Chromosomes (pink) in the fission yeast S. pombe, one of the model organisms used in our research.

Introduction

The eukaryotic cell division cycle comprises an ordered series of events, orchestrated by the activity of cyclin-dependent kinases (Cdks). In every cell cycle, chromosomes are first replicated during S-phase and later segregated during mitosis. How increasing Cdk activity during the cell cycle instructs ordered S phase and mitosis, as well as the sequential events during mitosis (metaphase, anaphase, telophase and cytokinesis) is not yet understood.

There are nine cyclins in budding yeast, analogous to G1, S and mitotic cyclins in human cells (Figure 1a). We still do not know in how far their individual characteristics (a qualitative model of Cdk control), or their collective ability to activate Cdk (a quantitative model of Cdk control), instruct ordered cell cycle progression. And what about phosphatases that counteract Cdk and other cell cycle kinases? Major cellular phosphatases preferentially target phospho-threonines, so that threonine phosphorylation is opposed and usually occurs late. Changing threonines to serines makes proteins invisible to these phosphatases and advances their phosphorylation (Figure 1b).

Despite this progress, crucial questions remain. What really distinguishes a cell cycle substrate that is phosphorylated in S phase from one that is phosphorylated in mitosis? What distinguishes a substrate that is dephosphorylated in anaphase to stabilise the elongating mitotic spindle from one that is dephosphorylated just a bit later, after the spindle has disassembled again, to trigger cytokinesis?

Phosphoproteomics has opened a renewed opportunity for us to survey time-resolved phosphorylation dynamics during cell cycle progression to elucidate the underlying control principles. Successful cell growth and division in all eukaryotes depend on the intricate orchestration of numerous ordered phosphorylation and dephosphorylation events.

Principles of cell cycle control

Figure 1: Principles of cell cycle control. a) 9 cyclins sequentially activate the budding yeast cyclin-dependent kinase (Cdc28). b) We still do not know what decides whether a Cdk substrate is phosphorylated in S phase or in mitosis. Threonines are phosphorylated later, because their phosphorylation is opposed by Cdk-counteracting phosphatases of the PP2A family. Yet, both serines and threonines are phosphorylated at widely different times between S phase and mitosis, suggesting that major determinants of phosphorylation timing remain unknown.