Editors' Choice

Science  25 Feb 2011:
Vol. 331, Issue 6020, pp. 988
  1. Policy

    Learning to Share Space

    1. Brad Wible
    CREDIT: ESA

    Though the Cold War inspired nightmares of nuclear incineration, it also spawned treaties that kept space from becoming a lawless, weaponized frontier. As more nations and commercial enterprises launch more crafts toward the heavens, though, new problems emerge, such as orbital debris, satellite crowding and collisions, and radiofrequency spectrum saturation. To maintain security, Robinson argues that we need renewed focus on transparency and confidence-building among spacefaring nations and that we must move beyond the long-standing, dominant, U.S.-Russian bilateral frameworks. Sharing of critical information with other nations and the public, engaging in consultative dialogues, and allowing monitors to verify compliance are among measures often used in promoting security. Some believe such efforts could be best updated by focusing on amending the 1967 U.N. Outer Space Treaty, whereas others promote newer movements, such as the Inter-Agency Space Debris Coordination Committee. The author proposes the E.U. Draft Code of Conduct as a promising alternative, deliberately structured outside existing multilateral institutions, to engage a broad range of nations in an ongoing process toward establishing standards and best practices.

    Space Policy 27, 10.1016/j.spacepol.2010.12.018 (2011).

  2. Microbiology

    Viral Escape Route

    1. Gilbert Chin
    CREDIT: TESSA QUAX AND DAVID PRANGISHVILI

    Archaea are generally less familiar to us and less well-studied than bacteria or eukaryotes, and the same is true for their viruses. Quax et al. report that the Sulfolobus islandicus rod-shaped virus 2 (SIRV2) employs an unusual means of escaping from its hyperthermophilic host. Using structural and biochemical analyses, these authors find that a pyramidal assembly—a heptamer of the 10-kilodalton viral protein P98—forms on the cell surface of the host cell. Each face of the pyramid is an isosceles triangle, whose base is roughly 90 nm wide and whose sides are 150 nm long. In the lytic phase of viral growth, the pyramid opens like the petals on a flower (shown above) to release the new wave of virions. Remarkably, expression of P98 in the bacterium Escherichia coli was sufficient to induce the formation of similar structures protruding from the inner membrane into the periplasmic space, although only closed pyramids were observed.

    Proc. Natl. Acad. Sci. U.S.A. 108, 10.1073/pnas.1018052108 (2011).

  3. Chemistry

    Capturing Paraquats

    1. Phil Szuromi

    Supramolecular polymers, in which the monomer units are bound by weak interactions, could have several potential advantages over covalently bonded chains. For example, their extent of association could be temperature-dependent, so they could have lower viscosities during processing steps and assemble longer chains after cooling. However, supramolecular polymers tend to be short oligomers with only about 10 repeating units. Niu et al. report on a supramolecular polymer made from two monomers, one a conjugated chain bearing 32-crown-10 cryptand cages at both ends, and the other bearing complementary paraquat rods at both ends. When these were mixed in organic solvent, the solution turned deep yellow, reflecting charge-transfer interactions between the different monomer units. Proton nuclear magnetic resonance spectra signaled the formation of longer pseudorotaxane polymers. An analysis indicated that at concentrations of ∼0.3 molar, the polymer chains had over 40 subunits.

    J. Am. Chem. Soc. 133, 10.1021/ja110384v (2011).

  4. Biomedicine

    Interfer(on)-ing in Melanoma

    1. Kristen L. Mueller

    Malignant melanoma is a particularly deadly form of skin cancer, for which the major risk factor is exposure to UV solar radiation, especially at a young age. How UV exposure eventually leads to melanoma is not well understood. Using a mouse model of UV light–induced melanoma where melanocytes can be visualized fluorescently, Zaidi et al. find that the cytokine interferon-γ (IFN-γ) is an important driver of tumorigenesis. Gene expression analysis of melanocytes from neonatal mice recently exposed to UV light revealed an IFN gene signature.

