bluuebunny/crossref_metadata_2025 huggingface dataset:
Conventional solar flare theory re-examined
Authors: Sudan R. N. , Spicer D. S.
Published: July 2002

Major objections are raised to the fundamental paradigm underlying conventional solar flare theory, viz., that the required free energy can be stored in situ at the requisite density in the corona in nonpotential magnetic fields by the action of photospheric convective motion, that it can be released rapidly through magnetic reconnection by a triggering event, and that a significant fraction of this energy is converted to x rays. An alternative explanation that avoids these difficulties requires that magnetic energy in the form of strongly sheared flux tubes be stored subphotospherically prior to the flare. It is the emergence of this subphotospherically stored magnetic shear energy into the photosphere and above, and its rapid conversion through internal magnetic reconnection to other forms that constitutes an actual flare.
Optimization of power density by decreasing the length of tungsten wire array Z pinches
Authors: Deeney C. , Peterson D. L. , Spielman R. B. , Struve K. W. , Chandler G. A.
Published: July 2002

Reducing the length of 30 mm diam tungsten wire arrays on the 20 MA Z pulsed power accelerator [R. B. Spielman, S. F. Breeze, C. Deeney et al., Proceedings of the 11th International Conference on Particle Beams, Prague, Czech Republic, edited by K. Junwirth and J. Ullschmied (Czech Academy of Sciences, Prague, 1996), p. 150] from 2 to 0.75 cm has shown that the radiated powers are energies that remain constant at 170±30 TW and 1600±150 kJ. The length-independent nature of the power and energy results in the radiated power per unit length increasing from 85±10 to 240±30 TW/cm. These high-power densities should result in approximately a 20% increase in radiation temperatures produced by a Z-pinch-driven vacuum or internal hohlarums. Two-dimensional radiation magnetohydrodynamic calculations indicate that the constant radiated energies with varying pinch lengths is consistent with the energy input being due to the work done by the Lorentz forces during the radial collapse, resulting in kinetic energy and during the on-axis pinch phase, resulting in pdV or compressional heating.
Explicit spectrally optimized Fourier series for nested magnetic surfaces
Authors: Hirshman S. P. , Breslau J.
Published: July 2002

The nonuniqueness of the poloidal angle θ in the parametric representation of a space curve x(θ)=[R(θ),Z(θ)] can be exploited to condense the Fourier spectra of R and Z. The nonlinear equation describing this spectral condensation was previously derived and solved numerically using Lagrange multipliers. Here a special case of the condensation equation is shown to be exactly solvable, leading to an explicit representation for x. A family of such representations is generated that possesses increasingly condensed spectra as a parameter is varied. Applications to a variety of curves are considered as models for three-dimensional magnetohydrodynamic (MHD) equilibria with nested flux surfaces. A substantial improvement occurs in spectral convergence compared with a polar representation, while retaining the numerical simplicity of the polar constraint. The asymptotic behavior for the R and Z spectral coefficients near a magnetic axis is analyzed. Implications for improvements of MHD equilibrium calculations are discussed.
Current filamentation in astrophysical magnetohydrodynamic jets
Authors: Wiechen H. , Birk G. T. , Lesch H.
Published: July 2002

Astrophysical plasmas typically have to be considered as highly collision-free from a kinetic point of view or highly ideal in a fluid description. This refers to near-Earth space plasmas, like the magnetosphere, or extragalactic objects like radio jets, as well. On the other hand, magnetic reconnection is discussed to play an important role as an effective mechanism for conversion of magnetic energy into kinetic energy by particle acceleration. As magnetic reconnection demands for local deviations from adiabatic plasma conditions, in the highly collision-free plasma regime inertia driven reconnection should be expected. In the framework of a magnetohydrodynamic description, anomalous resistivity due to current driven microinstabilities is discussed to provide sufficient nonidealness. Both classes of processes demand for sufficiently thin current sheets. This constraint can be fulfilled by current filamentation. In the present paper numerical magnetohydrodynamical studies are used to show that ideal boundary perturbations (like, e.g., Kelvin–Helmholtz modes) can result in filamentary current structures. This fundamental process is discussed with respect to extragalactic jets.
The feedout process: Rayleigh–Taylor and Richtmyer–Meshkov instabilities in uniform, radiation-driven foils
Authors: Smitherman D. Palmer , Chrien Robert E. , Hoffman Nelson M. , Magelssen Glenn R.
Published: July 2002

Observation of perturbation coupling between a Richtmyer–Meshkov-unstable interface on the cold surface of a radiatively-driven foil and the Rayleigh–Taylor-unstable hot surface is reported. For the 50 μm wavelength studied, the combination of pulse length and foils thickness was found to affect the strength of instability coupling. Thick (86 μm) foils with a 2.2 ns long pulse showed weak coupling between the two instabilities, while thin (35 μm) foils showed strong, fast coupling. An intermediate (50 μm) foil thickness with a cooler, 4.5 ns pulse showed a transition from weak to strong coupling during the pulse duration. Radiation-hydrodynamic simulations are in agreement with the experiments and provide insight into the coupling phenomenon.
Dynamic plasma screening effects on electron-ion Coulomb bremsstrahlung in the Lorentzian distribution plasma
Authors: Yoon Jung-Sik , Jung Young-Dae
Published: July 2002

