CDK-activating kinase

In the second step, CAK must phosphorylate the cyclin-Cdk complex on the threonine residue 160, which is located in the Cdk activation segment.

Phosphorylation is generally considered a reversible modification used to change enzyme activity in different conditions.

In fact, CAK activity remains high throughout the cell cycle and is not regulated by any known cell-cycle control mechanism.

In budding yeast, activating phosphorylation by CAK can take place before cyclin binding.

In both humans and yeast, cyclin binding is the rate limiting step in the activation of Cdk.

Therefore, phosphorylation of Cdk by CAK is considered a post-translational modification that is necessary for enzyme activity.

In fact, the Cdk7 subunit of vertebrate CAK phosphorylates several components of the transcriptional machinery.

Msc6 and Msc2 complex not only activates cell cycle Cdks but also regulates gene expression because it is part of the transcription factor TFIIH.

Credit to: Oxford University Press "Morgan: The Cell Cycle" The conformation of the Cdk2 active site changes dramatically upon cyclin binding and CAK phosphorylation.

In its inactive form, Cdk2 cannot bind substrate because the entrance of its active site is blocked by the T-loop.

Credit to: Oxford University Press "Morgan: The Cell Cycle" In addition to activating Cdks, CAK also regulates transcription.

CAK associated with TFIIH phosphorylates proteins involved in transcription including RNA polymerase II.

More specifically, associated CAK is involved in promoter clearance and progression of transcription from the preinitiation to the initiation stage.

In leukemic cells, where DNA is damaged, CAK’s ability to phosphorylate retinoic acid and estrogen receptors is decreased.

[1] The activity of CAK associated with TFIIH decreases when DNA is damaged by UV irradiation.

Cyclin binding alone causes partial activation of Cdks, but complete activation also requires activating phosphorylation by CAK. In animal cells, CAK phosphorylates the Cdk subunit only after cyclin binding, and so the two steps in Cdk activation are usually ordered as shown here, with cyclin binding occurring first. Budding yeast contains a different version of CAK that can phosphorylate the Cdk even in the absence of cyclin, and so the two activation steps can occur in either order. In all cases, CAK tends to be in constant excess in the cell, so that cyclin binding is the rate-limiting step in Cdk activation.
In animals (for example, H. sapiens, left), a trimeric CAK enzyme containing Cdk7 functions both in the activation of Cdks and in the regulation of transcription by RNA polymerase II. In the budding yeast S. cerevisiae (right) the homologous enzyme, Kin28, does not contribute to Cdk activation but is focused entirely on control of transcription. In this species, an unrelated protein kinase, Cak1, activates Cdks. The fission yeast S. pombe (center) occupies an intermediate position, in which Cdk activation can be achieved both by the Cdk7 homolog Mcs6 and by a Cak1 homolog, Csk1. Cdk7, Kin28 and Mcs6 are all Cdks whose activities are also enhanced by phosphorylation of residues in their T-loops. In budding and fission yeasts, this phosphorylation is carried out by Cak1 and Csk1, respectively. The kinase that phosphorylates Cdk7 in animals is not clear.