Université PSL



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Pushing off the walls: a mechanism of cell motility in confinement
Hawkins RJ, Piel M, Faure-Andre G, Lennon-Dumenil AM, Joanny JF, Prost J, Voituriez R
Phys. Rev. Lett. - 102(5) :58103 - DOI:10.1103/PhysRevLett.102.058103 - 2009
We propose a novel mechanism of cell motility, which relies on the coupling of actin polymerization at the cell membrane to geometric confinement. We consider a polymerizing viscoelastic cytoskeletal gel confined in a narrow channel, and show analytically that spontaneous motion occurs. Interestingly, this does not require specific adhesion with the channel walls, and yields velocities potentially larger than the polymerization velocity. The contractile activity of myosin motors is not necessary to trigger motility in this mechanism, but is shown quantitatively to increase the velocity. Our model qualitatively accounts for recent experiments which show that cells without specific adhesion proteins are motile only in confined environments while they are unable to move on a flat surface, and could help in understanding the mechanisms of cell migration in more complex confined geometries such as living tissues.
Diffusion through colloidal shells under stress
J. Guery, J. Baudry, D. A. Weitz, P. M. Chaikin, J. Bibette
Phys. Rev. E - 79 :060402 - DOI:10.1103/PhysRevE.79.060402 - 2009
The permeability of solids has long been associated with a diffusive process involving activated mechanism as originally envisioned by Eyring. Tensile stress can affect the activation energy but definitive experiments of the diffusion rate of species through a stressed solid are lacking. Here we use core-shell (liquid core–solid shell) colloidal particles that are sensitive to osmotic pressure to follow the permeation of encapsulated probes at various stresses. We unambiguously show that the tensile stress applied on colloidal shells linearly reduces the local energy barrier for diffusion.
Breaking of an Emulsion under an ac Electric Field
A. R. Thiam, N. Bremond, J. Bibette
Phys. Rev. Lett. - 102(18) :18304 - DOI:10.1103/PhysRevLett.102.188304 - 2009
By using microfluidic chips, we investigate the stability regarding coalescence of droplet pairs under an electric field as a function of drop separation and ac field intensity. Three different regimes are found: stable, coalescence, and partial merging. From this, we identify the two breaking scenarios of a one dimensional train of droplets: in one case the coalescence front propagates; in the other case, in which pairs belong to the partial merging regime, the coalescence front can become heterogeneous. From these findings, we can propose a destruction mechanism for a macroscopic emulsion, which includes the packing condition for which total and immediate destruction is effective.
Direct observation of twisting steps during Rad51 polymerization on DNA
Arata H, Dupont A, Miné-Hattab J, Disseau L, Renodon-Cornière A, Takahashi M, Viovy JL, Cappello G
Proc. Nat. Acad. Sci. USA - 106(46) :19239-44 - DOI:10.1073/pnas.0902234106 - 2009
The human recombinase hRad51 is a key protein for the maintenance of genome integrity and for cancer development. Polymerization and depolymerization of hRad51 on duplex DNA were studied here using a new generation of magnetic tweezers, measuring DNA twist in real time with a resolution of 5°. Our results combined with earlier structural information suggest that DNA is somewhat less extended by hRad51 than by RecA (4.5 vs. 5.1 Å per base pair) and untwisted by 18.2° per base pair. They also confirm a stoichiometry of 3–4 bp per protein in the hRad51-dsDNA nucleoprotein filament. At odds with earlier claims, we show that after initial deposition of a multimeric nucleus, nucleoprotein filament growth occurs by addition/release of single proteins, involving DNA twisting steps of 65° ± 5°. Simple numeric simulations show that this mechanism is an efficient way to minimize nucleoprotein filament defects. Nucleoprotein filament growth from a preformed nucleus was observed at hRad51 concentrations down to 10 nM, whereas nucleation was never observed below 100 nM in the same buffer. This behavior can be associated with the different stoichiometries of nucleation and growth. It may be instrumental in vivo to permit efficient continuation of strand exchange by hRad51 alone while requiring additional proteins such as Rad52 for its initiation, thus keeping the latter under the strict control of regulatory pathways.
Droplet breakup in microfluidic T-junctions at small capillary numbers
M. C Jullien, M. J Tsang Mui Ching, C. Cohen, L. Ménétrier, P. Tabeling
Phys. Fluids - 21( 7) :7200-61 - DOI:10.1063/1.3170983 - 2009
We perform experimental studies of droplet breakup in microfluidic T-junctions in a range of capillary numbers lying between 4×10-4 and 2×10-1 and for two viscosity ratios of the fluids forming the dispersed and continuous phases. The present paper extends the range of capillary numbers explored by previous investigators by two orders of magnitude. We single out two different regimes of breakup. In a first regime, a gap exists between the droplet and the wall before breakup occurs. In this case, the breakup process agrees well with the analytical theory of Leshansky and Pismen [Phys. Fluids 21, 023303 (2009) ]. In a second regime, droplets keep obstructing the T-junction before breakup. Using physical arguments, we introduce a critical droplet extension for describing the breakup process in this case.