Dynamic plasma screening effects are investigated on classical bremsstrahlung electron-ion Coulomb scattering in a generalization Lorentzian (kappa) velocity distribution plasma. The electron-ion interaction potential is obtained by introduction of the plasma dielectric function. A semiclassical straight-line trajectory method is applied to the path of the projectile electron. The results show that the differential bremsstrahlung radiation cross section including the dynamic plasma screening effect is increased as an increase of the spectral index κ. In other words, the radiation spectrum in a thermal distribution plasma is found to be stronger than that in the nonthermal distribution plasma. It is also found that the plasma screening effects on the bremsstrahlung are more effective for the hard photon radiation. When the projectile velocity is greater than the electron thermal velocity, then, the interaction potential is almost unshielded.
Study on expanding recombination plasma
Authors: Lan Ke , Zhang Yuquan , Yu Min
Published: July 2002

The behavior of Lagrangian cells in expanding recombination plasma from laser-irradiated carbon fiber are studied. Approximate expressions of electron density, electron temperature, and fractional population of bare nuclei, the escape probability of 2-1 transition and gain for 3d5/2–2p3/2 transition in H-like C ions obtained in this paper are in agreement with numerical simulation.
Two variational principles for incompressible plasmas
Authors: Spiess Peter , Elsässer Klaus
Published: July 2002

A canonical Hamiltonian theory is used to investigate the stability of equilibria of incompressible plasmas with nonconstant density. Two different variational approaches are used and it is shown how to derive them systematically with the help of Clebsch-variables. Both methods lead to an indefinite second variation of the Lyapunov functional for three-dimensional equilibria. It is therefore shown that all three-dimensional equilibria of the model are unstable.
Modeling of the shear effects on the thermal ion transport in advanced tokamak scenarios
Authors: Voitsekhovitch I. , Garbet X. , Moreau D. , Bush C. E. , Litaudon X. , Budny R. V. , Gohil P. , Kinsey J. , Taylor T. S. , Stallard B. W.
Published: July 2002

Plasma shear effects on thermal ion transport in present advanced tokamak scenarios are analyzed. In order to overcome some limitations encountered from previous shear dependent models [e.g., V. Parail et al., Plasma Phys. Controlled Fusion 40, 805 (1998), and references therein], a semiempirical model combining the effects of the magnetic and E×B rotation shears is proposed. These shear dependences are based on simple theoretical arguments from turbulence studies. The dominant stabilizing term of our shear correction is the shear in toroidal rotation. The predictive modeling of the formation and evolution of thermal ion internal transport barriers in relevant advanced scenarios of Tokamak Fusion Test Reactor [E. J. Synakowski et al., Phys. Plasmas 4, 1736 (1997)], Doublet III-D (DIII-D) [B. W. Rice et al., Nucl. Fusion 36, 1271 (1996)] and Joint European Torus (JET) [F. X. Söldner and the JET Team, Plasma Phys. Controlled Fusion 39, B353 (1997)] with our shear correction is presented. A multimachine test of the model performed here provides a basis for predictive simulations of future experiments and for the optimization of advanced scenarios through plasma control.
Quantum, many-body, finite-temperature perturbation theory for an electron–ion system
Authors: Baker George A. , Johnson J. D.
Published: July 2002

The expansion in powers of the electron charge, e, for a neutral system of electrons (fermions) and ions (Maxwell–Boltzmann particles) is extended to order e4 for arbitrary values of temperature and density. The methods of calculation of the series terms will be illustrated, and some of the consequences of these results will be discussed. The ionization profile so derived, at least at high temperatures, will be contrasted with Saha theory. Some special features of hydrogen related to the possible plasma phase transition will be noted.
Effect of externally imposed and self-generated flows on turbulence and magnetohydrodynamic activity in tokamak plasmas
Authors: Leboeuf J.-N. , Dawson J. M. , Decyk V. K. , Kissick M. W. , Rhodes T. L. , Sydora R. D.
Published: July 2002

The effects of externally imposed and self-generated poloidal flows on turbulence and magnetohydrodynamic (MHD) activity are examined in the context of the possible Electric Tokamak (ET) [Phys. Plasmas 6, 4722 (1999)] plasmas and (circularized) DIII-D-like [Fusion Technol. 8, 441 (1985)] discharges. Global gyrokinetic particle simulations and reduced MHD calculations respectively show that ion temperature gradient driven turbulence (ITGDT) and resistive internal kink MHD activity can be reduced and/or suppressed with experimentally achievable externally imposed flows for possible ET start-up plasmas. Global gyrokinetic particle simulations of ITGDT also serve to demonstrate that self-generated flows are necessary to yield experimentally relevant radial correlation lengths in the case of DIII-D-like discharges.
X-ray initiated high pressure glow discharges
Authors: Jayaram Krishnaswamy , Alcock A. J.
Published: July 2002

Pulsed x rays with a dosage of 1–2 mrad/pulse at a mean energy of ∼50 keV are shown to be capable of initiating glow discharges in various gas mixtures up to and above atmospheric pressure. dc voltages (up to 32 kV) have been switched in this way to form glow discharges in CO2, N2, He, and their mixtures with the resulting generation of 10.6-μm laser radiation when an appropriate cavity was used.
High-power closed-cycle subsonic cw CO laser excited by a transverse self-sustained discharge
Authors: Sato Shun-ichi , Fujioka Tomoo , Saito Hideaki
Published: July 2002

Design and performance characteristics of a closed-cycle subsonic cw CO laser excited by a transverse self-sustained dc glow discharge are described. The maximum laser output power obtained to date is 380 W or 950 W per meter of discharge length, corresponding electrical conversion efficiency and specific output energy being 10.2% and 64.7 J/g, respectively.
Room-temperature 1.3-μm optical time domain reflectometer using a photon counting InGaAs/InP avalanche detector
Authors: Levine B. F. , Bethea C. G. , Campbell J. C.
Published: July 2002

