1,925 research outputs found
HEAT repeats – versatile arrays of amphiphilic helices working in crowded environments?
Cellular proteins do not work alone in diluted conditions. They often function as part of large macromolecular complexes, which are transported and concentrated into specific cellular compartments and function in their highly crowded environments. A central theme of modern cell biology is to understand how cellular proteins might achieve these challenging tasks efficiently and faithfully. In this Opinion article, we will focus on HEAT repeats, flexible arrays of amphiphilic helices found in many eukaryotic proteins such as karyopherins and condensins, and discuss how this uniquely designed helical repeats might underlie dynamic protein-protein interactions and support cellular functions in crowded environments. We will make bold speculations on functional similarities between HEAT repeats and intrinsically disordered regions (IDRs) in macromolecular phase separation. Potential contributions of HEAT-HEAT interactions, as well as cooperation between HEATs and IDRs, to mesoscale organelle assembly will be discussed
Standing Accretion Shocks in the Supernova Core: Effects of Convection and Realistic EOS
We investigated the structure of the spherically symmetric accretion flows
through the standing shock wave onto the proto-neutron star in the post-bounce
phase of the collapse-driven supernova. We assume that the accretion flow is in
a steady state controlled by the neutrino luminosity and mass accretion rate
that are kept constant. We obtain solutions of the steady Euler equations for a
wide range of neutrino luminosity and mass accretion rate. We employ a
realistic EOS and neutrino-heating rates. More importantly, we take into
account the effect of convection phenomenologically. For each mass accretion
rate, we find the critical neutrino luminosity, above which there exists no
steady solution. These critical points are supposed to mark the onset of the
shock revival. As the neutrino luminosity increases for a given mass accretion
rate, there appears a convectively unstable region at some point before the
critical value is reached. We introduce a phenomenological energy flux by
convection so that the negative entropy gradient should be canceled out. We
find that the convection lowers the critical neutrino luminosity substantially.
We also consider the effect of the self-gravity. It is found that the
self-gravity is important only when the neutrino luminosity is high. The
critical luminosity, however, is little affected if the energy transport by
convection is taken into account.Comment: accepted by ApJ, 20 pages, 8 figure
A Polynomial-time, Truthful, Individually Rational and Budget Balanced Ridesharing Mechanism
Ridesharing has great potential to improve transportation efficiency while
reducing congestion and pollution. To realize this potential, mechanisms are
needed that allocate vehicles optimally and provide the right incentives to
riders. However, many existing approaches consider restricted settings (e.g.,
only one rider per vehicle or a common origin for all riders). Moreover, naive
applications of standard approaches, such as the Vickrey-Clarke-Groves or
greedy mechanisms, cannot achieve a polynomial-time, truthful, individually
rational and budget balanced mechanism. To address this, we formulate a general
ridesharing problem and apply mechanism design to develop a novel mechanism
which satisfies all four properties and whose social cost is within 8.6% of the
optimal on average
Effects of rotation on the revival of a stalled shock in supernova explosions
In order to infer the effects of rotation on the revival of a stalled shock
in supernova explosions, we investigated steady accretion flows with a standing
shock. We first obtained a series of solutions for equations describing
non-rotating spherically symmetric flows and confirmed the results of preceding
papers that, for a given mass accretion rate, there is a critical luminosity of
irradiating neutrinos, above which there exists no steady solution. Below the
critical value, we found two branches of solutions; one is stable and the other
is unstable against radial perturbations. With a simple argument based on the
Riemann problem, we can identify the critical luminosity as the one, at which
the stalled shock revives. We also obtained the condition satisfied by the flow
velocity for the critical luminosity, which can be easily applied to the
rotational case. If a collapsing star rotates, the accretion flow is
non-spherical due to centrifugal forces. Flows are accelerated near the
rotation axis whereas they are decelerated near the equatorial plane. As a
result, the critical luminosity is lowered, that is, rotation assists the
revival of a stalled shock. According to our calculations, the critical
luminosity is % lower for the mass accretion rate of 1M/sec
and the rotation frequency of 0.1 Hz at a radius of 1000 km than that of the
spherically symmetric flow with the same mass accretion rate. We found that the
condition of the flow velocity at the critical luminosity is first satisfied at
the rotation axis. This suggests that the shock revival is triggered on the
rotation axis and a jet-like explosion ensues.Comment: 26 pages, 10 figures, submitted to Ap
Separability of Rotational Effects on a Gravitational Lens
We derive the deflection angle up to due to a Kerr gravitational
lens with mass and specific angular momentum . It is known that at the
linear order in and the Kerr lens is observationally equivalent to the
Schwarzschild one because of the invariance under the global translation of the
center of the lens mass. We show, however, nonlinear couplings break the
degeneracy so that the rotational effect becomes in principle separable for
multiple images of a single source. Furthermore, it is distinguishable also for
each image of an extended source and/or a point source in orbital motion. In
practice, the correction at becomes for the
supermassive black hole in our galactic center. Hence, these nonlinear
gravitational lensing effects are too small to detect by near-future
observations.Comment: 12 pages (RevTeX); accepted for publication in Phys. Rev.
