Disordered Cdk Substrates Act as Multi-Input Signal Processors to Control the Key Decision Points in the Cell Cycle

Abstract

The decision points between different cell fates involve systems that process alternative signals into binary choices. At the beginning of the cell cycle the decision to commit to division is made at the point where the cyclin-dependent kinase (Cdk) signal overrides its antagonists. In Saccharomyces cerevisiae the cuescontrolling thecommitment to the Start decision are the mating pheromone-induced signal via MAPK Fus3 and the Start signals via G1- and S-CDK complexes. These three signals are integrated by a multi-phosphorylated and disordered Cdk1 inhibitor protein Far1 that serves as a signal processor molecule to calculate the thresholds for the commitment switch. The kinase signals are processed by a network of phosphorylation and docking motifs to form a double-negative feedback loop. At the next decision point of the cell cycle, the G1/S transition, another disordered Cdk1 inhibitor protein Sic1 inhibits the S-phase specific Clb5-Cdk1 complex, while Clb5-Cdk1 also targets Sic1 for destruction by phosphorylation. We show that the timing and the shape of Sic1 degradation switch is controlled by a similar principle of a multi-branched processor. Rewiring the multisite phosphorylation networks in Far1 and Sic1 can alter the timing and shape of both Start and G1/S switches. In conclusion, intrinsically disordered phospho-regulated proteins can serve as sophisticated signal processors controlling key cell cycle transitions. Moreover, the idea that integration and processing of multiple input signals can be performed by a single unstructured protein is opposed to the current thought that such signal processing requires entire genetic circuits or pathways. Similar principles of sequential signal processing via multisite phosphorylation networks can be applied to synthetic circuit design.