Peter, Nisha (2017) Characterisation of the phosphatase control system that prevents premature mitotic entry in mammalian cells. Doctoral thesis (PhD), University of Sussex.

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Abstract

Accurate chromosome segregation during mitosis prevents aneuploidy and cancer. The Wee1-Cdc25-Cdk1 feedback loop ensures cells enter mitosis in a timely and orderly manner. This signalling system has been proposed to work as a bistable switch that is maintained by the counterbalancing action of kinases and phosphatases. However, the phosphatases critical for this transition in mammalian cells are yet to be identified. Cdc14 is the major phosphatase antagonising CDK in yeast. But its eukaryotic homologue does not appear to play an essential role in mitotic control. Studies in Drosophila and Xenopus have shown that PP2A/B55 inhibition is crucial for MPF activation, and that this is brought about by Greatwall kinase and its substrates, the closely related 17kDa proteins Ensa and Arpp19. However, it remains unclear, if PP2A/B55 inhibition by phosphorylated Ensa/Arpp19 is a critical event in regulating mitotic entry in somatic cells.
In this thesis I have performed a cell biological, genetic and biochemical characterisation of Ensa/Arpp19. I have generated and optimised tools to study the function of these two proteins and characterised their localisation, their depletion phenotypes, the nature of their interaction with the PP2A/B55 phosphatase and their protein interactome in metaphase and anaphase in mammalian cells. In parallel I have been optimising a novel protein transfection based method to study the effects of constitutively phosphorylated Ensa/Arpp19 in cells.
The experiments described in this thesis show a surprising divergence of the functions of Ensa/Arpp19 and their upstream kinase Greatwall, suggesting a more complex signaling cascade involving these proteins. Our biochemical characterization of the Ensa/B55 interaction also suggests a novel mechanism of action for these proteins. Overall this work sheds light on how the balanced activity of the Cdk1 activation switch and its counteracting phosphatases contribute to the regulation of mitotic entry.

Item Type:	Thesis (Doctoral)
Schools and Departments:	School of Life Sciences > Biochemistry
Subjects:	Q Science > QD Chemistry > QD0241 Organic chemistry > QD0415 Biochemistry Q Science > QH Natural history > QH0301 Biology > QH0573 Cytology
Depositing User:	Library Cataloguing
Date Deposited:	21 Mar 2017 15:54
Last Modified:	25 Apr 2019 08:17
URI:	http://sro.sussex.ac.uk/id/eprint/67138

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