135 research outputs found
Shape Changes of Self-Assembled Actin Bilayer Composite Membranes
We report the self-assembly of thin actin shells beneath the membranes of
giant vesicles. Ion-carrier mediated influx of Mg2+ induces actin
polymerization in the initially spherical vesicles. Buckling of the vesicles
and the formation of blisters after thermally induced bilayer expansion is
demonstrated. Bilayer flickering is dominated by tension generated by its
coupling to the actin cortex. Quantitative flicker analysis suggests the
bilayer and the actin cortex are separated by 0.4 \mum to 0.5 \mum due to
undulation forces.Comment: pdf-file, has been accepted by PR
Non-equilibrium hydrodynamics of a rotating filament
The nonlinear dynamics of an elastic filament that is forced to rotate at its
base is studied by hydrodynamic simulation techniques; coupling between
stretch, bend, twist elasticity and thermal fluctuations is included. The
twirling-overwhirling transition is located and found to be strongly
discontinuous. For finite bend and twist persistence length, thermal
fluctuations lower the threshold rotational frequency, for infinite persistence
length the threshold agrees with previous analytical predictions
On Shape Transformations and Shape Fluctuations of Cellular Compartments and Vesicles
We discuss the shape formation and shape transitions of simple bilayer vesicles in context with their role in biology. In the first part several classes of shape changes of vesicles of one lipid component are described and it is shown that these can be explained in terms of the bending energy concept in particular augmented by the bilayer coupling hypothesis. In the second
part shape changes and vesicle fission of vesicles composed of membranes of lipid mixtures are reported. These are explained in terms of coupling between local curvature and phase separation
Finite Size Polyelectrolyte Bundles at Thermodynamic Equilibrium
We present the results of extensive computer simulations performed on
solutions of monodisperse charged rod-like polyelectrolytes in the presence of
trivalent counterions. To overcome energy barriers we used a combination of
parallel tempering and hybrid Monte Carlo techniques. Our results show that for
small values of the electrostatic interaction the solution mostly consists of
dispersed single rods. The potential of mean force between the polyelectrolyte
monomers yields an attractive interaction at short distances. For a range of
larger values of the Bjerrum length, we find finite size polyelectrolyte
bundles at thermodynamic equilibrium. Further increase of the Bjerrum length
eventually leads to phase separation and precipitation. We discuss the origin
of the observed thermodynamic stability of the finite size aggregates
The origin of stiffening in cross-linked semiflexible networks
Strain stiffening of protein networks is explored by means of a finite strain
analysis of a two-dimensional network model of cross-linked semiflexible
filaments. The results show that stiffening is caused by non-affine network
rearrangements that govern a transition from a bending dominated response at
small strains to a stretching dominated response at large strains.
Thermally-induced filament undulations only have a minor effect; they merely
postpone the transition.Comment: 5 pages, 5 figure
Elasticity of Semiflexible Biopolymer Networks
We develop a model for gels and entangled solutions of semiflexible
biopolymers such as F-actin. Such networks play a crucial structural role in
the cytoskeleton of cells. We show that the rheologic properties of these
networks can result from nonclassical rubber elasticity. This model can explain
a number of elastic properties of such networks {\em in vitro}, including the
concentration dependence of the storage modulus and yield strain.Comment: Uses RevTeX, full postscript with figures available at
http://www.umich.edu/~fcm/preprints/agel/agel.htm
Giant vesicles at the prolate-oblate transition: A macroscopic bistable system
Giant phospholipid vesicles are shown to exhibit thermally activated
transitions between a prolate and an oblate shape on a time scale of several
seconds. From the fluctuating contour of such a vesicle we extract ellipticity
as an effective reaction coordinate whose temporal probability distribution is
bimodal. We then reconstruct the effective potential from which we derive an
activation energy of the order of in agreement with theoretical
calculations. The dynamics of this transition is well described within a
Kramers model of overdamped diffusion in a bistable potential. Thus, this
system can serve as a model for macroscopic bistability.Comment: 10 pages, LaTeX, epsfig, 4 eps figures included, to appear in
Europhys. Let
Statistical mechanics of double-stranded semi-flexible polymers
We study the statistical mechanics of double-stranded semi-flexible polymers
using both analytical techniques and simulation. We find a transition at some
finite temperature, from a type of short range order to a fundamentally
different sort of short range order. In the high temperature regime, the
2-point correlation functions of the object are identical to worm-like chains,
while in the low temperature regime they are different due to a twist
structure. In the low temperature phase, the polymers develop a kink-rod
structure which could clarify some recent puzzling experiments on actin.Comment: 4 pages, 3 figures; final version for publication - slight
modifications to text and figure
Entanglement, elasticity and viscous relaxation of actin solutions
We have investigated the viscosity and the plateau modulus of actin solutions
with a magnetically driven rotating disc rheometer. For entangled solutions we
observed a scaling of the plateau modulus versus concentration with a power of
7/5. The measured terminal relaxation time increases with a power 3/2 as a
function of polymer length. We interpret the entanglement transition and the
scaling of the plateau modulus in terms of the tube model for semiflexible
polymers.Comment: 5 pages, 4 figures, published versio
Tug-of-war in motility assay experiments
The dynamics of two groups of molecular motors pulling in opposite directions
on a rigid filament is studied theoretically. To this end we first consider the
behavior of one set of motors pulling in a single direction against an external
force using a new mean-field approach. Based on these results we analyze a
similar setup with two sets of motors pulling in opposite directions in a
tug-of-war in the presence of an external force. In both cases we find that the
interplay of fluid friction and protein friction leads to a complex phase
diagram where the force-velocity relations can exhibit regions of bistability
and spontaneous symmetry breaking. Finally, motivated by recent work, we turn
to the case of motility assay experiments where motors bound to a surface push
on a bundle of filaments. We find that, depending on the absence or the
presence of a bistability in the force-velocity curve at zero force, the bundle
exhibits anomalous or biased diffusion on long-time and large-length scales
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