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PKC phosphorylates HEXIM1 and regulates P-TEFb activity.
Koh Fujinaga1, Matjaz Barboric, Qintong Li
1Departments of Medicine, Microbiology and Immunology, Rosalind Russell Research Center, University of California, San Francisco, San Francisco, CA 94143-0703, USA.
Nucleic Acids Research
|July 24, 2012
Summary
Protein kinase C (PKC) phosphorylates HEXIM1, disrupting its binding to 7SK snRNA and P-TEFb. This phosphorylation releases P-TEFb, increasing transcription and impacting cellular regulation.
Area of Science:
- Molecular Biology
- Cellular Regulation
- Gene Transcription
Background:
- Positive transcription elongation factor b (P-TEFb) controls RNA polymerase II elongation.
- P-TEFb exists in active and inactive states, with HEXIM1 and 7SK snRNA forming inactive 7SK snRNP.
- Cellular stimuli can alter the equilibrium between active and inactive P-TEFb.
Purpose of the Study:
- To investigate the role of protein kinase C (PKC) in regulating P-TEFb activity.
- To elucidate the mechanism by which PKC influences the 7SK snRNP complex formation and P-TEFb inhibition.
Main Methods:
- Investigated the effect of PKC on HEXIM1 phosphorylation at serine 158 (S158).
- Assessed the binding of phosphorylated HEXIM1 to 7SK snRNA and its inhibitory effect on P-TEFb.
- Utilized phorbol esters, T cell antigen receptor engagement, and constitutively active (CA) PKCθ to activate PKC signaling.
- Employed kinase-negative PKCθ and S158A mutant HEXIM1 to validate the regulatory role of phosphorylation.
Main Results:
- PKC phosphorylates HEXIM1 at S158, preventing its binding to 7SK snRNA and P-TEFb.
- PKC activation by phorbol esters, T cell receptor engagement, or CA PKCθ reduces 7SK snRNP formation.
- These stimuli lead to increased P-TEFb-dependent transcription.
- Kinase-inactive PKCθ and S158A HEXIM1 mutant abolish the effects of PKC-activating stimuli.
Conclusions:
- PKC-mediated phosphorylation of HEXIM1 at S158 is a critical regulatory mechanism.
- This phosphorylation event releases P-TEFb, thereby enhancing P-TEFb-dependent transcription.
- PKC acts as a key regulator of P-TEFb activity through HEXIM1 modification.
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