Activation of the Bur1-Bur2 cyclin-dependent kinase complex by Cak1.

Cyclin-dependent kinases (Cdks) were originally identified as regulators of eukaryotic cell cycle progression, but several Cdks were subsequently shown to perform important roles as transcriptional regulators. While the mechanisms regulating the Cdks involved in cell cycle progression are well documented, much less is known regarding how the Cdks that are involved in transcription are regulated. In Saccharomyces cerevisiae, Bur1 and Bur2 comprise a Cdk complex that is involved in transcriptional regulation, presumably mediated by its phosphorylation of the carboxy-terminal domain (CTD) of the largest subunit of RNA polymerase II. To investigate the regulation of Bur1 in vivo, we searched for high-copy-number suppressors of a bur1 temperature-sensitive mutation, identifying a single gene, CAK1. Cak1 is known to activate two other Cdks in yeast by phosphorylating a threonine within their conserved T-loop domains. Bur1 also has the conserved threonine within its T loop and is therefore a potential direct target of Cak1. Additional tests establish a direct functional interaction between Cak1 and the Bur1-Bur2 Cdk complex: Bur1 is phosphorylated in vivo, both the conserved Bur1 T-loop threonine and Cak1 are required for phosphorylation and Bur1 function in vivo, and recombinant Cak1 stimulates CTD kinase activity of the purified Bur1-Bur2 complex in vitro. Thus, both genetic and biochemical evidence demonstrate that Cak1 is a physiological regulator of the Bur1 kinase.