This Week in Science

Science  17 Feb 2012:
Vol. 335, Issue 6070, pp. 773
  1. Nanorobots Deliver


    DNA aptamers are short strands that have high binding affinity for a target protein that can be used as triggers for releasing cargo from delivery vehicles. Douglas et al. (p. 831) used this strategy to design DNA origami “nanorobots”—complex shaped structures created by manipulating a long DNA strand through binding with shorter “staple” strands—that could deliver payloads such as gold nanoparticles or fluorescently labeled antibody fragments. These nanorobots were designed to open in response to specific cell-surface proteins, releasing molecules that triggered cell signaling.

  2. Supramolecular Polymers Explained

    While polymers are constructed from chemically bonded units, supramolecular polymers arise through reversible linkages, such as hydrogen bonding and electrostatic interactions. Recent advances in the field of supramolecular polymer science have moved from a fundamental understanding of assembly properties to the introduction of functionality, in order to exploit the particular features of this class of materials. Aida et al. (p. 813) review the specific features of supramolecular polymers that can lead to applications in a variety of fields, including: materials—in which processability and self-healing properties are of interest; biomedicine—in which the concerns are dynamic functionality and biodegradability; and hierarchical assembly and electronic systems—with an interest in unidirectionality of charge flow.

  3. Life of Li

    Because lithium primarily exists in silicate minerals on continents, the variations of Li isotopes in marine sedimentary rocks reflect the degree of silicate weathering and formation of new sediments on the seafloor, which in turn are largely controlled by climate and tectonic forces over geologic time. By analyzing sedimentary cores from eight drill sites around the world's oceans, Misra and Froelich (p. 818, published online 26 January; see the cover; see the Perspective by Paytan) assembled a 68-million-year record of the Li isotope composition of seawater. The record reveals a stepwise change in the Li isotopic ratio, suggesting that several intense episodes of tectonic uplift increased continental weathering rates and delivery of sediments to the oceans. Abrupt swings in the record at specific points in Earth's history, for example, around the time when dinosaurs became extinct, imply major changes in ocean chemistry—but their mechanisms remain enigmatic.

  4. Driving Forces


    The physical properties that control the motions of Earth's tectonic plates are difficult to predict accurately on a global scale. Large-scale geodynamic models are continually becoming more powerful, but controversies remain regarding plate-driving forces. Ghosh and Holt (p. 838) present a global geodynamic model that achieves a high level of accuracy in fitting surface plate motions, plate boundary deformation, and intraplate stress fields. The model identifies regions where mantle drag drives plate motion and regions where the drag resists plate motion. Based on the relative magnitude of mantle drag forces and lithospheric buoyancies contributing to the stress field, the model also quantifies absolute viscosity—a parameter that has been notoriously difficult to predict for Earth's interior.

  5. Defective Gene Detective

    Identifying genes that give rise to diseases is one of the major goals of sequencing human genomes. However, putative loss-of-function genes, which are often some of the first identified targets of genome and exome sequencing, have often turned out to be sequencing errors rather than true genetic variants. In order to identify the true scope of loss-of-function genes within the human genome, MacArthur et al. (p. 823; see the Perspective by Quintana-Murci) extensively validated the genomes from the 1000 Genomes Project, as well as an additional European individual, and found that the average person has about 100 true loss-of-function alleles of which approximately 20 have two copies within an individual. Because many known disease-causing genes were identified in “normal” individuals, the process of clinical sequencing needs to reassess how to identify likely causative alleles.

  6. Toward Quantum Computing

    Quantum computers are expected to be able to tackle problems that would take classical computers many lifetimes to solve. Nonabelian states of matter can store quantum information in their topology, making them immune to environmental perturbations. A physical system expected to possess this unusual property is the fractional quantum Hall state at filling factor ν = 5/2. Tiemann et al. (p. 828, published online 26 January) used nuclear magnetic resonance to measure the polarization of the 5/2 state and found that it is fully polarized—a finding consistent with a nonabelian state—keeping hopes for topological fault-tolerant quantum computing alive.

  7. From Plant to Plastic

    Petroleum is primarily used as fuel, but it is also used in the production of plastics. Thus, if biomass were to replace petroleum as society's carbon feedstock, a means of deriving ethylene and propylene—the principal building blocks of today's commodity plastics— would be helpful. Well-known Fischer-Tropsch (FT) catalysts can transform gasified biomass into a range of hydrocarbon derivatives, but ethylene and propylene tend to constitute a small fraction of the overall product distribution. Torres Galvis et al. (p. 835) now demonstrate a class of iron catalysts on relatively passive supports (carbon nanofibers or α-alumina) that robustly directed the FT process toward light olefins.

