589 research outputs found
New Approaches Needed for Water Quality Gains in the 1990s
In the land of 15,000 lakes, 91,944 river miles and a trillion gallons of ground water, the summer of 1988 saw the unlikely come to pass. The mighty Mississippi dropped to less than one-tenth of its normal flow. Crops withered, grass turned brown, and well pumps burned out in the most hellish heat wave since the 1930s.
While the drought was primarily an issue of insufficient water quantity, it also served to focus additional attention on water quality. Water quality concerns are not new to Minnesota, which has led the nation in protecting the environment for two decades. Nonetheless, the drought has prompted us to take stock of how far we have come in protecting our waters, and where we need to go from here
Cdk1-dependent phosphoinhibition of a formin-F-BAR interaction opposes cytokinetic contractile ring formation
© 2018 Willet et al. In Schizosaccharomyces pombe, cytokinesis requires the assembly and constriction of an actomyosin-based contractile ring (CR). A single essential formin, Cdc12, localizes to the cell middle upon mitotic onset and nucleates the F-actin of the CR. Cdc12 medial recruitment is mediated in part by its direct binding to the F-BAR scaffold Cdc15. Given that Cdc12 is hyperphosphorylated in M phase, we explored whether Cdc12 phosphoregulation impacts its association with Cdc15 during mitosis. We found that Cdk1, a major mitotic kinase, phosphorylates Cdc12 on six N-terminal residues near the Cdc15-binding site, and phosphorylation on these sites inhibits its interaction with the Cdc15 F-BAR domain. Consistent with this finding, a cdc12 mutant with all six Cdk1 sites changed to phosphomimetic residues (cdc12-6D) displays phenotypes similar to cdc12-P31A, in which the Cdc15-binding motif is disrupted; both show reduced Cdc12 at the CR and delayed CR formation. Together, these results indicate that Cdk1 phosphorylation of formin Cdc12 antagonizes its interaction with Cdc15 and thereby opposes Cdc12\u27s CR localization. These results are consistent with a general role of Cdk1 in inhibiting cytokinesis until chromosome segregation is complete
Formin-based control of the actin cytoskeleton during cytokinesis
Cytokinesis, the terminal event in the canonical cell cycle, physically separates daughter cells following mitosis. For cleavage to occur in many eukaryotes, a cytokinetic ring must assemble and constrict between divided genomes. Although dozens of different molecules localize to and participate within the cytokinetic ring, the core machinery comprises linear actin filaments. Accordingly, formins, which nucleate and elongate F-actin (filamentous actin) for the cytokinetic ring, are required for cytokinesis in diverse species. In the present article, we discuss specific modes of formin-based actin regulation during cell division and highlight emerging mechanisms and questions on this topic. © 2013 Biochemical Society
Instantons and the spectral function of electrons in the half-filled Landau level
We calculate the instanton-anti-instanton action in
the gauge theory of the half-filled Landau level. It is found that for a class of interactions between electrons. This means that the instanton-anti-instanton
pairs are confining so that a well defined `charged' composite fermion can
exist. It is also shown that can be used to calculate
the spectral function of electrons from the microscopic theory within a
semiclassical approximation. The resulting spectral function varies as at low
energies.Comment: 13 pages, Plain Tex, MIT-CMT-APR-9
Specific heat and validity of quasiparticle approximation in the half-filled Landau level
We calculate the specific heat of composite fermion system in the half-filled
Landau level. Two different methods are used to examine validity of the
quasiparticle approximation when the two-body interaction is given by (). The singular part of the specific heat
is calculated from the free energy of the gauge field, which is compared with
the specific heat calculated from the quasiparticle approximation via the
singular self-energy correction due to the gauge field fluctuations. It turns
out that two results are in general different and they coincide only for the
case of the Coulomb interaction (). This result supports the fact
that the quasiparticle approximation is valid only for the case of the Coulomb
interaction. It is emphasized that this result is obtained by looking at a
gauge-invariant quantity -- the specific heat.Comment: 8 pages, Revte
Surface acoustic wave attenuation by a two-dimensional electron gas in a strong magnetic field
The propagation of a surface acoustic wave (SAW) on GaAs/AlGaAs
heterostructures is studied in the case where the two-dimensional electron gas
(2DEG) is subject to a strong magnetic field and a smooth random potential with
correlation length Lambda and amplitude Delta. The electron wave functions are
described in a quasiclassical picture using results of percolation theory for
two-dimensional systems. In accordance with the experimental situation, Lambda
is assumed to be much smaller than the sound wavelength 2*pi/q. This restricts
