Irreversibility of mitotic exit is the consequence of systems-level feedback

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Abstract
The eukaryotic cell cycle comprises an ordered series of events orchestrated by cyclin-dependent kinases (Cdks), with unidirectional cell-cycle transitions being required for its successful completion. Proteolytic degradation of cyclins has been assumed to be responsible for the irreversible transitions. Here, the contribution of cyclin proteolysis to the irreversibility of mitotic exit has been examined with a combination of experiments in budding yeast and mathematical modelling. Although forced cyclin degradation can drive mitotic exit, it is not sufficient for irreversibility, due to the re-synthesis of cyclin. Mitotic exit becomes irreversible only after longer periods of cyclin degradation and activation of a double negative feedback loop involving the Cdk inhibitor Sic1. The eukaryotic cell cycle comprises an ordered series of events orchestrated by cyclin-dependent kinases (Cdks), with unidirectional cell-cycle transitions being required for its successful completion. Proteolytic degradation of cyclins has been assumed to be responsible for the irreversible transitions, but here it is shown that, although forced cyclin degradation can drive mitotic exit, the re-synthesis of cyclin means that this is not sufficient for irreversibility. Rather, mitotic exit only become irreversible after activation of a double-negative feedback loop. The eukaryotic cell cycle comprises an ordered series of events, orchestrated by the activity of cyclin-dependent kinases (Cdks), leading from chromosome replication during S phase to their segregation in mitosis. The unidirectionality of cell-cycle transitions is fundamental for the successful completion of this cycle. It is thought that irrevocable proteolytic degradation of key cell-cycle regulators makes cell-cycle transitions irreversible, thereby enforcing directionality1,2,3. Here we have experimentally examined the contribution of cyclin proteolysis to the irreversibility of mitotic exit, the transition from high mitotic Cdk activity back to low activity in G1. We show that forced cyclin destruction in mitotic budding yeast cells efficiently drives mitotic exit events. However, these remain reversible after termination of cyclin proteolysis, with recovery of the mitotic state and cyclin levels. Mitotic exit becomes irreversible only after longer periods of cyclin degradation, owing to activation of a double-negative feedback loop involving the Cdk inhibitor Sic1 (refs 4, 5). Quantitative modelling suggests that feedback is required to maintain low Cdk activity and to prevent cyclin resynthesis. Our findings demonstrate that the unidirectionality of mitotic exit is not the consequence of proteolysis but of systems-level feedback required to maintain the cell cycle in a new stable state.