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The amazing structural variety of cells is matched only by their functional diversity, and reflects the complex interplay between biochemical and mechanical regulation. How both regulatory layers generate specifically shaped cellular domains is not fully understood. Here, we report how cell growth domains are shaped in fission yeast. Based on quantitative analysis of cell wall expansion and elasticity, we develop a model for how mechanics and cell wall assembly interact and use it to look for factors underpinning growth domain morphogenesis. Surprisingly, we find that neither the global cell shape regulators Cdc42-Scd1-Scd2 nor the major cell wall synthesis regulators Bgs1-Bgs4-Rgf1 are reliable predictors of growth domain geometry. Instead, their geometry can be defined by cell wall mechanics and the cortical localization pattern of the exocytic factors Sec6-Syb1-Exo70. Forceful re-directioning of exocytic vesicle fusion to broader cortical areas induces proportional shape changes to growth domains, demonstrating that both features are causally linked., Cell shape is determined by a combination of biochemical regulation and mechanical forces. By imaging the dynamic behaviour of growth regulatory proteins in fission yeast and integrating these data within a mechanical model, Abenza et al. find that exocytosis plays a dominant role in shaping growth domains.

The small GTPase Ran, bound to GTP, is required for the induction of spindle formation by chromosomes in M phase. High concentrations of Ran.GTP are proposed to surround M phase chromatin. We show that the action of Ran.GTP in spindle formation requires TPX2, a microtubule-associated protein previously known to target a motor protein, Xklp2, to microtubules. TPX2 is normally inactivated by binding to the nuclear import factor, importin alpha, and is displaced from importin alpha by the action of Ran.GTP. TPX2 is required for Ran.GTP and chromatin-induced microtubule assembly in M phase extracts and mediates spontaneous microtubule assembly when present in excess over free importin alpha. Thus, components of the nuclear transport machinery serve to regulate spindle formation in M phase.

Delegates at scientific meetings can come from diverse backgrounds and use very different methods in their research. Promoting interactions between these ‘distant’ delegates is challenging but such interactions could lead to novel interdisciplinary collaborations and unexpected breakthroughs. We have developed a network-based ‘speed dating’ approach that allows us to initiate such distant interactions by pairing every delegate with another delegate who might be of interest to them, but whom they might never have encountered otherwise. Here we describe our approach and its algorithmic implementation.

SummaryUnderstanding cells as integrated systems requires that we systematically decipher how single genes affect multiple biological processes and how processes are functionally linked. Here, we used multiprocess phenotypic profiling, combining high-resolution 3D confocal microscopy and multiparametric image analysis, to simultaneously survey the fission yeast genome with respect to three key cellular processes: cell shape, microtubule organization, and cell-cycle progression. We identify, validate, and functionally annotate 262 genes controlling specific aspects of those processes. Of these, 62% had not been linked to these processes before and 35% are implicated in multiple processes. Importantly, we identify a conserved role for DNA-damage responses in controlling microtubule stability. In addition, we investigate how the processes are functionally linked. We show unexpectedly that disruption of cell-cycle progression does not necessarily affect cell size control and that distinct aspects of cell shape regulate microtubules and vice versa, identifying important systems-level links across these processes.

Cell polarity is regulated by evolutionarily conserved polarity factors whose precise higher-order organization at the cell cortex is largely unknown. Here we image frontally the cortex of live fission yeast cells using time-lapse and super-resolution microscopy. Interestingly, we find that polarity factors are organized in discrete cortical clusters resolvable to ~50–100 nm in size, which can form and become cortically enriched by oligomerization. We show that forced co-localization of the polarity factors Tea1 and Tea3 results in polarity defects, suggesting that the maintenance of both factors in distinct clusters is required for polarity. However, during mitosis, their co-localization increases, and Tea3 helps to retain the cortical localization of the Tea1 growth landmark in preparation for growth reactivation following mitosis. Thus, regulated spatial segregation of polarity factor clusters provides a means to spatio-temporally control cell polarity at the cell cortex. We observe similar clusters in Saccharomyces cerevisiae and Caenorhabditis elegans cells, indicating this could be a universal regulatory feature., Cell polarity is generated and maintained by the spatial accumulation of polarity factors. By imaging fission yeast cells ‘end-on’, the authors show that the polarity factors Tea1 and Tea3 segregate into distinct clusters, and that surprisingly, their segregation is critical for cell polarization.

