The yeast CTDK-I complex has been implicated in
phosphorylation of the RNA polymerase II CTD and in transcription
control. It is a tripartite cyclin-dependent kinase complex that, in
addition of a kinase subunit (Ctk1) and a C-type cyclin subunit
(Ctk2), contains a third factor of unknown function, (Ctk3), that
displays no homology with known proteins. We previously showed that
the Ctk2 cyclin is, similarly to cell cycle cyclin, phosphorylated and
rapidly degraded by the ubiquitin-proteasome pathway (Hautbergue and
Goguel, 1999). Interestingly, Ctk3 is also an unstable protein whose
destruction by the ubiquitin-mediated proteasome pathway requires the
integrity of a PEST sequence. Our in vitro results show that Ctk3 is
not required for the association of Ctk1 with the Ctk2 cyclin,
indicating that it is not an assembly factor. Furthermore, Ctk3
interacts directly with the Ctk1 kinase, and is absolutely required
for its CTD kinase activity. Altogether, our results suggest that, in
contrast to other cyclin-dependent kinases, Ctk1 is not solely
activated by its cyclin, but by the Ctk2/Ctk3 heterodimer.
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