Transcriptional Regulation Is a Major Controller of Cell Cycle Transition Dynamics

DNA replication, mitosis and mitotic exit are critical transitions of the cell cycle which normally occur only once per cycle. A universal control mechanism was proposed for the regulation of mitotic entry in which Cdk helps its own activation through two positive feedback loops. Recent discoveries in various organisms showed the importance of positive feedbacks in other transitions as well. Here we investigate if a universal control system with transcriptional regulation(s) and post-translational positive feedback(s) can be proposed for the regulation of all cell cycle transitions. Through computational modeling, we analyze the transition dynamics in all possible combinations of transcriptional and post-translational regulations. We find that some combinations lead to ‘sloppy’ transitions, while others give very precise control. The periodic transcriptional regulation through the activator or the inhibitor leads to radically different dynamics. Experimental evidence shows that in cell cycle transitions of organisms investigated for cell cycle dependent periodic transcription, only the inhibitor OR the activator is under cyclic control and never both of them. Based on these observations, we propose two transcriptional control modes of cell cycle regulation that either STOP or let the cycle GO in case of a transcriptional failure. We discuss the biological relevance of such differences

Multisite phosphorylation networks as signal processors for Cdk1

The order and timing of cell cycle events is controlled by changing substrate specificity and different activity thresholds of cyclin-dependent kinases (CDK). However, it is not understood how a single protein kinase can trigger hundreds of switches in a sufficiently time-resolved fashion. We show that the cyclin-Cdk1-Cks1-dependent phosphorylation of multisite targets in Saccharomyces cerevisiae is controlled by key substrate parameters including distances between phosphorylation sites, the distribution of serines and threonines as phospho-acceptors, and the positioning of cyclin-docking motifs. The component mediating the key interactions in this process is Cks1, the phospho-adaptor subunit of the cyclin-Cdk1-Cks1 complex. We propose that variation of these parameters within the networks of phosphorylation sites in different targets provides a wide range of possibilities for the differential amplification of Cdk1 signals, providing a mechanism to generate a wide range of thresholds in the cell cycle

Cks confers specificity to phosphorylation-dependent CDK signaling pathways.

Cks is an evolutionarily conserved protein that regulates cyclin-dependent kinase (CDK) activity. Clarifying the underlying mechanisms and cellular contexts of Cks function is critical because Cks is essential for proper cell growth, and its overexpression has been linked to cancer. We observe that budding-yeast Cks associates with select phosphorylated sequences in cell cycle-regulatory proteins. We characterize the molecular interactions responsible for this specificity and demonstrate that Cks enhances CDK activity in response to specific priming phosphosites. Identification of the binding consensus sequence allows us to identify putative Cks-directed CDK substrates and binding partners. We characterize new Cks-binding sites in the mitotic regulator Wee1 and discover a new role for Cks in regulating CDK activity at mitotic entry. Together, our results portray Cks as a multifunctional phosphoadaptor that serves as a specificity factor for CDK activity