Drosophila TFIIH component Xpd down-regulates Cdk7's Cak activity and mitotic progression. J. Chen 1, S. Larochelle 1,2, R. Fisher 2, B. Suter 1. 1) Dept Biol, McGill Univ, Montreal, PQ, Canada; 2) Cell Biology Program, Memmorial Sloan-Kettering Cancer Center, New York, NY 10021, USA.
Cyclin-dependent-kinases (Cdks) are key players in cell cycle regulation. Cdk7 is a catalytic subunit of the Cdk-activating-kinase (Cak) that activates other Cdks through phosphorylation of a highly conserved threonine residue in their T-loops. In addition to its Cak function, Cdk7 also serves as the kinase subunit of TFIIH, the general transcription/DNA repair factor IIH. TFIIH is required for general transcription of the protein coding genes and possibly for DNA repair. Thus Cdk7 is involved in multiple events: cell cycle control, transcriptional regulation and perhaps DNA reapir. In a genetic screen we identified xpd/Ercc2 (Xeroderma Pigmentosum disorder group D/ Excision repair complementing group C) as a strong dominant suppressor of a Drosophila cdk7 allele that shows only cell cycle defects. Xpd is a helicase subunit of TFIIH and known to physically interact with the Cdk7 containing Cak complex. While reduction of Xpd activity facilitates the cdk7 cell cycle function, its over-expression in embryos results in the reduction of Cak activity. Xpd therefore acts as a negative regulator of the Cak function of Cdk7. In order to visualize the consequences of the reduction of the Cak activity, we analyzed the mitotic marker phospho-Histone H3 and mitotic spindles by immunostaining embryos that over-express Xpd. These embryos show cell cycle arrests and fewer cell divisions, and they are not viable. Our results suggests that Xpd recruits Cdk7 into TFIIH at the expense of Cak activity. Since mitosis and transcription are two mutually exclusive functions that require Cdk7, the regulation of Cdk7 activity through Xpd could contribute to a regulatory mechanism that prevents mitosis and transcription from occurring at the same time.