Switches and latches: a biochemical tug-of-war between the kinases and phosphatases that control mitosis

Abstract

Activation of the cyclin-dependent kinase (Cdk1) cyclin B (CycB) complex (Cdk1:CycB) in mitosis brings about a remarkable extent of protein phosphorylation. Cdk1:CycB activation is switch-like, controlled by two auto-amplification loops--Cdk1:CycB activates its activating phosphatase, Cdc25, and inhibits its inhibiting kinase, Wee1. Recent experimental evidence suggests that parallel to Cdk1:CycB activation during mitosis, there is inhibition of its counteracting phosphatase activity. We argue that the downregulation of the phosphatase is not just a simple latch that suppresses futile cycles of phosphorylation/dephosphorylation during mitosis. Instead, we propose that phosphatase regulation creates coherent feed-forward loops and adds extra amplification loops to the Cdk1:CycB regulatory network, thus forming an integral part of the mitotic switch. These network motifs further strengthen the bistable characteristic of the mitotic switch, which is based on the antagonistic interaction of two groups of proteins: M-phase promoting factors (Cdk1:CycB, Cdc25, Greatwall and Endosulfine/Arpp19) and interphase promoting factors (Wee1, PP2A-B55 and a Greatwall counteracting phosphatase, probably PP1). The bistable character of the switch implies the existence of a CycB threshold for entry into mitosis. The end of G2 phase is determined by the point where CycB level crosses the CycB threshold for Cdk1 activation.

Figure 1.

Influence diagram of mitotic regulators that make up the mitotic switch. Arrows represent activating interactions, and blunt-ended lines inhibition of activity of the target protein. Green- and red-labelled proteins and interactions are active in mitosis and interphase, respectively. Dashed lines represent proposed, but as yet unproven, interactions.

Figure 2.

The antagonistic and self-activating interaction between MPFs and IPFs. The protein molecules in the mitotic switch belong to one of the following two categories: mitosis promoting factors (MPFs) and interphase promoting factors (IPFs). Molecules in each group activate each other and inhibit the members of the other group represented by dashed lines. MPFs and IPFs have antagonistic effects on the phosphorylation of downstream substrates.

Figure 3.

Signal response curve of the mitotic switch. Phosphorylation state of downstream Cdk1–PP2A–B55 substrates are plotted as a function of cyclin B level (schematic). There are two stable states, interphase and mitosis, a CycB threshold for activation of MPFs and inhibition of IPFs (ON-threshold) and a different threshold for inactivation of MPFs and activation of IPFs (OFF-threshold). At these thresholds, the mitotic control system can jump from one stable state to the other. Cyclin synthesis drives the system from the left to the right on this diagram, while cyclin degradation pushes it in the reverse direction.

Figure 4.

The timing of mitosis and the length of G2 are determined by the Cdk1:CycB activation threshold and CycB synthesis rate. The red and green curves represent the change in the ON-threshold and CycB levels with time, respectively. For simplicity, we ignore APC/C-dependent CycB degradation which turns on at the end of mitosis. The intersection point of the two curves determines the commitment point for entering into mitosis. The value of the CycB threshold depends on the mitotic switch, which is influenced by DNA replication and cell size checkpoint mechanisms. Several scenarios are possible. (a) The cyclin threshold remains constant throughout interphase because checkpoints are inactive or already satisfied. CycB increases gradually in interphase and its rate of accumulation (the slope of the green line) determines the timing of mitotic entry. This situation is probably realized in embryonic cell cycles. (b) The cyclin threshold drops when checkpoints are turned off, but this occurs after CycB has reached its maximum level. In this case, the silencing of the checkpoint is rate-limiting for mitotic entry. This situation probably occurs in cells recovering from some types of checkpoint arrest. (c) Checkpoints are silenced while the CycB level is still increasing. Here, both the rate of CycB accumulation and checkpoint silencing determine the timing of mitotic entry. This is most likely the case in many somatic and free-living cells. Cases (a) and (b) can be seen as the two extremes of this picture. (d) This picture represents a stable interphase arrest owing to permanent activation of checkpoints. The cyclin threshold remains higher than the maximum physiological CycB level so that there is no mitotic entry. Eventual silencing of the checkpoint would result in case (b).