We demonstrate the first room-temperature photon counting optical time domain reflectometer at λ=1.3 μm. A high sensitivity (10−14 W) equal to that of a Ge avalanche diode cooled to 77 K has been realized.
Inverted base-collector tunnel transistors
Authors: Bonnefoi A. R. , Chow D. H. , McGill T. C.
Published: July 2002

Two novel three-terminal devices based on tunneling in quantum well and quantum barrier heterostructures are proposed and analyzed theoretically. In both devices, the relative positions of the base and collector are interchanged from conventional emitter-base-collector sequence. This provides a means for obtaining negligible base currents and large current transfer ratios. In both cases, a base voltage controls the emitter-collector tunneling current by shifting the resonances in a quantum well. Calculations indicate that significant variations in the emitter-collector current-voltage characteristics can be obtained for reasonable base-emitter voltages. We call the two devices a Stark effect transistor and a negative resistance Stark effect transistor, respectively.
Technological applications of scanning tunneling microscopy at atmospheric pressure
Authors: Miranda R. , García N. , Baró A. M. , García R. , Peña J. L. , Rohrer H.
Published: July 2002

We present experimental evidence of the capability of the scanning tunneling microscope (STM) to image the microstructure of surfaces with atomic resolution even at atmospheric pressure. Some examples of different types of materials (metal single crystals, graphite, semiconductor, oxide and metal films) are shown. We propose the STM as a highly sensitive standard for surface roughness determination of industrial finishing at atmospheric pressure. In our opinion, these results may open new perspectives in technology and industrial applications.
Nitrogen effect on oxygen precipitation in Czochralski silicon
Authors: Shimura F. , Hockett R. S.
Published: July 2002

The nitrogen effect on enhancement of oxygen precipitation in Czochralski-grown silicon wafers has been investigated by means of a preferential chemical etching technique and secondary ion mass spectroscopy. The precipitate enhancement is evident in an oxygen out-diffused region in which oxygen precipitation does not normally occur. Incorporation of the nitrogen atoms in substitutional sites to generate very stable microdefects in Czochralski-grown silicon crystals can explain the nitrogen effect on oxygen precipitation.
Ga2O3: The origin of growth-induced oval defects in GaAs molecular beam epitaxy
Authors: Weng Shang-Lin
Published: July 2002

Based on thermodynamic and other quantitative results, we show that the formation of growth-induced oval defects in GaAs molecular beam epitaxy originates from the decomposition of Ga2O3 in the Ga melt and the subsequent evaporation of the resultant suboxide Ga2O molecules. After landing on the epilayer surface, these suboxide species most likely combine to form Ga2O3 plus Ga atoms which then act as nucleation centers for the formation of oval defects.
Submicron conducting channels defined by shallow mesa etch in GaAs-AlGaAs heterojunctions
Authors: van Houten H. , van Wees B. J. , Heijman M. G. J. , André J. P.
Published: July 2002

A new approach to the lateral confinement of electrons in the two-dimensional electron gas of GaAs-AlGaAs heterojunctions has been developed. The electrons are electrostatically confined by a shallow mesa structure etched in the upper n-doped AlGaAs layer. This structure is fabricated using electron beam lithography and reactive ion etching. The undoped AlGaAs spacer layer is not removed in order to avoid mobility degradation and channel depletion. Long narrow channels have been made for the study of electrical transport properties. The effective channel width in the submicron range is smaller than the width of the mesa structure. Preliminary low-temperature magnetoresistance data are presented.
Photoluminescence study of the shallow donor neutralization in GaAs(Si) by atomic hydrogen
Authors: Weber Jörg , Pearton S. J. , Dautremont-Smith W. C.
Published: July 2002

Atomic hydrogen in silicon-implanted GaAs samples is studied by photoluminescence. The near-band-gap luminescence of GaAs samples subjected to a hydrogen plasma reveals the neutralization effect of the shallow donors. The photoluminescence intensity increases after hydrogen plasma treatment. However, the donor-related luminescence is drastically reduced, indicating a smaller shallow donor concentration. After a thermal anneal (400 °C, 15 min), the original intensities of donor-related lines are restored. We confirm the model of an electrically inactive hydrogen-donor complex and rule out compensating defects created by the plasma treatment.
Degenerate four-wave mixing from layered semiconductor clusters in the quantum size regime
Authors: Sarid Dror , Rhee Bum Ku , McGinnis Brian P. , Sandroff Claude J.
Published: July 2002

We report the first measurement of the third-order nonlinear susceptibility χ(3) in layered semiconductor clusters exhibiting pronounced quantum size effects at room temperature. BiI3 clusters prepared in colloidal form in acetonitrile had a thickness of ≂7 Å and lateral dimensions between 60 and 90 Å. Using degenerate four-wave mixing, we observed that the conjugate pulses from the small and the large gratings had comparable intensities, verifying the electronic origin of the nonlinearity. The nonlinear susceptibility was found to be 2.3×10−11 esu for a colloid with a cluster volume fraction of 10−5.
Self-consistent analysis of resonant tunneling current
Authors: Ohnishi Hiroaki , Inata Tsuguo , Muto Shunichi , Yokoyama Naoki , Shibatomi Akihiko
Published: July 2002

We investigated the current-voltage characteristics of the double barrier, resonant tunneling structure, using a self-consistent method. We note the significance of the effects of band bending and buildup of space charge in the quantum well. For the peak current, our calculated results agree with the measured results very well. However, the measured valley current is much greater than the calculated values.
Laser photochemical etching of molybdenum and tungsten thin films by surface halogenation
Authors: Rothschild M. , Sedlacek J. H. C. , Ehrlich D. J.
Published: July 2002

