Role of phosphatases in the regulation of mitotic exit

In this project we are investigating the regulation of cell division. Tight control of this process is essential, and failure to do so can lead to cancer. For many years, the focus of attention has been laid in a group of proteins known as Cyclin-dependent kinases, which work as an engine to drive cell division. These kinases are able to modify other proteins in order to change their function or activity. However, a different sort of proteins known as protein phosphatases can revert the action of these kinases, and hence can also have a major role in controling cell division. We have centered our studies in one of them: PP2A. We have found that it is a central component in the regulation of cell division and differentiation, especially in conditions of nutritional stress.

This project aims at deciphering which phosphatases regulate mitosis and mitotic exit in the fission yeast Schizosaccharomyces pombe. Mitotic phosphatases have been poorly studied in the field of cell cycle regulation, and only in the case of budding yeas t its identity is clear, Cdc14. In higher eukaryotes this remains a controversial issue, with studies hinting at PP1 whereas other suggest PP2A as the main driver of mitotic exit. In this project I suggest to use fission yeast as a model, as it is a genet ically amenable system, and of well studied physiology. Besides these technical advantages, it has also been shown to still conserve the same mechanisms of division also present in more complex organisms, providing an invaluable tool for cell cycle resea rch. This project is divided in three parts. The first one focuses in the identification of the mitotic phosphatase(s). For this, we will take advantage of a collection of phosphatases deletions, as well as conditional alleles generated by me during my po st-doctoral research. We will challenge these strains for their ability to normally exit mitosis in a modified strain background where cyclin dependent kinase (cdk) downregulation is impaired. In the second project, we will investigate how the phosphatas e activity is regulated during the cell cycle. In order to achieve sharp cell cycle transitions, the activity of the phosphatase should rise as cdk activity declines, however, these should only happen once the cell is ready to divide, i.e. when the mitoti c checkpoint is satisified. We will analyse the contribution of both events to phosphatase activation, from a proteomic point of view. Finally, we will investigate how phosphatase activity impinges on the maintenance of a G1/G0 state when they need to dif ferentiate. using the same collection as in the first point, we will investigate the ability of the different mutants to arrest in G1 upon treatment with a differentiation stimulus.