CDK7 pathway

CDK7 is a cyclin-dependent kinase shown to be not easily classified. CDK7 is both a CDK-activating kinase (CAK) and a component of the general transcription factor TFIIH.

Introduction

An intricate network of cyclin-dependent kinases (CDKs) are organized in a pathway to ensure that each cell accurately replicates its DNA and segregate it equally between the two daughter cells [Morgan DO. (2007). The Cell Cycle: Principles of Control. New Science Press Ltd: London, UK] . One CDK–the CDK7 complex–cannot be so easily classified. CDK7 is both a CDK-activating kinase (CAK), which phosphorylates cell-cycle CDKs within the activation segment (T-loop), and a component of the general transcription factor TFIIH, which phosphorylates the C-terminal domain (CTD) of the largest subunit of Pol II [Harper, J. W., Elledge, S. J., Keyomarski, K., Dynlacht, B., Tsai, L.-H., Zhang, P., Dobrowolski, S., Bai, C., Connell-Crowley, L., Swindell, E. et al. (1995). Inhibition of cyclin-dependent kinases by p21. Mol. Biol. Cell 6, 387-400] . A proposed mode of CDK7 inhibition is the phosphorylation of cyclin H by CDK7 itself [Lolli, G., Lowe, E. D., Brown, N. R. and Johnson, L. N. (2004). The crystal structure of human CDK7 and its protein recognition properties. Structure 12, 2067-2079] or by another kinase [Akoulitchev, S. and Reinberg, D. (1998). The molecular mechanism of mitotic inhibition of TFIIH is mediated by phosphorylation of CDK7. Genes Dev. 12, 3541-3550] .

The T-loop

In order to be active, most CDKs require not only a cyclin partner but also phosphorylation at one particular site, which corresponds to Thr161 in human CDK1, and which is located within the so-called T-loop of kinase subdomain VIII [Morgan DO: Principles of CDK regulation. Nature 1995, 374:131-134] [Solomon MJ: The function(s) of CAK, the p34cdc2 activating kinase. Trends Biochem Sci 1994,19:496-500] . CDKl, CDK2 and CDK4 all require T-loop phosphorylation for maximum activity [Connell-Cowley L, Solomon MJ, Wei N, Harper JW: Phosphorylation independent activation of human cyclindependent kinase 2 by cyclin A in vitro. Mol Biol Cell 1993, 4:79-92] [Matsuoka M, Kate JY, Fisher RP, Mor of cyclin-dependent kinase 4 (cdk4 by B mouse M015-an associated klnase. Mol Cell Biol 1994, 14:7265-7275.] .

Dual activity

An entirely new perspective on CDK7 function was opened when CDK7 was identified as a subunit of transcription factor IIH (TFIIH) and shown to phosphorylate the carboxy-terminal domain (CTD) of RNA polymerase II (RNAPII) [Roy R, Adamczewski JP, Seroz T, Vermeulen W, Tassan JP, Schaeffer L, Nigg EA, Hoejimakers JHJ, Egly JM: The MO15 cell cycle kinase is associated with the TFIIH transcription-DNA repair factor. Cell 1994, 79:1093-1101] . TFIIH is a multiprotein complex required not only for class II transcription but also for nucleotide-excision repair [Seroz T, Hwang JR, Moncollin V, Egly JM: TFIIH: a link between transcription, DNA repair and cell cycle regulation. Gun Opin Gener Dev 1995, 5:217-221] . Its associated CTD-kinase activity is considered important for the promoter-clearance step of transcription, but the precise structural consequences of the phosphorylation of the CTD remain the subject of debate [Dahmus ME: The role of multisite phosphorylatlon in the regulation of RNA polymerase II activity. Prog Nucleic Acid Res Mol Biol 1994, 48: 143-179] . Cyclin H and MAT1 are also present in TFIIH [Shiekhattar R, Mermelstein F, Fisher R, Drapkin R, Dynlacht B, Wessling HC, Morgan DO, Reinberg D: Cdk-activating kinase complex is a component of human transcription factor TFIIH. Nature 1995, 374:203-287] , and it is not known what, if anything, distinguishes the TFIIH-associated form of CDK7 from the quantitatively predominant free form. Whether CDK7 really displays dual-substrate specificity remains to be further explored, but there is no question that the CDK7-cyclin H-MAT1 complex is able to phosphorylate both the T-loop of CDKs and the YSPTSPS (single-letter code for amino acids) repeats of the RNAPII CTD "in vitro".

HIV latency

It has been demonstrated that TFIIH is a rate-limiting factor for HIV transcription in unactivated T-cells by using a combination of in vivo ChIP experiments and cell-free transcription studies [Kim YK et al., Recruitment of TFIIH to the HIV LTR is a rate-limiting step in the emergence of HIV from latency. EMBO J. 2006 Aug 9;25(15):3596-604] . The ability of NFkB to rapidly recruit TFIIH during HIV activation in T-cells is an unexpected discovery; however, there are several precedents in the literature of cellular genes that are activated through the recruitment of TFIIH. In an early and influential paper [Blau J , Xiao H , McCracken S , O'Hare P , Greenblatt J , Bentley D (1996) Three functional classes of transcriptional activation domains. Mol Cell Biol 16: 2044–2055] , demonstrated that type I activators such as Sp1 and CTF, which were able to support initiation but were unable to support efficient elongation, were also unable to bind TFIIH. By contrast, type II activators such as VP16, p53 and E2F1, which supported both initiation and elongation, were able to bind to TFIIH. In one of the most thoroughly characterized transcription systems [Spilianakis C , Kretsovali A , Agalioti T , Makatounakis T , Thanos D , Papamatheakis J (2003) CIITA regulates transcription onset via Ser5-phosphorylation of RNA Pol II. EMBO J 22: 5125–5136] , have studied the temporal order of recruitment of transcription factors during the activation of the major histocompatibility class II (MHC II) DRA gene by IFN-gamma. Following induction of the CIITA transcription factor by IFN-gamma, there was recruitment of both CDK7 and CDK9 causing RNAP CTD phosphorylation and elongation. Finally, Nissen and Yamamoto (2000) [Nissen RM , Yamamoto KR (2000) The glucocorticoid receptor inhibits NFkB by interfering with serine-2 phosphorylation of the RNA polymerase II carboxy-terminal domain. Genes Dev 14: 2314–2329] in their studies of the activation of the IL-8 and ICAM-1 promoters observed enhanced CDK7 recruitment and RNAP II CTD phosphorylation in response to NFkB activation by TNF.

References

See also

* Cyclin
* Cell cycle
* Wee (cell cycle)
* Cell cycle checkpoint