3,505 research outputs found
Motor cooperation in bi-directional early endosome motility
In mammalian cells and fungi, early endosomes form a dynamic compartment that undergoes bi-directional motility along microtubules. Previous work has shown that in the model system Ustilago maydis early endosome motility involves the opposing motor proteins dynein and kinesin-3. Here I performed a detailed analysis of the role of the motors in early endosome motility, using quantitative live cell imaging of kinesin-3, dynein and the endosomal GTPase Rab5a. In the first part of my work, I analysed the role of dynein at MT plus-ends, where the motor forms a strong accumulation that was thought to be involved in capturing early endosomes. I could demonstrate that ~55 dynein motors build up the dynein accumulation. In collaboration with Ms. Congping Lin and Prof. Peter Ashwin (Institute for Mathematics, Exeter), I found theoretical evidence that ~25 dynein motors concentrate and leave the plus-ends stochastically. In addition, dynein motors are captured by an interaction of dynactin and the plus-end binding protein EB1. Together both mechanisms increase the number of motors, which ensures that EEs will be loaded onto dynein before they reach the end of their track. In a second project, I provide evidence that loading of dynein is not restricted to the plus-ends. Instead, dynein leaves the plus-ends and is able to bind to kinesin-3 delivered early endosomes, which changes their transport direction from anterograde to retrograde. Kinesin-3 remains bound to these retrograde EEs. When dynein leaves the organelle, it switches back to anterograde motility. Interestingly, a single dynein wins over three to five kinesin-3 motors. I discuss these findings in the light of current motor cooperation concepts. In a third part, I demonstrated that kinesin-3 has an unexpected role in long-range retrograde endosome motility. In contrast, dynein is only responsible for the distal 10-20 µm. This is possible because most of the hyphal cells contain a symmetric and bi-polar MT array. This MT organization is reminiscent of that in dendrites. Kinesin-3-based retrograde motility is required to mix the organelles and might support long-range communication between both cell poles
The EU’s Rule of Law Initiative for Central Asia: From Initiative to More Substance? EUCAM Policy Brief No. 18, June 2011
All five Central Asian states are weak in terms of rule of law, good governance and democracy. The EU chose to devote specific attention to the rule of law through a regional initiative with Central Asian partners' participation. What is the current state of the initiative and is the EU on track
Transducer-Based Rewriting Games for Active XML
Context-free games are two-player rewriting games that are played on nested
strings representing XML documents with embedded function symbols. These games
were introduced to model rewriting processes for intensional documents in the
Active XML framework, where input documents are to be rewritten into a given
target schema by calls to external services.
This paper studies the setting where dependencies between inputs and outputs
of service calls are modelled by transducers, which has not been examined
previously. It defines transducer models operating on nested words and studies
their properties, as well as the computational complexity of the winning
problem for transducer-based context-free games in several scenarios. While the
complexity of this problem is quite high in most settings (ranging from
NP-complete to undecidable), some tractable restrictions are also identified.Comment: Extended version of MFCS 2016 conference pape
Observational evidence for a broken Li Spite plateau and mass-dependent Li depletion
We present NLTE Li abundances for 88 stars in the metallicity range -3.5 <
[Fe/H] < -1.0. The effective temperatures are based on the infrared flux method
with improved E(B-V) values obtained mostly from interstellar NaI D lines. The
Li abundances were derived through MARCS models and high-quality UVES+VLT,
HIRES+Keck and FIES+NOT spectra, and complemented with reliable equivalent
widths from the literature. The less-depleted stars with [Fe/H] < -2.5 and
[Fe/H] > -2.5 fall into two well-defined plateaus of A_{Li} = 2.18 (sigma =
0.04) and A_{Li} = 2.27 (sigma = 0.05), respectively. We show that the two
plateaus are flat, unlike previous claims for a steep monotonic decrease in Li
abundances with decreasing metallicities. At all metallicities we uncover a
fine-structure in the Li abundances of Spite plateau stars, which we trace to
Li depletion that depends on both metallicity and mass. Models including atomic
diffusion and turbulent mixing seem to reproduce the observed Li depletion
assuming a primordial Li abundance A_{Li} = 2.64, which agrees well with
current predictions (A_{Li} = 2.72) from standard Big Bang nucleosynthesis.
Adopting the Kurucz overshooting model atmospheres increases the Li abundance
by +0.08 dex to A_{Li} = 2.72, which perfectly agrees with BBN+WMAP.Comment: A&A Letters, in pres
Mapping of the energetically lowest exciton in bulk -HfS
By combining electron energy-loss spectroscopy and state-of-the-art
computational methods, we were able to provide an extensive picture of the
excitonic processes in -HfS. The results differ significantly from the
properties of the more scrutinized group VI semiconducting transition metal
dichalcogenides such as MoS and WSe. The measurements revealed a
parabolic exciton dispersion for finite momentum parallel to the
K direction which allowed the determination of the effective exciton
mass. The dispersion decreases monotonically for momentum exchanges parallel to
the M high symmetry line. To gain further insight into the excitation
mechanisms, we solved the ab-initio Bethe-Salpeter equation for the system. The
results matched the experimental loss spectra closely, thereby confirming the
excitonic nature of the observed transitions, and produced the
momentumdependent binding energies. The simulations also demonstrated that the
excitonic transitions for || M occur exactly along that
particular high symmetry line. For || K on the other hand,
the excitations traverse the Brillouin zone crossing various high symmetry
lines. A particular interesting aspect of our findings was that the calculation
of the electron probability density revealed that the exciton assumes a
six-pointed star-like shape along the real space crystal planes indicating a
mixed Frenkel-Wannier character.Comment: 12 pages, 10 figure
Tragedy of the Commons in the Chemostat
We present a proof of principle for the phenomenon of the tragedy of the
commons that is at the center of many theories on the evolution of cooperation.
We establish the tragedy in the context of a general chemostat model with two
species, the cooperator and the cheater. Both species have the same growth rate
function and yield constant, but the cooperator allocates a portion of the
nutrient uptake towards the production of a public good -the "Commons" in the
Tragedy- which is needed to digest the externally supplied nutrient. The
cheater on the other hand does not produce this enzyme, and allocates all
nutrient uptake towards its own growth. We prove that when the cheater is
present initially, both the cooperator and the cheater will eventually go
extinct, hereby confirming the occurrence of the tragedy. We also show that
without the cheater, the cooperator can survive indefinitely, provided that at
least a low level of public good or processed nutrient is available initially.
Our results provide a predictive framework for the analysis of
cooperator-cheater dynamics in a powerful model system of experimental
evolution.Comment: 23 pages, 4 figure
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