Tight junctions regulate paracellular permeability size and charge selectively. Models have been proposed for the molecular architecture of tight junction strands and paracellular channels. However, ...they are not fully consistent with experimental and structural data. Here, we analysed the architecture of claudin-based tight junction strands and channels by cellular reconstitution of strands, structure-guided mutagenesis, in silico protein docking and oligomer modelling. Prototypic channel- (Cldn10b) and barrier-forming (Cldn3) claudins were analysed. Förster resonance energy transfer (FRET) assays indicated multistep claudin polymerisation, starting with cis-oligomerization specific to the claudin subtype, followed by trans-interaction-triggered cis-polymerisation. Alternative protomer interfaces were modelled in silico and tested by cysteine-mediated crosslinking, confocal- and freeze fracture EM-based analysis of strand formation. The analysed claudin mutants included also mutations causing the HELIX syndrome. The results indicated that protomers in Cldn10b and Cldn3 strands form similar antiparallel double rows, as has been suggested for Cldn15. Mutually stabilising ‐hydrophilic and hydrophobic ‐ cis- and trans-interfaces were identified that contained novel key residues of extracellular segments ECS1 and ECS2.
Hydrophobic clustering of the flexible ECS1 β1β2 loops together with ECS2–ECS2 trans-interaction is suggested to be the driving force for conjunction of tetrameric building blocks into claudin polymers. Cldn10b and Cldn3 are indicated to share this polymerisation mechanism. However, in the paracellular centre of tetramers, electrostatic repulsion may lead to formation of pores (Cldn10b) and electrostatic attraction to barriers (Cldn3). Combining in vitro data and in silico modelling, this study improves mechanistic understanding of paracellular permeability regulation by elucidating claudin assembly and its pathologic alteration as in HELIX syndrome.
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•Multistep polymerisation of claudins is driven by mutual stabilisation of cis- and trans-interfaces.•Claudin-3 and -10b differ in oligomerization but share similar face-to-face double-row architecture in strands.•Strand polymerisation occurs independent of channel versus barrier conformation.•Structural keys for claudin assembly determining paracellular permeability were identified.
We have performed time-dependent numerical simulations of the interstellar medium (ISM) which account for galactic shear and magnetic fields, vertical gravity, and a radiative cooling function for ...atomic gas. This allows us to study the magnetorotational instability (MRI) in cloudy, vertically stratified disks. As in previous unstratified models, we find that thermal instability interacts with MRI-driven turbulence and galactic shear to produce a network of cold, dense, filamentary clouds embedded in a warm diffuse ambient medium. There is significant thermally unstable gas, but the density and temperature distributions retain the twin peaks of the classical two-phase ISM. Independent of the total gas surface density and vertical gravity levels adopted, the midplane ratios of thermal to magnetic pressure are beta = 0.3-0.6, when the mean vertical magnetic field is 0.26 mu G. We analyze the vertical distributions of density and various pressure terms and address what supports the ISM vertically. All models become differentially stratified by temperature; only when the cold mass fraction is small does turbulent mixing maintain a large cold-medium scale height. Turbulent velocities of the cold gas also increase as the cold mass fraction decreases, but are generally low ( similar to 1-3 km s super(-1)) near the midplane; they increase to >5 km s super(-1) at high degree 'z degree '. Turbulent amplitudes are higher in the warm gas. The central thermal pressure is similar for all models even though the total weight varies by a factor of 7 for a range of imposed vertical gravity; in higher gravity models the increased weight is supported by increased magnetic pressure gradients. Approximate vertical equilibrium holds for all models. Finally, we argue that in the outer parts of galactic disks, MRI is likely able to prevent the development of self-gravitating instabilities and hence suppress star formation, even if cold gas is present.
