PKC phosphorylates HEXIM1 and regulates P-TEFb activity

Nucleic Acids Res. 2012 Oct;40(18):9160-70. doi: 10.1093/nar/gks682. Epub 2012 Jul 20.


The positive transcription elongation factor b (P-TEFb) regulates RNA polymerase II elongation. In cells, P-TEFb partitions between small active and larger inactive states. In the latter, HEXIM1 binds to 7SK snRNA and recruits as well as inactivates P-TEFb in the 7SK snRNP. Several stimuli can affect this P-TEFb equilibrium. In this study, we demonstrate that protein kinase C (PKC) phosphorylates the serine at position158 (S158) in HEXIM1. This phosphorylated HEXIM1 protein neither binds to 7SK snRNA nor inhibits P-TEFb. Phorbol esters or the engagement of the T cell antigen receptor, which activate PKC and the expression of the constitutively active (CA) PKCθ protein, which is found in T cells, inhibit the formation of the 7SK snRNP. All these stimuli increase P-TEFb-dependent transcription. In contrast, the kinase-negative PKCθ and the mutant HEXIM1 (S158A) proteins block effects of these PKC-activating stimuli. These results indicate that the phosphorylation of HEXIM1 by PKC represents a major regulatory step of P-TEFb activity in cells.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amino Acid Sequence
  • Cell Line
  • Humans
  • Jurkat Cells
  • Molecular Sequence Data
  • NF-kappa B / metabolism
  • Phosphorylation
  • Positive Transcriptional Elongation Factor B / metabolism*
  • Protein Kinase C / metabolism
  • Protein Kinase C-delta / metabolism*
  • RNA, Small Nuclear / metabolism
  • RNA-Binding Proteins / chemistry
  • RNA-Binding Proteins / metabolism*
  • Receptors, Antigen, T-Cell / immunology
  • Serine / metabolism


  • HEXIM1 protein, human
  • NF-kappa B
  • RNA, Small Nuclear
  • RNA-Binding Proteins
  • Receptors, Antigen, T-Cell
  • Serine
  • Positive Transcriptional Elongation Factor B
  • Protein Kinase C
  • Protein Kinase C-delta