171 research outputs found
Quantifying protein diffusion and capture on filaments
The functional relevance of regulating proteins is often limited to specific
binding sites such as the ends of microtubules or actin-filaments. A
localization of proteins on these functional sites is of great importance. We
present a quantitative theory for a diffusion and capture process, where
proteins diffuse on a filament and stop diffusing when reaching the filament's
end. It is found that end-association after one-dimensional diffusion is the
main source for tip-localization of such proteins. As a consequence, diffusion
and capture is highly efficient in enhancing the reaction velocity of enzymatic
reactions, where proteins and filament ends are to each other as enzyme and
substrate. We show that the reaction velocity can effectively be described
within a Michaelis-Menten framework. Together one-dimensional diffusion and
capture beats the (three-dimensional) Smoluchowski diffusion limit for the rate
of protein association to filament ends.Comment: 13 pages, 7 figure
Cooperative effects enhance the transport properties of molecular spider teams
Molecular spiders are synthetic molecular motors based on DNA nanotechnology. While natural molecular motors have evolved towards very high efficiency, it remains a major challenge to develop efficient designs for man-made molecular motors. Inspired by biological motor proteins such as kinesin and myosin, molecular spiders comprise a body and several legs. The legs walk on a lattice that is coated with substrate which can be cleaved catalytically. We propose a molecular spider design in which n spiders form a team. Our theoretical considerations show that coupling several spiders together alters the dynamics of the resulting team significantly. Although spiders operate at a scale where diffusion is dominant, spider teams can be tuned to behave nearly ballistic, which results in fast and predictable motion. Based on the separation of time scales of substrate and product dwell times, we develop a theory which utilizes equivalence classes to coarse-grain the microstate space. In addition, we calculate diffusion coefficients of the spider teams, employing a mapping of an n-spider team to an n-dimensional random walker on a confined lattice. We validate these results with Monte Carlo simulations and predict optimal parameters of the molecular spider team architecture which makes their motion most directed and maximally predictable
Microtubule Length-Regulation by Molecular Motors
Length-regulation of microtubules (MTs) is essential for many cellular
processes. Molecular motors like kinesin 8, which move along MTs and also act
as depolymerases, are known as key players in MT dynamics. However, the
regulatory mechanisms of length control remain elusive. Here, we investigate a
stochastic model accounting for the interplay between polymerization kinetics
and motor-induced depolymerization. We determine the dependence of MT length
and variance on rate constants and motor concentration. Moreover, our analyses
reveal how collective phenomena lead to a well-defined MT length.Comment: 7 pages (5 p. letter, 3 p. supplementary information), 4 figures (3
f. letter, 1 f. supplementary information
The Atacama Cosmology Telescope: A Measurement of the Thermal Sunyaev-Zel'dovich Effect Using the Skewness of the CMB Temperature Distribution
We present a detection of the unnormalized skewness induced by the
thermal Sunyaev-Zel'dovich (tSZ) effect in filtered Atacama Cosmology Telescope
(ACT) 148 GHz cosmic microwave background temperature maps. Contamination due
to infrared and radio sources is minimized by template subtraction of resolved
sources and by constructing a mask using outlying values in the 218 GHz
(tSZ-null) ACT maps. We measure = -31 +- 6 \mu K^3 (measurement error
only) or +- 14 \mu K^3 (including cosmic variance error) in the filtered ACT
data, a 5-sigma detection. We show that the skewness is a sensitive probe of
sigma_8, and use analytic calculations and tSZ simulations to obtain
cosmological constraints from this measurement. From this signal alone we infer
a value of sigma_8= 0.79 +0.03 -0.03 (68 % C.L.) +0.06 -0.06 (95 % C.L.). Our
results demonstrate that measurements of non-Gaussianity can be a useful method
for characterizing the tSZ effect and extracting the underlying cosmological
information.Comment: 9 pages, 5 figures. Replaced with version accepted by Phys. Rev. D,
with improvements to the likelihood function and the IR source treatment;
only minor changes in the result
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