This Week in Science

Science  05 May 2006:
Vol. 312, Issue 5774, pp. 653
  1. Titan's Dunes


    Cassini radar observations of Titan by Lorenz et al. (p. 724; see the Perspective by Lancaster) show that large regions of its surface contain parallel or ‘seif,’ dune fields, similar to those in the Sahara Desert on Earth. The height of the dunes is about 150 meters, and they occur in regions up to 1500 by 200 kilometers in extent, especially near the equator. The east-west alignment of the dunes and other flow features suggest that they form from the actions of easterly surface winds. The presence of dunes indicates that processes create sand-sized particles, and that Titan's equatorial surface lacks persistent liquids that would otherwise trap the sand.

  2. Shuttle Service for Polymer Synthesis

    In a “living” polymerization, each polymer chain grows at a steady rate from a single catalyst site. The successive removal and addition of different monomers to the reactor yields block copolymers with structurally distinct chain segments. Arriola et al. (p. 714; see the Perspective by Gibson) present an alternative strategy for building block copolymers in which diverse monomers are all present at once and a molecular shuttle transfers growing polymer chains back and forth between catalysts with differing selectivities. The shuttling technique tolerates high temperatures (to maintain polymer solubility) and is amenable to economically efficient continuous flow conditions. Screening of a wide range of catalyst and shuttle combinations revealed useful elastomeric copolymers in which polyethylene blocks alternate with high and low levels of a higher olefin, 1-octene, with a high degree of block intermixing.

  3. Turbulent Route to Extreme Magnetic Fields

    Short gamma-ray bursts are thought to result when two compact neutron stars merge to form a black hole. Strong magnetism has been thought to produce the gamma rays, but the mechanism is not clear. Price and Rosswog (p. 719, published online 30 March; see the cover) have modeled the death throes of a binary pair of neutron stars and included the effects of magnetism as well as hydrodynamics and gravity. In their simulations, a strong shear layer develops at the interface between the two stars during the first few milliseconds of the merger. Turbulent eddies within this layer amplify the magnetic fields to greater than 1015 Gauss, which is even stronger than those in known magnetic stars. This process occurs very quickly, so that the gamma-ray bursts may be emitted before the black hole is actually formed.

  4. Primitive Organics


    Interplanetary dust particles are particularly carbon rich, and are thought to have their origins outside the solar system in interstellar material and comets. This primitive material was thought to have been largely lost in more heavily processed meteorites. Using hydrogen and nitrogen isotopes, Busemann et al. (p. 727; see the Perspective by Marty) show that the organic matter in carbonaceous meteorites is as primitive as that in the dust grains. These results imply that temperatures of the protosolar nebula in the region of the asteroid belt were low and that the meteoritic material has suffered little alteration since then.

  5. Holding On into the Cambrian

    Vendobionts are one of several enigmatic animals from the Ediacaran, the time before the Cambrian radiation of animals, and represent some of Earth's earliest animals. Details of their relation to later animals have been unclear; indeed, most Ediacaran animals seem to have no later or extant relatives. Shu et al. (p. 731) now describe a vendobiont from the Lower Cambrian of China in which the preservation is sufficient to see internal soft tissues. The fossil suggests that they lived attached to the sea floor by a stalk. These animals not only survived the Ediacaran but also show some similarity to younger Cambrian animals.

  6. Starved for Energy

    During times of food deprivation or increased energy demand, mammals use intracellular triglycerides stored in fat tissue as a primary energy source. Mobilization of these stores requires activation of lipid-degrading enzymes. Haemmerle et al. (p. 734) characterized mice deficient in one such enzyme, adipose triglyceride lipase (ATGL). Its absence had major metabolic consequences, including alterations in glucose tolerance and insulin sensitivity, defective thermogenesis, and massive accumulation of lipids in the heart that resulted in cardiomyopathy and premature death.

  7. First, Collect Antigens

    Antibodies are produced by B cells after antigens stimulate receptors on their surface and other appropriate signals have been received. If antigens are bound to the surface of another cell, activation signals can be particularly strong and allow B cells to discriminate among a wide range of antigen affinities. Fleire et al. (p. 738; see the Perspective by Harnett) show that B cells can actually focus antigen into aggregates that resemble the well-characterized immune synapses of T cells. After initial contact, B cells spread themselves over the other cell and then contract, gathering up antigens in the process. This response depended on both antigen affinity and ligand occupancy, suggesting how both parameters might be used to optimize an evolving antibody response.