Laser direct-write etching of the refractory metals Mo and W has been developed using reactions in Cl2 and NF3 vapors. Rates and high spatial resolution are simultaneously optimized using a two-vapor halogenation/development sequence, based on surface modification. Local-area laser chlorination of the metal surface is used to predispose areas to subsequent bulk etching.
Multilayer step formation after As adsorption on Si (100): Nucleation of GaAs on vicinal Si
Authors: Pukite P. R. , Cohen P. I.
Published: July 2002

We have used reflection high-energy electron diffraction to characterize the initial surface of misoriented Si (100) and then to follow the nucleation of GaAs. Measurement of the diffracted intensity along the length of the specular streak shows sharp structure due to an ordered array of steps. The initial surface contains monolayer steps. However, after exposing to an As4 flux above 650 °C, the surface morphology changes to multilayer steps with four times the original period. In contrast, below 650 °C, surface migration is inhibited and monolayer steps are retained. Subsequent growth of GaAs on either the monolayer- or multilayer-stepped surfaces yields single domain films. However, GaAs grown on the monolayer steps is misoriented toward the (111)A while GaAs grown on the multilayer steps is misoriented toward the (111)B.
Pair of local vibration mode absorption bands related to EL2 defects in semi-insulating GaAs
Authors: Song Chunying , Ge Weikun , Jiang Desheng , Hsu Chenchia
Published: July 2002

A pair of correlated bands, which show the same fine structure at low temperature, is discovered in infrared absorption spectra of semi-insulating GaAs crystals. At 80 K the bands peak at 730 cm−1 and 714 cm−1, respectively. According to the fine structures, the spectral linewidths, and the temperature dependence of frequency shifts, the two absorption bands are identified to be induced by local vibration modes of the same defect. As the interconversion of the two bands under some conditions of illumination and temperature variation is very similar to the transformation between the ground state and the metastable state of the EL2 level, these absorptions are ascribed to be related to the EL2 defect.
Importance of space-charge effects in resonant tunneling devices
Authors: Cahay M. , McLennan M. , Datta S. , Lundstrom M. S.
Published: July 2002

The consideration of space charge in the analysis of resonant tunneling devices leads to a substantial modification of the current-voltage relationship. The region of negative differential resistance (NDR) is shifted to a higher voltage, and broadened along the voltage axis. Moreover, the peak value of current prior to NDR is reduced, leading to a reduction in the predicted peak-to-valley ratio. An approach is presented to include space-charge effects, and a recently fabricated GaAs-AlxGa1−xAs structure is analyzed, to underscore the importance of a self-consistent electrostatic potential in theoretical calculations.
Rapid annealing and the anomalous diffusion of ion implanted boron into silicon
Authors: Michel A. E. , Rausch W. , Ronsheim P. A. , Kastl R. H.
Published: July 2002

The anomalous diffusion of ion implanted boron into silicon is shown to be a transient effect with a decay time that decreases rapidly with increasing anneal temperature. The decay time is approximately 45 min at 800 °C and decreases to the order of a second at 1000 °C. The anomalous displacement in the low concentration region is greater at low temperatures but a larger fraction of the boron is redistributed at high temperature. Sheet resistance measurements agree with the idea that the moving fraction of the boron atoms is electrically active and limited to the intrinsic carrier concentration at the anneal temperature. The activation energy for the decay of the transient is greater than that for the diffusion coefficient, which makes an appropriate rapid thermal anneal cycle an important practical process in the fabrication of shallow p-n junctions.
Process dependence of AlAs/GaAs superlattice mixing induced by silicon implantation
Authors: Schwarz S. A. , Venkatesan T. , Hwang D. M. , Yoon H. W. , Bhat R. , Arakawa Y.
Published: July 2002

Silicon implantation induced mixing of a 400-Å period AlAs/GaAs superlattice grown by organometallic chemical vapor deposition is examined using secondary ion mass spectrometry and transmission electron microscopy. The depth dependence of Al and Si diffusion for a 180-kV 3×1015 cm−2 Si+ implant is measured as a function of implantation temperature, annealing temperature, and annealing time. The room-temperature implanted sample exhibits a near-surface mixing inhibition following anneal. No mixing inhibition is observed in samples implanted at higher temperatures. Si segregates rapidly into the GaAs layers during the anneal. Si diffusion is inhibited near the peak of the implant. Mixing depths and defect distributions are strongly dependent on the processing conditions employed. The results are consistent with a divacancy model for Al diffusion.
Evidence for a magnetic permeability relaxation of dissipative type in amorphous ferromagnetic alloys
Authors: Allia P. , Beatrice C. , Mazzetti P. , Vinai F.
Published: July 2002

A strong relaxation of the magnetic permeability is observed in amorphous ferromagnetic alloys when times shorter than 10−3 s after impulsive sample demagnetization are explored. This effect is not related to an activated ordering process. It is shown that the motion of domain walls is dominated, at the measuring frequency, by dissipative effects. As a consequence, the fast relaxation of the magnetic permeability is related to a change with time of the power dissipated by the system, rather than to a change of the restoring force acting on the walls, responsible instead for the conventional permeability aftereffect. A mechanism for the power dissipation change is proposed.
London penetration depth of superconductors having lifetime broadened density of states
Authors: Takada J. , Bando Y. , Mazaki M.
Published: July 2002

