480 research outputs found
Cooperative molecular motors moving back and forth
We use a two-state ratchet model to study the cooperative bidirectional
motion of molecular motors on cytoskeletal tracks with randomly alternating
polarities. Our model is based on a previously proposed model [Badoual et al.,
{\em Proc. Natl. Acad. Sci. USA} {\bf 99}, 6696 (2002)] for collective motor
dynamics and, in addition, takes into account the cooperativity effect arising
from the elastic tension that develops in the cytoskeletal track due to the
joint action of the walking motors. We show, both computationally and
analytically, that this additional cooperativity effect leads to a dramatic
reduction in the characteristic reversal time of the bidirectional motion,
especially in systems with a large number of motors. We also find that
bidirectional motion takes place only on (almost) a-polar tracks, while on even
slightly polar tracks the motion is unidirectional. We argue that the origin of
these observations is the sensitive dependence of the cooperative dynamics on
the difference between the number of motors typically working in and against
the instantaneous direction of motion.Comment: Accepted for publication in Phys. Rev.
Bidirectional cooperative motion of myosin-II motors on actin tracks with randomly alternating polarities
The cooperative action of many molecular motors is essential for dynamic
processes such as cell motility and mitosis. This action can be studied by
using motility assays in which the motion of cytoskeletal filaments over a
surface coated with motor proteins is tracked. In previous studies of
actin-myosin II systems, fast directional motion was observed, reflecting the
tendency of myosin II motors to propagate unidirectionally along actin
filaments. Here, we present a motility assay with actin bundles consisting of
short filamentous segments with randomly alternating polarities. These actin
tracks exhibit bidirectional motion with macroscopically large time intervals
(of the order of several seconds) between direction reversals. Analysis of this
bidirectional motion reveals that the characteristic reversal time,
, does not depend on the size of the moving bundle or on the number
of motors, . This observation contradicts previous theoretical calculations
based on a two-state ratchet model [Badoual et al., Proc. Natl. Acad. Sci. USA,
vol. 99, p. 6696 (2002)], predicting an exponential increase of
with . We present a modified version of this model that takes into account
the elastic energy due to the stretching of the actin track by the myosin II
motors. The new model yields a very good quantitative agreement with the
experimental results.Comment: A slightly revised version. Figures 2 and 7 were modified. Accepted
for publication in "Soft Matter
Robust spatial coherence 5m from a room-temperature atom chip
We study spatial coherence near a classical environment by loading a
Bose-Einstein condensate into a magnetic lattice potential and observing
diffraction. Even very close to a surface (5m), and even when the
surface is at room temperature, spatial coherence persists for a relatively
long time (500ms). In addition, the observed spatial coherence extends
over several lattice sites, a significantly greater distance than the
atom-surface separation. This opens the door for atomic circuits, and may help
elucidate the interplay between spatial dephasing, inter-atomic interactions,
and external noise.Comment: 15 pages, 14 figures, revised for final publication. This manuscript
includes in-depth analysis of the data presented in arXiv:1502.0160
Fluorescence Correlation Spectroscopy analysis of segmental dynamics in Actin filaments
We adapt Fluorescence Correlation spectroscopy (FCS) formalism to the studies
of the dynamics of semi-flexible polymers and derive expressions relating FCS
correlation function to the longitudinal and transverse mean square
displacements of polymer segments. We use the derived expressions to measure
the dynamics of actin filaments in two experimental situations: filaments
labeled at distinct positions and homogeneously labeled filaments. Both
approaches give consistent results and allow to measure the temporal dependence
of the segmental mean-square displacement (MSD) over almost five decades in
time, from ~0.04ms to 2s. These noninvasive measurements allow for a detailed
quantitative comparison of the experimental data to the current theories of
semi-flexible polymer dynamics. Good quantitative agreement is found between
the experimental results and theories explicitly accounting for the
hydrodynamic interactions between polymer segments
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