  8. Plastid Origins

    The glaucophytes, represented by the alga Cyanophora paradoxa, are the putative sister group of red and green algae and plants, which together comprise the founding group of photosynthetic eukaryotes, the Plantae. In their analysis of the genome of C. paradoxa, Price et al. (p. 843; see the Perspective by Spiegel) demonstrate a unique origin for the plastid in the ancestor of this supergroup, which retains much of the ancestral diversity in genes involved in carbohydrate metabolism and fermentation, as well as in the gene content of the mitochondrial genome. Moreover, about 3.3% of nuclear genes in C. paradoxa seem to carry a signal of cyanobacterial ancestry, and key genes involved in starch biosynthesis are derived from energy parasites such as Chlamydiae. Rapid radiation, reticulate evolution via horizontal gene transfer, high rates of gene divergence, loss, and replacement, may have diffused the evolutionary signals within this supergroup, which perhaps explains previous difficulties in resolving its evolutionary history.

  9. Adapting to the Cold

    The gating of potassium channels is temperature sensitive, suggesting that these channels must adapt to function efficiently in the extreme cold. Garrett and Rosenthal (p. 848, published online 5 January; see the Perspective by Öhman) show that the coding sequences for delayed rectifier potassium channels from an Antarctic and a tropical octopus differed at only four positions and gave functionally identical channels when expressed in Xenopus oocytes. A variation in temperature responses instead came from extensive messenger RNA editing. In particular, an edit that recoded an isoleucine to a valine in the pore of the Antarctic octopus channel greatly accelerated gating kinetics.

  10. From NO to Complement

    To complete their development in the mosquito, ookinetes—reproductive stages of the malaria parasite Plasmodium—must traverse the midgut epithelium and avoid being detected and lysed by the mosquito complement system thioester-containing protein 1 (TEP1). Oliveira et al. (p. 856, published online 26 January) identified mosquito heme peroxidase (HPX2) and a reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 5 (NOX5) as key enzymes that are induced in midgut cells during ookinete invasion that, together with nitric oxide synthase, mediate protein nitration. The HPX2-NOX5 system potentiates nitric oxide toxicity and is critical for mosquitoes to mount an effective antiplasmodial response. Epithelial nitration and TEP1-mediated lysis appear to act sequentially in parasite killing, and epithelial nitration may help to promote the mosquito complement cascade.

  11. Flagellin Takes Its Toll


    The immune system recognizes bacterial infections by binding to conserved molecular fragments derived from the invading bacteria. Molecular mimics of these bacterial determinants have the potential to boost the immunogenicity of vaccines. Yoon et al. (p. 859) now report the crystal structure of the D1/D2 fragment of Salmonella flagellin, a protein critical for the motility of flagellated bacteria, with the ectodomain of zebrafish Toll-like receptor 5 (TLR5), the host receptor that binds to flagellin and signals the immune system to react. Two TLR5-flagellin heterodimers dimerized into a 2:2 tail-to-tail signaling complex. Mutational analysis and use of human TLR5 validated the signaling mechanism, which is conserved from zebrafish to humans.

  12. Faculty Fates

    Academic careers in STEM (science, technology, engineering, and math) include challenges that may differ depending on gender. Kaminski and Geisler (p. 864) analyzed the retention rates for tenure-track faculty at 14 U.S. universities in various disciplines. College catalogs, departmental Web sites, and other public sources of information were used to follow the academic careers of nearly 3000 STEM faculty. Except in mathematics faculties—where women left a couple of years earlier than men—generally, women and men were retained and promoted at the same rate.

  13. A Lipid-Sensing GPCR

    Sphingosine 1-phosphate (S1P) is a sphingolipid that binds to the G protein–coupled receptor subtype 1 (S1P1) to activate signaling pathways involved in regulation of the vascular and immune systems. Hanson et al. (p. 851) determined the crystal structure of S1PR in complex with an antagonist sphingolipid mimic. Ligand access to the receptor from the extracellular milieu is occluded, and a gap between helices I and VII may provide ligand access from within the membrane. The structural information, together with mutagenesis and structure activity relationship data, provides insight into the molecular recognition events that modulate signaling.

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