This Week in Science

Science  05 Aug 2005:
Vol. 309, Issue 5736, pp. 847
  1. Squeezed Silica


    The existence of a high-pressure form of silica with the pyrite structure has long been speculated. Kuwayama et al. (p. 923) report experimental evidence of a new high-pressure polymorph of SiO2 with a structure that matches the theoretical predictions. Although it is unlikely that this polymorph plays a role in the core of the Earth, this structure has implications for the existence of SiO2 in the deep planetary interiors of gas giants such as Uranus and Neptune.

  2. A Forensic Analysis

    The legal system frequently faces situations in which scientifically valid data would help determine the outcome of the case. Saks and Koehler (p. 892) review the state of forensic science and find it to be in transition. Some areas, such as DNA fingerprinting, are increasingly well grounded in scientific principles, whereas other areas are more subjective. The authors discuss the various sources of error and offer some proposals for improving the rigor of forensic science.

  3. Flex and Rise

    Earth models that have attempted to simulate the sea-level rise from ice sheet melting after the Last Glacial Maximum have failed to reproduce the changes recorded at the so-called “far-field” sites, such as Tahiti and the Sunda Shelf. Bassett et al. (p. 925, published online 23 June 2005) have used a model that combines a high-viscosity lower mantle and a significant contribution from the Antarctic ice sheet to meltwater formation. The reconstructed record and the data agree well, and these results also provide another line of evidence that Antarctic ice was responsible for more of the deglacial sea-level rise than was thought until recently.

  4. Softer at the Edges

    Metals become harder as grain sizes decrease, but at some point the grains become so small that the deformation mechanisms change. Nanostructured ceramics also show enhanced properties relative to their coarser-grained counterparts, but do similar changes in deformation mechanisms occur in these more brittle materials? Szlufarska et al. (p. 911) show that these ceramics can be thought of as composites of hard nanoscale grains bounded by softer, amorphous-like grain boundaries. A massive molecular dynamics simulation shows that nanoindentation of a nanostructured silicon carbide goes through four deformation regimes. The deformation changes from cooperative grain sliding to a process dominated by amorphization of the crystalline grains.

  5. Patterns of Stress


    During the fabrication of nanoparticles consisting of a silver core surrounded by a silica shell, Li et al. (p. 909) found that controlling the cooling rate could induce stresses in the silica that cause it to form a dimpled pattern on the core sphere. The silica bumps take on either a triangular or Fibonacci sequence pattern that minimizes the total strain energy. These patterns are highly reminiscent of those seen in the development of flowers and plants.

  6. A Little Light Work

    Light-driven structural changes in proteins that are required for function are likely the result of photoexcitation processes redistributing charges. However, measuring changes in charge distribution on the time scale of the structural changes is challenging. Schenkl et al. (p. 917) have used Trp residues that are close to the retinal-binding pocket in bacteriorhodopsin to probe electric field changes. From the observed changes in Trp absorbance, they calculate that the retinal dipole moment increases during the first 200 femtoseconds after excitation. This change in charge distribution precedes, and likely drives, isomerization.

  7. Eukaryotic Potassium Channel Structure

    Voltage-gated K+ channels open in response to cell depolarization, reacting to the change in potential by movement of four charged Arg residues, which opens the pore and allows only K+ ions to exit the cell. X-ray crystallographic structures of bacterial channels have revealed the basis of the K+ selectivity. Forming crystals of the larger, multisubunit eukaryotic K+ channels has been more challenging, but Long et al. (pp. 897 and 903, published online 7 July 2005; see the cover and the news story by Service) now present in two papers a 2.9-angstrom-resolution crystal structure and a mechanistic analysis for eukaryotic Kv1.2 channels from the Shaker family. The crystals, which were formed by adding lipids during crystallization, include the oxido-reductase β subunit and are probably in a native, open state. The β subunits are positioned directly below the intracellular opening to the pore but far enough away to allow the K+ ions access to the pore through four large side portals. The voltage-sensor domains act as almost independent regions positioned within the membrane beside the cylindrical pore, with at least one of the charge-sensing arginines in direct contact with lipid. Movement of the voltage sensor causes pore opening through the S4-S5 linker helix, which constricts and dilates the S6 “inner” helices around the pore. This structure explains many apparently contradictory results reported to date on K+ channel structure and function.

  8. Avoiding Too Much of a Good Thing

    Certain plants carry resistance (R) genes variants that match a particular pathogen's virulence factor. However, too much or too little of the R protein can send the plant's immune response haywire. Holt et al. (p. 929, published online 23 June 2005) now provide a genetic analysis of some of the factors that keep the immune response in Arabidopsis primed for a rapid deployment but not running rampant. One component, RAR1, somehow promotes the accumulation of the R proteins, and another, SGT1, interacts with RAR1, antagonizing its activity. SGT1 does double duty in infected plants by regulating the cell death response that limits the damage done by some pathogens.

