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Cell cycle analysis of the activity, subcellular localization, and subunit composition of human CAK (CDK-activating kinase)

Tassan, J. P. and Schultz, S. J. and Bartek, J. and Nigg, E. A.. (1994) Cell cycle analysis of the activity, subcellular localization, and subunit composition of human CAK (CDK-activating kinase). The Journal of cell biology, Vol. 127, H. 2. pp. 467-478.

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Official URL: http://edoc.unibas.ch/dok/A5249461

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Abstract

The activity of cyclin-dependent kinases (cdks) depends on the phosphorylation of a residue corresponding to threonine 161 in human p34cdc2. One enzyme responsible for phosphorylating this critical residue has recently been purified from Xenopus and starfish. It was termed CAK (for cdk-activating kinase), and it was shown to contain p40MO15 as its catalytic subunit. In view of the cardinal role of cdks in cell cycle control, it is important to learn if and how CAK activity is regulated during the somatic cell cycle. Here, we report a molecular characterization of a human p40MO15 homologue and its associated CAK activity. We have cloned and sequenced a cDNA coding for human p40MO15, and raised specific polyclonal and monoclonal antibodies against the corresponding protein expressed in Escherichia coli. These tools were then used to demonstrate that p40MO15 protein expression and CAK activity are constant throughout the somatic cell cycle. Gel filtration suggests that active CAK is a multiprotein complex, and immunoprecipitation experiments identify two polypeptides of 34 and 32 kD as likely complex partners of p40MO15. The association of the three proteins is near stoichiometric and invariant throughout the cell cycle. Immunocytochemistry and biochemical enucleation experiments both demonstrate that p40MO15 is nuclear at all stages of the cell cycle (except for mitosis, when the protein redistributes throughout the cell), although the p34cdc2/cyclin B complex, one of the major purported substrates of CAK, occurs in the cytoplasm until shortly before mitosis. The absence of obvious changes in CAK activity in exponentially growing cells constitutes a surprise. It suggests that the phosphorylation state of threonine 161 in p34cdc2 (and the corresponding residue in other cdks) may be regulated primarily by the availability of the cdk/cyclin substrates, and by phosphatase(s).
Faculties and Departments:05 Faculty of Science > Departement Biozentrum
05 Faculty of Science > Departement Biozentrum > Former Organization Units Biozentrum > Cell Biology (Nigg)
UniBasel Contributors:Nigg, Erich A.
Item Type:Article, refereed
Article Subtype:Research Article
Bibsysno:Link to catalogue
Publisher:Rockefeller University Press
ISSN:0021-9525
Note:Publication type according to Uni Basel Research Database: Journal article
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Last Modified:05 Jan 2016 08:46
Deposited On:22 Mar 2012 13:17

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