title: Human and yeast cdk-activating kinases (CAKs) display distinct substrate specificities.
abstract: Cell cycle progression is controlled by the sequential functions of cyclin-dependent kinases (cdks). Cdk activation requires phosphorylation of a key residue (on sites equivalent to Thr-160 in human cdk2) carried out by the cdk-activating kinase (CAK). Human CAK has been identified as a p40(MO15)/cyclin H/MAT1 complex that also functions as part of transcription factor IIH (TFIIH) where it phosphorylates multiple transcriptional components including the C-terminal domain (CTD) of the large subunit of RNA polymerase II. In contrast, CAK from budding yeast consists of a single polypeptide (Cak1p), is not a component of TFIIH, and lacks CTD kinase activity. Here we report that Cak1p and p40(MO15) have strikingly different substrate specificities. Cak1p preferentially phosphorylated monomeric cdks, whereas p40(MO15) preferentially phosphorylated cdk/cyclin complexes. Furthermore, p40(MO15) only phosphorylated cdk6 bound to cyclin D3, whereas Cak1p recognized monomeric cdk6 and cdk6 bound to cyclin D1, D2, or D3. We also found that cdk inhibitors, including p21(CIP1), p27(KIP1), p57(KIP2), p16(INK4a), and p18(INK4c), could block phosphorylation by p40(MO15) but not phosphorylation by Cak1p. Our results demonstrate that although both Cak1p and p40(MO15) activate cdks by phosphorylating the same residue, the structural mechanisms underlying the enzyme-substrate recognition differ greatly. Structural and physiological implications of these findings will be discussed.
authors.affiliation: Yale University School of Medicine, Department of Molecular Biophysics and Biochemistry, New Haven, Connecticut 06520-8024, USA.
authors.forename: P
authors.initials: P
authors.lastname: Kaldis
authors.forename: A A
authors.initials: AA
authors.lastname: Russo
authors.forename: H S
authors.initials: HS
authors.lastname: Chou
authors.forename: N P
authors.initials: NP
authors.lastname: Pavletich
authors.forename: M J
authors.initials: MJ
authors.lastname: Solomon
mesh_terms: D042846:CDC2-CDC28 Kinases; D051357:Cyclin-Dependent Kinase 2; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D016213:Cyclins / Q000378:metabolism; D004789:Enzyme Activation; D006801:Humans; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D013379:Substrate Specificity; D015003:Yeasts / Q000201:enzymology; D000095043:Cyclin-Dependent Kinase-Activating Kinase
chemical_list: D016213:Cyclins; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; C495911:CDK6 protein, human; D051357:Cyclin-Dependent Kinase 2; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases; D000095043:Cyclin-Dependent Kinase-Activating Kinase