London penetration depth λL was calculated for superconductors having lifetime broadened density of states. With the increase of gap states, λL increases and the temperature dependence deviates from the ordinary form. The model gives a T2 dependence of λL in the case of weak-coupling limit of the Bardeen–Cooper–Schrieffer theory. The results are discussed referring to some experimental results of magnetic field penetration depth of oxide superconductors.
Induced defenses in response to an invading crab predator: An explanation of historical and geographic phenotypic change
Authors: Trussell Geoffrey C. , Smith L. David
Published: July 2002

The expression of defensive morphologies in prey often is correlated with predator abundance or diversity over a range of temporal and spatial scales. These patterns are assumed to reflect natural selection via differential predation on genetically determined, fixed phenotypes. Phenotypic variation, however, also can reflect within-generation developmental responses to environmental cues (phenotypic plasticity). For example, water-borne effluents from predators can induce the production of defensive morphologies in many prey taxa. This phenomenon, however, has been examined only on narrow scales. Here, we demonstrate adaptive phenotypic plasticity in prey from geographically separated populations that were reared in the presence of an introduced predator. Marine snails exposed to predatory crab effluent in the field increased shell thickness rapidly compared with controls. Induced changes were comparable to (i) historical transitions in thickness previously attributed to selection by the invading predator and (ii) present-day clinal variation predicted from water temperature differences. Thus, predator-induced phenotypic plasticity may explain broad-scale geographic and temporal phenotypic variation. If inducible defenses are heritable, then selection on the reaction norm may influence coevolution between predator and prey. Trade-offs may explain why inducible rather than constitutive defenses have evolved in several gastropod species.
Competitive regulation of CaT-like-mediated Ca
2+
entry by protein kinase C and calmodulin
Authors: Niemeyer Barbara A. , Bergs Christiane , Wissenbach Ulrich , Flockerzi Veit , Trost Claudia
Published: July 2002

A finely tuned Ca
2+
signaling system is essential for
cells to transduce extracellular stimuli, to regulate growth, and to
differentiate. We have recently cloned CaT-like (CaT-L), a highly
selective Ca
2+
channel closely related to the epithelial
calcium channels (ECaC) and the calcium transport protein CaT1. CaT-L
is expressed in selected exocrine tissues, and its expression also
strikingly correlates with the malignancy of prostate cancer. The
expression pattern and selective Ca
2+
permeation properties
suggest an important function in Ca
2+
uptake and a role in
tumor progression, but not much is known about the regulation of this
subfamily of ion channels. We now demonstrate a biochemical and
functional mechanism by which cells can control CaT-L activity. CaT-L
is regulated by means of a unique calmodulin binding site,
which, at the same time, is a target for protein kinase C-dependent
phosphorylation. We show that Ca
2+
-dependent
calmodulin binding to CaT-L, which facilitates channel
inactivation, can be counteracted by protein kinase C-mediated
phosphorylation of the calmodulin binding site.
Intercellular trafficking of a
KNOTTED1
green fluorescent protein fusion in the leaf and shoot meristem of
Arabidopsis
Authors: Kim Jae Yean , Yuan Zhuang , Cilia Michelle , Khalfan-Jagani Zainab , Jackson David
Published: July 2002

Dominant mutations in the maize homeobox gene
knotted1
(
kn1
) act nonautonomously during maize leaf development, indicating that
Kn1
is involved in the generation or transmission of a developmental signal that passes from the inner layers of the leaf to epidermal cells. We previously found that this nonautonomous activity is correlated with the presence of KN1 protein in leaf epidermal cells, where KN1 mRNA could not be detected. Furthermore, KN1 protein expressed in
Escherichia coli
and labeled with a fluorescent dye can traffic between leaf mesophyll cells in microinjection assays. Here we show that green fluorescent protein (GFP)-tagged KN1 is able to traffic between epidermal cells of
Arabidopsis
and onion. When expressed
in vivo
, the GFP∼KN1 fusion trafficked from internal tissues of the leaf to the epidermis, providing the first direct evidence, to our knowledge, that KN1 can traffic across different tissue layers in the leaf. Control GFP fusions did not show this intercellular trafficking ability. GFP∼KN1 also trafficked in the shoot apical meristem, suggesting that cell-to-cell trafficking of KN1 may be involved in its normal function in meristem initiation and maintenance.
The role of members of the pertussis toxin-sensitive family of G proteins in coupling receptors to the activation of the G protein-gated inwardly rectifying potassium channel
Authors: Leaney Joanne Louise , Tinker Andrew
Published: July 2002

Inwardly rectifying potassium (K
+
) channels gated by G proteins (Kir3.x family) are widely distributed in neuronal, atrial, and endocrine tissues and play key roles in generating late inhibitory postsynaptic potentials, slowing the heart rate and modulating hormone release. They are directly activated by G
βγ
subunits released from G protein heterotrimers of the G
i/o
family upon appropriate receptor stimulation. Here we examine the role of isoforms of pertussis toxin (PTx)-sensitive G protein α subunits (G
iα1–3
and G
oαA
) in mediating coupling between various receptor systems (A
1
, α
2A
, D
2S
, M
4
, GABA
B
1a+2, and GABA
B
1b+2) and the cloned counterpart of the neuronal channel (Kir3.1+3.2A). The expression of mutant PTx-resistant G
i/oα
subunits in PTx-treated HEK293 cells stably expressing Kir3.1+3.2A allows us to selectively investigate that coupling. We find that, for those receptors (A
1
, α
2A
) known to interact with all isoforms, G
iα1–3
and G
oαA
can all support a significant degree of coupling to Kir3.1+3.2A. The M
4
receptor appears to preferentially couple to G
iα2
while another group of receptors (D
2S
, GABA
B
1a+2, GABA
B
1b+2) activates the channel predominantly through G
βγ
liberated from G
oA
heterotrimers. Interestingly, we have also found a distinct difference in G protein coupling between the two splice variants of GABA
B
1. Our data reveal selective pathways of receptor activation through different G
i/oα
isoforms for stimulation of the G protein-gated inwardly rectifying K
+
channel.
Proteomic dissection of dome formation in a mammary cell line: Role of tropomyosin-5b and maspin
Authors: Zucchi I. , Bini L. , Valaperta R. , Ginestra A. , Albani D. , Susani L. , Sanchez J. C. , Liberatori S. , Magi B. , Raggiaschi R. , Hochstrasser D. F. , Pallini V. , Vezzoni P. , Dulbecco R.
Published: July 2002

