This Week in Science

Science  26 Jan 2001:
Vol. 291, Issue 5504, pp. 549
  1. Imaging the Magnetosphere

    Earth's magnetosphere, which shields us from the interplanetary magnetic field and the solar wind, is dominated by the geomagnetic field. The magnetosphere is populated by plasma that cannot be seen with typical observing technologies. Now, the IMAGE spacecraft (Imager for Magnetopause-to-Aurora Global Exploration) has overcome these technical challenges and has been imaging the plasma in the magnetosphere since its launch in March 2000. Burch et al. (p. 619; see the cover) show how the magnetosphere changes during several geomagnetic storms and relates these images to our understanding of the sun-Earth interactions that can affect our ground-based and orbiting electromagnetic communications.

  2. Planetary Nebulae Not so Hot?

    Planetary nebulae are the highly ionized gaseous shells created during the collapse of a low-mass star (1 to 10 solar masses) to a white dwarf. The surface of the shell is very luminous and thought to be maintained at very high temperatures through the absorption of energetic ultraviolet photons from the still-burning central star and their re-emission at longer wavelengths from recombined hydrogen, helium, and other elements. Johansson and Letokhov (p. 625) suggest that the bright emission is actually a resonance-enhanced two-photon ionization of heavier elements such as silicon by hydrogen Lyman α radiation. This different mechanism to explain the spectral lines observed in planetary nebulae removes the need for high temperatures on the surface of the nebulae and possibly the need for additional fusion in the dying central star.

  3. Droplets Make a Fast Break

    Gradients at a surface, such as in temperature or surface energy, can lead to liquid flows. This movement of fluid through unbalanced surface tension, also known as the “tears of wine” or the Maragoni effect, tends to move fluids slowly, however. Daniel et al. (p. 633; see the Perspective by Wasan et al.) now show that when surfaces designed with a surface energy gradient are exposed to high fluxes of condensing vapor, the droplets that form grow and merge with one another to accelerate their movement. The droplets flow at speeds hundreds to thousands of times faster than Maragoni flows. This effect could be used to improve the efficiency of heat exchangers and heat pipes.

  4. Slow Particles Can Also Shine

    Although the speed of light is constant in a vacuum, it travels more slowly through matter. If a charged particle can move through a medium faster than light can, radiation will be emitted. It is this Cherenkov radiation that gives a nuclear reactor its blue glow, and it is also used in nuclear and high-energy physics to detect, count, and identify relativistic particles. It was previously thought there was a one-to-one relation between the speed of the particle and the cone angle at which radiation was emitted relative to the particle. Theoretical and experimental work by Stevens et al. (p. 627) now show that Cherenkov radiation can also be emitted by particles moving subluminally, or slower than the speed of light.

  5. Silencing Is Replication-Free

    Gene silencing is a mechanism by which cells shut down large sections of chromosomal DNA by incorporating it into heterochromatin, a process that is important for the differentiation of the many different cell types during the life cycle of a eukaryotic organism. Silencing of the two yeast mating-type loci, HML and HMR, is established during the S, or DNA synthesis, phase of the cell cycle, and it has therefore been assumed that DNA replication is involved in this process. Kirchmaier and Rine (p. 646) and Li et al. (p. 650) now show that a portion of DNA containing the HMR locus that was excised from the chromosome and thus unable to replicate is nevertheless effectively silenced, which indicates that replication is not required for the establishment of silencing. In a Perspective, Smith and Boeke point out that replication could nevertheless still play a role for the maintenance or inheritance (or both) of the silenced state.

  6. Nanotubes Go with the Flow

    The alignment and patterning of individual nanoscale components will be a crucial requirement for developing molecular electronics. Huang et al. (p. 630) describe a process for the hierarchical assembly of nanowire and nanotube functional structures based on the combination of fluid flow and chemical patterning of surfaces. They demonstrate alignment of the nanowires with fluid flow on the length scales of nanometers to millimeters and the controlled assembly of grids of crossed nanowires. Complex crossed structures were prepared by using a layer-by-layer strategy in which distinct flows were used for each sequential step.

  7. Dynamics Within Nanocavities

    Transmembrane protein pores have previously been fitted internally with cyclodextrin (CD) molecules that can greatly reduce the pore opening and hence their ion conductance. Gu et al. (p. 636) now extend this approach by identifying mutants of α-hemolysin that can bind two different CDs in different locations along the pore to create a nanocavity with a volume of ∼4400 cubic angstroms. Singly charged guest molecules that pass through one CD but not the other can be introduced into the cavity with an electrical potential, where they reside for hundreds of milliseconds. This process allows the differential trapping of molecules in the cavity.