Mol Syst Biol. 3: 95 We want to see how, in some cases at least, the forms of living things, and of the parts of living things, can be explained by physical considerations.…D'Arcy Thompson (1917) Proper organization of microtubule polymers is crucial to the form and function of all eukaryotic cells. Whether growing, dividing or polarizing, different cell types—be they neurons, plant cells or fungal cells—organize specialized microtubule patterns appropriate for their needs. Is it possible to identify the set of factors sufficient to organize microtubules in a specific cell, and to understand how the cell regulates those factors spatially and temporally such that they collectively sustain that particular cellular pattern of microtubules? In a recent issue of Cell , Marcel Janson and collaborators (Janson et al , 2007) use an elegant combination of quantitative microscopy, in vitro assays and computer simulations to try to elucidate the minimal set of components required to stably organize microtubule patterns in a unicellular eukaryote, the fission yeast Schizosaccharomyces pombe . Microtubule organization in a given cell is …

The importin alpha-regulated microtubule-associated protein TPX2 is known to be critical for meiotic and mitotic spindle formation in vertebrates, but its detailed mechanism of action and regulation is not understood. Here, the site of interaction on TPX2 for importin alpha is mapped. A TPX2 mutant that cannot bind importin alpha is constitutively active in the induction of microtubule-containing aster-like structures in Xenopus egg extract, demonstrating that no other importin alpha or RanGTPase target is required to mediate microtubule assembly in this system. Further, recombinant TPX2 is shown to induce the formation and bundling of microtubules in dilute solutions of pure tubulin. In this purified system, importin alpha prevents TPX2-induced microtubule formation, but not TPX2-tubulin interaction or microtubule bundling. This demonstrates that TPX2 has more than one mode of interaction with tubulin and that only one of these types of interaction is abolished by importin alpha. The data suggest that the critical early function in spindle formation regulated by importin alpha is TPX2-mediated microtubule nucleation.

Cell polarity—the intrinsic property of cells to take different shape, migration or growth patterns suited to their function—is one of those things intuitively we are all aware of. It is part of our collective imagination, and indeed if prompted all of us immediately think of cells as

La segregation correcte du materiel genetique au moment de la mitose depend de la formation du fuseau mitotique, assemblage de microtubules qui se reorientent autour des chromosomes. Les mecanismes qui controlent l'assemblage et la reorientation de cette structure dynamique sont tres complexes et peu connus. Une GTPase nucleaire de la famille de Ras, Ran, impliquee jusqu'a maintenant dans le transport nucleo-cytoplasmique, s'avere un acteur essentiel de ce processus. Pendant l'interphase, la forme active, Ran-GTP, est nucleaire, et la forme inactive, Ran-GDP, cytoplasmique. Lors de la mitose, Ran-GTP induit la polymerisation des microtubules et leur organisation en un fuseau bipolaire, probablement sous l'effet inducteur de la chromatine elle-meme. Ran-GTP, agirait durant la mitose comme au cours du transport nucleocytoplasmique lors de l'interphase, d'echange, agirait en liberant des proteines nucleaires directement responsables de l'assemblage des polymeres de tubuline. Ces donnees, en suggerant l'implication directe de la chromatine dans l'assemblage des microtubules par l'intermediaire de Ran, confirment que la formation du fuseau est un processus en perpetuelle activite.

The mitotic spindle of animal cells is a bipolar array of microtubules that guides chromosome segregation during cell division [1]. It has been proposed that during spindle assembly chromatin can positively influence microtubule stability at a distance from its surface throughout its neighboring cytoplasm [2]. However, such an "a distance" effect has never been visualized directly. Here, we have used centrosomal microtubules and chromatin beads to probe the regulation of microtubule behavior around chromatin in Xenopus egg extracts. We show that, in this system, chromatin does affect microtubule formation at a distance, inducing preferential orientation of centrosomal microtubules in its direction. Moreover, this asymmetric distribution of microtubules is translated into a directional migration of centrosomal asters toward chromatin and their steady-state repositioning within 10 μm of chromatin. To our knowledge, this is the first direct evidence of a long-range guidance effect at the sub-cellular level.

Les microtubules sont, à chaque division cellulaire, organisés en fuseau bipolaire pour assurer la transmission des chromosomes aux cellules filles. Dès la sortie d’interphase, les microtubules deviennent extrêmement labiles, ce qui facilite leur réorganisation. Parallèlement, des mécanismes de restabilisation locale des microtubules doivent être mis en place pour l’assemblage du fuseau. Tout d’abord, la chromatine favorise la polymérisation et la stabilisation des microtubules autour d’elle. Cet « effet chromatine » est principalement relayé par une enzyme, la GTPase Ran. Il est renforcé par l’activité de moteurs moléculaires liés à la chromatine, qui ancrent des microtubules aux bras des chromosomes. Enfin, les kinétochores, structures protéiques accolées à une région limitée de la chromatine, stabilisent d’autres microtubules qui s’organisent en « fibres kinétochoriennes » robustes et contribuent au maintien du fuseau bipolaire dans la cellule., The partition of the genetic material of cells in two identical lots during cell division relies on the assembly of a structure composed of microtubules, the bipolar spindle. As a cell prepares its division, changes in the state of its cytoplasm lead to the disassembly and to the global instability of its microtubule network. However, in order for the spindle to form, its microtubules must persist amid this destabilizing cytoplasm. A large body of experimental data now shows that the chromosomes themselves are the source of this persistence. They carry out this task through a chromatin-induced reversal of the dominating microtubule depolymerization regime, thereby resulting in a net microtubule polymerization locally around chromosomes, and through the capture, by kinetochores, of microtubules associated to each spindle pole and the assembly of kinetochore-associated microtubule bundles (K-fibres). In this review, we discuss the mechanisms and molecules that allow chromosomes to locally induce microtubule polymerization and stabilization during spindle assembly.