This Week in Science

Science  11 Jul 2008:
Vol. 321, Issue 5886, pp. 172
  1. Making Avalanches


    Slab avalanches arise when weather changes cause a layer of low-density snow or ice to form between the base snow and a deep cover of snow. Such a layer can collapse under the weight of the snow cover, leading to a “wumph.” However, under other conditions, the snow cover can slide off in a slab, producing a slab avalanche. It has been thought that the angle of the slope determined whether a wumph or an avalanche would occur. Heierli et al. (p. 240) analyze the snow fracture process using the concept of anticracks, where material displaces in an opposite manner to normal crack propagation, leading to a loss of cohesion and local density changes. The results of the analysis suggest that the angle of the slope is not the only factor in determining the avalanche type. Long-distance fracturing may be a cause for remote triggering of some avalanches.

  2. Enzymes at Work

    Recent technological advances in structural biology are now furnishing our first glimpses of the proteins that reside and work at the hydrophobic-hydrophilic boundaries provided by the lipid bilayers that constitute cellular membranes. Forneris and Mattevi (p. 213) review the sparse collection of membrane-associated enzymes whose crystal structures have been determined and classify them within a function-based framework that highlights how a diverse group of proteins have arrived at common strategies to cope with the challenges they face—of carrying out hydrolysis reactions within the interior of the membrane or of mediating the transfer of a hydrophobic substrate from the membrane into the enzyme's active site.

  3. The Makings of Autism

    Autism is a common developmental disorder that impairs the acquisition of social skills and communication in children. The underlying causes of autism are unclear, but are likely to involve diverse and complex genetic factors. Morrow et al. (p. 218, cover; see the Perspective by Sutcliffe) have now used homozygosity mapping to identify genetic loci that correlate with susceptibility to autism. Results from a population enriched in consanguineous marriages highlighted the importance of autosomal recessive genes. The findings suggest that defects in activity-regulated gene expression may link many seemingly diverse autism mutations.

  4. Going Supernova

    Supernovae are massive stellar explosions that occur when a star runs out of nuclear fuel and collapses. Most are detected after the explosion has begun, which limits the amount of information available on how the explosion began and on the passage of the shock wave through the outer stellar material. Schawinski et al. (p. 223, published online 12 June) exploited an optical survey aimed at finding supernovae to search for ultraviolet light that preceded the optical light curve from the same star. For one recent supernova, a strong signal was observed 2 weeks before the optical detection. The data reveal the explosion just as it passes the shock wave at the star's surface and suggest that the supernova formed from a large red giant precursor star.

  5. Excitonic Logic


    In contrast to electrons, excitons (coupled electrons and holes held together by the Coulomb attraction) are optically active, which allows direct coupling between light and electronics. Compared to direct excitons that occur in bulk semiconductors, indirect excitons that are formed in coupled quantum well structures have longer lifetimes and can be spatially manipulated. High et al. (p. 229, published online 19 June) now show that “excitonic circuits” that exploit indirect excitons can be formed using patterned electrical contacts and that these circuits can perform simple logic processes using optical input and output signals.

  6. Improving Solar Cells

    Solar cells could generate more electricity if sunlight were to be collected over a wider area and concentrated on the cell. The use of mirrors can provide one solution, but can require tracking and lead to unwanted heating by also concentrating infrared radiation. An alternative approach is to use luminescent solar concentrators, waveguides that contain dye molecules that absorb light and then reemit at wavelengths that avoid reabsorption. In practice, self-absorption losses can be high. Now Currie et al. (p. 226) have created organic luminescent concentrators that avoid many of these losses by taking advantage of Förster energy transfer to lower dye concentrations and phosphorescent dyes that absorb more weakly in their emissive state.

  7. The Geography of Rodinia

    250 million years ago, all of the continents were assembled into a large supercontinent, Pangea. Its breakup had large effects on the geological evolution of the various fragments and affected evolution and Earth's climate. About 1 billion years ago, all the continents were also thought to be assembled into a supercontinent, Rodinia. The breakup of Rodinia controlled the geological and evolutionary history of the Paleozoic. The geography of Rodinia, and particularly connections across the western margin of North America, has been uncertain and widely debated. Goodge et al. (p. 235) present geochemical data on detritus in the Transantarctic Mountains and identify a rock clast there, which provides a link to a set of distinctive granitic rocks exposed in North America. The data support a model in which Antarctica (to the south) and Australia were juxtaposed along western North America.

