This Week in Science

Science  08 Aug 2008:
Vol. 321, Issue 5890, pp. 741
  1. Symporter Structure Solved


    Solute sodium symporters (SSS) are a large family of proteins that couple the transport of sodium out of the cell with the transport of nutrients into the cell, but the molecular mechanism of this symport has remained unclear. Now Faham et al. (p. 810, published online 3 July; see the Perspective by Karpowich and Wang) have determined the structure of the sodium galactose symporter from Vibrio parahaemolyticus (vSGLT). The structure obtained is in an inward facing conformation with galactose bound and blocked from exit by a gating residue. Surprisingly the core structure has a similar topology to the core structure of LeuT, a member of the neurotransmitters sodium symporter (NSS) family that has no significant sequence similarity to the SSS family. Based on the LeuT structure an outward facing conformation could be modeled for vSGLT that, together with biophysical data, provides insight into the mechanism of active transport.

  2. A Question of Trust

    Patients with borderline personality disorder (BPD) have difficulty in maintaining social relationships. King-Casas et al. (p. 806; see the Perspective by Meyer-Lindenberg) studied the behaviors and neural activities of BPD patients participating in an economic exchange game—the so-called Trust game—with healthy partners. Patients were less likely to maintain the level of trust required: as trustees paying back less of the profit that results from a tripling of the investments made by healthy subjects, thus causing the investors to scale back the amount that they would risk. In addition, the patients were less able to repair these breaches of trust—which would require offering disgruntled investors overgenerous payouts to induce a return to a cooperative mode of play. Coupled to these behaviors, neural activity in the anterior insula region of the brain indicated that patients did not seem to process the offer of trust (a high investment) any differently than an expression of distrust (a low investment).

  3. Modeling the Birth of Planetary Systems

    A large number of planetary systems have now been discovered around other stars. Many of these contain giant planets in a close orbit, but others contain a different geometry. Thommes et al. (p. 814; see the Perspective by Papaloizou) now present a model of planetary evolution that examines the entire process from the initial formation of planets from a protostellar disk through their subsequent evolution. The model confirms that a wide range of distributions are possible but suggests that the final distribution is particularly sensitive to the viscosity and density of the original protostellar disk. Furthermore, it seems that the production of a planetary geometry like our own solar system is unusual.

  4. Nanoscale Standards


    A challenge in making nanoscale materials is being able to control overall dimensions in a manner that also ensures uniformity of the end product. Yin et al. (p. 824) describe the programming of monodispersed molecular tubes of predefined circumferences from simple single-stranded DNA motifs. Simple programs are written by specifying complementary relationships between ports and nodes, which define the way the strands will connect together into larger tubular structures. The programs ensure that only one diameter tube will form from a set of starting DNA strands, thus guaranteeing the size that forms and allowing for long tubes to form.

  5. Flattening the Spin Landscape

    Quantum dots are attractive candidates as the basic building blocks for quantum information processing and quantum computation. However, the material of choice, gallium arsenide, causes problems due to the large and fluctuating magnetic background landscape of the Ga atoms. Each electron is coupled to more than a million background spins, resulting in decoherence that limits the spin lifetime to several nanoseconds. Reilly et al. (p. 817, published online 10 July) now show that a series of carefully applied voltage pulses can diminish the influence of the background spins thereby allowing the spin lifetime (measured in terms of dephasing time) to be extended to more than a microsecond. This lifetime extension should provide a sufficiently wide window to perform millions of operations on the quantum dots.

  6. Breaking in Three

    Though bimolecular reactions are fairly common, it is clear from simple collision probabilities that reactions requiring the simultaneous merger of three different molecules will be comparatively rare. What about the reverse process though? What factors might drive a compound to fly apart into three pieces after an injection of energy? Savee et al. (p. 826) explore this question at the quantum mechanical level in a combined experimental and theoretical study of sym-triazine dissociation. The hexagonal compound, which consists of alternating HC and N sites, breaks apart into three HCN products. A partitioning between two competing pathways could be observed—one in which the bonds all break simultaneously, and one in which the bonds break sequentially—based on the nature of the initially populated electronic excited state.

  7. Mars Rocks

    Clay minerals, which contain some water in their structure, have been seen in the oldest areas of Mars. New spectroscopic observations from the Mars Reconnaissance Orbiter, with a resolution as low as 18 meters per pixel, allow mapping of the distribution of several clay minerals and reveal a sequence of alteration in one of the oldest outflow valleys of Mars. Bishop et al. (p. 830) show that the oldest rocks contain abundant smectite rich in iron and magnesium; stratigraphically higher rocks contain abundant kaolinite and montmorillonite, more Al-rich clay minerals, and hydrated silica. These differences may reflect different chemistries of the host rocks or a change in the chemistry and distribution of groundwater over time.

