title: Crystal structure of human cyclin K, a positive regulator of cyclin-dependent kinase 9.
abstract: Cyclin K and the closely related cyclins T1, T2a, and T2b interact with cyclin-dependent kinase 9 (CDK9) forming multiple nuclear complexes, referred to collectively as positive transcription elongation factor b (P-TEFb). Through phosphorylation of the C-terminal domain of the RNA polymerase II largest subunit, distinct P-TEFb species regulate the transcriptional elongation of specific genes that play central roles in human physiology and disease development, including cardiac hypertrophy and human immunodeficiency virus-1 pathogenesis. We have determined the crystal structure of human cyclin K (residues 11-267) at 1.5 A resolution, which represents the first atomic structure of a P-TEFb subunit. The cyclin K fold comprises two typical cyclin boxes with two short helices preceding the N-terminal box. A prominent feature of cyclin K is an additional helix (H4a) in the first cyclin box that obstructs the binding pocket for the cell-cycle inhibitor p27(Kip1). Modeling of CDK9 bound to cyclin K provides insights into the structural determinants underlying the formation and regulation of this complex. A homology model of human cyclin T1 generated using the cyclin K structure as a template reveals that the two proteins have similar structures, as expected from their high level of sequence identity. Nevertheless, their CDK9-interacting surfaces display significant structural differences, which could potentially be exploited for the design of cyclin-targeted inhibitors of the CDK9-cyclin K and CDK9-cyclin T1 complexes.
authors.affiliation: Molecular Medicine Laboratory and Macromolecular Crystallography Unit, Division of Experimental Medicine, Harvard Institutes of Medicine, Harvard Medical School, Boston, MA 02115, USA.
authors.forename: Kyuwon
authors.initials: K
authors.lastname: Baek
authors.forename: Raymond S
authors.initials: RS
authors.lastname: Brown
authors.forename: Gabriel
authors.initials: G
authors.lastname: Birrane
authors.forename: John A A
authors.initials: JA
authors.lastname: Ladias
mesh_terms: D000595:Amino Acid Sequence; D018360:Crystallography, X-Ray; D042863:Cyclin-Dependent Kinase 9 / Q000737:chemistry*; D016213:Cyclins / Q000737:chemistry*; D006801:Humans; D008958:Models, Molecular; D008969:Molecular Sequence Data; D046912:Multiprotein Complexes / Q000737:chemistry; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D017386:Sequence Homology, Amino Acid; D013329:Structure-Activity Relationship; D014157:Transcription Factors / Q000737:chemistry
chemical_list: C113028:CCNK protein, human; D016213:Cyclins; D046912:Multiprotein Complexes; D014157:Transcription Factors; D042863:Cyclin-Dependent Kinase 9