Helical Stripes Research Articles

Hard to Fence You in: Computational Approaches to Explore the Hypothesis that Actin Filaments Impede PIP2 Diffusion in Membranes

Experiments described by Golebiewska et al. at this meeting suggest the existence of a ‘fence’that impedes the diffusion of phosphatidylinositol 4,5-bisphosphate (PIP2) into and out of nascent phagosomes in macrophages. Although the nature of the fence remains an enigma, actin filaments are plausible components. They are highly negatively charged (as is PIP2), are swept away from the central region and are concentrated at the perimeter of the forming phagosome. To explore the actin fence hypothesis, we have used (1) Poisson-Boltzmann continuum electrostatics and a grid-based repulsive potential to describe a fence model made of a single layer of actin filaments, and (2) Brownian dynamics to describe the diffusion of PIP2 molecules modeled as single spheres. The simulations with actin filaments positioned parallel to the membrane indicate that a single filament without attached proteins does not significantly impede the diffusion of PIP2. A helical stripe of basic residues on the acidic actin filament provides a hole in the putative fence through which PIP2 can diffuse, no matter how close the filament is positioned to the membrane. Results from simulations of PIP2 diffusion out of corrals formed of multiple layers of actin filaments, and mazes of non-electrostatic barriers will also be presented.

Sus1, Cdc31, and the Sac3 CID Region Form a Conserved Interaction Platform that Promotes Nuclear Pore Association and mRNA Export

SummaryThe yeast Sac3:Cdc31:Sus1:Thp1 (TREX-2) complex facilitates the repositioning and association of actively transcribing genes with nuclear pores (NPCs)—“gene gating”—that is central to integrating transcription, processing, and mRNA nuclear export. We present here the crystal structure of Sus1 and Cdc31 bound to a central region of Sac3 (the CID domain) that is crucial for its function. Sac3CID forms a long, gently undulating α helix around which one Cdc31 and two Sus1 chains are wrapped. Sus1 has an articulated helical hairpin fold that facilitates its wrapping around Sac3. In vivo studies using engineered mutations that selectively disrupted binding of individual chains to Sac3 indicated that Sus1 and Cdc31 function synergistically to promote NPC association of TREX-2 and mRNA nuclear export. These data indicate Sac3CID provides a scaffold within TREX-2 to integrate interactions between protein complexes to facilitate the coupling of transcription and mRNA export during gene expression.

Stability of a compact three-period stellarator with quasiaxial symmetry features

The ideal magnetohydrodynamic stability properties of a three-field period stellarator with quasiaxial symmetry features proposed by Garabedian and Ku [Phys. Plasmas 6, 645 (1999)] are investigated. The stability limit imposed by local ballooning and Mercier modes allows β∼5.3% to be achieved by tailoring the pressure profile. For this system, the global external kink modes are shown to be stable. To demonstrate this, a sequence of configurations has been developed that varies from the reference system proposed to the equivalent axisymmetric tokamak. A scan of kink stability reveals that the configuration with a plasma boundary shape that corresponds to 80% of the basic three-period stellarator and 20% of the tokamak becomes marginal to global external modes. The normal magnetic field line curvature and the local magnetic shear play crucial roles in the stability properties. The local magnetic shear develops a helical stripe with increasing three-dimensional plasma deformation that inhibits the formation of global structures. The normal magnetic field line curvature, however, becomes locally amplified by the three-dimensional plasma deformation in regions where the local magnetic shear vanishes. As a result, the susceptibility to localized ballooning mode destabilization is enhanced.

Flux-flow-like state of a Pb-Tl rod in a longitudinal field

Experiments and calculations have been made for the flux-flow state of the longitudinal field problem. As one of the results from these, an experimental formula was obtained, which is useful for knowing the characteristics not only of the flux-flow but also of the enhancement of the critical current. For many existing data on the critical current curve for the samples such as Pb alloy good fit with this formula is obtained. A helical stripes structure has been found experimentally, and one of the stripes domain proved to be a superconducting state. The calculations assuming the superconducting and normal stripes were qualitatively in good agreement with the experiments. Therefore, the other stripes domain is thought to be a normal state. Finally, it is expected that the problem of critical current enhancement may be solved quantitatively by the use of this helical stripes model. (auth)

Helical and Other Stripe Structures in Single-Crystal Permalloy Platelets

Four primary stripe patterns are observed in Permalloy single-crystal platelets with {100} surfaces in different regions of thickness and composition. All four structures involve magnetic moment rotations between easy 〈111〉 axes parallel to {110} planes. The first to appear with increasing thickness (0.3<D<1.0 μm) has 〈100〉 stripes and 〈100〉 net magnetization. The second and third coexist from 0.8–1.4 μm, have fine 〈110〉 and broad 〈100〉 stripes, respectively, and have different magnitudes of net magnetization. Both involve 71° rotations parallel to vertical {110} planes. The fourth (D>0.9 μm) achieves appreciable internal flux closure by forming a helical structure through rotations toward alternate 〈111〉 directions in passing from top to bottom surfaces at any stripe, and through having the planar surface component in alternate stripes perpendicular to the stripes. Models for 180° and 90° walls readily follow from the above scheme in conjunction with applied-field observations.

Viscous Frictional Moment between Eccentric Rotating Cylinders when Outer Cylinder Rotates

Although many studies on journal bearings have been reported as regards eccentric rotating cylinders, there is no frictional moment (torque) measurement which has been done for various eccentricity ratios and radius ratios in condition that there is sufficient fluid to fill the clearance in journal bearing. Recently, Oguro, Coles and Van Atta reported that under certain conditions a pattern of alternating helical stripes of laminar and turbulent flow could be observed in the gap between concentric rotating cylinders, with the outer cylinder rotating. However the critical Reynolds number has not been investigated in detail. In this paper the torque acting on the inner cylinder in eccentric rotating cylinders is measured over the range from the laminar flow to the fully developed turbulent flow, with the outer cylinder rotating. Torque measurements are done using water, glycerine-water solution and spindle oil, and increasing the fluid pressure in order to prevent the occurrence of cavitation. And we studied experimentally the relation between torque coefficient and a few parameters, such as rotating Reynolds number Rω, eccentricity ratio etc., and obtained Rω at which transition to turbulent flow occurs.

Exploratory measurements in spiral turbulence

The present experiments lay a foundation for a detailed study of interface propagation in a mixed laminar–turbulent flow between counter-rotating concentric cylinders. Such mixed flows, including one particularly well-organized pattern called spiral turbulence, are found to be a dominant feature of transition in Couette flow. In spiral turbulence, the laminar and turbulent regions of the flow form an alternating pattern of helical stripes, rotating with approximately the mean angular velocity of the two cylinders. Stable right-handed or left-handed spirals occur with equal probability when the flow is established from rest. Hot-wire measurements have been made of the mean cross-sectional shape of the interfaces in the axial mid-plane for a spiral-turbulent flow having low dispersion in the interface position. A nose of turbulence associated with the leading interface projects into the laminar region near the outer cylinder, while a corresponding tail is associated with the trailing interface near the inner cylinder. Fluctuations in interface position and in apparent length of the turbulent region are closely Gaussian, and the fluctuations are rapid compared with the period of rotation of either cylinder.