This Week in Science

Science  03 Apr 1998:
Vol. 280, Issue 5360, pp. 9
  1. Inside view of a potassium channel

    Ion channels are protein gates that permit certain ions rapid passage into and out of cells. Our understanding of their structure, which has been frustratingly indirect, has been largely derived from mutagenesis studies. Doyle et al. (p. 69; see the cover) now present the x-ray structure of a potassium channel from the bacteria Streptomyces lividans, which reveals an architecture of the pore itself that can be applied to all potassium channels (see the commentary by Armstrong, p. 56). A ring of main chain carbonyl oxygen atoms allows the passage of potassium but not sodium ions through the pore. An enlarged chamber within the pore at the center of the membrane bilayer plus helix dipoles allow the ions to easily traverse the electrostatic forces in the bilayer. Localization of two potassium ions within the selectivity filter indicates that the pore promotes rapid throughput of many ions by exploiting interion repulsive forces to overcome the attraction between potassium and the selectivity filter. MacKinnon et al. (p. 106) report that a toxin from scorpion venom (agitoxin2) that binds to the pore vestibule of the Streptomyces channel was shown by modeling studies to also bind the similar eukaryotic channels, verifying that the prokaryotic potassium channel has the same pore structure as eukaryotic channels.

  2. Mechanical imaging with ultrasound

    The mechanical properties of an object can be determined by its vibrational frequency, which is normally in the acoustic range. Fatemi and Greenleaf (p. 82) have developed an ultrasound technique in which beams at two slightly different frequencies are focused to a point and scanned over the object. The interference of the beams produces a sinusoidal variation of the ultrasound energy, effectively vibrating the object. The object's vibrational response can then be recorded acoustically and converted into an image. The method can distinguish between identical materials with different mechanical properties (tuning forks of different pitches) and detect small hard objects in a soft background (glass beads in latex). Images of excised arteries reveal calcified versus noncalcified regions.

  3. Ancient pollinators

    The evolution of angiosperms (flowering plants) corresponded with the evolution of pollinating insects, as discussed in a research commentary by Labandeira (p. 57). Angiosperm fossils are first seen in the fossil record perhaps in the Early Cretaceous (about 120 million years ago), although their exact origin is debated. Some of the earliest insect pollinators were flies, wasps, and beetles. Ren (p. 85) describes fossils of several Brachycera flies preserved in rocks that may be as old as Late Jurassic (about 145 million years ago) in age from the Yixian Formation, China. The flies have a long mouthpart designed for extracting nectar, perhaps from flowers.

  4. A ring around Jupiter

    The dust detector on the Galileo spacecraft measures the impacts of particles and allows the mass and velocity of each particle to be estimated. Colwell et al. (p. 88) used these impacts to identify a faint ring around Jupiter. The ring is composed of 0.5- to 1.5-micrometer particles, mostly in retrograde orbits from 3 to 20 Jupiter radii away from the planet with relatively low eccentricities but a wide range of inclinations. They believe this ring was formed mostly from interplanetary dust with some minor interstellar dust. It is also possible that random events, such as the collision of comet Shoemaker-Levy 9 with Jupiter, added material to this ring.

  5. Crawling the Web

    The World Wide Web has become both research tool and arena for studies of research behavior. Most users navigate the Web via one or more search engines, but how much of the Web do these utilities actually encompass? Lawrence and Giles (p. 98) have analyzed the coverage and freshness of some of the more popular search tools and find that no one engine indexes more than about one-third of the Web; strategies that combine results of many search engines yield far better results. How do users actually surf the Web? Huberman et al. (p. 95) have studied patterns of behavior as information foragers move from one hyperlinked document to the next. The authors find strong regularities in surf behavior that may find applications in better Web design for specific research communities.

  6. Earth's initial interior conditions

    Mantle convection models have improved to include depth-dependent viscosity, mineral phase changes, heating from the core, and, perhaps most importantly, the coupling of mantle motions to tectonic plate motions. However, because we do not know the velocity and geometry of tectonic plates beyond 119 million years ago with enough accuracy, it has been difficult to model how patterns of convection in the mantle have changed over time. Bunge et al. (p. 91) have modeled time-dependent convection by starting with the plate motions from 119 to 100 million years ago and running their convection model forward for 2 billion years to establish some initial convection conditions. They then run their model using the current plate motions and these initial convection conditions for 500 million years to provide a simulated snapshot of the convection in the mantle today. Their final convection model is consistent with global tomography models except that the convection model does not produce large, hot thermal anomalies at the core-mantle boundary as predicted by tomography. These differences may require a reinterpretation of the global tomography models, specifically whether slow velocity anomalies represent thermal or chemical anomalies at the core-mantle boundary.

  7. Stimuli awareness and memory

    The hippocampus is necessary for declarative memory-the conscious recollection of facts and events-and is not required for classical conditioning, a simple form of associative learning in which a tone that is paired with a puff of air acquires the power to elicit an eyeblink even without the air puff. Trace conditioning, a variant paradigm in which a short interval (of about 1 second) is interposed between the tone and the air puff, is known to require an intact hippocampus. Clark and Squire (p. 77; see the commentary by Schacter, p. 59) present data that explain this puzzle by examining the performance and awareness of amnesic patients (with hippocampal damage) and normal subjects during classical and trace conditioning tasks. They find that the hippocampus is required for the development of conscious knowledge of the relation between the stimuli; thus the patients do not acquire trace conditioning because they never learn the relation of events.

