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J Biol Chem. 2008 Nov 28;283(48):33578-84. doi: 10.1074/jbc.M807495200. Epub 2008 Oct 1.

Phosphatase PPM1A regulates phosphorylation of Thr-186 in the Cdk9 T-loop.

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  • 1Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas 77030, USA.

Abstract

Cdk9 is the catalytic subunit of a general RNA polymerase II elongation factor known as positive transcription elongation factor b (P-TEFb). The kinase function of P-TEFb requires phosphorylation of Thr-186 in the T-loop of Cdk9 to allow substrates to access the catalytic core of the enzyme. To identify human phosphatases that dephosphorylate the T-loop of Cdk9, we used a Thr-186-phosphospecific antiserum to screen a phosphatase expression library. Overexpression of PPM1A and the related PPM1B greatly reduced Cdk9 T-loop phosphorylation in vivo. PPM1A and Cdk9 appear to associate in vivo as the proteins could be co-immunoprecipitated. The short hairpin RNA depletion of PPM1A resulted in an increase in Cdk9 T-loop phosphorylation. In phosphatase reactions in vitro, purified PPM1A could dephosphorylate Thr-186 both with and without the association of 7SK RNA, a small nuclear RNA that is bound to approximately 50% of total cellular P-TEFb. PPM1B only efficiently dephosphorylated Cdk9 Thr-186 in vitro when 7SK RNA was depleted from P-TEFb. Taken together, our data indicate that PPM1A and to some extent PPM1B are important negative regulators of P-TEFb function.

PMID:
18829461
[PubMed - indexed for MEDLINE]
PMCID:
PMC2586277
Free PMC Article
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