• E/E35 contains only glutamic acid at both e and g positions; K/K35, only lysine, E/E35 and K/K35 were designed to form a stable coiled coil heterodimer when combined at neutral pH [15].
• RESULTS: OMP analysis revealed that cefoxitin and ceftazidime resistance was mediated by loss of a porin Omp K35 in the isolates of K.pneumoniae and E.coli [16].

Physical interactions of CDK8

• We have identified at least two distinct cyclin C/CDK8 containing complexes within the cell, a larger complex over 500 kD in size, that also contains the largest subunit of RNA polymerase II, and a smaller 170 kD species [17].

Other interactions of CDK8

• Cyclin C/CDK8 is a novel CTD kinase associated with RNA polymerase II [17].
• Additionally, the motifs showed CDK8 to be lower relative to ICAM1, and ANXA5 to be relatively high by itself in node positive tumors [18].
• We provide in vitro and in vivo evidence that VP16 activates transcription through a specific MED25-associated Mediator, which is deficient in CDK8 [19].
• Kinases responsible for such CTD phosphorylation that are associated with RNA polymerase II at distinct steps of transcription, such as cdk7 and cdk8, also phosphorylate some other components of the transcription machinery in a regulatory manner [20].

Analytical, diagnostic and therapeutic context of CDK8

• We use electron microscopy and single-particle reconstruction to demonstrate that the Cdk8 module forms a distinct structural entity that binds to the head and middle region of Mediator, thereby sterically blocking interactions with pol II [7].

References