This paper reports on three-dimensional numerical simulations of dynamics and thermodynamics in the diffuse interstellar medium (ISM). Our models are local, account for sheared galactic rotation, ...magnetic fields, and realistic cooling, and resolve scales -1-200 pc. This combination permits the study of quasi-steady state turbulence in a cloudy medium representing the warm/cold atomic ISM. Turbulence is driven by the magnetorotational instability (MRI); our models are the first to study the saturated state of MRI under strongly inhomogeneous conditions, with cloud/intercloud density and temperature contrasts of 6100. For volume-averaged densities = 0.25-4 cm super(-3), the mean saturated-state velocity dispersion ranges from 8 to 1 km s super(-1), with a scaling du 8 super(-0.77). The MRI is therefore likely quite important in driving turbulence in low-density regions of the ISM, both away from the midplane in the inner Galaxy (as observed at high latitudes) and throughout the far outer Galaxy (where the mean density drops and the disk flares). The MRI may even be key to suppressing star formation at large radii in spiral galaxies, where the pressure can be high enough that without MRI-driven turbulence, a gravitationally unstable cold layer would form. As expected, we find that turbulence affects the thermal structure of the ISM. In all our simulations, the fraction of thermally unstable gas increases as the MRI develops and in the saturated state is largest in high-du models. The mass fractions of warm stable and unstable gas are typically comparable, in agreement with observations. While inclusion of resistive dissipation of magnetic fields could enhance the amount of thermally unstable gas compared to current models, our present results indicate that even high levels of turbulence cannot wipe out the signature of thermal instability and that a shift to a "phase continuum" description is probably unwarranted. Instead, we find that temperature and density PDFs are broadened (and include extreme departures from equilibrium), but retain the bimodal character of the classical two-phase description. Our presentation also includes results on the distribution of clump masses (the mass spectrum peaks at 6100 M sub( )), comparisons of saturated-state MRI scalings with single-phase simulation results (we find that (B super(2)) is independent of ), and examples of synthetic H I line profile maps (showing that physical clumps are not easily distinguished in velocity components and vice versa).
The structure and dynamics of diffuse gas in the Milky Way and other disk galaxies may be strongly influenced by thermal and magnetorotational instabilities (TI and MRI, respectively) on scales ...approx1-100 pc. We initiate a study of these processes, using two-dimensional numerical hydrodynamic and magnetohydrodynamic simulations with conditions appropriate for the atomic interstellar medium (ISM). Our simulations incorporate thermal conduction and adopt local "shearing-periodic" equations of motion and boundary conditions to study dynamics of a (100 pc) super( 2) radial-vertical section of the disk. We demonstrate, consistent with previous work, that nonlinear development of "pure TI" produces a network of filaments that condense into cold clouds at their intersections, yielding a distinct two-phase warm/cold medium within approx20 Myr. TI-driven turbulent motions of the clouds and warm intercloud medium are present but saturate at quite subsonic amplitudes for uniform initial P/k = 2000 K cm super(-3). MRI has previously been studied in near-uniform media; our simulations include both TI+MRI models, which begin from uniform- density conditions, and cloud+MRI models, which begin with a two-phase cloudy medium. Both the TI+MRI and cloud+MRI models show that MRI develops within a few galactic orbital times, just as for a uniform medium. The mean separation between clouds can affect which MRI mode dominates the evolution. Provided intercloud separations do not exceed half the MRI wavelength, we find the MRI growth rates are similar to those for the corresponding uniform medium. This opens the possibility that if low cloud volume filling factors increase MRI dissipation times compared to those in a uniform medium, then MRI-driven motions in the ISM could reach amplitudes comparable to observed H I turbulent line widths.
Aggressive vertical scaling of SiGe HBTs has yielded impressive values for the cut-off frequencies (f T ), but these HBTs often suffer from too high current gains. This leads to low values for the ...open-base breakdown voltage (BV CEO ). In this letter we demonstrate the use of a SiGe spike in the emitter as a practical method to increase the base current. Hence, the breakdown voltage is increased. At the same time, the device RF performance is not affected, which leads to a significant improvement in the f T xBV CEO product
As device scaling for high-performance bipolar transistors continues, not only the vertical scaling but also the lateral scaling with reduction of the parasitics can have an important impact on the ...reliability of the HBT. In this work we will present the impact on the RF performance and the reliability of the vertical and lateral scaling of a SiGe:C HBT with a low-complexity QSA (quasi self-aligned) architecture. The reliability will be assessed for three different stress modes.
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