3,570 research outputs found
Simulated single molecule microscopy with SMeagol
SMeagol is a software tool to simulate highly realistic microscopy data based
on spatial systems biology models, in order to facilitate development,
validation, and optimization of advanced analysis methods for live cell single
molecule microscopy data. Availability and Implementation: SMeagol runs on
Matlab R2014 and later, and uses compiled binaries in C for reaction-diffusion
simulations. Documentation, source code, and binaries for recent versions of
Mac OS, Windows, and Ubuntu Linux can be downloaded from
http://smeagol.sourceforge.net.Comment: v2: 14 pages including supplementary text. Pre-copyedited,
author-produced version of an application note published in Bioinformatics
following peer review. The version of record, and additional supplementary
material is available online at:
https://academic.oup.com/bioinformatics/article-lookup/doi/10.1093/bioinformatics/btw10
Force generation in small ensembles of Brownian motors
The motility of certain gram-negative bacteria is mediated by retraction of
type IV pili surface filaments, which are essential for infectivity. The
retraction is powered by a strong molecular motor protein, PilT, producing very
high forces that can exceed 150 pN. The molecular details of the motor
mechanism are still largely unknown, while other features have been identified,
such as the ring-shaped protein structure of the PilT motor. The surprisingly
high forces generated by the PilT system motivate a model investigation of the
generation of large forces in molecular motors. We propose a simple model,
involving a small ensemble of motor subunits interacting through the
deformations on a circular backbone with finite stiffness. The model describes
the motor subunits in terms of diffusing particles in an asymmetric,
time-dependent binding potential (flashing ratchet potential), roughly
corresponding to the ATP hydrolysis cycle. We compute force-velocity relations
in a subset of the parameter space and explore how the maximum force (stall
force) is determined by stiffness, binding strength, ensemble size, and degree
of asymmetry. We identify two qualitatively different regimes of operation
depending on the relation between ensemble size and asymmetry. In the
transition between these two regimes, the stall force depends nonlinearly on
the number of motor subunits. Compared to its constituents without
interactions, we find higher efficiency and qualitatively different
force-velocity relations. The model captures several of the qualitative
features obtained in experiments on pilus retraction forces, such as roughly
constant velocity at low applied forces and insensitivity in the stall force to
changes in the ATP concentration.Comment: RevTex 9 pages, 4 figures. Revised version, new subsections in Sec.
III, removed typo
Anisotropic membrane curvature sensing by amphipathic peptides
Many proteins and peptides have an intrinsic capacity to sense and induce
membrane curvature, and play crucial roles for organizing and remodelling cell
membranes. However, the molecular driving forces behind these processes are not
well understood. Here, we describe a new approach to study curvature sensing,
by simulating the direction-dependent interactions of single molecules with a
buckled lipid bilayer. We analyse three amphipathic antimicrobial peptides, a
class of membrane-associated molecules that specifically target and destabilize
bacterial membranes, and find qualitatively different sensing characteristics
that would be difficult to resolve with other methods. These findings provide
new insights into the curvature sensing mechanisms of amphipathic peptides and
challenge existing theories of hydrophobic insertion. Our approach is generally
applicable to a wide range of curvature sensing molecules, and our results
provide strong motivation to develop new experimental methods to track position
and orientation of membrane proteins.Comment: 14 pages. Supplementary movies available on request. Accepted
version, with corrected description lipid bilayer compositio
Gas phase thermometry of hot turbulent jets using laser induced phosphorescence
This article is made available through the Brunel Open Access Publishing Fund. Copyright @ 2013 OSAThe temperature distributions of heated turbulent jets of air were determined using two dimensional (planar) laser induced phosphorescence. The jets were heated to specific temperature increments, ranging from 300 – 850 K and several Reynolds numbers were investigated at each temperature. The spectral ratio technique was used in conjunction with thermographic phosphors BAM and YAG:Dy, individually. Single shot and time averaged results are presented as two dimensional stacked images of turbulent jets. YAG:Dy did not produce a high enough signal for single shot measurements. The results allowed for a direct comparison between BAM and YAG:Dy, revealing that BAM is more suitable for relatively lower temperature, fast and turbulent regimes and that YAG:Dy is more suited to relatively higher temperature, steady flow situations
Decay times in turnover statistics of single enzymes
The first passage times for enzymatic turnovers in non-equilibrium steady
state display a statistical symmetry property related to non-equilibrium
fluctuation theorems, that makes it possible to extract the chemical driving
force from single molecule trajectories in non-equilibrium steady state. Below,
we show that this system violates the general expectation that the number of
decay constants needed to fit a first passage time distribution reflects the
number of states in the escape problem. In fact, the structure of the kinetic
mechanism makes half of the decay times vanish identically from the turnover
time distribution. The terms that cancel out correspond to the eigenvalues of a
certain sub-matrix of the master equation matrix for the first exit time
problem. We discuss how these results make modeling and data analysis easier
for such systems, and how the turnovers can be measured.Comment: 4 pages, 1 figure v2: Published version, minor corrections in
response to referee comment
Multiple Lac-mediated loops revealed by Bayesian statistics and tethered particle motion
The bacterial transcription factor LacI loops DNA by binding to two separate
locations on the DNA simultaneously. Despite being one of the best-studied
model systems for transcriptional regulation, the number and conformations of
loop structures accessible to LacI remain unclear, though the importance of
multiple co-existing loops has been implicated in interactions between LacI and
other cellular regulators of gene expression. To probe this issue, we have
developed a new analysis method for tethered particle motion, a versatile and
commonly-used in vitro single-molecule technique. Our method, vbTPM, performs
variational Bayesian inference in hidden Markov models. It learns the number of
distinct states (i.e., DNA-protein conformations) directly from tethered
particle motion data with better resolution than existing methods, while easily
correcting for common experimental artifacts. Studying short (roughly 100 bp)
LacI-mediated loops, we provide evidence for three distinct loop structures,
more than previously reported in single-molecule studies. Moreover, our results
confirm that changes in LacI conformation and DNA binding topology both
contribute to the repertoire of LacI-mediated loops formed in vitro, and
provide qualitatively new input for models of looping and transcriptional
regulation. We expect vbTPM to be broadly useful for probing complex
protein-nucleic acid interactions.Comment: 34 pages, 25 figures, including Supporting information. To appear in
Nucleic Acids Research. Accompanying open-source software:
http://sourceforge.net/projects/vbtpm
- …