Investigating the roles of lipin phosphatase Spo7-Nem1 and lipid flux in mitotic nuclear envelope remodelling

Student thesis: Doctoral ThesisDoctor of Philosophy


Eukaryotes utilise a range of mitotic nuclear envelope (NE) reorganisation strategies to accommodate chromosome segregation and the formation of the daughter nuclei during mitosis. These include closed mitosis where the NE remains intact throughout cell division, open mitosis whereby the NE is broken down during prophase and reforms around the segregated daughter genomes at telophase, as well as a host of additional strategies that are intermediate between these two extremes. The model yeast Schizosaccharomyces pombe undergoes closed mitosis, keeping its NE intact whereas its cousin species Schizosaccharomyces japonicus breaks its NE during mitosis. It was previously shown that this functional divergence results from differences in trans-regulatory networks regulating the phosphatidic acid (PA) phosphatase lipin activity. CDK1 phosphorylation of lipin was shown to drive expansion of the NE during mitosis in S. pombe but not in S. japonicus. NE breakage is thus inevitable due to insufficient membrane availability in S. japonicus. In this thesis, we characterized the positive regulator of lipin activity, the lipin phosphatase complex Spo7-Nem1, as well as the distribution of the substrate and product of lipin, PA and diacylglycerol (DG), in the two fission yeast species.
Our results indicate that the Spo7-Nem1 phosphatase complex from the two related fission yeast species have divergent properties. The loss of Spo7, Nem1 and Ned1 resulted in differing responses in S. pombe and S. japonicus including cellular lipidome changes, protein stability, oxidative stress tolerance, hyphae transition and lipid droplet morphology. We also identified a novel role of the diacylglycerol kinase Dgk1, which catalyzes the reverse reaction of lipin, during mitosis in S. pombe. We have also developed a set of PA and DG probes for use in fission yeasts, allowing us to track changes in the distribution and levels of PA and DG with a spatial and temporal resolution. This technique complemented our biochemical and -omics based work in characterizing the Spo7-Nem1 phosphatase of S. pombe and S. japonicus, showing significant differences in lipid flux at the NE during mitotic nuclear remodelling. S. pombe actively removes DG from its NE during mitosis, while S. japonicus does not, suggesting a potential role of DG at the NE during remodelling.
Together with additional work utilizing mass spectrometry based proteomics, this study has identified future avenues of investigation into the regulation and novel functions of Spo7-Nem1, including sporulation, sterol biosynthesis and lipid partitioning at the NE, with potential relevance for higher eukaryotes.
Date of Award1 Aug 2021
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorSnezhana Oliferenko (Supervisor) & Markus R. Wenk (Supervisor)

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