The Budding Yeast Nuclear Envelope Adjacent to the Nucleolus Serves as a Membrane Sink during Mitotic Delay

Abstract

Summary The mechanisms that dictate nuclear shape are largely unknown. Here we screened the budding yeast deletion collection for mutants with abnormal nuclear shape. A common phenotype was the appearance of a nuclear extension, particularly in mutants in DNA repair and chromosome segregation genes. Our data suggest that these mutations led to the abnormal nuclear morphology indirectly, by causing a checkpoint-induced cell cycle delay. Indeed, delaying cells in mitosis by other means also led to the appearance of nuclear extensions, while inactivating the DNA damage checkpoint pathway in a DNA repair mutant reduced the fraction of cells with nuclear extensions. Formation of a nuclear extension was specific to a mitotic delay, as cells arrested in S or G2 had round nuclei. Moreover, the nuclear extension always coincided with the nucleolus, while the morphology of DNA mass remained largely unchanged. Finally, we found that phospholipid synthesis continues unperturbed when cells delay in mitosis, and inhibiting phospholipid synthesis abolished the formation of nuclear extensions. Our data suggest a mechanism that promotes nuclear envelope expansion during mitosis. When mitotic progression is delayed, cells sequester the added membrane to the nuclear envelope associated with the nucleolus, possibly to avoid disruption of intra-nuclear organization.

Evidence for nucleolar subcompartments in Dictyostelium

“…This phenomenon is strikingly similar to the mitotic arrest-induced nuclear envelope extensions that occur in budding yeast [31]. These extensions are thought to result from the accumulation of excess phospholipids during mitotic arrest [31]. This also fits with recent work demonstrating that nucleoli behave like liquid-like droplets rather than solids [32].…”

“…The NumA1-occupied area decreases in size whereas CBP4a, Snf12, and FhkA appear to bud off from the nucleus [15][16][17]. This phenomenon is strikingly similar to the mitotic arrest-induced nuclear envelope extensions that occur in budding yeast [31]. These extensions are thought to result from the accumulation of excess phospholipids during mitotic arrest [31].…”

Recent advances in understanding nuclear size and shape

“…131 In yeast, formation of NE flares adjacent to the nucleolus in response to excess membrane production or mitotic arrest is dependent on polo kinase Cdc5. 132,133 A question for future research is precisely how kinases with well-established roles in mitosis, including cyclin-dependent kinases, polo kinases, and PKC, contribute to the maintenance of proper nuclear morphology. Given the potential impact of the cell cycle on nuclear morphology, it is worth considering the interplay between size scaling of mitotic structures and the nucleus.…”

Lipin Family Proteins - Key Regulators in Lipid Metabolism

“…Deletion of Smp2 or its dephosphorylated form causes transcriptional upregulation of genes involved in phospholipid biosynthesis concurrent with a massive expansion of the nuclear envelope. Conversely, constitutive dephosphorylation of Smp2 represses de novo phospholipid synthesis [28] and inhibits cell division [29], although the underlying mechanisms remain to be determined. In addition, RNAi-​mediated down regulation of lipin1 in C. elegans causes nuclear envelope breakdown and alterations in the organization of the endoplasmic reticulum membrane, resulting in the appearance of large membrane sheets [30,31].…”