    After UV exposure, macrophages entered the exposed skin and produced IFN-γ. When these macrophages were isolated and mixed with melanoma cells and then implanted into mice, tumors mixed with macrophages grew faster and exhibited less cell death than did tumor cells implanted without macrophages. These results may be of clinical relevance, because macrophages that expressed IFN-γ were found in tissues from melanoma patients. Melanomas, therefore, may arise in part because of an ineffective immune response to damaged tissue. After UV exposure, the immune system most likely eliminates many of the damaged cells; however, the remaining cells perhaps contain mutations that allow for immune evasion and over time give rise to melanomas.

    Nature 469, 548 (2011).

  5. Cell Signaling

    Receptor-Free Signaling

    1. L. Bryan Ray

    Heterotrimeric guanine nucleotide–binding proteins (G proteins) are central regulators of many physiological processes in mammals and are thus prime targets for the development of therapeutics. In animals, G proteins are activated through an interaction with G protein–coupled receptors (GPCRs). GPCRs trigger the exchange of guanosine diphosphate for guanosine triphosphate, which activates the G protein alpha subunit and frees it from the receptor complex.

    Arabidopsis thaliana plants have a G protein alpha subunit (AtGPA1) but do not have GPCRs. Instead, guanine nucleotide exchange by AtGPA1 is constitutively active. Jones et al. used x-ray crystallography, molecular dynamics simulations, and biochemical analyses to explore the structural basis for this unusual property. Their results showed the presence of a helical domain in AtGPA1 that is more disordered and dynamic than its mammalian counterpart. In mammals, the stability of interactions between the helical domain and the catalytic domain prevents nucleotide exchange; however, the plant protein has no such restriction. A mammalian G alpha protein lost its stability—and nucleotide exchange increased by over 150-fold—when the helical domain from the plant protein was swapped in. Understanding the properties of AtGPA1 may help unravel the mechanisms controlling receptor-activated G proteins in mammals.

    Sci. Signal. 4, ra8 (2011).

  6. Geophysics

    Fields of Old

    1. Brooks Hanson
    CREDIT: M. DE WIT

    Earth's magnetic field is sustained by fluid motion in the planet's liquid iron outer core, spurred today by latent heat released as the solid inner core grows. The timing of the field's appearance has been uncertain, as back calculations imply that close to 2 billion years may have been needed for Earth's internal temperatures to cool sufficiently to allow a solid core to form. Biggin et al. report paleomagnetic measurements, and improved new dates, from rocks in South Africa that provide further evidence that Earth had a stable magnetic field by 3.5 billion years ago. As a check that the original magnetic signature had not been reset, the authors showed that cobbles in a nearby conglomerate preserved random magnetic orientations. These data thus further strengthen the inference that early Earth had a field and that some other process aside from solidification of the inner core was driving the geodynamo then. The data further hint that the early field was capable of reversing.

    Earth Planet. Sci. Lett. 302, 314 (2011).

  7. Materials Science

    Grow with the Flow

    1. Marc S. Lavine

    Formation of the mineral component of bone occurs through an amorphous precursor, which is stabilized by a number of proteins. Maas et al. show that they can form collagen fibrils that incorporate calcium phosphate directly. They flowed an acidic feed solution containing calcium cations and monomolecular tropocollagen through a polycarbonate track-etched membrane into a basic solution containing phosphate anions. The change in acidity caused the tropocollagen to self-assemble into triple helix collagen fibrils. At the same time, the combination of the calcium and phosphate ions led to the formation of amorphous CaP, which was incorporated into the inside of the fibrils and could form overgrowths on the outside of the fibrils at higher concentrations. The authors seeded the mineralized fibrils with human adipose–derived stem cells and found that the inclusion of calcium phosphate enhanced cell proliferation. These cells also showed an increase in alkaline phosphatase activity, which is an early indicator of bone cell differentiation, suggesting that this mineralized collagen could be used for in situ bone healing.

    Nano Lett. 11, 10.1021/nl200116d (2011).

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