A Cdk7-Cdk4 T-Loop Phosphorylation Cascade Promotes G1 Progression

Abstract

Summary Eukaryotic cell division is controlled by cyclin-dependent kinases (CDKs), which require phosphorylation by a CDK-activating kinase (CAK) for full activity. Chemical genetics uncovered requirements for the metazoan CAK Cdk7 in determining cyclin-specificity and activation order of Cdk2 and Cdk1 during S and G2 phases. It was unknown if Cdk7 also activates Cdk4 and Cdk6 to promote passage of the Restriction (R) Point, when continued cell-cycle progression becomes mitogen-independent; or if CDK-activating phosphorylation regulates G1 progression. Here we show that Cdk7 is a Cdk4- and Cdk6-activating kinase in human cells, required to maintain activity, not just to establish the active state, as is the case for Cdk1 and Cdk2. Activating phosphorylation of Cdk7 rises concurrently with that of Cdk4 as cells exit quiescence, and accelerates Cdk4 activation in vitro. Therefore, mitogen signaling drives a CDK-activation cascade during G1 progression, and CAK might be rate-limiting for R-point passage.

Cyclin-Dependent Kinase 7 Controls mRNA Synthesis by Affecting Stability of Preinitiation Complexes, Leading to Altered Gene Expression, Cell Cycle Progression, and Survival of Tumor Cells

“…We find it noteworthy that under the conditions of limited CDK7 inhibition, effects on CDK1 and CDK2 T-loop phosphorylation were not detectable in asynchronously growing tumor cells. We reason that such effects may be detectable only in cells released from serum starvation (8). This finding, together with the late cell cycle response to the CDK7 inhibitors (after several days), suggests that the cell-type-specific alterations result at least in part from alterations in the gene expression program, which may eventually also dictate tumor cell survival.…”

“…Specifically, CDK7 forms the CDK-activating kinase (CAK) with two other TFIIH subunits, cyclin H and MAT1. The CAK activates downstream cell cycle CDKs, including cdc-2/CDK1, CDK2, CDK4, and CDK6, by phosphorylating key threonine residues in a process known as T-loop activation (7,8). In transcription, as RNAPII begins to lose contact with many of the general transcription factors (GTFs) during promoter escape, CDK7, functioning as part of the TFIIH complex, phosphorylates RNAPII and allows the elongation complex to move downstream away from the transcription start site (TSS) (reviewed in reference 9).…”

The CDK4/CDK6 inhibitor PD0332991 paradoxically stabilizes activated cyclin D3-CDK4/6 complexes

“…3,[7][8][9] In contrast to the weak T177 phosphorylation of CDK6, our previous work has identified the activating T172 phosphorylation of CDK4 as the last highly regulated step determining CDK4 activity. 8,[10][11][12][13] Whereas CDK7, the catalytic component of CDK-activating kinase (CAK), is clearly involved in CDK4/6 activation, 14,15 other proline-directed kinases could phosphorylate CDK4 but not CDK6 which lacks the adjacent proline present in the phosphoacceptor domain of CDK4. 13,15 The impacts of p21 and p27 on CDK4/6 activation are complex and remain much debated.…”

Pause, play, repeat

“…25 The dependence of Cdk9 on Cdk7 for full activity also implies that a subset of the effects of Cdk7 inactivation might be due to impaired Cdk9 function, as discussed below. Finally, Cdk7 is also a CDKactivating kinase (CAK) for CDKs that drive cell cycle progression; 27,38,39 the demonstration that it plays a similar role in the transcription cycle supports a unified model of the metazoan CDK network, and suggests potential ways to coordinate gene expression and cell division.…”