3,814 research outputs found
Steady-state MreB helices inside bacteria: dynamics without motors
Within individual bacteria, we combine force-dependent polymerization
dynamics of individual MreB protofilaments with an elastic model of
protofilament bundles buckled into helical configurations. We use variational
techniques and stochastic simulations to relate the pitch of the MreB helix,
the total abundance of MreB, and the number of protofilaments. By comparing our
simulations with mean-field calculations, we find that stochastic fluctuations
are significant. We examine the quasi-static evolution of the helical pitch
with cell growth, as well as timescales of helix turnover and denovo
establishment. We find that while the body of a polarized MreB helix treadmills
towards its slow-growing end, the fast-growing tips of laterally associated
protofilaments move towards the opposite fast-growing end of the MreB helix.
This offers a possible mechanism for targeted polar localization without
cytoplasmic motor proteins.Comment: 7 figures, 1 tabl
Helicase activity on DNA as a propagating front
We develop a propagating front analysis, in terms of a local probability of
zipping, for the helicase activity of opening up a double stranded DNA (dsDNA).
In a fixed-distance ensemble (conjugate to the fixed-force ensemble) the front
separates the zipped and unzipped phases of a dsDNA and a drive acts locally
around the front. Bounds from variational analysis and numerical estimates for
the speed of a helicase are obtained. Different types of helicase behaviours
can be distinguished by the nature of the drive.Comment: 5 pages, 5 eps figures; replaced by the published versio
Surface tension in bilayer membranes with fixed projected area
We study the elastic response of bilayer membranes with fixed projected area
to both stretching and shape deformations. A surface tension is associated to
each of these deformations. By using model amphiphilic membranes and computer
simulations, we are able to observe both the types of deformation, and thus,
both the surface tensions, related to each type of deformation, are measured
for the same system. These surface tensions are found to assume different
values in the same bilayer membrane: in particular they vanish for different
values of the projected area. We introduce a simple theory which relates the
two quantities and successfully apply it to the data obtained with computer
simulations
Spicules and the effect of rigid rods on enclosing membrane tubes
Membrane tubes (spicules) arise in cells, or artificial membranes, in the
nonlinear deformation regime due to, e.g. the growth of microtubules, actin
filaments or sickle hemoglobin fibers towards a membrane. We calculate the
axial force exerted by the cylindrical membrane tube, and its average radius,
by taking into account steric interactions between the fluctuating membrane and
the enclosed rod. The force required to confine a fluctuating membrane near the
surface of the enclosed rod diverges as the separation approaches zero. This
results in a smooth crossover of the axial force between a square root and a
linear dependence on the membrane tension as the tension increases and the tube
radius shrinks. This crossover can occur at the most physiologically relevant
membrane tensions. Our work may be important in (i) interpreting experiments in
which axial force is related to the tube radius or membrane tension (ii)
dynamical theories for biopolymer growth in narrow tubes where these
fluctuation effects control the tube radius.Comment: 10 pages, 1 figur
Fluctuation-Driven Molecular Transport in an Asymmetric Membrane Channel
Channel proteins, that selectively conduct molecules across cell membranes,
often exhibit an asymmetric structure. By means of a stochastic model, we argue
that channel asymmetry in the presence of non-equilibrium fluctuations, fueled
by the cell's metabolism as observed recently, can dramatically influence the
transport through such channels by a ratchet-like mechanism. For an
aquaglyceroporin that conducts water and glycerol we show that a previously
determined asymmetric glycerol potential leads to enhanced inward transport of
glycerol, but for unfavorably high glycerol concentrations also to enhanced
outward transport that protects a cell against poisoning.Comment: REVTeX4, 4 pages, 3 figures; Accepted for publication in Phys. Rev.
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Equilibrium Chemical Engines
An equilibrium reversible cycle with a certain engine to transduce the energy
of any chemical reaction into mechanical energy is proposed. The efficiency for
chemical energy transduction is also defined so as to be compared with Carnot
efficiency. Relevance to the study of protein motors is discussed. KEYWORDS:
Chemical thermodynamics, Engine, Efficiency, Molecular machine.Comment: 5 pages, late
Anomalous lateral diffusion in a viscous membrane surrounded by viscoelastic media
We investigate the lateral dynamics in a purely viscous lipid membrane
surrounded by viscoelastic media such as polymeric solutions. We first obtain
the generalized frequency-dependent mobility tensor and focus on the case when
the solvent is sandwiched by hard walls. Due to the viscoelasticity of the
solvent, the mean square displacement of a disk embedded in the membrane
exhibits an anomalous diffusion. An useful relation which connects the mean
square displacement and the solvent modulus is provided. We also calculate the
cross-correlation of the particle displacements which can be applied for
two-particle tracking experiments.Comment: 6 pages, 2 figure
Rubidazone vs adriamycin: an evaluation of their differential toxicity in the spleen colony assay system.
Rubidazone, the new semi-synthetic benzol hydrazone hydrochloride derivative of dauorubicin, has proved on a molecular weight basis to be less toxic than adriamycin and similar to daunorubicin in cardiac toxicity studies in the hamster as well as in other in vivo and in vitro test systems. It has proven effectiveness against several animal tumours and human acute leukaemias. We have compared the inhibitory effect of rubidazone to that of adriamycin on P388 leukaemia and normal bone marrow colony-forming units (CFU) using the spleen colony assay system in male DBA2 mice. The efficacy ratios (i.e., the ratio of the slopes of the normal bone marrow CFU to leukaemic CFU dose-survival curves) in the spleen colony assay system for rubidazone and adriamycin were 7-8 and 7-5 respectively. This near identity of efficacy ratios fro rubidazone and adriamycin correlated with the results of median survival time studies in the leukaemic mice. Their dose-median survival time curves were almost parallel, having nearly identical slopes. Rubidazone's equal therapeutic index as compared to adriamycin in the spleen colony assay system together with its known decreased toxicity to cardiac muscle cells makes it an extremely promising new anthracycline derivative to study in comparison to adriamycin in human malignancies
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