1,439 research outputs found
Novel steady state of a microtubule assembly in a confined geometry
We study the steady state of an assembly of microtubules in a confined
volume, analogous to the situation inside a cell where the cell boundary forms
a natural barrier to growth. We show that the dynamical equations for growing
and shrinking microtubules predict the existence of two steady states, with
either exponentially decaying or exponentially increasing distribution of
microtubule lengths. We identify the regimes in parameter space corresponding
to these steady states. In the latter case, the apparent catastrophe frequency
near the boundary was found to be significantly larger than that in the
interior. Both the exponential distribution of lengths and the increase in the
catastrophe frequency near the cell margin is in excellent agreement with
recent experimental observations.Comment: 8 pages, submitted to Phys. Rev.
Dynamic concentration of motors in microtubule arrays
We present experimental and theoretical studies of the dynamics of molecular
motors in microtubule arrays and asters. By solving a convection-diffusion
equation we find that the density profile of motors in a two-dimensional aster
is characterized by continuously varying exponents. Simulations are used to
verify the assumptions of the continuum model. We observe the concentration
profiles of kinesin moving in quasi two-dimensional artificial asters by
fluorescent microscopy and compare with our theoretical results.Comment: 4pages, 4 figures revte
Antipolar ordering of topological defects in active liquid crystals
ATP-driven microtubule-kinesin bundles can self-assemble into two-dimensional
active liquid crystals (ALCs) that exhibit a rich creation and annihilation
dynamics of topological defects, reminiscent of particle-pair production
processes in quantum systems. This recent discovery has sparked considerable
interest but a quantitative theoretical description is still lacking. We
present and validate a minimal continuum theory for this new class of active
matter systems by generalizing the classical Landau-de Gennes free-energy to
account for the experimentally observed spontaneous buckling of motor-driven
extensile microtubule bundles. The resulting model agrees with recently
published data and predicts a regime of antipolar order. Our analysis implies
that ALCs are governed by the same generic ordering principles that determine
the non-equilibrium dynamics of dense bacterial suspensions and elastic bilayer
materials. Moreover, the theory manifests an energetic analogy with strongly
interacting quantum gases. Generally, our results suggest that complex
non-equilibrium pattern-formation phenomena might be predictable from a few
fundamental symmetry-breaking and scale-selection principles.Comment: final accepted journal version; SI text and movies available at
article on iop.or
Stochastic models of intracellular transport
The interior of a living cell is a crowded, heterogenuous, fluctuating environment. Hence, a major challenge in modeling intracellular transport is to analyze stochastic processes within complex environments. Broadly speaking, there are two basic mechanisms for intracellular transport: passive diffusion and motor-driven active transport. Diffusive transport can be formulated in terms of the motion of an over-damped Brownian particle. On the other hand, active transport requires chemical energy, usually in the form of ATP hydrolysis, and can be direction specific, allowing biomolecules to be transported long distances; this is particularly important in neurons due to their complex geometry. In this review we present a wide range of analytical methods and models of intracellular transport. In the case of diffusive transport, we consider narrow escape problems, diffusion to a small target, confined and single-file diffusion, homogenization theory, and fractional diffusion. In the case of active transport, we consider Brownian ratchets, random walk models, exclusion processes, random intermittent search processes, quasi-steady-state reduction methods, and mean field approximations. Applications include receptor trafficking, axonal transport, membrane diffusion, nuclear transport, protein-DNA interactions, virus trafficking, and the self–organization of subcellular structures
Intracellular transport driven by cytoskeletal motors: General mechanisms and defects
Cells are strongly out-of-equilibrium systems driven by continuous energy
supply. They carry out many vital functions requiring active transport of
various ingredients and organelles, some being small, others being large. The
cytoskeleton, composed of three types of filaments, determines the shape of the
cell and plays a role in cell motion. It also serves as a road network for the
so-called cytoskeletal motors. These molecules can attach to a cytoskeletal
filament, perform directed motion, possibly carrying along some cargo, and then
detach. It is a central issue to understand how intracellular transport driven
by molecular motors is regulated, in particular because its breakdown is one of
the signatures of some neuronal diseases like the Alzheimer.
We give a survey of the current knowledge on microtubule based intracellular
transport. We first review some biological facts obtained from experiments, and
present some modeling attempts based on cellular automata. We start with
background knowledge on the original and variants of the TASEP (Totally
Asymmetric Simple Exclusion Process), before turning to more application
oriented models. After addressing microtubule based transport in general, with
a focus on in vitro experiments, and on cooperative effects in the
transportation of large cargos by multiple motors, we concentrate on axonal
transport, because of its relevance for neuronal diseases. It is a challenge to
understand how this transport is organized, given that it takes place in a
confined environment and that several types of motors moving in opposite
directions are involved. We review several features that could contribute to
the efficiency of this transport, including the role of motor-motor
interactions and of the dynamics of the underlying microtubule network.
Finally, we discuss some still open questions.Comment: 74 pages, 43 figure
- …