This Week in Science

Science  07 Feb 2014:
Vol. 343, Issue 6171, pp. 577
  1. Observing the Upturn

    When two parallel conducting wires are separated by a small insulating barrier, a current in one wire can generate a net charge displacement in the other by virtue of electron-electron interactions. Some of the models for this process predict a nonmonotonic temperature dependence of the resulting Coulomb drag voltage, with an upturn occurring at a certain low temperature T*. Laroche et al. (p. 631, published online 23 January) observed this upturn in a pair of vertically integrated quantum wires separated by a 15-nanometer-wide barrier.

  2. Curving Crystals

    When a material with a different set of lattice parameters is grown on the surface of a crystal of a second material, the stresses at the interface can affect the growing crystal. Meng et al. (p. 634) studied the growth of colloidal crystals on top of a curved water droplet. Owing to the elastic stress caused by the bending of the crystal, strong distortions occurred in the growing crystal, but, nonetheless, large single-crystalline domains with no topological defects were formed.

  3. Predictable Behavior

    Few internally forced large-scale atmospheric circulation patterns exhibit periodic behavior, and those that do are centered in the tropics. Identifying these periodic processes is important for understanding the dynamics of weather. Thompson and Barnes (p. 641) report the discovery of a 20- to 30-day periodicity in the atmospheric circulation in the Southern Hemisphere. The oscillation could potentially drive large-scale climate variability throughout much of the mid-latitude Southern Hemisphere.

  4. LEAFY Evolution

    It is generally believed that redundancy across gene copies allows for the evolution of novel function in proteins. However, it is less clear how single-copy genes with crucial function may evolve. Sayou et al. (p. 645, published online 16 January; see the Perspective by Kovach and Lamb) examined the evolution of the essential plant transcription factor LEAFY, which is generally found as a single-copy gene. LEAFY homologs in taxa representing the major evolutionary branches of the land plants and algae exhibited three classes of LEAFY binding sites. Structural analysis identified amino acid changes in the proteins, that were responsible for contacts with specific DNA motifs and allowed the likely effects of specific amino acid changes over the evolution of land plants to be resolved.

  5. Sculpting Actomyosin

    The sculpting of embryos during development involves coordinated movement of cells in large groups. How actomyosin is controlled during such collective cell movement remains poorly understood. Working with developing Xenopus mesoderm, Shindo and Wallingford (p. 649) found that planar cell polarity proteins and septins interface with the actomyosin machinery to control collective cell movement.

  6. Easy M

    CREDIT: CHRISTINA CORBACI AND RAJESH K. GROVER

    Our immune systems can produce a vastly diverse repertoire of antibody molecules that each recognize and bind to a specific foreign antigen via a hypervariable region. However, there are a few bacterial antigens—such as Protein A, Protein G, and Protein L—that instead bind to the antibody's conserved regions and can bind to a large number of different antibodies. These high-affinity broad-spectrum antibody-binding properties have been widely exploited both in the laboratory and in industry for purifying, immobilizing, and detecting antibodies. Grover et al. (p. 656) have now identified Protein M found on the surface of human mycoplasma, which displays even broader antibody-binding specificity. The crystal structure of Protein M revealed how Protein-M binding blocks the antibody's antigen binding site. This mechanism may be exploited by mycoplasma to escape the humoral immune response.

  7. Causing Chloride Changes

    Because intracellular chloride concentrations largely determine the direction and magnitude of current flow through GABAA channels, the stability of intracellular chloride concentration is important to maintain consistent synaptic inhibition. Glykys et al. (p. 670) examined the mechanisms by which chloride gradients in neurons are established, using chloride imaging with transgenically expressed clomeleon dye. Surprisingly, intracellular chloride was not primarily determined by transporters. Instead, subcellular gradients of immobile anions generated inverse chloride gradients.

  8. The Switch That Doesn't

    In mammals, a class of neurons in the brain normally switches from excitatory to inhibitory functions at birth. Tyzio et al. (p. 675; see the Perspective by Zimmerman and Connors) studied how these neurons function in rat and mouse models of autism. The results show that oxytocin normally accelerates the switch in function, but in these two animal models, the switch fails. The dysfunction could be replicated in normal animals using an oxytocin receptor antagonist.

