Brd4 is a novel atypical kinase that phosphorylates the RNA Polymerase II C-terminal domain

Tuesday, October 25, 2011 — Poster Session II
Noon – 2:00 p.m.	Natcher Conference Center	NCI	EPIGEN/TRANS/CHROM-1

* FARE Award Winner

Authors

• D Ballachanda
• B Lewis
• N Cherman
• P Robey
• K Ozato
• D Singer

Abstract

Phosphorylation of the carboxy-terminal domain (CTD) of the largest RNA Pol II subunit, RPB1 is critical for recruitment of mRNA capping and splicing factors during transcription. The bromodomain protein Brd4 plays a critical role in CTD phosphorylation, thought to be due to its recruitment of PTEFb, currently considered the principal CTD Ser2 kinase. Despite being documented to regulate transcription of HIV, HPV as well as oncogenes, the precise role of Brd4 in transcription has remained unclear. We report that Brd4 has intrinsic kinase activity and is an atypical kinase which directly binds to the Pol II CTD and phosphorylates it, both in vitro and in vivo. Brd4 specifically phosphorylated CTD Ser2 sites. Brd4 kinase activity is distinct from all other known CTD kinases. In vivo, CTD Ser2 phosphorylation levels increased with increasing levels of an exogenous Brd4 mutant that retained kinase activity but was unable to recruit PTEFb. In a physiological context, Ser2 phosphorylation remains unchanged in bone marrow stromal stem cells and mouse embryonic fibroblasts which have drastically reduced PTEFb, but normal Brd4 levels. Taken together, our results identify Brd4 as a novel Pol II CTD Ser2 kinase and shed new light on its role in transcription.

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