the absorption of surface phonons at a filling factor \bar{\nu} approx 1/2 to
electrons occupying extended trajectories of fractal structure. Both
piezoelectric and deformation potential interactions of surface acoustic
phonons with electrons are considered and the corresponding interaction
vertices are derived. These vertices are found to differ from those valid for
three-dimensional bulk phonon systems with respect to the phonon wave vector
dependence. We derive the appropriate dielectric function varepsilon(omega,q)
to describe the effect of screening on the electron-phonon coupling. In the low
temperature, high frequency regime T << Delta (omega_q*Lambda
/v_D)^{alpha/2/nu}, where omega_q is the SAW frequency and v_D is the electron
drift velocity, both the attenuation coefficient Gamma and varepsilon(omega,q)
are independent of temperature. The classical percolation indices give
alpha/2/nu=3/7. The width of the region where a strong absorption of the SAW
occurs is found to be given by the scaling law |Delta \bar{\nu}| approx
(omega_q*Lambda/v_D)^{alpha/2/nu}. The dependence of the electron-phonon
coupling and the screening due to the 2DEG on the filling factor leads to a
double-peak structure for Gamma(\bar{\nu}).Comment: 17 pages, 3 Postscript figures, minor changes mad
Composite fermions traversing a potential barrier
Using a composite fermion picture, we study the lateral transport between two
two-dimensional electron gases, at filling factor 1/2, separated by a potential
barrier. In the mean field approximation, composite fermions far from the
barrier do not feel a magnetic field while in the barrier region the effective
magnetic field is different from zero. This produces a cutoff in the
conductance when represented as a function of the thickness and height of the
barrier. There is a range of barrier heights for which an incompressible
liquid, at , exists in the barrier region.Comment: 3 pages, latex, 4 figures available upon request from
[email protected]. To appear in Physical Review B (RC) June 15t
Quantum Boltzmann equation of composite fermions interacting with a gauge field
We derive the quantum Boltzmann equation (QBE) of composite fermions at/near
the state using the non-equilibrium Green's function technique. The
lowest order perturbative correction to the self-energy due to the strong gauge
field fluctuations suggests that there is no well defined
Landau-quasi-particle. Therefore, we cannot assume the existence of the
Landau-quasi-particles {\it a priori} in the derivation of the QBE. Using an
alternative formulation, we derive the QBE for the generalized Fermi surface
displacement which corresponds to the local variation of the chemical potential
in momentum space. {}From this QBE, one can understand in a unified fashion the
Fermi-liquid behaviors of the density-density and the current-current
correlation functions at (in the long wave length and the low
frequency limits) and the singular behavior of the energy gap obtained from the
finite temperature activation behavior of the compressibility near .
Implications of these results to the recent experiments are also discussed.Comment: 44 pages, Plain Tex, 5 figures (ps files) available upon reques
Composite Fermion Theory, Edge Currents and the Fractional Quantum Hall Effect
We present a mean field theory of composite fermion edge channel transport in
the fractional and integer quantum Hall regimes. An expression relating the
electro-chemical potentials of composite fermions at the edges of a sample to
those of the corresponding electrons is obtained and a plausible form is
assumed for the composite fermion Landau level energies near the edges. The
theory yields the observed fractionally quantized Hall conductances and also
explains other experimental results. We also discuss some experiments that are
relevant to the question whether fractional edge states in real devices should
be described as Fermi or Luttinger liquids.Comment: 4 pages + 1 figure. Talk presented at EP2DSXI in Nottingham in
August, 1995. Self-unpacking uuencoded postscript. Unpacking instructions at
beginning of fil
Gauge-invariant response functions of fermions coupled to a gauge field
We study a model of fermions interacting with a gauge field and calculate
gauge-invariant two-particle Green's functions or response functions. The
leading singular contributions from the self-energy correction are found to be
cancelled by those from the vertex correction for small and . As a
result, the remaining contributions are not singular enough to change the
leading order results of the random phase approximation. It is also shown that
the gauge field propagator is not renormalized up to two-loop order. We examine
the resulting gauge-invariant two-particle Green's functions for small and
, but for all ratios of and we conclude that they can
be described by Fermi liquid forms without a diverging effective mass.Comment: Plain Tex, 35 pages, 5 figures available upon request, Revised
Version (Expanded discussion), To be published in Physical Review B 50,
(1994) (December 15 issue
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