To identify near complete sets of genes required for the cell cycle and cell shape, we have visually screened a genome-wide gene deletion library of 4843 fission yeast deletion mutants (95.7% of total protein encoding genes) for their effects on these processes. A total of 513 genes have been identified as being required for cell cycle progression, 276 of which have not been previously described as cell cycle genes. Deletions of a further 333 genes lead to specific alterations in cell shape and another 524 genes result in generally misshapen cells. Here, we provide the first eukaryotic resource of gene deletions, which describes a near genome-wide set of genes required for the cell cycle and cell shape.

Primary culture rat astrocytes exposed to the long acting nitric oxide donor (Z)‐1‐[2‐aminoethyl)‐N‐(2‐ammonioethyl)amino]diazen‐1‐ium‐1,2‐diolate (DETA‐NO) for 24 h approximately double their concentration of glutathione (GSH) and show no sign of cell death. In contrast, GSH was depleted by 48%, and significant loss of mitochondrial respiratory chain complex activity and cell death were observed in primary culture rat neurones subjected to DETA‐NO for 18 h. Northern blot analysis suggested that mRNA amounts of both subunits of glutamate‐cysteine ligase (GCL), the rate‐limiting enzyme in GSH synthesis, were elevated in astrocytes following nitric oxide (NO) exposure. This correlated with an increase in astrocytic GCL activity. Neurones on the other hand did not exhibit increased GCL activity when exposed to NO. In addition, the rate of GSH efflux was doubled and γ‐glutamyltranspeptidase (γ‐GT) activity was increased by 42% in astrocytes treated with NO for 24 h. These results suggest that astrocytes, but not neurones, up‐regulate GSH synthesis as a defence mechanism against excess NO. It is possible that the increased rate of GSH release and activity of γ‐GT in astrocytes may have important implications for neuroprotection in vivo by optimizing the supply of GSH precursors to neurones in close proximity.

Self-organization of cellular structures is an emerging principle underlying cellular architecture. Properties of dynamic microtubules and microtubule-binding proteins contribute to the self-assembly of structures such as microtubule asters^1,2. In the fission yeast Schizosaccharomyces pombe, longitudinal arrays of cytoplasmic microtubule bundles regulate cell polarity and nuclear positioning^3,4,5. These bundles are thought to be organized from the nucleus at multiple interphase microtubule organizing centres (iMTOCs)³. Here, we find that microtubule bundles assemble even in cells that lack a nucleus. These bundles have normal organization, dynamics and orientation, and exhibit anti-parallel overlaps in the middle of the cell. The mechanisms that are responsible for formation of these microtubule bundles include cytoplasmic microtubule nucleation, microtubule release from the equatorial MTOC (eMTOC), and the dynamic fusion and splitting of microtubule bundles. Bundle formation and organization are dependent on mto1p (γ-TUC associated protein), ase1p (PRC1), klp2p (kinesin-14) and tip1p (CLIP-170). Positioning of nuclear fragments and polarity factors by these microtubules illustrates how self-organization of these bundles contributes to establishing global spatial order.

Upon acute heat stress (HS), overall mRNA transcription, processing, and export are inhibited, leading to cell growth arrest. However, how cells turn off mRNA metabolism is not fully understood. Here, we show that acute HS results in the segregation and aggregation of multiple nuclear and nucleolar proteins into ring-like structures located at the nucleolar periphery (nucleolar rings [NuRs]). NuRs sequester essential factors required for nuclear mRNA metabolism and nuclear pore complex function, as well as cell-cycle regulators. When cells are switched back to growing temperatures, NuRs disaggregate, and their components relocate to their functional environments in an Hsf1- and Hsp104-dependent manner, and concomitantly with the reinitiation of cell growth. These findings highlight the contribution of reversible protein aggregation to the inhibition of overall RNA-related activities in the nucleus and its functional relevance in the maintenance of cellular homeostasis during acute HS.

An important question in cell biology is how cellular organelles partition during cell division. In organisms undergoing closed mitosis, the elongation of an intranuclear spindle drives nuclear division, generating two identically sized nuclei [1, 2]. However, how the site of nuclear division is determined and the underlying mechanism driving nuclear envelope (NE) fission remain largely unknown. Here, using the fission yeast, we show that the microtubule bundler Ase1/PRC1 at the spindle midzone is required for the local concentration of nuclear pore complexes (NPCs) in the region of the NE in contact with the central spindle. As the spindle elongates during anaphase B, components of these NPCs are sequentially eliminated, and this is accompanied by the local remodeling of the NE. These two events lead to the eventual removal of NPCs and nuclear division. In the absence of importin α, NPCs remain stable in this region and no event of NE remodeling is observed. Consequently, cells fail to undergo nuclear division. Thus, our results highlight a new role of the central spindle as a spatial cue that determines the site of nuclear division and point to NPC removal as the triggering event.

Temperature fluctuation is one of the most frequent threats to which organisms are exposed in nature. The activation of gene expression programs that trigger the transcription of heat stress-protective genes is the main cellular response to resist high temperatures. In addition, reversible accumulation and compartmentalization of thermosensitive proteins in high-order molecular assemblies are emerging as critical mechanisms to ensure cellular protection upon heat stress. Here, we summarize representative examples of membrane-less intracellular bodies formed upon heat stress in yeasts and human cells and highlight how protein aggregation can be turned into a cytoprotective mechanism.