This Week in Science

Science  24 Jul 1998:
Vol. 281, Issue 5376, pp. 485
  1. Minute Bearing

    One route to the miniaturization of mechanical devices is the use of single molecules for performing the various tasks required in the device, in a manner analogous to biological molecular motors. Gimzewski et al. (p. 531; see the cover) have designed propeller-shaped molecules 1.5 nanometers in diameter and show that a scanning tunneling microscope tip can be used to switch the molecules between two nearby but different sites within an incomplete monolayer on a surface. In one site, the molecule is immobilized, whereas in the other the molecule rotates too fast to be clocked experimentally. The surrounding molecules thus constitute a bearing for a single rotating molecule. The rotation appears to be wearless and can be stopped very quickly because of the small inertia of the rotor.

  2. Kondo Effect Tuning in Quantum Dots

    Recent studies have presented evidence for the Kondo effect, the coupling of magnetic impurities to conduction electrons observed in bulk semiconductors, in quantum dots. Cronenwett et al. (p. 540; see the Perspective by Inoshita) show that quantum dots can be converted from a Kondo to a non-Kondo state by changing the number of electrons on the dot from odd to even, that is, from a magnetic state with one unpaired spin to a nonmagnetic state with all paired spins. The Kondo temperature could also be tuned by means of a gate voltage, and the observed magnetic field and temperature dependence of the effect was in good agreement with theoretical predictions.

  3. Fast Route to Macroporous Materials

    Macroporous materials (pore sizes greater than 25 nanometers) have been synthesized in a single fast step using polystyrene spheres as templates. Holland et al. (p. 538) formed millimeter-thick films of ∼0.5-micrometer spheres by vacuum filtration and then covered them with solutions of metal alkoxides; heating to 575°C created microcrystalline networks of titania, zirconia, or alumina containing close-packed spherical voids approximately 320 to 360 nm in diameter.

  4. Closing in on Kinase Inhibitors

    Selective kinase inhibitors are useful for analyzing signaling pathways and have potential therapeutic value. Gray et al. (p. 533) describe an approach for developing new inhibitors based on the purine olomoucine, which binds in an unusual fashion to the adenosine triphosphate site of cyclin-dependent kinases (CDKs). Screening of chemical libraries that modified the 2, 6, and 9 ring positions led to the identification of several strong CDK2 inhibitors, including purvalanol B, which is 30 times more potent than flavopiridol, an inhibitor in clinical trails. Structural studies suggest further synthetic targets, and cellular effects of these inhibitors were investigated by screening yeast genome messenger RNA expression well as in 60 human tumor cell lines.

  5. A Hasty Retreat

    The Pine Island Glacier in West Antarctica has been thought to be particularly susceptible to retreat because its bed is below sea level and it is not constrained by a large ice shelf, as is much of the West Antarctic Ice Sheet. Rignot (p. 549; see the news story by Kerr) presents satellite radar measurements showing that the hinge-line position of the Pine Island Glacier (marking the limit of tidal flexing of the glacier) retreated rapidly, by more than 1 kilometer per year, between 1992 to 1996 (the period of data coverage). The retreat is likely caused by an influx of warm seawater that enhanced melting at the base of the glacier.

  6. Early Distance Cues

    The early stages of cortical visual processing focus on features of visual stimuli, such as orientation and color, and the later stages begin to provide information about spatial relations of objects in the visual field and object identity. Dobbins et al. (p. 552; see the news story by Barinaga) uncover a surprising influence of viewing distance (that is, the spatial relation of objects to the observer's body) on the activity of neurons in V1, the earliest station in the visual cortex. Although it is known that the combination of input from the two eyes in the early stages allows for processing of binocular disparity, the effect of object distance remains even with monocular viewing, indicating that monocular depth or scene cues may be important. An additional possibility is that neurons may be tuned to respond preferentially to objects on the basis of their perceived distance within a scene.

  7. Unraveling Immune Receptor Responses

    How cells of the immune system can respond to ligands with widely varying affinities is the subject of two reports (see the Perspective by Malissen). Multichain antigen receptors must recruit kinases to their complex so that they can initiate signals to the cell when they bind their ligands. Low-affinity ligands can disrupt the signaling of high-affinity ligands, but the mechanism is not clear. With the use of the Fc receptor system, Torigoe et al. (p. 568) determined that this inhibition need not be direct competition for the same receptor. An excess of a non-crossreacting low-affinity ligand clustered receptors together to the exclusion of the receptors clustered by high-affinity ligand. This process efficiently horded the associated kinases, such that the more distal signaling events normally initiated by high-affinity ligand failed to proceed. This “selfish” seizing of resources that it cannot utilize, like Aesop's “Dog in the Manger,” may also help to explain the response to T cell receptors to antigens of different affinities. The ab antigen receptor on T cells contains the two antigen-binding chains and additional γ, δ, ϵ, and ζ subunits. The ζ subunit has six tyrosine phosphorylation sites. Because multiple forms of phosphorylated ζ can be detected, but could not previously be examined individually, the connection between various phosphorylation and activation states has not been clear. Neumeister Kersh et al. (p. 572) have generated antisera to each phosphorylation site and found that the tyrosines are phosphorylated in a particular order, with the phosphorylation of some sites dependent on the previous phosphorylation of others. Peptide antigens that do not fully activate T cells are not able to complete the series of ordered phosphorylations. This process may be the root of the signaling differences that lead to functional differences in T cells presented with altered peptide ligands.

