title: A novel cyclin associates with MO15/CDK7 to form the CDK-activating kinase.
abstract: Phosphorylation by the CDK-activating kinase (CAK) is a required step in the activation of cyclin-dependent kinases. We have purified CAK from mammalian cells; the enzyme comprises two major polypeptides of 42 and 37 kDa. Protein sequencing indicates that the 42 kDa subunit is the mammalian homolog of MO15, a protein kinase known to be a component of CAK in amphibians and echinoderms. Cloning of a cDNA encoding the 37 kDa subunit identifies it as a novel cyclin (cyclin H). We have reconstituted CAK in vitro with the MO15 catalytic subunit and cyclin H, demonstrating that MO15 is a cyclin-dependent kinase (CDK7). Like other CDKs, MO15/CDK7 contains a conserved threonine required for full activity; mutation of this residue severely reduces CAK activity. The CAK holoenzyme activates complexes of CDK2 and CDC2 with various cyclins and also phosphorylates CDK2, but not CDC2, in the absence of cyclin. Thus, CAK is a CDK-cyclin complex implicated in the control of multiple cell cycle transitions.
authors.affiliation: Department of Physiology, University of California, San Francisco 94143-0444.
authors.forename: R P
authors.initials: RP
authors.lastname: Fisher
authors.forename: D O
authors.initials: DO
authors.lastname: Morgan
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D003001:Cloning, Molecular; D056748:Cyclin H; D018844:Cyclin-Dependent Kinases; D016213:Cyclins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018076:DNA, Complementary; D006367:HeLa Cells; D006801:Humans; D051379:Mice; D008954:Models, Biological; D008969:Molecular Sequence Data; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000302:isolation & purification / Q000378:metabolism; D016415:Sequence Alignment; D017421:Sequence Analysis; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D000095043:Cyclin-Dependent Kinase-Activating Kinase
chemical_list: C534751:CCNH protein, human; C534752:Ccnh protein, mouse; D056748:Cyclin H; D016213:Cyclins; D018076:DNA, Complementary; D011993:Recombinant Fusion Proteins; D017346:Protein Serine-Threonine Kinases; D018844:Cyclin-Dependent Kinases; D000095043:Cyclin-Dependent Kinase-Activating Kinase