T7 RNA polymerase studied by force measurements varying cofactor concentration
P. Thomen, P. J. Lopez, U. Bockelmann, J. Guillerez, M. Dreyfus, and F. Heslot
Biophys. J. - 95(5) :2423–33 - DOI:10.1529/biophysj.107.125096 - 2009
RNA polymerases carry out the synthesis of an RNA copy from a DNA template. They move along DNA, incorporate nucleotide triphosphate (NTP) at the end of the growing RNA chain, and consume chemical energy. In a single-molecule assay using the T7 RNA polymerase, we study how a mechanical force opposing the forward motion of the enzyme along DNA affects the translocation rate. We also study the influence of nucleotide and magnesium concentration on this process. The experiment shows that the opposing mechanical force is a competitive inhibitor of nucleotide binding. Also, the single-molecule data suggest that magnesium ions are involved in a step that does not depend on the external load force. These kinetic results associated with known biochemical and mutagenic data, along with the static information obtained from crystallographic structures, shape a very coherent view of the catalytic cycle of the enzyme: translocation does not take place upon NTP binding nor upon NTP cleavage, but rather occurs after PPi release and before the next nucleotide binding event. Furthermore, the energetic bias associated with the forward motion of the enzyme is close to kT and represents only a small fraction of the free energy of nucleotide incorporation and pyrophosphate hydrolysis.
Probing DNA base pairing energy profiles using a nanopore
Virgile Viasnoff, Nicolas Chiaruttini, and Ulrich Bockelmann
European Biophysics Journal - 38(2) :263–9 - PMID:18836709 - 2009
We experimentally show that the voltage driven unzipping of long DNA duplexes by an a-hemolysin pore is sensitive to the shape of the base pairing energy landscape. Two sequences of equal global stability were investigated. The sequence with an homogeneous base pairing profile translocates faster than the one with alternative weak and strong regions. We could qualitatively account for theses observations by theoretically describing the voltage driven translocation as a biased random walk of the unzipping fork in the sequence dependent energy landscape.
A nanostructure made of a small non-coding RNA
Cayrol B., Nogues C., Dawid A., Sagi I., Silberzan P., Isambert H.
JACS - 131(47) :17270-6 - DOI:10.1021/ja906076e - 2009
Natural RNAs, unlike many proteins, have never been reported to form extended nanostructures, despite their wide variety of cellular functions. This is all the more striking, as synthetic DNA and RNA forming large nanostructures have long been successfully designed. Here, we show that DsrA, a 87-nt noncoding RNA of Escherishia coli, self-assembles into a hierarchy of nanostructures through antisense interactions of three contiguous self-complementary regions. Yet, the extended nanostructures, observed using atomic force microscopy (AFM) and fluorescence microscopy, are easily disrupted into >100 nm long helical bundles of DsrA filaments, including hundreds of DsrA monomers, and are surprisingly resistant to heat and urea denaturation. Molecular modeling demonstrates that this structural switch of DsrA nanostructures into filament bundles results from the relaxation of stored torsional constraints and suggests possible implications for DsrA regulatory function.
Mechanisms of chiral discrimination by topoisomerase IV
K. C. Neuman, G. Charvin, D. Bensimon and V. Croquette
Proc. Nat. Acad. Sci. USA - 106(17) :6986–91 - DOI:10.1073/pnas.0900574106 - 2009
Topoisomerase IV (Topo IV), an essential ATP-dependent bacterial type II topoisomerase, transports one segment of DNA through a transient double-strand break in a second segment of DNA. In vivo, Topo IV unlinks catenated chromosomes before cell division and relaxes positive supercoils generated during DNA replication. In vitro, Topo IV relaxes positive supercoils at least 20-fold faster than negative supercoils. The mechanisms underlying this chiral discrimination by Topo IV and other type II topoisomerases remain speculative. We used magnetic tweezers to measure the relaxation rates of single and multiple DNA crossings by Topo IV. These measurements allowed us to determine unambiguously the relative importance of DNA crossing geometry and enzymatic processivity in chiral discrimination by Topo IV. Our results indicate that Topo IV binds and passes DNA strands juxtaposed in a nearly perpendicular orientation and that relaxation of negative supercoiled DNA is perfectly distributive. Together, these results suggest that chiral discrimination arises primarily from dramatic differences in the processivity of relaxing positive and negative supercoiled DNA: Topo IV is highly processive on positively supercoiled DNA, whereas it is perfectly distributive on negatively supercoiled DNA. These results provide fresh insight into topoisomerase mechanisms and lead to a model that reconciles contradictory aspects of previous findings while providing a framework to interpret future results.
Measurement of the Torque on a Single Stretched and Twisted DNA Using Magnetic Tweezers
F. Mosconi, J.-F. Allemand, D. Bensimon and V. Croquette
Phys. Rev. Lett. - 102(7) :78301 - PMID:19257716 - 2009
We deduced the torque applied on a single stretched and twisted DNA by integrating the change in the molecule's extension with respect to force as it is coiled. While consistent with previous direct measurements of the torque at high forces (F > 1 pN), this method, which is simple and does not require a sophisticated setup, allows for lower force estimates. We used this approach to deduce the effective torsional modulus of DNA, which decreases with force, and to estimate the buckling torque of DNA as a function of force in various salt conditions.

346 publications.