In this work we extended the study of genes controlling the formation of specific differentiation structures called “domes” formed by the rat mammary adenocarcinoma cell line LA7 under the influence of DMSO. We have reported previously that an interferon-inducible gene, rat-8, and the β-subunit of the epithelial sodium channel (ENaC) play a fundamental role in this process. Now, we used a proteomic approach to identify proteins differentially expressed either in DMSO-induced LA7 or in 106A10 cells. Two differentially expressed proteins were investigated. The first, tropomyosin-5b, strongly expressed in DMSO-induced LA7 cells, is needed for dome formation because its synthesis inhibition by the antisense RNA technology abolished domes. The second protein, maspin, strongly expressed in the uninduced 106A10 cell line, inhibits dome formation because 106A10 cells, transfected with rat8 cDNA (the function of which is required for the organization of these structures), acquired the ability to develop domes when cultured in presence of an antimaspin antibody. Dome formation in these cultures are accompanied by ENaC β-subunit expression in the absence of DMSO. Therefore, dome formation requires the expression of tropomyosin-5b, in addition to the ENaC β-subunit and the rat8 proteins, and is under the negative control of maspin.
Allosteric, chiral-selective drug binding to DNA
Authors: Qu Xiaogang , Trent John O. , Fokt Izabela , Priebe Waldemar , Chaires Jonathan B.
Published: July 2002

The binding interactions of (−)-daunorubicin (WP900), a newly synthesized enantiomer of the anticancer drug (+)-daunorubicin, with right- and left-handed DNA, have been studied quantitatively by equilibrium dialysis, fluorescence spectroscopy, and circular dichroism. (+)-Daunorubicin binds selectively to right-handed DNA, whereas the enantiomeric WP900 ligand binds selectively to left-handed DNA. Further, binding of the enantiomeric pair to DNA is clearly chirally selective, and each of the enantiomers was found to act as an allosteric effector of DNA conformation. Under solution conditions that initially favored the left-handed conformation of [poly(dGdC)]2, (+)-daunorubicin allosterically converted the polynucleotide to a right-handed intercalated form. In contrast, under solution conditions that initially favored the right-handed conformation of [poly(dGdC)]2, WP900 converted the polynucleotide to a left-handed form. Molecular dynamics studies by using theamberforce field resulted in a stereochemically feasible model for the intercalation of WP900 into left-handed DNA. The chiral selectivity observed for the DNA binding of the daunorubicin/WP900 enantiomeric pair is far greater than the selectivity previously reported for a variety of chiral metal complexes. These results open a new avenue for the rational design of potential anticancer agents that target left-handed DNA.
A dibasic motif involved in parathyroid hormone-induced down-regulation of the type IIa NaPi cotransporter
Authors: Karim-Jimenez Zoubida , Hernando Nati , Biber Jürg , Murer Heini
Published: July 2002

Type II NaPi cotransporters are expressed in the apical membrane of
P
i
-(re)absorbing epithelia: the type IIa in renal proximal
tubule and the type IIb in small intestine. Parathyroid hormone (PTH)
leads to a retrieval from the apical membrane of the type IIa NaPi
cotransporter. The type IIa cotransporter is also expressed in opossum
kidney (OK) cells, and its expression is under the control of PTH. In
the present study, we identified the molecular “domains” involved
in the PTH-induced retrieval of the type IIa NaPi cotransporter.
Wild-type mouse type IIa (mIIa) and type IIb (mIIb) as well as several
mIIa-mIIb chimeras and site-directed mutants were fused to the enhanced
green fluorescent protein and transfected into OK cells. We found that
mIIa but not mIIb was internalized and degraded after incubation with
1–34 (or 3–34) PTH. Using chimeras, we found that the N and C termini
were not required in this effect, whereas a “domain” located
between residues 216 and 658 seemed to be necessary. This region
contains two putative intracellular loops with highly conserved
sequences between mIIa and mIIb; in the last intracellular loop, two
charged amino acids of type IIa (K
503
R
504
) are
replaced by uncharged residues in type IIb
(N
520
I
521
). We generated two mutants in which
these residues were interchanged: mIIaNI and mIIbKR. Similarly to mIIa,
the mIIbKR mutant was endocytosed in response to 1–34 PTH; in
contrast, mIIaNI behaved as mIIb and was not internalized. In
conclusion, a dibasic amino acid motif
(K
503
R
504
) located in the last intracellular
loop of the type IIa NaPi cotransporter is essential for its
PTH-induced retrieval.
Endogenously expressed estrogen receptor and coactivator AIB1 interact in MCF-7 human breast cancer cells
Authors: Tikkanen Minna K. , Carter Damien J. , Harris Alana M. , Le Hung M. , Azorsa David O. , Meltzer Paul S. , Murdoch Fern E.
Published: July 2002

