Structure and dynamics of a stabilized coiled-coil domain in the P-TEFb regulator Hexim1

J Mol Biol. 2011 Dec 16;414(5):639-53. doi: 10.1016/j.jmb.2011.10.022. Epub 2011 Oct 18.


The positive transcription elongation factor P-TEFb mediates the transition from transcription initiation to productive elongation by phosphorylation of the C-terminal domain of RNA polymerase II. P-TEFb is negatively regulated by the cellular protein Hexim1 (hexamethylene bisacetamide-inducible protein 1), which is highly conserved in higher eukaryotes. The C-terminal coiled-coil domain of Hexim1 recognizes the Cyclin T subunit of P-TEFb, whereas a central PYNT motif is required to inhibit the cyclin-dependent kinase Cdk9 by a yet unknown mechanism. Here, the crystal structure of the Cyclin T-binding domain (TBD) of human Hexim1 was determined at 2.1 Å resolution using a deletion mutant of three residues in its central stammer motif. The structure showed a continuous parallel coiled-coil domain of nine hepta-repeats with a preceding helix encompassing up to 15 residues. Two uncommon residues at heptad a positions in the N-terminal part of the coiled-coil structure, Lys284 and Tyr291, stabilize the preceding helix by a tight intermolecular hydrogen bond network with residues of the opposing chain. These interactions delineate a characteristic turn between both helices that is supposed to mediate binding to Cyclin T1. Stabilization of the coiled-coil domain by deletion of the stammer region was confirmed by NMR spectroscopic and backbone dynamic analyses analyzing wild-type TBD and three mutant variants. This study thus provides structural insights into the recognition of the regulator protein Hexim1 by P-TEFb and the modulation of coiled-coil dynamics by specific discontinuities.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Motifs
  • Amino Acid Sequence
  • Crystallography, X-Ray
  • Humans
  • Molecular Sequence Data
  • Positive Transcriptional Elongation Factor B / chemistry*
  • Protein Conformation
  • RNA-Binding Proteins / chemistry*
  • Transcription Factors


  • HEXIM1 protein, human
  • RNA-Binding Proteins
  • Transcription Factors
  • Positive Transcriptional Elongation Factor B

Associated data

  • PDB/3S9G