Cdk9 T-loop phosphorylation is regulated by the calcium signaling pathway

Abstract

Eukaryotic RNA polymerase II transcriptional elongation is a tightly regulated process and is dependent upon positive transcription elongation factor-b (P-TEFb). The core P-TEFb complex is composed of Cdk9 and Cyclin T and is essential for the expression of most protein coding genes. Cdk9 kinase function is dependent upon phosphorylation of Thr186 in its T-loop. In this study, we examined kinases and signaling pathways that influence Cdk9 T-loop phosphorylation. Using an RNAi screen in HeLa cells, we found that Cdk9 T-loop phosphorylation is regulated by Ca 2+/calmodulin-dependent kinase 1D (CaMK1D). Using small molecules inhibitors in HeLa cells and primary CD4 + T lymphocytes, we found that the Ca 2+ signaling pathway is required for Cdk9 T-loop phosphorylation. Inhibition of Ca 2+ signaling led to dephosphorylation of Thr186 on Cdk9. In reporter plasmid assays, inhibition of the Ca 2+ signaling pathway repressed the PCNA promoter and HIV-1 Tat transactivation of the HIV-1 LTR, but not HTLV-1 Tax transactivation of the HTLV-1 LTR, suggesting that perturbation of the Ca 2+ pathway and reduction of Cdk9 T-loop phosphorylation inhibits transcription units that have a rigorous requirement for P-TEFb function. © 2011 Wiley Periodicals, Inc.