  8. Genetics of the Sexes

    Sex determination is a fundamental biological trait in plants, directly coupled to the evolution of mating systems and with tremendous practical significance in fruit and hybrid seed production for many crop species. Changes in the ratio of male and female sexual organs are common in plants, although the underlying genetics are generally not well understood. Andromonoecious plants possess both hermaphroditic and male flowers and have been observed in many different species of plant. Boualem et al. (p. 836) now show that in melons (Cucumis melo) the andromonoecious (a) locus encodes the 1-aminocyclopropane-1-carboxylic acid synthase gene (ACS). The CmACS protein is important in the synthesis of ethylene, a plant hormone that influences the development of the plant's sexual organs.

  9. Axonal Pathfinding in Sight


    About 1 in 1000 people are afflicted with Duane's retraction syndrome (DRS), a complex congenital eye disorder characterized by a restricted ability to move the eye(s) outward or inward. The condition is thought to arise from faulty innervation of extraocular muscles by cranial motor neurons, which probably occurs early in embryogenesis. Miyake et al. (p. 839, published online 24 July) now provide genetic evidence that strongly supports this hypothesis. Studying families with a variant form of DRS, the authors discover that the mutations responsible for the disorder fall within a gene on chromosome 2 encoding α2-chimaerin, a RacGAP signaling protein previously implicated in axonal pathfinding of corticospinal nerves in mice. The human mutations cause α2-chimaerin to become overactive, and expression of the mutant protein in a chick embryo model did indeed disrupt oculomotor axon development.

  10. Autoimmune Fail-Safe Strategy

    The immune system detects and destroys foreign antigens that enter the body; at the same time it must avoid destroying the organism's own antigens, a process that can cause autoimmune disease. To this end, medullary epithelial cells in the thymus express the Autoimmune Regulator (Aire) gene, which promotes the expression of many of these self antigens. As immune cells mature in the thymus, the ones that recognize these self antigens are deleted. Gardner et al. (p. 843; see the Perspective by Kyewsky) now describe an auxiliary system in the lymph nodes and spleen that ensures that circulating immune cells remain tolerant. To accomplish this, the Aire gene triggers the expression of a different array of self antigens in the epithelium of these peripheral lymph organs. There, antigen-specific interactions occur between Aire-expressing cells and autoreactive T cells, presumably leading to deletion of any self-reactive T cells that have escaped deletion in the thymus.

  11. Noticing Is Remembering

    The dominant model of human visual working memory allows for the simultaneous represention of only three or four objects. With what precision is each visual object stored as a function of the number of items in a scene? Bays and Husain (p. 851) tested the ability of human subjects to remember the location and orientation of multiple visual items after a brief disappearance of the stimulus array, and found that visual working memory is a flexibly allocated resource. Making an eye movement toward an object, or directing covert attention to it, caused a greater proportion of memory resources to be allocated to that object, allowing the memory of its presence to be retained with far greater precision than other objects in the scene.

  12. Cooling with Ferroelectric Polymers

    Magnetocaloric effects have long been exploited to cool substances through a cycle in which magnetic domains absorb heat from a load disorder and then can be reordered with an external field. In a similar way, some materials that can undergo large changes in polarization with an applied electrical field (ferroelectrics) can exhibit analogous electrocaloric effects. However, in ceramic materials near room temperature, the isothermal changes in entropy are not as high as for magnetocaloric materials. Neese et al. (p. 821) now report that a copolymer of vinylidene fluoride and trifluoroethylene shows a large isothermal entropy change of 55 joules per kilogram per kelvin degree at 70°C, which is above its ferroelectric-paraelectric transition. In a terpolymer that adds a bulky monomer such as chlorofluoroethylene that increases disorder and that acts as a relaxor ferroelectric, similar effects were achieved near room temperature.

  13. Brown: The New Black

    Atmospheric aerosols have a significant effect on climate owing to the radiative forcing that they provide. Much of our understanding, and modeling, of the forcing supplied by carbonaceaous particles is based on a simple representation of those aerosols as a binary mixture of light-reflecting organics and light-absorbing soot, or black carbon. Alexander et al. (p. 833) now show that the real situation is more complex. Carbon spheres found in ambient aerosols collected above the Yellow Sea in the East Asian-Pacific outflow were brown in color, and not black, which will change how radiative forcing by this type of pollution is assessed.

  14. Modulating Striatal Synapses

    The role of dopamine in plasticity at glutamatergic synapses in the striatum is central to our understanding of basal ganglia functions and dopamine-dependent reward mechanisms. Long-term potentiation (LTP) and long-term depression (LTD) at these synapses are thought to be dependent on D1 and D2 dopamine receptors, respectively. However, the mechanisms of LTP and LTD in the striatum are controversial. Using brain slices from transgenic mice, Shen and Surmeier (p. 848) show that LTP and LTD can occur in both D1- and D2-expressing neurons but with different molecular mechanisms. Dopaminergic modulation of plasticity is receptor- and cell-type specific. The findings suggest that the control of bidirectional plasticity is not exerted through a monolithic mechanism, as previously asserted, but by cell-type-specific mechanisms depending on the subtype of dopamine receptor expressed.