Magnetic phase diagram of antiferroquadrupole ordering in HoB2C2
The magnetic phase diagram for antiferro-quadrupole (AFQ) ordering in
tetragonal HoBC has been investigated by measurements of elastic
constants , and in fields along the basal -
plane as well as the principal [001]-axis. The hybrid magnet (GAMA) in Tsukuba
Magnetic Laboratory was employed for high field measurements up to 30 T. The
AFQ phase is no longer observed above 26.3 T along the principal [001] axis in
contrast to the relatively small critical field of 3.9 T in fields applied
along the basal [110] axis. The quadrupolar intersite interaction of
and/or is consistent with the anisotropy in the magnetic phase diagram
of the AFQ phase in HoBC.Comment: Phys. Rev. B. (2005) in press. approx 8 pages, 10 figure
A novel Rac1-GSPT1 signaling pathway controls astrogliosis following central nervous system injury
Astrogliosis (i.e. glial scar), which is comprised primarily of proliferated astrocytes at the lesion site and migrated astrocytes from neighboring regions, is one of the key reactions in determining outcomes after CNS injury. In an effort to identify potential molecules/pathways that regulate astrogliosis, we sought to determine whether Rac/Rac-mediated signaling in astrocytes represents a novel candidate for therapeutic intervention following CNS injury. For these studies, we generated mice with Rac1 deletion under the control of the GFAP (glial fibrillary acidic protein) promoter (GFAP-Cre;Rac1(flox/flox)). GFAP-Cre;Rac1(flox/flox) (Rac1-KO) mice exhibited better recovery after spinal cord injury and exhibited reduced astrogliosis at the lesion site relative to control. Reduced astrogliosis was also observed in Rac1-KO mice following microbeam irradiation-induced injury. Moreover, knockdown (KD) or KO of Rac1 in astrocytes (LN229 cells, primary astrocytes, or primary astrocytes from Rac1-KO mice) led to delayed cell cycle progression and reduced cell migration. Rac1-KD or Rac1-KO astrocytes additionally had decreased levels of GSPT1 (G(1) to S phase transition 1) expression and reduced responses of IL-1β and GSPT1 to LPS treatment, indicating that IL-1β and GSPT1 are downstream molecules of Rac1 associated with inflammatory condition. Furthermore, GSPT1-KD astrocytes had cell cycle delay, with no effect on cell migration. The cell cycle delay induced by Rac1-KD was rescued by overexpression of GSPT1. Based on these results, we propose that Rac1-GSPT1 represents a novel signaling axis in astrocytes that accelerates proliferation in response to inflammation, which is one important factor in the development of astrogliosis/glial scar following CNS injury
Does governance play a role in the distribution of invasive alien species?
Invasive alien species (IAS) constitute a major threat to global biological diversity. In order to control their spread, a detailed understanding of the factors influencing their distribution is essential. Although international trade is regarded as a major force structuring spatial patterns of IAS, the role of other social factors remains unclear. Despite studies highlighting the importance of strong governance in slowing drivers of biodiversity loss such as logging, deforestation, and agricultural intensification, no study has yet analyzed its contribution to the issue of IAS. Using estimates of governance quality and comprehensive spatiotemporal IAS data, we performed multiple linear regressions to investigate the effect of governance quality upon the distribution of species listed under "100 of the worst" IAS in 38 Eurasian countries as defined by DASIE. Our model suggested that for countries with higher GDP, stronger governance was associated with a greater number of the worst IAS; in contrast, for the lowest GDP countries under analysis, stronger governance was associated with fewer of these IAS. We elucidate how the quality of governance within a country has implications for trade, tourism, transport, legislation, and economic development, all of which influence the spread of IAS. While our findings support the common assumption that strengthening governance benefits conservation interventions in countries of smaller economy, we find that this effect is not universal. Stronger governance alone cannot adequately address the problem of IAS, and targeted action is required in relatively high-GDP countries in order to stem the influx of IAS associated with high volumes of trade
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