  8. Putting Salmonella to Work

    To facilitate the production of oral vaccines, attenuated versions of the intracellular bacteria Salmonella typhimurium have been developed as “carriers” of the genes or genomes from other pathogenic organisms. Rüssmann et al. (p. 565) use the bacterium's own specialized secretion system, the type III system, to generate cytotoxic T cells that protect against an otherwise lethal viral infection in mice. The type III secretion system ensures that the “hitchhiking” viral proteins get injected into the cytoplasm of the host cell. The proteins are then processed and transported to the cell surface as peptides bound to class I major histocompatibility molecules, where they stimulate a cellular immune response. Salmonella now has the potential to be an easy-to-administer vaccine for generating cytotoxic protection.

  9. Beetle-Mania

    Insects of the order Coleoptera, beetles, are the most diverse of all animal and plant groups. Seeking an explanation for this diversity, Farrell (p. 555; see the news story by Morell) constructed the diversification history of a large clade of plant-eating beetles, the Phytophaga, using a combination of DNA sequences and morphological characters. This procedure identified a series of origins for angiosperm feeding, each of which was associated with dramatically enhanced beetle diversity. Such radiations support a coevolutionary model in which a proliferation of beetle life-history traits accompanies the development of flowering plant diversity.

  10. Viral Interference with Signaling

    Many genes in viruses are thought to have originated in their host's genomes, but viruses then turn their use around to further the propagation of the virus, often at the expense of the host. Miskin et al. (p. 562) have determined that a protein from African swine fever virus, A238L, can inhibit signaling pathways that are critical for the production of immunomodulatory cytokines by binding to the catalytic subunit of calcineurin. This finding implies that mammalian cells may contain homologous genes for proteins that would regulate cytokine production during immune responses.

  11. Working Fractions

    Typically, isotopic fractionation in nature is mass dependent—for example, the difference in the isotopic composition of gaseous and liquid water or of water and oxygen in a reaction would be governed by the relative mass difference between the isotopes involved. However, a number of important atmospheric trace gases show a mass-independent isotopic fractionation in oxygen isotopes, and this effect, if it were understood, could be used to follow the sources and reactions involving these trace gases. Röckmann et al. provide evidence that atmospheric carbon monoxide, an important oxidant in the atmosphere, undergoes mass-independent isotope fractionation. Experimental measurements suggest that the origin of the effect is through its reaction with another important trace gas, hydroxyl.

  12. Caught by the Snail

    Marine snails of the Conus family are predatory animals that immobilize and catch their prey by deadly poisons. These venoms are composed of substances with high specificity for certain ligand-gated or voltage-gated ion channels. After their discovery they soon became invaluable tools for the classification and analysis of many receptors. However, one receptor family, the serotonin receptors, seemed to escape the poisonous snail attack. Now England et al. provide evidence that serotonin receptors can also be targets for Conus toxins. They describe a new snail toxin that selectively binds to 5-HT3 receptors, a subtype of serotonin receptors. This finding should increase understanding of this important group of ion channels and may lead to the development of new and specific drugs to treat nervous system disorders that are caused by malfunctioning serotonin receptors.

  13. Laying down Tracks

    Like the interurban express, neuronal connections in the developing brain are made by axons crossing intervening areas to reach their chosen targets. Analyzing thalamocortical connections, Catalano and Shatz observed that growing axons send temporary branches to the subplate, a region that these axons pass en route. Normally, these connections are withdrawn in favor of connections with the final target. This pruning process is dependent on synaptic activity.

  14. Hot Spots over Slow Waves

    Convection in the mantle is a manifestation of the cooling of the Earth and drives plate tectonics, but even the basic geometry of the overall convection pattern is uncertain at depth. In one model, hot buoyant plumes are thought to rise from near the base of the mantle and produce volcanic hot spots on the surface, such as Hawaii. Williams et al. examined the spatial relation between surface hot spots and presumed hot or partially molten regions at the base of the mantle, as indicated by low seismic velocities (this region of the mantle is still not completely mapped by seismic waves). Most of the surface hot spots overlie regions of low seismic velocity near the mantle base, suggesting a plume connection.

  15. Beta-Chemokine MDC and HIV-1 Infection

    R. Pal et al. (Reports, 24 Oct., p. 695) found that macrophage-derived chemokine (MDC) “suppresses infection” of blood cells by some strains of the human immunodeficiency virus-type 1 (HIV-1). They suggested that β-chemokines could be “responsible for a major proportion of HIV-1-specific suppressor activity produced by primary T cells.”

    B. Lee et al. comment that, in their experiments, “even high concentrations of [recombinant] MDC did not inhibit productive infection of peripheral blood mononuclear cells, or macrophages by … HIV-1 strains.” F. Arenzana-Seisdedos et al. also found that MDC did not show “suppressive activity against [coreceptors] CCR5 or CXCR4-dependent HIV strains.”

    In response, A. L. DeVico et al. state that, in more recent experiments, their “findings are in agreement with those of the other groups.” But they ask “which molecule in the native MDC preparations is responsible for the antiviral effect” observed? They discuss several possibilities—including a contaminant, “a cryptic factor,” or “other isoforms” of MDC—that could be explored. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/281/5376/487a

Log in to view full text

Via your Institution

Log in through your institution

Log in through your institution