Differential localization and expression of the Cdk9 42k and 55k isoforms
Article first published online: 27 SEP 2004
Copyright © 2004 Wiley-Liss, Inc.
Journal of Cellular Physiology
Volume 203, Issue 1, pages 251–260, April 2005
How to Cite
Liu, H. and Herrmann, C. H. (2005), Differential localization and expression of the Cdk9 42k and 55k isoforms. J. Cell. Physiol., 203: 251–260. doi: 10.1002/jcp.20224
- Issue published online: 25 JAN 2005
- Article first published online: 27 SEP 2004
- Manuscript Accepted: 12 AUG 2004
- Manuscript Received: 29 JUN 2004
- National Institutes of Health. Grant Number: AI42558
Cdk9, a member of the cyclin-dependent kinase family, is the catalytic subunit of P-TEFb, a protein kinase complex that stimulates transcriptional elongation. Cdk9, complexed with its regulatory partner cyclin T1, serves as the cellular mediator of the transactivation function of the HIV Tat protein. There are two known isoforms of Cdk9: a 42 kDa protein (42k, originally identified as PITALRE) and a more recently identified 55 kDa form (55k). To investigate possible functional differences between the two isoforms, we examined their kinase activities, their subcellular distributions, and their expression levels in primary cells relevant to HIV infection. Both isoforms were found to hyper-phosphorylate the carboxyl-terminal domain of the largest subunit of RNA polymerase II and displayed identical phosphorylation patterns with 144 peptide substrates. Epitope-tagged transiently-expressed Cdk9 42k localized diffusely in the nucleoplasm, while Cdk9 55k accumulated in the nucleolus. In primary undifferentiated monocytes, Cdk9 55k expression was not detected although 42k was present at high levels; however, 55k expression was induced upon macrophage differentiation. In primary lymphocytes, the levels of 55k decreased or remained steady following activation, while the levels of 42k increased. The promoter for 42k was significantly stronger than that of 55k in HeLa cells, and only the 42k promoter was responsive to activation signals in primary lymphocytes. These results indicate that expression of the 42k and 55k isoforms is differentially regulated and suggest that functional differences between the 42k and 55k isoforms of Cdk9 are likely to depend on access to substrates based on their differential subcellular localization and expression patterns. © 2004 Wiley-Liss, Inc.