In yeast cells, a single Cdk protein binds all classes of cyclins and triggers different cell-cycle events by changing cyclin partners at different stages of the cycle. In vertebrate cells, by contrast, there are four Cdks. Two interact with G1-cyclins, one with G1/Sand S-cyclins, and one with Sand M-cyclins. In this chapter, we simply refer to the different cyclin–Cdk complexes as G1-Cdk, G1/S-Cdk, S-Cdk, and M-Cdk. Table 17–1 lists the names of the individual Cdks and cyclins.
How do different cyclin–Cdk complexes trigger different cell-cycle events? The answer, at least in part, seems to be that the cyclin protein does not simply activate its Cdk partner but also directs it to specific target proteins. As a result, each cyclin–Cdk complex phosphorylates a different set of substrate proteins. The same cyclin–Cdk complex can also induce different effects at different times in the cycle, probably because the accessibility of some Cdk substrates changes during the cell cycle. Certain proteins that function in mitosis, for example, may become available for phosphorylation only in G2.
Figure 17–11 Cyclin–Cdk complexes of the cell-cycle control system. The concentrations of the three major cyclin types oscillate during the cell cycle, while the concentrations of Cdks (not shown) do not change and exceed cyclin amounts. In late g1, rising g1/s-cyclin levels lead to the formation of g1/s-Cdk complexes that trigger progression through the start transition. s-Cdk complexes form at the start of s phase and trigger dna replication, as well as some early mitotic events. m-Cdk complexes form during g2 but are held in an inactive state; they are activated at the end of g2 and trigger entry into mitosis at the g2/m transition. a separate regulatory protein complex, the apC/C, initiates the metaphase-toanaphase transition, as we discuss later.
Studies of the three-dimensional structures of Cdk and cyclin proteins have revealed that, in the absence of cyclin, the active site in the Cdk protein is partly obscured by a protein loop, like a stone blocking the entrance to a cave (Figure 17–12A). Cyclin binding causes the loop to move away from the active site, resulting in partial activation of the Cdk enzyme (Figure 17–12B). Full activation of the cyclin–Cdk complex then occurs when a separate kinase, the Cdk-activating kinase (CAK), phosphorylates an amino acid near the entrance of the Cdk active site. This causes a small conformational change that further increases the activity of the Cdk, allowing the kinase to phosphorylate its target proteins effectively and thereby induce specific cell-cycle events (Figure 17–12C).
The rise and fall of cyclin levels is the primary determinant of Cdk activity during the cell cycle. Several additional mechanisms, however, help control Cdk activity at specific stages of the cycle.
Phosphorylation at a pair of amino acids in the roof of the kinase active site inhibits the activity of a cyclin–Cdk complex. Phosphorylation of these sites by a protein kinase known as Wee1 inhibits Cdk activity, while dephosphorylation of these sites by a phosphatase known as Cdc25 increases Cdk activity (Figure 17–13). We will see later that this regulatory mechanism is particularly important in the control of M-Cdk activity at the onset of mitosis.
Binding of Cdk inhibitor proteins (CKIs) inactivates cyclin–Cdk complexes. The three-dimensional structure of a cyclin–Cdk–CKI complex reveals that CKI binding stimulates a large rearrangement in the structure of the Cdk active site, rendering it inactive (Figure 17–14). Cells use CKIs primarily to help govern the activities of G1/Sand S-Cdks early in the cell cycle.
Regulated proteolysis Triggers the metaphase-to-anaphase Transition