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


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.

DOI: 10.1074/jbc.M807495200

Cite this paper

@article{Wang2008PhosphatasePR, title={Phosphatase PPM1A regulates phosphorylation of Thr-186 in the Cdk9 T-loop.}, author={Yan Wang and Eugene C Dow and Yao-Yun Liang and Rajesh Ramakrishnan and Hongbing Liu and T Sung and Xia Lin and Andrew P. Rice}, journal={The Journal of biological chemistry}, year={2008}, volume={283 48}, pages={33578-84} }