Investigating nutrient-sensing mechanisms that control cell size at division

  • Elizabeth Davie

    Student thesis: Phd


    Cell growth and cell cycle progression are tightly coordinated, allowingcells to adjust their size to the demands of proliferation in varying nutritionalenvironments. This coordination is regulated by Target of Rapamycin (TOR)signalling. Active TOR complex 1 (TORC1) promotes cell growth to delaymitosis and increase cell size at division, whereas under limited nutrientsTORC1 activity is down-regulated to advance mitosis and reduce cell size. Infission yeast Schizosaccharomyces pombe, a shift from rich to poor nitrogenenvironment - nitrogen stress - results in TOR inhibition, which activates MAPKsignalling to promote entry into mitosis at a reduced cell size. However, itremains unclear how nitrogen stress is sensed upstream of TOR signalling tocontrol this response.This study uses genetic and biochemical approaches to probe themechanisms of nitrogen sensing that control cell size at division. I propose amodel whereby nitrogen stress is sensed by two mechanisms that regulate theAMP-activated protein kinase (AMPK). I show that nitrogen stress reducescellular ATP levels, suggesting that advanced mitotic entry may be promoted bydetecting a reduction in cellular energy. Analysis of AMPK mutants revealedthat Ssp2 (AMPKalpha) is required for cells to reduce their size in response tonitrogen stress. Moreover, activating phosphorylation of Ssp2 on Thr189 isenhanced under nitrogen-poor conditions. However, I also observe that the β-and γ-subunits of AMPK are not essential for the nitrogen-stress response,providing evidence for an additional, energy-independent mechanism ofsensing changes in nitrogen quality. This work presents new evidence for theregulation of Ssp2 phosphorylation by Mph1 kinase on Thr189, and also byTORC1 on a newly characterised conserved phosphorylation site Ser367.Overall, the data suggests that activated Ssp2 can signal to modulate TORC1activity and control cell size at division in response to nitrogen stress.
    Date of Award1 Aug 2014
    Original languageEnglish
    Awarding Institution
    • The University of Manchester
    SupervisorPhilip Woodman (Supervisor) & Janni Petersen (Supervisor)

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