  9. Differentiation on the Rac


    Rac1, a member of the Rho family of guanosine triphosphatases (GTPases), is a pleiotropic regulator of many cellular processes, including the cell cycle, cell-cell adhesion, and motility, as well as a key regulator of epithelial differentiation. Aznar Benitah et al. (p. 933; see the Perspective by Dotto and Cotsarelis) show that Rac1 is expressed in the pro-liferative compartment of mammalian epidermis. In mice, conditional deletion of Rac1 produces a rapid transient proliferation of cells, followed by the depletion of epidermal stem cells and by a corresponding increase in cell differentiation. For its effect on the stem cell compartment, Rac1 acts through negative regulation of c-Myc. Thus, as Rac1 is down-regulated, cells can no longer adhere tightly to the substratum, which leads to an inefficient relay of signals from the stem cell niche and subsequent cell differentiation.

  10. Sweet Relations

    Bacteria can glycosylate proteins, but the mechanisms and spatial localization of glycosylation are poorly understood relative to those of eukaryotes. VanderVen et al. (p. 941) now describe a direct link between prokaryote protein glycosylation and the Sec translocation system, the primary protein export mechanism in bacteria. The association of protein O-mannosylation and Sec-translocation along with other known aspects of protein glycosylation in Mycobacterium tuberculosis present parallels with the O-mannosylation system of eukaryotes, in particular the well-studied protein mannosyltransferase system found in budding yeast. Thus, primitive prokaryotes have systems for O-protein glycosylation that are analogous to those present in eukaryotes.

  11. The Ultimate Glucose Monitor

    The brain, and in particular, the hypothalamus, controls liver glucose production, but the cellular and molecular mechanisms by which the brain senses glucose levels have been unclear. Lam et al. (p. 943) now show that, in rats, this process requires the conversion of glucose in the hypothalamus to lactate, which in turn stimulates pyruvate metabolism and adenosine triphosphate (ATP) production. Alterations in ATP levels control neuronal excitability through effects on ATP-sensitive potassium channels, which have been implicated in glucose output by the liver.

  12. Neuronal Oscillations and Brain Imaging

    Brain-imaging methods detect neuronal activity indirectly by measuring blood oxygenation level-dependent (BOLD) signals. Niessing et al. (p. 948) investigated the hemodynamic responses recorded by optical imaging and compared them to neuronal activity recorded with microelectrodes in anesthetized cats. The BOLD response was well correlated with the gamma-frequency components of the local field potential, but only weakly correlated with firing rate. Mukamel et al. (p. 951) compared electrophysiological measurements obtained from the auditory cortex of neurosurgical patients with functional magnetic resonance imaging (fMRI) signals obtained from the conscious human brains under identical sensory stimulation. A long-lasting coupling was observed between fMRI measurements and single unit activity. Thus, the fMRI signal reflects the firing rate of human cortical neurons during complex natural stimulation.

  13. Electrons Surf After All

    Charged water clusters have been used as experimental models to study bulk hydrated electrons, which are implicated in radiation damage and other reductive processes. However, researchers have wondered whether the clusters bind excess electrons at the surface or encapsulate them in a cavity more analogous to the bulk structure. Turi et al. (p. 914) have simulated anionic clusters ranging in size from 20 to 200 water molecules, with the water treated classically and the excess electron quantum mechanically. They find that below 200 water molecules, the surface-bound state predominates, and the calculated variations of the electron's absorption spectrum, kinetic energy, and radius with cluster size match those observed experimentally. It appears, therefore, that the experimental small cluster studies have involved surface-bound electrons.

  14. The Slightest Nudge

    When particles are randomly added to matrix, a concentration is eventually reached where a percolation network forms. For a conductive material, for example, the formation of a percolation network would manifest as a sudden change in the electrical resistivity. Not surprisingly, elongated or rod-like particles can have a lower percolation threshold than spherical ones. Vigolo et al. (p. 920) studied aqueous carbon nanotube dispersions that were charge stabilized by ionic surfactant molecules adsorbed onto their surface. The percolation threshold decreased considerably even for very weak rod-to-rod interactions.

  15. Regulatory Network Aerobics

    Most yeast species are strict aerobes, but Saccharomyces cerevisiae prefers to grow anaerobically. Ihmels et al. (p. 938) show that this difference can be seen at the level of transcriptional regulation. Levels of expression of genes encoding mitochondrial ribosomal proteins (MRP) in Candida albicans (an aerobe) were strongly correlated with genes encoding cytoplasmic ribosomal proteins. This situation is in contrast to what happens in S. cerevisiae and could be due to a motif (AATTTT) found only in the promoter of C. albicans MRP genes. This motif is overrepresented in all genomes that diverged from the S. cerevisiae lineage prior to the whole-genome duplication event and correspond to species that only grow aerobically.

  16. Structural Insights into the Carboxysome

    The carboxysome is a bacterial microcompartment made up of a protein shell that sequesters enzymes involved in carbon fixation reactions. Kerfield et al. (p. 936) have determined the crystal structures of two carboxysome shell proteins and show that, similar to some viral capsids, a hexameric unit is the basic building block of the polyhedral shell. Hexamer sheets suggest how the building blocks may assemble. Central pores and gaps between hexamers are positively charged and may be involved in regulating molecular transport across the shell.