  9. Weathering Heights

    CREDIT: ISAAC LARSEN

    The production of soil is the result of chemical weathering of rocks and minerals. In regions where tectonic uplift brings fresh material to Earth's surface, erosion and weathering can accelerate. Using chemical tracers, Larsen et al. (p. 637, published online 16 January; see the Perspective by Heimsath) measured soil production rates of over 2 millimeters per year in New Zealand's Southern Alps, which are some of the fastest uplifting mountains in the world. Because chemical weathering consumes CO2, these rapid rates may over time influence global climate.

  10. Stochasticity and Cell Fate

    CREDIT: ROBERT J. JOHNSTON

    Stochastic mechanisms can diversify cell fates in nervous systems. In the Drosophila retina, the stochastic distribution of color-sensing photoreceptors is controlled by the random, On/Off expression of the Spineless transcriptional regulator. Johnston and Desplan (p. 661) found that each allele of spineless makes an intrinsically random expression choice controlled by an enhancer and two silencer DNA elements acting at long range. spineless alleles communicate between chromosomes using activating and repressing mechanisms to determine the frequency of expression and coordinate expression state. These findings suggest critical roles for intrinsically random expression decisions and interchromosomal communication in stochastic cell fate specification.

  11. The Message in the Medium

    What is the point of autocrine signaling in which a cell produces a signal that activates receptors on its own cell surface? An internal signal seems simpler, unless there is value to allowing neighboring cells to know what other cells are up to. Youk and Lim (10.1126/science.1242782; see the Perspective by Lee and You) explored the broad range of signaling outcomes that can result in a system in which some yeast cells could secrete and sense a signal whereas others could only sense signals from their neighbors. The cells were engineered so that the response of the two cell types could be distinguished from one another. Experiments and mathematical modeling showed that depending on how circuits were constructed—for example, how much receptor was present, how the signal molecule was degraded, the presence of feedback, the density of the cell culture, and so on—a range of behaviors was possible: Some conditions favored activation of one type of cell over another. Others altered the timing or consistency of the response within a population. The principles revealed could also be used in other biological contexts or in the design of synthetic biological cell systems with desired regulatory properties.

  12. Interacting and Topological

    Topological insulators (TIs), which have a bandgap and a robust conducting surface state protected by time-reversal symmetry, are typically materials with weak electron-electron interaction and are well-described by band theory. A major experimental goal has been to observe such symmetry-protected topological (SPT) phases in interacting systems. Wang et al. (p. 629) used a theoretical approach to classify SPT phases of interacting fermions in three dimensions and found six other phases in addition to the noninteracting ones. The results lay the groundwork for future microscopic models and inform the experimental search for such materials.

  13. Viral ESCRT

    The ESCRT (Endosomal Sorting Complex Required for Transport) protein complex plays a role in budding into multivesicular bodies, cytokinesis, and HIV budding, but the details of how the ESCRTs facilitate viral budding are unclear. Now, using high-resolution light and electron microscopical imaging techniques, Van Engelenburg et al. (p. 653, published online 16 January) dissect the role for ESCRT proteins in HIV budding. The findings suggest that the ESCRT machinery required for the scission of HIV particles from infected cells is located within the core of the virus particle and not, as might have been expected based on previous work, on the cellular side of the membrane scission event involved in viral budding.

  14. From Channel to Sensor

    The coupling between voltage-activated calcium channels and calcium sensors of exocytosis on synaptic vesicles is a key factor that determines the timing and efficiency of transmitter release. It is still largely unclear how tight this coupling is at mature synapses in the central nervous system. Vyleta and Jonas (p. 665) found that mossy fiber boutons contain high concentrations of endogenous calcium buffers that normally limit the amount of calcium that reaches the calcium sensor responsible for neurotransmitter release. As a consequence, the calcium signal available to trigger release is small for the initial action potential. However, after high-frequency stimulation, the endogenous calcium buffer binds calcium and is less able to buffer calcium entry, which allows more calcium to reach the calcium sensor, increasing neurotransmitter release and synaptic facilitation.