19,499 research outputs found
Regulation of the Neuron-specific Ras GTPase-activating Protein, synGAP, by Ca2+/Calmodulin-dependent Protein Kinase II
synGAP is a neuron-specific Ras GTPase-activating protein found in high concentration in the postsynaptic density fraction from mammalian forebrain. Proteins in the postsynaptic density, including synGAP, are part of a signaling complex attached to the cytoplasmic tail of the N-methyl-D-aspartate-type glutamate receptor. synGAP can be phosphorylated by a second prominent component of the complex, Ca2+/calmodulin-dependent protein kinase II. Here we show that phosphorylation of synGAP by Ca2+/calmodulin-dependent protein kinase II increases its Ras GTPase-activating activity by 70-95%. We identify four major sites of phosphorylation, serines 1123, 1058, 750/751/756, and 764/765. These sites together with other minor phosphorylation sites in the carboxyl tail of synGAP control stimulation of GTPase-activating activity. When three of these sites and four other serines in the carboxyl tail are mutated, stimulation of GAP activity after phosphorylation is reduced to 21 ± 5% compared with 70-95% for the wild type protein. We used phosphosite-specific antibodies to show that, as predicted, phosphorylation of serines 765 and 1123 is increased in cultured cortical neurons after exposure of the neurons to the agonist N-methyl-D-aspartate
Generation of macroscopic superposition states with small nonlinearity
We suggest a scheme to generate a macroscopic superposition state
(Schrodinger cat state) of a free-propagating optical field using a beam
splitter, homodyne measurement and a very small Kerr nonlinear effect. Our
scheme makes it possible to considerably reduce the required nonlinear effect
to generate an optical cat state using simple and efficient optical elements.Comment: Significantly improved version, to be published in PRA as a Rapid
Communicatio
The first-mover advantage in scientific publication
Mathematical models of the scientific citation process predict a strong
"first-mover" effect under which the first papers in a field will, essentially
regardless of content, receive citations at a rate enormously higher than
papers published later. Moreover papers are expected to retain this advantage
in perpetuity -- they should receive more citations indefinitely, no matter how
many other papers are published after them. We test this conjecture against
data from a selection of fields and in several cases find a first-mover effect
of a magnitude similar to that predicted by the theory. Were we wearing our
cynical hat today, we might say that the scientist who wants to become famous
is better off -- by a wide margin -- writing a modest paper in next year's
hottest field than an outstanding paper in this year's. On the other hand,
there are some papers, albeit only a small fraction, that buck the trend and
attract significantly more citations than theory predicts despite having
relatively late publication dates. We suggest that papers of this kind, though
they often receive comparatively few citations overall, are probably worthy of
our attention.Comment: 7 pages, 3 figure
Preferential attachment in the protein network evolution
The Saccharomyces cerevisiae protein-protein interaction map, as well as many
natural and man-made networks, shares the scale-free topology. The preferential
attachment model was suggested as a generic network evolution model that yields
this universal topology. However, it is not clear that the model assumptions
hold for the protein interaction network. Using a cross genome comparison we
show that (a) the older a protein, the better connected it is, and (b) The
number of interactions a protein gains during its evolution is proportional to
its connectivity. Therefore, preferential attachment governs the protein
network evolution. The evolutionary mechanism leading to such preference and
some implications are discussed.Comment: Minor changes per referees requests; to appear in PR
Purification and detection of entangled coherent states
In [J. C. Howell and J. A. Yeazell, Phys. Rev. A 62, 012102 (2000)], a
proposal is made to generate entangled macroscopically distinguishable states
of two spatially separated traveling optical modes. We model the decoherence
due to light scattering during the propagation along an optical transmission
line and propose a setup allowing an entanglement purification from a number of
preparations which are partially decohered due to transmission. A purification
is achieved even without any manual intervention. We consider a nondemolition
configuration to measure the purity of the state as contrast of interference
fringes in a double-slit setup. Regarding the entangled coherent states as a
state of a bipartite quantum system, a close relationship between purity and
entanglement of formation can be obtained. In this way, the contrast of
interference fringes provides a direct means to measure entanglement.Comment: 9 pages, 6 figures, using Revtex
On a Matrix Model of Level Structure
We generalize the dimensionally reduced Yang-Mills matrix model by adding d=1
Chern-Simons term and terms for a bosonic vector. The coefficient, \kappa of
the Chern-Simons term must be integer, and hence the level structure. We show
at the bottom of the Yang-Mills potential, the low energy limit, only the
linear motion is allowed for D0 particles. Namely all the particles align
themselves on a single straight line subject to \kappa^2/r^2 repulsive
potential from each other. We argue the relevant brane configuration to be
D0-branes in a D4 after \kappa of D8's pass the system.Comment: 1+6 pages, No figure, LaTeX; Minor changes; To appear in Class.