Coactivators are believed to mediate estrogen-induced gene
responses via interaction with estrogen receptors (ER). Currently, a
major challenge is to determine the importance of each coactivator in a
specific cell type and promoter context in response to a particular
ligand. The potential of ER to interact with a growing list of
coactivators has been shown in a variety of
in vitro
and
gene transfer assays, yet very few data have demonstrated the
interaction of endogenous coactivators with ER in intact
cells. We report here a ligand-specific interaction of
endogenous human ER (hER) and the AIB1 coactivator in MCF-7
human breast cancer cells by using immunoprecipitation analyses.
Complexes between endogenously expressed hER and AIB1 were
detected in estradiol-treated cells and to a much lesser extent in
cells treated with the partial agonist, monohydroxytamoxifen. We were
unable to detect an hER–SRC-1 complex in our immunoprecipitations from
MCF-7 cells. The
in vitro-
binding affinity for mouse ER
interaction with AIB1 was estimated to be 40–120 nM. We conclude that
AIB1 is a major coactivator for hER in MCF-7 human breast cancer cells.
Structure of the sodium channel pore revealed by serial cysteine mutagenesis.
Authors: Pérez-García M T , Chiamvimonvat N , Marban E , Tomaselli G F
Published: July 2002

The pores of voltage-gated cation channels are formed by four intramembrane segments that impart selectivity and conductance. Remarkably little is known about the higher order structure of these critical pore-lining or P segments. Serial cysteine mutagenesis reveals a pattern of side-chain accessibility that contradicts currently favored structural models based on alpha-helices or beta-strands. Like the active sites of many enzymes of known structure, the sodium channel pore consists of irregular loop regions.
Receptor stimulation causes slow inhibition of IRK1 inwardly rectifying K+ channels by direct protein kinase A-mediated phosphorylation.
Authors: Wischmeyer E , Karschin A
Published: July 2002

Strongly rectifying IRK-type inwardly rectifying K+ channels are involved in the control of neuronal excitability in the mammalian brain. Whole-cell patch-clamp experiments show that cloned rat IRK1 (Kir 2.1) channels, when heterologously expressed in mammalian COS-7 cells, are inhibited following the activation of coexpressed serotonin (5-hydroxytryptamine) type 1A receptors by receptor agonists. Inhibition is mimicked by internal perfusion with GTP[gamma-S] and elevation of internal cAMP concentrations. Addition of the catalytic subunits of protein kinase A (PKA) to the internal recording solution causes complete inhibition of wild-type IRK1 channels, but not of mutant IRK1(S425N) channels in which a C-terminal PKA phosphorylation site has been removed. Our data suggest that in the nervous system serotonin may negatively control IRK1 channel activity by direct PKA-mediated phosphorylation.
A family of lysozyme-like virulence factors in bacterial pathogens of plants and animals.
Authors: Mushegian A R , Fullner K J , Koonin E V , Nester E W
Published: July 2002

We describe a conserved family of bacterial gene products that includes the VirB1 virulence factor encoded by tumor-inducing plasmids of Agrobacterium spp., proteins involved in conjugative DNA transfer of broad-host-range bacterial plasmids, and gene products that may be involved in invasion by Shigella spp. and Salmonella enterica. Sequence analysis and structural modeling show that the proteins in this group are related to chicken egg white lysozyme and are likely to adopt a lysozyme-like structural fold. Based on their similarity to lysozyme, we predict that these proteins have glycosidase activity. Iterative data base searches with three conserved sequence motifs from this protein family detect a more distant relationship to bacterial and bacteriophage lytic transglycosylases, and goose egg white lysozyme. Two acidic residues in the VirB1 protein of Agrobacterium tumefaciens form a putative catalytic dyad, Each of these residues was changed into the corresponding amide by site-directed mutagenesis. Strains of A. tumefaciens that express mutated VirB1 proteins have a significantly reduced virulence. We hypothesize that many bacterial proteins involved in export of macromolecules belong to a widespread class of hydrolases and cleave beta-1,4-glycosidic bonds as part of their function.
Cytosolic Na+ controls and epithelial Na+ channel via the Go guanine nucleotide-binding regulatory protein.
Authors: Komwatana P , Dinudom A , Young J A , Cook D I
Published: July 2002

In tight Na+-absorbing epithelial cells, the fate of Na+ entry through amiloride-sensitive apical membrane Na+ channels is matched to basolateral Na+ extrusion so that cell Na+ concentration and volume remain steady. Control of this process by regulation of apical Na+ channels has been attributed to changes in cytosolic Ca2+ concentration or pH, secondary to changes in cytosolic Na+ concentration, although cytosolic Cl- seems also to be involved. Using mouse mandibular gland duct cells, we now demonstrate that increasing cytosolic Na+ concentration inhibits apical Na+ channels independent of changes in cytosolic Ca2+, pH, or Cl-, and the effect is blocked by GDP-beta-S, pertussis toxin, and antibodies against the alpha-subunits of guanine nucleotide-binding regulatory proteins (Go). In contrast, the inhibitory effect of cytosolic anions is blocked by antibodies to inhibitory guanine nucleotide-binding regulatory proteins (Gi1/Gi2. It thus appears that apical Na+ channels are regulated by Go and Gi proteins, the activities of which are controlled, respectively, by cytosolic Na+ and Cl-.
Characterization of the osmotic response element of the human aldose reductase gene promoter.
Authors: Ruepp B , Bohren K M , Gabbay K H
Published: July 2002

