1,675 research outputs found
Positive Feedback Regulation Results in Spatial Clustering and Fast Spreading of Active Signaling Molecules on a Cell Membrane
Positive feedback regulation is ubiquitous in cell signaling networks, often
leading to binary outcomes in response to graded stimuli. However, the role of
such feedbacks in clustering, and in spatial spreading of activated molecules,
has come to be appreciated only recently. We focus on the latter, using a
simple model developed in the context of Ras activation with competing negative
and positive feedback mechanisms. We find that positive feedback, in the
presence of slow diffusion, results in clustering of activated molecules on the
plasma membrane, and rapid spatial spreading as the front of the cluster
propagates with a constant velocity (dependent on the feedback strength). The
advancing fronts of the clusters of the activated species are rough, with
scaling consistent with the Kardar-Parisi-Zhang (KPZ) equation in one
dimension. Our minimal model is general enough to describe signal transduction
in a wide variety of biological networks where activity in the
membrane-proximal region is subject to feedback regulation.Comment: 37 pages, 8 figures. Journal of Chemical Physics (in press
Specialization at an expanding front
As a population grows, spreading to new environments may favor
specialization. In this paper, we introduce and explore a model for
specialization at the front of a colony expanding synchronously into new
territory. We show through numerical simulations that, by gaining fitness
through accumulating mutations, progeny of the initial seed population can
differentiate into distinct specialists. With competition and selection limited
to the growth front, the emerging specialists first segregate into sectors,
which then expand to dominate the entire population. We quantify the scaling of
the fixation time with the size of the population and observe different
behaviors corresponding to distinct universality classes: unbounded and bounded
gains in fitness lead to superdiffusive () and diffusive ()
stochastic wanderings of the sector boundaries, respectively.Comment: 6+3 pages, 3+3 figure
Material dependence of Casimir forces: gradient expansion beyond proximity
A widely used method for estimating Casimir interactions [H. B. G. Casimir,
Proc. K. Ned. Akad. Wet. 51, 793 (1948)] between gently curved material
surfaces at short distances is the proximity force approximation (PFA). While
this approximation is asymptotically exact at vanishing separations,
quantifying corrections to PFA has been notoriously difficult. Here we use a
derivative expansion to compute the leading curvature correction to PFA for
metals (gold) and insulators (SiO) at room temperature. We derive an
explicit expression for the amplitude of the PFA correction to
the force gradient for axially symmetric surfaces. In the non-retarded limit,
the corrections to the Casimir free energy are found to scale logarithmically
with distance. For gold, has an unusually large temperature
dependence.Comment: 4 pages, 2 figure
Casimir Force at a Knife's Edge
The Casimir force has been computed exactly for only a few simple geometries,
such as infinite plates, cylinders, and spheres. We show that a parabolic
cylinder, for which analytic solutions to the Helmholtz equation are available,
is another case where such a calculation is possible. We compute the
interaction energy of a parabolic cylinder and an infinite plate (both perfect
mirrors), as a function of their separation and inclination, and ,
and the cylinder's parabolic radius . As , the proximity force
approximation becomes exact. The opposite limit of corresponds to a
semi-infinite plate, where the effects of edge and inclination can be probed.Comment: 5 pages, 3 figures, uses RevTeX; v2: expanded conclusions; v3: fixed
missing factor in Eq. (3) and incorrect diagram label (no changes to
results); v4: fix similar factor in Eq. (16) (again no changes to results
Spontaneous emission by rotating objects: A scattering approach
We study the quantum electrodynamics (QED) vacuum in the presence of a body
rotating along its axis of symmetry and show that the object spontaneously
emits energy if it is lossy. The radiated power is expressed as a general trace
formula solely in terms of the scattering matrix, making an explicit connection
to the conjecture of Zel'dovich [JETP Lett. 14, 180 (1971)] on rotating
objects. We further show that a rotating body drags along nearby objects while
making them spin parallel to its own rotation axis
First Passage Distributions in a Collective Model of Anomalous Diffusion with Tunable Exponent
We consider a model system in which anomalous diffusion is generated by
superposition of underlying linear modes with a broad range of relaxation
times. In the language of Gaussian polymers, our model corresponds to Rouse
(Fourier) modes whose friction coefficients scale as wavenumber to the power
. A single (tagged) monomer then executes subdiffusion over a broad range
of time scales, and its mean square displacement increases as with
. To demonstrate non-trivial aspects of the model, we numerically
study the absorption of the tagged particle in one dimension near an absorbing
boundary or in the interval between two such boundaries. We obtain absorption
probability densities as a function of time, as well as the position-dependent
distribution for unabsorbed particles, at several values of . Each of
these properties has features characterized by exponents that depend on
. Characteristic distributions found for different values of
have similar qualitative features, but are not simply related quantitatively.
Comparison of the motion of translocation coordinate of a polymer moving
through a pore in a membrane with the diffusing tagged monomer with identical
also reveals quantitative differences.Comment: LaTeX, 10 pages, 8 eps figure
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