Chromosome Segregation: Organizing Overlap at the Midzone

SummarySets of overlapping microtubules support the segregation of chromosomes by linking the poles of mitotic spindles. Recent work examines the effect of putting these linkages under pressure by the activation of dicentric chromosomes and sheds new light on the structural role of several well-known spindle midzone proteins

Dynamic instability of microtubules is regulated by force

Microtubules are long filamentous protein structures that randomly alternate between periods of elongation and shortening in a process termed dynamic instability. The average time a microtubule spends in an elongation phase, known as the catastrophe time, is regulated by the biochemical machinery of the cell throughout the cell cycle. In this light, observed changes in the catastrophe time near cellular boundaries (Brunner, D., and P. Nurse. 2000. Cell. 102:695–704; Komarova, Y.A., I.A. Vorobjev, and G.G. Borisy. 2002. J. Cell Sci. 115:3527–3539) may be attributed to regulatory effects of localized proteins. Here, we argue that the pushing force generated by a microtubule when growing against a cellular object may itself provide a regulatory mechanism of the catastrophe time. We observed an up to 20-fold, force-dependent decrease in the catastrophe time when microtubules grown from purified tubulin were polymerizing against microfabricated barriers. Comparison with catastrophe times for microtubules growing freely at different tubulin concentrations leads us to conclude that force reduces the catastrophe time only by limiting the rate of tubulin addition

Shortening of microtubule overlap regions defines membrane delivery sites during plant cytokinesis

© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Current Biology 27 (2017): 514-520, doi:10.1016/j.cub.2016.12.043.Different from animal cells that divide by constriction of the cortex inwards, cells of land plants divide by initiating a new cell wall segment from their centre. For this, a disk-shaped, membrane-enclosed precursor termed the cell plate is formed that radially expands towards the parental cell wall. The synthesis of the plate starts with the fusion of vesicles into a tubulo-vesicular network. Vesicles are putatively delivered to the division plane by transport along microtubules of the bipolar phragmoplast network that guides plate assembly. How vesicle immobilisation and fusion are then locally triggered is unclear. In general, a framework for how the cytoskeleton spatially defines cell plate formation is lacking. Here we show that membranous material for cell plate formation initially accumulates along regions of microtubule overlap in the phragmoplast of the moss Physcomitrella patens. Kinesin-4 mediated shortening of these overlaps at the onset of cytokinesis proved to be required to spatially confine membrane accumulation. Without shortening, the wider cell plate membrane depositions evolved into cell walls that were thick and irregularly shaped. Phragmoplast assembly thus provides a regular lattice of short overlaps on which a new cell wall segment can be scaffolded. Since similar patterns of overlaps form in central spindles of animal cells, involving the activity of orthologous proteins, we anticipate that our results will help uncover universal features underlying membrane-cytoskeleton coordination during cytokinesis.The work has been financially supported by HFSP grant RGP0026/2011 to MEJ and GG.2018-01-2

DIX Domain Polymerization Drives Assembly of Plant Cell Polarity Complexes

The identities of cell polarity determinants are not conserved between animals and plants; however, characterization of a DIX-domain containing protein in land plants reveals that the physical principles of polar complex assembly are preserved across eukaryotes.</p

Nitrogen fixation and transfer in open ocean diatom–cyanobacterial symbioses

Many diatoms that inhabit low-nutrient waters of the open ocean live in close association with cyanobacteria. Some of these associations are believed to be mutualistic, where N2-fixing cyanobacterial symbionts provide N for the diatoms. Rates of N2 fixation by symbiotic cyanobacteria and the N transfer to their diatom partners were measured using a high-resolution nanometer scale secondary ion mass spectrometry approach in natural populations. Cell-specific rates of N2 fixation (1.15–71.5 fmol N per cell h−1) were similar amongst the symbioses and rapid transfer (within 30 min) of fixed N was also measured. Similar growth rates for the diatoms and their symbionts were determined and the symbiotic growth rates were higher than those estimated for free-living cells. The N2 fixation rates estimated for Richelia and Calothrix symbionts were 171–420 times higher when the cells were symbiotic compared with the rates estimated for the cells living freely. When combined, the latter two results suggest that the diatom partners influence the growth and metabolism of their cyanobacterial symbionts. We estimated that Richelia fix 81–744% more N than needed for their own growth and up to 97.3% of the fixed N is transferred to the diatom partners. This study provides new information on the mechanisms controlling N input into the open ocean by symbiotic microorganisms, which are widespread and important for oceanic primary production. Further, this is the first demonstration of N transfer from an N2 fixer to a unicellular partner. These symbioses are important models for molecular regulation and nutrient exchange in symbiotic systems

Bacterial Gut Symbionts Contribute to Seed Digestion in an Omnivorous Beetle

Obligate bacterial symbionts alter the diets of host animals in numerous ways, but the ecological roles of facultative bacterial residents that colonize insect guts remain unclear. Carabid beetles are a common group of beneficial insects appreciated for their ability to consume insect prey and seeds, but the contributions of microbes to diet diversification in this and similar groups of facultative granivores are largely unknown.Using 16S rRNA gene clone libraries and terminal restriction fragment (tRF) length polymorphism analyses of these genes, we examined the bacterial communities within the guts of facultatively granivorous, adult Harpalus pensylvanicus (Carabidae), fed one of five dietary treatments: 1) an untreated Field population, 2) Seeds with antibiotics (seeds were from Chenopodium album), 3) Seeds without antibiotics, 4) Prey with antibiotics (prey were Acheta domesticus eggs), and 5) Prey without antibiotics. The number of seeds and prey consumed by each beetle were recorded following treatment. Harpalus pensylvanicus possessed a fairly simple gut community of approximately 3-4 bacterial operational taxonomic units (OTU) per beetle that were affiliated with the Gammaproteobacteria, Bacilli, Alphaproteobacteria, and Mollicutes. Bacterial communities of the host varied among the diet and antibiotic treatments. The field population and beetles fed seeds without antibiotics had the closest matching bacterial communities, and the communities in the beetles fed antibiotics were more closely related to each other than to those of the beetles that did not receive antibiotics. Antibiotics reduced and altered the bacterial communities found in the beetle guts. Moreover, beetles fed antibiotics ate fewer seeds, and those beetles that harbored the bacterium Enterococcus faecalis consumed more seeds on average than those lacking this symbiont.We conclude that the relationships between the bacterium E. faecalis and this factultative granivore's ability to consume seeds merit further investigation, and that facultative associations with symbiotic bacteria have important implications for the nutritional ecology of their hosts

Forced apart : a microtubule-based mechanism for equidistant positioning of multiple nuclei in single cells

Cells can position multiple copies of components like carboxysomes, nucleoids, and nuclei at regular intervals. By controlling positions, cells, for example, ensure equal partitioning of organelles over daughter cells and, in the case of nuclei, control cell sizes during cellularization. Mechanisms that generate regular patterns are as yet poorly understood. We used fission yeast cell cycle mutants to investigate the dispersion of multiple nuclei by microtubule-generated forces in single cells. After removing internuclear attractive forces by microtubule-based molecular motors, we observed the establishment of regular patterns of nuclei. Based on live-cell imaging, we hypothesized that microtubule growth within internuclear spaces pushes neighbouring nuclei apart. In the proposed mechanism, which was validated by stochastic simulations, the repulsive force weakens with increasing separation because stochastic shortening events limit the extent over which microtubules generate forces. Our results, therefore, demonstrate how cells can exploit the dynamics of microtubule growth for the equidistant positioning of organelles