  8. Repressing transcription

    Regulation of gene expression involves multiple protein-protein and protein-DNA interactions. To better understand the mechanism of transcription inhibition by two short-range repressors in the Drosophila early embryo, Knirps and Snail, Nibu et al. (p. 101) have identified a protein that interacts with these repressors in vitro. Additional support for these interactions is gained through in vivo gene-dosage analyses. The identified corepressor is a homolog of the human CtBP protein, which interacts with the adenovirus E1A protein to inhibit E1A-mediated transcription activation and tumorigenesis. Thus, a conserved mechanism of transcription repression is found in humans and Drosophila.

  9. Cold-hardy plants

    Like heat shock, cold shock induces physiological changes in plants that make them better able to tolerate subsequent temperature extremes further along the thermometer. Jaglo-Ottosen et al. (p. 104; see the news story by Pennisi, p. 36) have shown that overexpression of a single transcription factor, CBF1, in Arabidopsis plants can induce the expression of a suite of cold-responsive genes even in the absence of the normal stimulus of low temperatures. These transgenic plants are also more resistant to the damaging effects of low temperatures than are unmodified plants.

  10. Ras and PKC signaling

    Members of the protein kinase C (PKC) family contribute to cellular responses to a broad spectrum of signals and control multiple cellular processes, including cell growth and differentiation. Many actions of PKC are brought about by activation of mitogen-activated protein (MAP) kinases. However, the mechanism by which PKC influences activity of MAP kinases has been unclear. Marais et al. (p. 109) found that the small guanine nucleotide binding protein Ras is required for this process. The proto-oncogene product Ras is known to mediate activation of MAP kinases in response to receptor tyrosine kinases, but the authors show that activation of Ras in response to PKC occurs by a different mechanism than that initiated by the receptor tyrosine kinases.

  11. Less means more

    Agricultural intensification in developing regions continues apace, but management practice is poorly developed, and environmental consequences have not been well studied. Matson et al. (p. 112) have assessed the biogeochemical, socioeconomic, and crop yield implications of alternative strategies for inorganic nitrogen application in Mexico's Yaqui Valley. One approach, involving the addition of fertilizer in lower quantities and later in the crop cycle than typical farmer practice, proved to be a winner: Crop yield was maintained in concert with reductions in nitrous and nitric oxide emissions and significant cost savings for farmers.

  12. Lens formation

    Development of the eye requires the participation of multiple proteins in a genetic cascade. Several transcription factors have been shown to be involved in eye formation. Ogino and Yasuda (p. 115) report the identification of a novel chick lens-specific gene that is homologous to the maf proto-oncogene family; hence, the name L-maf (lens-specific maf). L-maf induces lens differentiation through binding to lens-specific genes. Unlike other genes involved in eye development, L-maf is expressed almost exclusively in the lens.

  13. Chondrules seeing waves

    Chondrules are rounded aggregates of mainly silicate minerals found in chondritic meteorites. They are assumed to have formed in the solar nebula, a protoplanetary disk of gas and dust. Explaining how chondrules may have formed in the solar nebula has been difficult because no two chondrules are alike and no formation model can explain the great variety of chemical compositions and textures observed in these inclusions. Connolly and Love review the chemical and textural characteristics of chondrules. They describe how the estimated peak temperatures, cooling rates, sizes, chondrule rims, estimated oxygen fugacity and nebular gas pressure inferred or derived directly from the characteristics of chondrules can be generally explained with a simplified model of one or more planar shock waves moving through the solar nebula. The exact mechanism or mechanisms of formation are probably more complex and the source of the shock waves remains uncertain, but this review provides a synthesis of our current observations of chondrule characteristics and how they fit the proposed shock-wave model.

  14. Quasiparticles and Thermal Conductivity

    K. Krishana et al. (Reports, 4 July, p. 83) studied the behavior of the excited-state “quasi-particles” (QP) of the superconductor Bi2Sr2CaCu2O8 in a magnetic field by measuring thermal conductivity. They observed evidence of “phase transition of the condensate to a state in which the QP current is zero (the system remains superconducting).”

    H. Aubin et al. replicate the results in the report and go on to ask, “Is the field-independent thermal conductivity of the high-field state insensitive to the way the magnetic field is applied?” They study the thermal conductivity of a single superconducting crystal as a function of a variable magnetic field, and focused on hysteresis effects, or how thermal conductivity depends on the manner in which the magnetic field is applied. Their results indicate that the background thermal conductivity “depends on the field profile of the sample” as it relates to the vortex scattering of photons.

    In response, Krishana et al. provide further data so as “to look at the hysteresis in perspective.” They state that it seems unlikely that “the plateau feature could depend in an essential way on the phase of the vortex system or the magnetization history of the sample.”

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