Aldose reductase (EC 1.1.1.21) catalyzes the NADPH-mediated conversion of glucose to sorbitol. The hyperglycemia of diabetes increases sorbitol production primarily through substrate availability and is thought to contribute to the pathogenesis of many diabetic complications. Increased sorbitol production can also occur at normoglycemic levels via rapid increases in aldose reductase transcription and expression, which have been shown to occur upon exposure of many cell types to hyperosmotic conditions. The induction of aldose reductase transcription and the accumulation of sorbitol, an organic osmolyte, have been shown to be part of the physiological osmoregulatory mechanism whereby renal tubular cells adjust to the intraluminal hyperosmolality during urinary concentration. Previously, to explore the mechanism regulating aldose reductase levels, we partially characterized the human aldose reductase gene promoter present in a 4.2-kb fragment upstream of the transcription initiation start site. A fragment (-192 to +31 bp) was shown to contain several elements that control the basal expression of the enzyme. In this study, we examined the entire 4.2-kb human AR gene promoter fragment by deletion mutagenesis and transfection studies for the presence of osmotic response enhancer elements. An 11-bp nucleotide sequence (TGGAAAATTAC) was located 3.7 kb upstream of the transcription initiation site that mediates hypertonicity-responsive enhancer activity. This osmotic response element (ORE) increased the expression of the chloramphenicol acetyltransferase reporter gene product 2-fold in transfected HepG2 cells exposed to hypertonic NaCl media as compared with isoosmotic media. A more distal homologous sequence is also described; however, this sequence has no osmotic enhancer activity in transfected cells. Specific ORE mutant constructs, gel shift, and DNA fragment competition studies confirm the nature of the element and identify specific nucleotides essential for enhancer activity. A plasmid construct containing three repeat OREs and a heterologous promoter increased expression 8-fold in isoosmotic media and an additional 4-fold when the transfected cells are subjected to hyperosmotic stress (total approximately 30-fold). These findings will permit future studies to identify the transcription factors involved in the normal regulatory response mechanism to hypertonicity and to identify whether and how this response is altered in a variety of pathologic states, including diabetes.
Urea-inducible Egr-1 transcription in renal inner medullary collecting duct (mIMCD3) cells is mediated by extracellular signal-regulated kinase activation.
Authors: Cohen D M
Published: July 2002

Urea (200-400 milliosmolar) activates transcription, translation of, and trans-activation by the immediate-early gene transcription factor Egr-1 in a renal epithelial cell-specific fashion. The effect at the transcriptional level has been attributed to multiple serum response elements and their adjacent Ets motifs located within the Egr-1 promoter. Elk-1, a principal ternary complex factor and Ets domain-containing protein, is a substrate of the extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases. In the renal medullary mIMCD3 cell line, urea (200-400 milliosmolar) activated both ERK1 and ERK2 as determined by in-gel kinase assay and immune-complex kinase assay of epitope-tagged] ERK1 and ERK2. Importantly, urea did not affect abundance of either ERK. Urea-inducible Egr-1 transcription was a consequence of ERK activation because the ERK-specific inhibitor, PD98059, abrogated transcription from the murine Egr-1 promoter in a luciferase reported gene assay. In addition, activators of protein kinase A, including forskolin and 8-Br-cAMP, which are known to inhibit ERK-mediated events, also inhibited urea-inducible Egr-1 transcription. Furthermore, urea-inducible activation of the physiological ERK substrate and transcription factor, Elk-1, was demonstrated through transient cotransfection of a chimeric Elk-1/GAL4 expression plasmid and a GAL4-driven luciferase reporter plasmid. Taken together, these data indicate that, in mIMCD3 cells, urea activates ERKs and the ERK substrate, Elk-1, and that ERK inhibition abrogates urea-inducible Egr-1 transcription. These data are consistent with a model of urea-inducible renal medullary gene expression wherein sequential activation of ERKs and Elk-1 results in increased transcription of Egr-1 through serum response element/Ets motifs.
Cystic fibrosis transmembrane conductance regulator activation stimulates endosome fusion in vivo.
Authors: Biwersi J , Emans N , Verkman A S
Published: July 2002

Previous studies have suggested a role for cystic fibrosis transmembrane conductance regulator (CFTR) in the regulation of intracellular vesicular trafficking. A quantitative fluorescence method was used to test the hypothesis that CFTR expression and activation affects endosome-endosome fusion in intact cells. Endosomes from CFTR-expressing and control (vector-transfected) Swiss 3T3 fibroblasts were labeled by internalization with 4,4-difluoro-5,7-dimethyl-4-bora-3a, 4a-diaza-s-indacene (Bodipy)-avidin, a fluid-phase marker whose fluorescence increases approximately 8-fold upon biotin binding. Cells were washed, chased, and then labeled with biotin-albumin or biotin-transferrin. The fraction of Bodipy-avidin-labeled endosomes that fused with biotin-containing endosomes (f(fusion)) was quantified by ratio imaging microfluorimetry. Endosome fusion in unstimulated CFTR-expressing cells was similar to that in control cells. However, in CFTR-expressing cells activated by forskolin, ffusion was increased by 1.30 +/- 0.18- and 2.65 +/- 0.17-fold for a 0 and 10 min chase time between avidin and biotin-albumin pulses; f(fusion) also increased (1.32 +/- 0.11-fold) when biotin-transferrin replaced biotin-albumin. The stimulation of endosome fusion was not due to differences in rates of endocytosis or endosomal acidification. Endosome fusion was not stimulated by forskolin in Cl--depleted CFTR-expressing cells, suggesting that the increase in endosome fusion is due to the CFTR chloride channel activity. These results provide evidence that CFTR is involved in the regulation of endosome fusion and, thus, a possible basis for the cellular defects associated with cystic fibrosis.