  8. Ran's Importin Partner

    The small guanosine triphosphatase Ran is involved in transport of proteins into the nucleus and has been recognized more recently as a regulator of formation of the mitotic spindle. Wiese et al. (p. 653) show that Ran uses the same binding partner, importin-β, to control microtubule formation that it uses to promote protein transport. Importin-β carries cargo proteins to the nucleus and releases them upon interaction with guanosine triphosphate (GTP)-bound Ran. In the case of spindle assembly, importin-β interacts with the NuMA (nuclear-mitotic apparatus) protein, which helps organize microtubules at the spindle poles. Activated Ran causes release of NuMA from importin-β and thus promotes microtubule assembly.

  9. Checking Back?

    Encoding and retrieval of declarative memories in the primate brain depends on the interaction between the medial temporal lobe system and the neocortex. Naya et al. (p. 661) recorded nerve cell activity in awake, behaving monkeys from area 36 of the perirhinal cortex and area TE, two morphologically distinct but mutually interconnected areas of the temporal cortex. During a visual pair-association task, which requires retrieval of a target from long-term memory, they observed not only a perceptual signal that spread in the normal forward direction in the brain but also a memory signal that propagated backward. That may serve to extract stored knowledge about objects.

  10. More than a Supporting Role

    Glia are brain cells often described as providing “support” for the less numerous, but more famous, neuronal brain cells. Now, Ullian et al. (p. 657; see the news story by Helmuth) show that glia in culture control the number of synapses on neurons and that they are necessary for the maintenance of proper synaptic electrophysiological responses. In the newborn rat, synapses are formed in the superior colliculus at the end of the first week after birth, at precisely the same time that atrocytic glia appear and proliferate. The authors suggest that glia may trigger immature and highly plastic synapses in the developing brain to increase and stabilize in order to lock synaptic circuitry in place.

  11. Exposing Lipids

    NK-T cells are a small and unusual class of T cells that recognize lipid, rather than protein-derived antigen. The structures responsible for presenting lipid antigens—the CD1 molecules—have been studied for some time, yet the intracellular requirements for this mode of antigen presentation have not been defined. Prigozy et al. (p. 664) report that as for protein antigen, intracellular processing may be an obligate part of presentation of lipids to T cells. Using a precursor of a model glycosphingolipid antigen, the authors observed that only when the enzymatic machinery of the lysosmal compartment was intact could antigen recognition by NK-T cells take place. This process involved the removal of a terminal sugar group by the enzyme α-galactosidase, thus permitting the association of the modified lipid with the presenting molecule CD1d.

  12. Crawling Along

    Motor proteins like kinesin generally possess two “heads” attached to a filamentous tail. The motor proteins use energy to translocate themselves (and any attached cargo) along intracellular tracks known as microtubules. Kawaguchi and Ishiwata (p. 667) have examined this process in isolation using sophisticated biophysical techniques to analyze how and when the head domains use energy from adenosine triphosphate as they crawl along microtubules. Their findings support the notion that, as the motor moves along, alternately one and then two heads are bound to the microtubule.

  13. Ups and Downs in Lake Titicaca

    Lake Titicaca, the only large deep lake in South America, is an important archive of climate history because it records a tropical signal that is representative of the adjacent Amazon basin. Its importance for climate reconstruction is enhanced by the relative paucity of data from other sites on the continent and by the many contradictions between existing records. Baker et al. (p. 640) have used sediment cores from the lake to evaluate whether the last glacial maximum in the tropical Andes was recorded at the same time as at high latitudes, when there were periods of extreme wetness or aridity in tropical South America, and if there was a consistent relation between wet-dry periods in tropical South America and variations in sea surface temperature in the tropical Atlantic Ocean. Their results point to a strong connection between Amazonian rainfall and sea surface temperatures in the North Atlantic Ocean.

  14. Airing on the Side of Caution

    Although aerobes rely on oxygen metabolization to sustain life, they must also contend with the toxic by-product, reactive oxygen species, which has been suggested to function in processes such as aging and oxidative stress. Antioxidant defense systems act to eliminate this detrimental molecule. Two enzymes commonly found in these antioxidant defense systems are glutathione reductase and thioredoxin reductase. Genome analysis reveals that Drosophila melanogaster lacks glutathione reductase. Kanzok et al. (p. 643) now report that the fruit fly, and possibly other insects, instead uses a thioredoxin system to reduce glutathione disulfide.