The role of Cdk7 in CAK function, a retro-retrospective.

Most transitions in the eukaryotic cell cycle are catalyzed by cyclin-dependent kinases (Cdks, CDKs), complexes formed between a member of the Cdk family of protein kinases and a regulatory cyclin subunit. CDKs are among the most highly regulated enzymes known: Their activities are controlled through multiple mechanisms that include cyclin association, positive and negative phosphorylation events, negative regulation through association with Cdk inhibitors (CKIs), and association with accessory proteins such as Cks/Suc1 (Fig. 1) (reviewed in Morgan 1997). In addition to controlling Cdk activation, the cyclin subunit may also contribute to substrate specificity. These elaborate regulatory pathways reflect the critical roles of cyclin–kinases in the life of a cell; alterations that generate unregulated cyclin– kinase activity can promote improper proliferation that can result in developmental defects or proliferative diseases such as cancer (Sherr 1996). The Cdk subunit alone is inactive and requires both association with a cyclin and phosphorylation on a conserved threonine residue (T161 in human Cdc2 and T160 in human Cdk2) for full activation (Gould et al. 1991; Desai et al. 1992; Solomon et al. 1992; Connell-Crowley et al. 1993). In the case of Cdk2, these two events account for an increase in activity of greater than seven orders of magnitude (Connell-Crowley et al. 1993; Russo 1997). Cyclin A promotes Cdk2 activation by inducing two major structural changes in the kinase that are likely to be conserved in other cyclin/Cdk pairs (Jeffrey et al. 1996; Russo et al. 1996). First, cyclin binding leads to a conformational change in the amino-terminal PSTAIRE/helix that reorients the catalytic residue glutamate-51. This residue is conserved in all eukaryotic protein kinases and is involved in ATP binding. In addition, cyclin binding leads to a change in the orientation of the T-loop, the region of Cdk2 containing the activating threonine phosphorylation site T160. In the absence of cyclin, the T-loop blocks substrate access to the active site but the conformational changes induced by cyclin largely relieve this steric block. This conformational change in the T-loop may also be critical for the activating phosphorylation event because T160 is not particularly solvent-exposed in monomeric Cdk2 but is well exposed in the cyclin/Cdk2 complex. Completion of the Cdk activation process involves phosphorylation of the T-loop threonine by Cdk activating kinase (CAK), a modification that promotes further structural changes in the T-loop (Russo et al. 1996).