title: Cyclin box structure of the P-TEFb subunit cyclin T1 derived from a fusion complex with EIAV tat.
abstract: The positive transcription elongation factor b (P-TEFb) is an essential regulator of viral gene expression during the life cycle of human immunodeficiency virus type 1 (HIV-1). Its cyclin T1 subunit forms a ternary complex with the viral transcriptional transactivator (Tat) protein and the transactivation response (TAR) RNA element thereby activating cyclin dependent kinase 9 (Cdk9), which stimulates transcription at the level of chain elongation. We report the structure of the cyclin box domain of human cyclin T1 at a resolution of 2.67 A. The structure was obtained by crystallographic analysis of a fusion protein composed of cyclin T1 linked to the transactivator protein Tat from equine infectious anemia virus (EIAV), which is functionally and structurally related to HIV-1 Tat. The conserved cyclin box domain of cyclin T1 exhibits structural features for interaction with physiological binding partners such as Cdk9. A recognition site for Cdk/Cyclin substrates is partly covered by a cyclin T-specific insert, suggesting specific interactions with regulatory factors. The previously identified Tat/TAR recognition motif (TRM) forms a C-terminal helix that is partly occluded in the cyclin box repeat interface, while cysteine 261 is accessible to form an intermolecular zinc finger with Tat. Residues of the TRM contribute to a positively charged groove that may directly attract RNA molecules. The EIAV Tat protein instead appeared undefined from the electron density map suggesting that it is highly disordered. Functional experiments confirmed the TAR binding properties of the fusion protein and suggested residues on the second cyclin box repeat to contribute to Tat stimulated transcription.
authors.affiliation: EMBL Heidelberg, Structural and Computational Biology & Developmental Biology Unit, 69117 Heidelberg, Germany.
authors.forename: Kanchan
authors.initials: K
authors.lastname: Anand
authors.forename: Antje
authors.initials: A
authors.lastname: Schulte
authors.forename: Koh
authors.initials: K
authors.lastname: Fujinaga
authors.forename: Klaus
authors.initials: K
authors.lastname: Scheffzek
authors.forename: Matthias
authors.initials: M
authors.lastname: Geyer
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D018360:Crystallography, X-Ray; D056906:Cyclin T; D042863:Cyclin-Dependent Kinase 9 / Q000737:chemistry; D016213:Cyclins / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics; D024202:Electrophoretic Mobility Shift Assay; D015696:Gene Products, tat / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics; D006801:Humans; D004860:Infectious Anemia Virus, Equine / Q000235:genetics*; D051379:Mice; D008958:Models, Molecular; D008969:Molecular Sequence Data; D041681:NIH 3T3 Cells; D011487:Protein Conformation; D021122:Protein Subunits / Q000096:biosynthesis / Q000737:chemistry / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000737:chemistry* / Q000235:genetics
chemical_list: C503684:CCNT1 protein, human; D056906:Cyclin T; D016213:Cyclins; D015696:Gene Products, tat; D021122:Protein Subunits; D011993:Recombinant Fusion Proteins; D042863:Cyclin-Dependent Kinase 9