Phosphorylation by Cak1 Regulates the C-Terminal Domain Kinase Ctk1 in Saccharomyces cerevisiae

Abstract

Ctk1 is a Saccharomyces cerevisiae cyclin-dependent protein kinase (CDK) that assembles with Ctk2 and Ctk3 to form an active protein kinase complex, CTDK-I. CTDK-I phosphorylates Ser2 within the RNA polymerase II C-terminal domain, an activity that is required for efficient transcriptional elongation and 3′ RNA processing. Ctk1 contains a conserved T loop, which undergoes activating phosphorylation in other CDKs. We show that Ctk1 is phosphorylated on Thr-338 within the T loop. Mutation of this residue abolished Ctk1 kinase activity in vitro and resulted in a cold-sensitive phenotype. As with other yeast CDKs undergoing T-loop phosphorylation, Ctk1 phosphorylation on Thr-338 was dependent on the Cak1 protein kinase. Ctk1 isolated from cak1Δ cells was unphosphorylated and exhibited low protein kinase activity. Moreover, Cak1 directly phosphorylated Ctk1 in vitro. Unlike wild-type cells, cells expressing Ctk1^T338A delayed growth at early stationary phase, did not show the increase in Ser2 phosphorylation that normally accompanies the transition from rapid growth to stationary phase, and had compromised transcriptional activation of two stationary-phase genes, CTT1 and SPI1. Therefore, Ctk1 phosphorylation on Thr-338 is carried out by Cak1 and is required for normal gene transcription during the transition into stationary phase.

ACKNOWLEDGMENTS

We thank Janet Burton and Aiyang Cheng for support, discussions, and critical reading of the manuscript. We thank Vasiliki Tsakraklides for providing recombinant GST-Cak1 protein and Jingsheng Si for technical assistance. We thank Ken Nelson for assistance with microarray printing.

The microarray facility was supported by grant CA77808 from the National Institutes of Health to Michael Snyder. This work was supported by grant GM47830 (to M.J.S.) from the National Institutes of Health.