Quant. Gra
Anisotropic strains and magnetoresistance of La_{0.7}Ca_{0.3}MnO_{3}
Thin films of perovskite manganite La_{0.7}Ca_{0.3}MnO_{3} were grown
epitaxially on SrTiO_3(100), MgO(100) and LaAlO_3(100) substrates by the pulsed
laser deposition method. Microscopic structures of these thin film samples as
well as a bulk sample were fully determined by x-ray diffraction measurements.
The unit cells of the three films have different shapes, i.e., contracted
tetragonal, cubic, and elongated tetragonal for SrTiO_3, MgO, and LaAlO_3
cases, respectively, while the unit cell of the bulk is cubic. It is found that
the samples with cubic unit cell show smaller peak magnetoresistance than the
noncubic ones do. The present result demonstrates that the magnetoresistance of
La_{0.7}Ca_{0.3}MnO_{3} can be controlled by lattice distortion via externally
imposed strains.Comment: Revtex, 10 pages, 2 figure
Charge states and magnetic ordering in LaMnO3/SrTiO3 superlattices
We investigated the magnetic and optical properties of
[(LaMnO3)n/(SrTiO3)8]20 (n = 1, 2, and 8) superlattices grown by pulsed laser
deposition. We found a weak ferromagnetic and semiconducting state developed in
all superlattices. An analysis of the optical conductivity showed that the
LaMnO3 layers in the superlattices were slightly doped. The amount of doping
was almost identical regardless of the LaMnO3 layer thickness up to eight unit
cells, suggesting that the effect is not limited to the interface. On the other
hand, the magnetic ordering became less stable as the LaMnO3 layer thickness
decreased, probably due to a dimensional effect.Comment: 17 pages including 4 figures, accepted for publication in Phys. Rev.
Effects of aging and links removal on epidemic dynamics in scale-free networks
We study the combined effects of aging and links removal on epidemic dynamics
in the Barab\'{a}si-Albert scale-free networks. The epidemic is described by a
susceptible-infected-refractory (SIR) model. The aging effect of a node
introduced at time is described by an aging factor of the form
in the probability of being connected to newly added nodes
in a growing network under the preferential attachment scheme based on
popularity of the existing nodes. SIR dynamics is studied in networks with a
fraction of the links removed. Extensive numerical simulations reveal
that there exists a threshold such that for , epidemic
breaks out in the network. For , only a local spread results. The
dependence of on is studied in detail. The function
separates the space formed by and into regions
corresponding to local and global spreads, respectively.Comment: 8 pages, 3 figures, revtex, corrected Ref.[11
Quadrature domains and kernel function zipping
It is proved that quadrature domains are ubiquitous in a very strong sense in
the realm of smoothly bounded multiply connected domains in the plane. In fact,
they are so dense that one might as well assume that any given smooth domain
one is dealing with is a quadrature domain, and this allows access to a host of
strong conditions on the classical kernel functions associated to the domain.
Following this string of ideas leads to the discovery that the Bergman kernel
can be zipped down to a strikingly small data set. It is also proved that the
kernel functions associated to a quadrature domain must be algebraic.Comment: 13 pages, to appear in Arkiv for matemati
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