title: Characterization of Cdk9 T-loop phosphorylation in resting and activated CD4(+) T lymphocytes.
abstract: The cellular kinase complex P-TEFb is composed of Cdk9 and cyclin T, and it is required for expression of most protein-coding genes by RNAP II. Cdk9 has been shown recently to be activated in cis by autophosphorylation of Thr186 in its T-loop. Using a phosphospecific Cdk9 antibody, we examined the level of Cdk9 T-loop phosphorylation in resting and activated CD4(+) T lymphocytes. Cdk9 T-loop phosphorylation was found to be low-to-undetectable in resting CD4(+) T lymphocytes, and upon activation by distinct stimuli, there is a rapid (<1 h) increase in pCdk9 that does not require protein synthesis. The low level of Cdk9 T-loop phosphorylation was not to be a result of the absence of an associated regulatory cyclin partner. These observations suggest that autophosphorylation of the Cdk9 T-loop is repressed in resting CD4(+) T lymphocytes. The low level of T-loop phosphorylation in resting cells is also reflected in a low level of phosphorylation of Ser2 in the carboxyl terminal domain of RNAP II, suggesting that lack of Cdk9 T-loop autophosphorylation may limit RNAP II elongation in quiescent CD4(+) T lymphocytes.
authors.affiliation: Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas 77030, USA.
authors.forename: Rajesh
authors.initials: R
authors.lastname: Ramakrishnan
authors.forename: Eugene C
authors.initials: EC
authors.lastname: Dow
authors.forename: Andrew P
authors.initials: AP
authors.lastname: Rice
mesh_terms: D015496:CD4-Positive T-Lymphocytes / Q000201:enzymology / Q000276:immunology*; D056906:Cyclin T / Q000235:genetics / Q000378:metabolism; D042863:Cyclin-Dependent Kinase 9 / Q000276:immunology* / Q000378:metabolism; D005786:Gene Expression Regulation / Q000276:immunology*; D006801:Humans; D008213:Lymphocyte Activation / Q000276:immunology*; D010766:Phosphorylation / Q000276:immunology; D042925:Positive Transcriptional Elongation Factor B / Q000235:genetics / Q000378:metabolism; D017434:Protein Structure, Tertiary / Q000502:physiology; D012319:RNA Polymerase II / Q000276:immunology* / Q000378:metabolism
chemical_list: D056906:Cyclin T; D042925:Positive Transcriptional Elongation Factor B; C467969:CDK9 protein, human; D042863:Cyclin-Dependent Kinase 9; D012319:RNA Polymerase II