  8. Who Let the Drugs Out?

    Bacteria achieve multidrug resistance partly through families of integral membrane transporters that efflux drugs. Transporters in one such family, the Major Facilitator Superfamily (MFS), export a broad spectrum of hydrophobic compounds. Now Yin et al. (p. 741) have determined a 3.5 angstrom resolution structure for EmrD, a proton-dependent MFS transporter in the inner membrane of Escherichia coli. Two long loops extend into the inner leaflet of the membrane bilayer where they likely recognize and bind substrate.

  9. Makings of a Modified Histone Binding Module

    To understand how histone modifications affect processes such as transcriptional regulation and epigenetic inheritance, we must understand how these modifications are recognized and deciphered. Huang et al. (p. 748; published online 6 April) determined crystal structures of the double tudor domain of the histone demethylase JMJD2A alone and in complex with its substrate, a methylated histone peptide. The two domains form an interdigitated structure that is required for binding to the methylated histone peptide. This reveals the potential for forming new histone binding modules from familiar effector domains.

  10. A Small, But Not So Simple, Heart


    Vertebrates hearts begin as a valveless tube that has generally been described as a peristaltic pump. Using confocal laser scanning microscopy and time-resolved three-dimensional visualization methods, Forouhar et al. (p. 751) followed the heart wall and blood cells in the zebrafish embryo. The embryonic heart tube did not show the properties expected of a peristaltic pump. Instead, elastic wave propagation and reflection in the heart tube produced an action that is more consistent with a hydroimpedance pump influenced by mechanical features of the heart tube, such as its diameter, length, and elasticity.

  11. Defining the Neural Basis of Dread

    Deciding between two choices can be difficult, particularly when they are separated in time. Economic theory accommodates the calculation by discounting the future outcome by the amount of time, most simply via a hyperbolic function. An additional factor is the cost of waiting, which can be represented clearly when the outcomes are unpleasant (electric shocks to one's foot), and the choice is between a stronger shock in a few seconds versus a weaker shock a half minute later. Many people will opt to “get it over with,” primarily, one assumes, to avoid the anticipation of future pain, which is used as an operational definition by Berns et al. (p. 754) in examining the neural basis of dread. Areas within the cortical pain matrix respond in a fashion that can be associated with the extent of dread expressed across individuals.

  12. The Grid in the Brain

    Spatial navigation depends on several brain regions that interface with the medial entorhinal cortex (MEC), whose layer II cells express an environment-independent, gridlike coordinate system. The interaction of these grid cells with other cell types in the enthorhinal cortex is not well understood. Sargolini et al. (p. 758; see the Perspective by Loewenstein) analyzed the firing properties of neurons from different layers in the most posterior portion of the medial entorhinal cortex in rats that explored a square arena. The neurons of layer II were predominately grid cells, but in deeper layers, the grid cells commingled with head-direction cells. Some cells in the deep layers signal a conjunction of both head direction and spatial grid information. Cell activity in all layers was modulated by running speed.

  13. A Stable Triangular Carbene

    Interstellar space contains numerous electronically unsaturated small molecules that would quickly decompose through collisions in denser environments. One such molecule is the triangular carbon ring cyclopropenylidene, C(CH)2, which also has a singlet ground state that is electronic aromatic. Lavallo et al. (p. 722, published online 13 April) have stabilized a derivative of this carbene compound by appending electron-rich amino groups to the carbon skeleton at room temperature. Its core structure is only slightly perturbed from the calculated geometry of the unsubstituted parent ring. Unlike earlier isolable carbenes, this compound did not require the amine donors to bond directly to the unsaturated carbene center.

  14. Deciphering Signal Recognition

    Secretory and membrane proteins contain so-called signal sequences that are recognized by the signal recognition particle (SRP) as they emerge from the ribosome. The nascent proteins are then targeted to the membrane by docking with the SRP receptor, which facilitates transfer of the ribosome to the translocon and subsequent translocation of the protein across the membrane. Halic et al. (p. 745) used cryo-electron microscopy to generate a structure at approximately 8 angstrom resolution of a docking complex consisting of an SRP-bound 80S ribosome and SRP receptor.