  8. Enough's Enough

    Achieving the efficient and specific uptake of extracellular molecules is only half the battle for any cell—there also needs to be a regulatory system that halts excessive accumulation. The bacterial membrane transporter ModBC hydrolyzes ATP to bring the oxyanions, molybdate and tungstate, into the cytoplasm. Gerber et al. (p. 246, published online 29 May) now show that a regulatory domain at the C-terminal end of the ModC subunit binds molybdate and in doing so blocks ATP hydrolysis by the rest of the ModC subunit. Within the crystal structure of the molybdate-inhibited ModBC complex, two molybdate ions are located at the interface between the two regulatory domains. Because the ATP-binding site is formed by the interface between these domains, the molybdate-bound ModBC cannot bind ATP.

  9. Adapting from Co- to Posttranslational Membrane Targeting


    Most membrane proteins are cotranslationally inserted into membranes by the Signal Recognition Particle (SRP) that has a universally conserved ribonucleoprotein core. However, the most abundant family of membrane proteins, light harvesting chlorophyll a/b-binding proteins (LHCPs), are imported into the chloroplast (cp) and then must be targeted to the thylakoid membrane by cpSRP. The latter contains no RNA, but is a heterodimer between cpSRP54, the chloroplast homolog of the SRP core protein, and cpSRP43, a protein dedicated to this targeting complex. Now Stengel et al. (p. 253) report the high-resolution structure of cpSRP43 alone and in complex with an internal signal peptide from LHCP. The overall shape and charge distribution of cpSRP43 resembles SRP RNA. In addition, SRP43 recognizes a specific motif in the LHCP peptide to adapt the conserved SRP system to achieve posttranslational targeting of LHCP proteins.

  10. Know Thyself

    Cells of the pathogenic bacterium Proteus mirabilis clump together to form colonies that, on meeting other colonies, can distinguish self from foreigners. Proteus detects then deters interlopers of the same species using polypeptide weapons called proticines. Gibbs et al. (p. 256) investigated the genetic basis of this self-nonself recognition system in Proteus and discovered a locus of six genes that seem to constitute general and specific recognition determinants, as well as accessory genes. Within infected hosts, P. mirabilis infections are usually clonal, and so this recognition system could be a way for an established clone to prevent superinfection by another clone and to avoid competition.

  11. Resist or Persist

    Many of the key immune pathways initiated in response to infection begin with the activation of the transcription factor NF-κb. Thus, it would not be unreasonable to expect that at least some pathogens would have evolved the means by which to impede this pathway. Kravchenko et al. (p. 259, published online 19 June) demonstrate that the opportunistic bacteria Pseudomonas aeruginosa produces a small signaling protein, C12, which impairs the turnover of proteins critical to the regulation of NF-κb activity. The resulting reduction in transcription of key immune response genes could have significant influence on persistent infection by this and other bacteria.

  12. Exploiting Happy Coincidences

    Treatment of human patients with therapeutic drugs and the annotation of side effects provide a large body of information about how these chemical agents affect human physiology. Campillos et al. (p. 263) analyzed data on more than 700 drugs currently on the market to determine whether shared side effects might be a useful predictor of drugs that, though chemically dissimilar, might share a similar mechanism of action on a target protein. Indeed, 13 of 20 pairs of drugs predicted to share a target were confirmed to show in vitro binding to a target protein. Although further analysis will be necessary, it seems existing drugs that have already been established to be safe in humans should possibly be able to be used for additional therapeutic purposes.

  13. Voiding Vibrations

    Laser cooling techniques can slow samples of atoms down to a near standstill, opening the door to wide-ranging manipulations of coherent matter. Applying the same techniques to molecules has proved more challenging because molecules tend to retain a significant amount of rotational and vibrational energy despite efficient translational cooling. Viteau et al. (p. 232) describe a method involving electronic excitation that allows the fast and efficient accumulation of Cs2 molecules in the lowest vibrational level.

  14. Gentle Dissection

    The most common uses of mass spectrometry rely upon analyzing the original components in a sample by uniquely identifying fragments of the components and then reconstructing the original molecules on the basis of well-established fragmentation behaviors. Barrera et al. (p. 243, published online 12 June) describe a remarkable step in the other direction, surpassing previous mass spectrometry studies of ribosomes and viruses, by documenting the analysis of a membrane protein complex embedded within a detergent micelle. The conditions are gentle enough that the complex (the vitamin B12 membrane transporter) retains the capacity to bind ATP specifically.

  15. Position Matters

    In product inhibition, a build-up of the product of an enzyme-catalyzed reaction competes with the substrate for binding at the active site, thereby reducing the forward flux through the pathway. But how could this work when the product and the substrate are the same species, differing only in their location like when nutrients are taken into a cell? Kadaba et al. (p. 250) have determined the crystal structure of one such system, the bacterial methionine importer MetNI, which is a member of the large family of ATP-binding cassette transporter proteins. A methionine-binding site located within a cytoplasmic domain of MetN adopts a conformation that blocks ATP hydrolysis, which in turn blocks movement of methionine through the transmembrane component MetI.

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