A second mechanism that suppresses Cdk activity in G1 depends on the increased production of CKIs, the Cdk inhibitor proteins discussed earlier. Budding yeast cells, in which this mechanism is best understood, contain a CKI protein called Sic1, which binds to and inactivates M-Cdk in late mitosis and G1 (see
Figure 17–52 The creation of a G1 phase by stable Cdk inhibition after mitosis. (a) In early embryonic cell cycles, Cdc20–apC/C activity rises at the end of metaphase, triggering m-cyclin destruction. Because m-Cdk activity stimulates Cdc20–apC/C activity, the loss of m-cyclin leads to apC/C inactivation after mitosis, which allows m-cyclins to begin accumulating again.
(B) In cells that have a g1 phase, the drop in m-Cdk activity in late mitosis leads to the activation of Cdh1–apC/C (as well as to the accumulation of Cdk inhibitor proteins; not shown). This ensures a continued suppression of Cdk activity after mitosis, as required for a g1 phase.
(A) embryonic cells with no G1 phase Cdc20–APC/C activity (B) cells with G1 phase Cdc20–APC/C activity
Table 17–2). Like Cdh1, Sic1 is inhibited by M-Cdk, which phosphorylates Sic1 during mitosis and thereby promotes its ubiquitylation by SCF. Thus, Sic1 and M-Cdk, like Cdh1 and M-Cdk, inhibit each other. As a result, the decline in M-Cdk activity that occurs in late mitosis causes the Sic1 protein to accumulate, and this CKI helps keep M-Cdk activity low after mitosis. A CKI protein called p27 (see Figure 17–14) may serve similar functions in animal cells.
In most cells, decreased transcription of M-cyclin genes also inactivates M-Cdks in late mitosis. In budding yeast, for example, M-Cdk promotes the expression of these genes, resulting in a positive feedback loop. This loop is turned off as cells exit from mitosis: the inactivation of M-Cdk by Cdh1 and Sic1 leads to decreased M-cyclin gene transcription and thus decreased M-cyclin synthesis. Gene regulatory proteins that promote the expression of G1/Sand S-cyclins are also inhibited during G1.
Thus, Cdh1–APC/C activation, CKI accumulation, and decreased cyclin gene expression act together to ensure that the early G1 phase is a time when essentially all Cdk activity is suppressed. As in many other aspects of cell-cycle control, the use of multiple regulatory mechanisms allows the system to operate with reasonable efficiency even if one mechanism fails. So how does the cell escape from this stable G1 state to initiate a new cell cycle? The answer is that G1/S-Cdk activity, which rises in late G1, releases all the braking mechanisms that suppress Cdk activity, as we describe later, in the last section of this chapter.
After mitosis completes the formation of a pair of daughter nuclei, cytokinesis finishes the cell cycle by dividing the cell itself. Cytokinesis depends on a ring of actin and myosin filaments that contracts in late mitosis at a site midway between the segregated chromosomes. In animal cells, the positioning of the contractile ring is determined by signals emanating from the microtubules of the anaphase spindle. Dephosphorylation of Cdk targets, which results from Cdk inactivation in anaphase, triggers cytokinesis at the correct time after anaphase. After cytokinesis, the cell enters a stable G1 state of low Cdk activity, where it awaits signals to enter a new cell cycle.