Dynamics of myosin, microtubules, and Kinesin-6 at the cortex during cytokinesis in Drosophila S2 cells

© The Authors, 2009 . This article is distributed under the terms of the Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License. The definitive version was published in Journal of Cell Biology 186 (2009): 727-738, doi:10.1083/jcb.200902083.Signals from the mitotic spindle during anaphase specify the location of the actomyosin contractile ring during cytokinesis, but the detailed mechanism remains unresolved. Here, we have imaged the dynamics of green fluorescent protein–tagged myosin filaments, microtubules, and Kinesin-6 (which carries activators of Rho guanosine triphosphatase) at the cell cortex using total internal reflection fluorescence microscopy in flattened Drosophila S2 cells. At anaphase onset, Kinesin-6 relocalizes to microtubule plus ends that grow toward the cortex, but refines its localization over time so that it concentrates on a subset of stable microtubules and along a diffuse cortical band at the equator. The pattern of Kinesin-6 localization closely resembles where new myosin filaments appear at the cortex by de novo assembly. While accumulating at the equator, myosin filaments disappear from the poles of the cell, a process that also requires Kinesin-6 as well as possibly other signals that emanate from the elongating spindle. These results suggest models for how Kinesin-6 might define the position of cortical myosin during cytokinesis.This work was supported by a National Institutes of Health grant NIH 38499 to R.D. Vale

Directional instability of microtubule transport in the presence of kinesin and dynein, two opposite polarity motor proteins.

Kinesin and dynein are motor proteins that move in opposite directions along microtubules. In this study, we examine the consequences of having kinesin and dynein (ciliary outer arm or cytoplasmic) bound to glass surfaces interacting with the same microtubule in vitro. Although one might expect a balance of opposing forces to produce little or no net movement, we find instead that microtubules move unidirectionally for several microns (corresponding to hundreds of ATPase cycles by a motor) but continually switch between kinesin-directed and dynein-directed transport. The velocities in the plus-end (0.2-0.3 microns/s) and minus-end (3.5-4 microns/s) directions were approximately half those produced by kinesin (0.5 microns/s) and ciliary dynein (6.7 microns/s) alone, indicating that the motors not contributing to movement can interact with and impose a drag upon the microtubule. By comparing two dyneins with different duty ratios (percentage of time spent in a strongly bound state during the ATPase cycle) and varying the nucleotide conditions, we show that the microtubule attachment times of the two opposing motors as well as their relative numbers determine which motor predominates in this assay. Together, these findings are consistent with a model in which kinesin-induced movement of a microtubule induces a negative strain in attached dyneins which causes them to dissociate before entering a force-generating state (and vice versa); reversals in the direction of transport may require the temporary dissociation of the transporting motor from the microtubule. The bidirectional movements described here are also remarkably similar to the back-and-forth movements of chromosomes during mitosis and membrane vesicles in fibroblasts. These results suggest that the underlying mechanical properties of motor proteins, at least in part, may be responsible for reversals in microtubule-based transport observed in cells

Insights into centriole geometry revealed by cryotomography of doublet and triplet centrioles.

Centrioles are cylindrical assemblies comprised of 9 singlet, doublet, or triplet microtubules, essential for the formation of motile and sensory cilia. While the structure of the cilium is being defined at increasing resolution, centriolar structure remains poorly understood. Here, we used electron cryo-tomography to determine the structure of mammalian (triplet) and Drosophila (doublet) centrioles. Mammalian centrioles have two distinct domains: a 200 nm proximal core region connected by A-C linkers, and a distal domain where the C-tubule is incomplete and a pair of novel linkages stabilize the assembly producing a geometry more closely resembling the ciliary axoneme. Drosophila centrioles resemble the mammalian core, but with their doublet microtubules linked through the A tubules. The commonality of core-region length, and the abrupt transition in mammalian centrioles, suggests a conserved length-setting mechanism. The unexpected linker diversity suggests how unique centriolar architectures arise in different tissues and organisms

Amplification and squeezing of quantum noise with a tunable Josephson metamaterial

It has recently become possible to encode the quantum state of superconducting qubits and the position of nanomechanical oscillators into the states of microwave fields. However, to make an ideal measurement of the state of a qubit, or to detect the position of a mechanical oscillator with quantum-limited sensitivity requires an amplifier that adds no noise. If an amplifier adds less than half a quantum of noise, it can also squeeze the quantum noise of the electromagnetic vacuum. Highly squeezed states of the vacuum serve as an important quantum information resource. They can be used to generate entanglement or to realize back-action-evading measurements of position. Here we introduce a general purpose parametric device, which operates in a frequency band between 4 and 8 GHz. It is a subquantum-limited microwave amplifier, it amplifies quantum noise above the added noise of commercial amplifiers, and it squeezes quantum fluctuations by 10 dB.Comment: 13 pages, 4 figure

Cellular aspect ratio and cell division mechanics underlie the patterning of cell progeny in diverse mammalian epithelia.

Cell division is essential to expand, shape, and replenish epithelia. In the adult small intestine, cells from a common progenitor intermix with other lineages, whereas cell progeny in many other epithelia form contiguous patches. The mechanisms that generate these distinct patterns of progeny are poorly understood. Using light sheet and confocal imaging of intestinal organoids, we show that lineages intersperse during cytokinesis, when elongated interphase cells insert between apically displaced daughters. Reducing the cellular aspect ratio to minimize the height difference between interphase and mitotic cells disrupts interspersion, producing contiguous patches. Cellular aspect ratio is similarly a key parameter for division-coupled interspersion in the early mouse embryo, suggesting that this physical mechanism for patterning progeny may pertain to many mammalian epithelia. Our results reveal that the process of cytokinesis in elongated mammalian epithelia allows lineages to intermix and that cellular aspect ratio is a critical modulator of the progeny pattern

Bragg spectroscopy of a strongly interacting Fermi gas

We present a comprehensive study of the Bose-Einstein condensate to Bardeen-Cooper-Schrieffer (BEC-BCS) crossover in fermionic $^6$Li using Bragg spectroscopy. A smooth transition from molecular to atomic spectra is observed with a clear signature of pairing at and above unitarity. These spectra probe the dynamic and static structure factors of the gas and provide a direct link to two-body correlations. We have characterised these correlations and measured their density dependence across the broad Feshbach resonance at 834 G.Comment: Replaced with published versio