3,622 research outputs found
Dynamics of protein-protein encounter: a Langevin equation approach with reaction patches
We study the formation of protein-protein encounter complexes with a Langevin
equation approach that considers direct, steric and thermal forces. As three
model systems with distinctly different properties we consider the pairs
barnase:barstar, cytochrome c:cytochrome c peroxidase and p53:MDM2. In each
case, proteins are modeled either as spherical particles, as dipolar spheres or
as collection of several small beads with one dipole. Spherical reaction
patches are placed on the model proteins according to the known experimental
structures of the protein complexes. In the computer simulations, concentration
is varied by changing box size. Encounter is defined as overlap of the reaction
patches and the corresponding first passage times are recorded together with
the number of unsuccessful contacts before encounter. We find that encounter
frequency scales linearly with protein concentration, thus proving that our
microscopic model results in a well-defined macroscopic encounter rate. The
number of unsuccessful contacts before encounter decreases with increasing
encounter rate and ranges from 20-9000. For all three models, encounter rates
are obtained within one order of magnitude of the experimentally measured
association rates. Electrostatic steering enhances association up to 50-fold.
If diffusional encounter is dominant (p53:MDM2) or similarly important as
electrostatic steering (barnase:barstar), then encounter rate decreases with
decreasing patch radius. More detailed modeling of protein shapes decreases
encounter rates by 5-95 percent. Our study shows how generic principles of
protein-protein association are modulated by molecular features of the systems
under consideration. Moreover it allows us to assess different coarse-graining
strategies for the future modelling of the dynamics of large protein complexes
Towards Understanding the Early Events in the Conformational Transition of Amyloid Beta Peptides
Apparent facilitation of an invasive mealybug by an invasive ant
Summary: In the southeast United States, the invasive ant Solenopsis invicta is known to derive important carbohydrate (honeydew) resources from mealybugs utilizing grasses. Most important appears to be an invasive mealybug, Antonina graminis. We studied whether this mealybug and a similar native species also benefit from association with S. invicta. We found that mealybug occurrence increases significantly with increasing proximity to S. invicta mounds, suggesting that mealybugs benefit as well. Mutual benefits derived by S. invicta and A. graminis are consistent with a hypothesis proposing that associations among invasive species can be important in their success at introduced location
Reverse Khas'minskii condition
The aim of this paper is to present and discuss some equivalent
characterizations of p-parabolicity in terms of existence of special exhaustion
functions. In particular, Khas'minskii in [K] proved that if there exists a
2-superharmonic function k defined outside a compact set such that , then R is 2-parabolic, and Sario and Nakai in [SN] were
able to improve this result by showing that R is 2-parabolic if and only if
there exists an Evans potential, i.e. a 2-harmonic function with \lim_{x\to \infty} \E(x)=\infty. In this paper, we will prove a
reverse Khas'minskii condition valid for any p>1 and discuss the existence of
Evans potentials in the nonlinear case.Comment: final version of the article available at http://www.springer.co
На граматичних вершинах
On the Ivan Vykhovanets’life and career the famous Ukrainian linguist and founder of Ukrainian school grammarians
Division of labor and the evolution of task sharing in queen associations of the harvester ant Pogonomyrmex califonicus
Exploring barriers in expertise seeking : why don\u27t they ask an expert?
This paper reports findings from a research project that explores reasons why some employees prefer to seek expertise to resolve work-related problems from direct colleagues rather than designated internal experts. Several studies suggest that while an expert generally provides a higher quality solution in a shorter time, workers tend to ask friendly or proximate colleagues to help with knowledge-based problems at work. Prior research provides only fragmented insights into understanding the barriers to asking a designated internal expert for help at work. To address this gap, we asked post-graduate students enrolled in a knowledge management subject at a large Australian university to share their perspectives in an online discussion forum. Content analysis of the collected perspectives enabled identification of twenty-one factors that may limit the seeking of expertise from a designated internal expert. The factors are grouped in four categories: environment, accessibility, communication and personality. In addition one context variable is described, determining the extent to which the barriers are influential in a specific situation. By synthesising the results, we have proposed two models of expertise-seeking barriers. A literature review helps validate the barriers identified by the study. Key theoretical and practical implications are also discussed.<br /
Modeling the ballistic-to-diffusive transition in nematode motility reveals variation in exploratory behavior across species
A quantitative understanding of organism-level behavior requires predictive
models that can capture the richness of behavioral phenotypes, yet are simple
enough to connect with underlying mechanistic processes. Here we investigate
the motile behavior of nematodes at the level of their translational motion on
surfaces driven by undulatory propulsion. We broadly sample the nematode
behavioral repertoire by measuring motile trajectories of the canonical lab
strain N2 as well as wild strains and distant species. We focus on
trajectory dynamics over timescales spanning the transition from ballistic
(straight) to diffusive (random) movement and find that salient features of the
motility statistics are captured by a random walk model with independent
dynamics in the speed, bearing and reversal events. We show that the model
parameters vary among species in a correlated, low-dimensional manner
suggestive of a common mode of behavioral control and a trade-off between
exploration and exploitation. The distribution of phenotypes along this primary
mode of variation reveals that not only the mean but also the variance varies
considerably across strains, suggesting that these nematode lineages employ
contrasting ``bet-hedging'' strategies for foraging.Comment: 46 pages, 18 figures, 6 table
Differentiation of multipotent vascular stem cells contributes to vascular diseases.
It is generally accepted that the de-differentiation of smooth muscle cells, from the contractile to the proliferative/synthetic phenotype, has an important role during vascular remodelling and diseases. Here we provide evidence that challenges this theory. We identify a new type of stem cell in the blood vessel wall, named multipotent vascular stem cells. Multipotent vascular stem cells express markers, including Sox17, Sox10 and S100β, are cloneable, have telomerase activity, and can differentiate into neural cells and mesenchymal stem cell-like cells that subsequently differentiate into smooth muscle cells. On the other hand, we perform lineage tracing with smooth muscle myosin heavy chain as a marker and find that multipotent vascular stem cells and proliferative or synthetic smooth muscle cells do not arise from the de-differentiation of mature smooth muscle cells. In response to vascular injuries, multipotent vascular stem cells, instead of smooth muscle cells, become proliferative, and differentiate into smooth muscle cells and chondrogenic cells, thus contributing to vascular remodelling and neointimal hyperplasia. These findings support a new hypothesis that the differentiation of multipotent vascular stem cells, rather than the de-differentiation of smooth muscle cells, contributes to vascular remodelling and diseases
Response of selected microorganisms to experimental planetary environments
The anaerobic utilization of phosphite or phosphine and the significance of this conversion to potential contamination of Jupiter were investigated. A sporeforming organism was isolated from Cape Canaveral soil which anaerobically converts hypophosphite to phosphate. This conversion coincides with an increase in turbidity of the culture and with phosphate accumulation in the medium. Investigations of omnitherms (organisms which grow over a broad temperature range, i.e. 3 -55 C were also conducted. The cellular morphology of 28 of these isolates was investigated, and all were demonstrated to be sporeformers. Biochemical characterizations are also presented. Procedures for replicate plating were evaluated, and those results are also presented. The procedures for different replicate-plating techniques are presented, and these are evaluated on the basis of reproducibility, percentage of viable transfer, and ease of use. Standardized procedures for the enumeration of microbial populations from ocean-dredge samples from Cape Canaveral are also presented
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