CDK9 Is Constitutively Expressed throughout the Cell Cycle, and Its Steady-State Expression Is Independent of SKP2

Abstract

CDK9 is a CDC2-related kinase and the catalytic subunit of the positive-transcription elongation factor b and the Tat-activating kinase. It has recently been reported that CDK9 is a short-lived protein whose levels are regulated during the cell cycle by the SCF SKP2 ubiquitin ligase complex (R. E. Kiernan et al., Mol. Cell. Biol. 21:7956-7970, 2001). The results presented here are in contrast to those observations. CDK9 protein levels remained unchanged in human cells entering and progressing through the cell cycle from G 0 , despite dramatic changes in SKP2 expression. CDK9 levels also remained unchanged in cells exiting from mitosis and progressing through the next cell cycle. Similarly, the levels of CDK9 protein did not change as cells exited the cell cycle and differentiated along various lineages. In keeping with these observations, the kinase activity associated with CDK9 was found to not be regulated during the cell cycle. We have also found that endogenous CDK9 is a very stable protein with a half-life (t 1/2 ) of 4 to 7 h, depending on the cell type. In contrast, when CDK9 is overexpressed, it is not stabilized and is rapidly degraded, with a t 1/2 of less than 1 h, depending on the level of expression. Treatment of cells with proteasome inhibitors blocked the degradation of short-lived proteins, such as p27, but did not affect the expression of endogenous CDK9. Ectopic overexpression of SKP2 led to reduction of p27 protein levels but had no effect on the expression of endogenous CDK9. Finally, downregulation of endogenous SKP2 gene expression by interfering RNA had no effect on CDK9 protein levels, whereas p27 protein levels increased dramatically. Therefore, the SCF SKP2 ubiquitin ligase does not regulate CDK9 expression in a cell cycle-dependent manner.CDK9 is the catalytic subunit of the positive-transcription elongation factor b (P-TEFb) (30). CDK9 was first identified as a CDC2-related kinase named PITALRE, which associated with a number of cellular polypeptides and exhibited a kinase activity that was not regulated during the cell cycle (14). CDK9 associates with either cyclin T1, T2a, T2b, or K to form active P-TEFb complexes, which activate transcriptional elongation by phosphorylating the C-terminal domain (CTD) of RNA polymerase II (RNAPII) as well as negative-transcription elongation factors (reviewed in reference 26). CDK9 is tethered to promoters by certain transcription factors, such as NF-B, the androgen receptor, and c-myc, resulting in stimulation of transcription (2, 8, 18). CDK9/cyclin T1 complexes are also the target of the human immunodeficiency virus transcriptional activator Tat, which recruits this complex to the transactivation response structure in the nascent human immunodeficiency virus RNA and positions the kinase complex to phosphorylate the CTD of the RNAPII, and other transcription factors, resulting in stimulation of transcriptional elongation that would otherwise be interrupted (26).CDK9 has also been found to associate with components of a chaperone pathway, includin...

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