This Week in Science

Science  26 Feb 1999:
Vol. 283, Issue 5406, pp. 1225
  1. Carbon Dust in Exploding Stars

    Carbon-based particles, such as graphite, diamond, and silicon carbide, have been found in meteorites. Isotopic analysis of some of these particles indicates that they may have formed in supernovae. One problem with these models is that carbon will react with oxygen to form carbon monoxide (CO) molecules, which makes it difficult to form carbon-based solids in the supernova except under specific circumstances. Clayton et al. (p. 1290) show that carbon-based solids can condense in supernovae without any special restrictions because energetic electrons and ions effectively circumvent the CO formation mechanism. This unequilibrated atomic carbon may then rapidly condense into solid particles that are eventually ejected from the supernova.

  2. Light Storage

    Optical signal processing would benefit from methods that would allow a light signal to be stored and retrieved at a later time; doing this directly (for example, with mirrors or an optical fiber loop) can require large setups and fixed delay times. Zimmermann et al. (p. 1292) show how a light pulse can be “stored” in a quantum well structure at 100 kelvin. A light pulse generates pairs of electrons and holes that are spatially separated; after delays as long as 30 microseconds (five orders of magnitude longer than the normal electron-hole pair decay time), a change in voltage across the well releases the charges, which reemits a light pulse. The authors suggest ways to increase the lifetimes and to achieve operation at room temperature.

  3. Anticipating a Ceramic Failure

    Glass can be cut (broken) to shape by first scoring its surface, but less extreme surface damage can still cause brittle materials like ceramics to fail catastrophically in structural applications. Worse still, the fracture strength can vary widely for the same material. Crack propagation can be slowed and failure strength variability reduced by adding fibers or whiskers, but this approach often reduces the materials' overall strength. Green et al. (p. 1295; see the news story by Hellemans) show that greater, less variable strength can be achieved in silica glass by modifying its surface stress profile through ion exchange. The material undergoes multiple cracking before it fails, thus allowing examination and replacement, rather than failure without warning through a single crack.

  4. Death from (Telomere) Exposure

    Why are natural chromosome ends not detected by the checkpoints that search for DNA damage elsewhere in the genome? Karlseder et al. (p. 1321) show that, in mammalian cells, a telomere binding protein, TRF2, appears to “hide” telomeres (repeat sequences at the ends of chromosomes) from these checkpoints. Inhibition of TRF2 resulted in rapid cell death by an apoptotic pathway that involved the checkpoint proteins p53 and ATM. These results suggest that telomere malfunction may induce apoptosis rather than senescence in certain cell types.

  5. Finding Clouds Before They Form

    Why do thunderstorms form in one place and not another along a weather front? The reason may reflect subtle variations in the distribution of water vapor in the atmosphere. Water vapor is also one of Earth's major greenhouse gases and is important in controlling temperatures and global climate. Hanssen et al. (p. 1297) show how radar interferometry can be used to map these distributions of water vapor in the atmosphere and discuss the effects of variations on cloud formation and the dynamics of weather fronts.

  6. Holding Up Tibet

    The continent-continent collision between India and Asia about 50 million years ago created the high-standing Tibetan Plateau. Kosarev et al. (p. 1306) imaged the structure of the crust and uppermost mantle across the plateau using teleseismic P waves that were converted by changes in structural features into S waves. These images allow them to infer a continuous north-dipping structure within the Indian lithosphere that may be related to stable subduction and a discontinuous, south-dipping structure within the Asian lithosphere that suggests a weak region that is being broken up by the collision. These structures suggest that the rheology and composition of the Indian lithosphere are different from the Asian lithosphere.

  7. Missing What Wasn't There?

    Molecular clock estimates of the divergence times of major animal groups can be much older than the earliest fossil record. Foote et al. (p. 1310) present a model of the origination, evolution, and preservation of species aimed at evaluating whether this missing gap is likely. Preservation is calibrated on the basis of the known fossil record. They apply their approach specifically to the placental mammals, for which one molecular divergence time of 120 million years ago is much greater than the fossil evidence of first appearance about 70 million years ago. The analysis suggests that such an early origination is unlikely; one possible explanation is that the rates of molecular evolution are accelerated during speciation events.

  8. More Predictable Magmas

    A little bit of water in a magma can alter its density and viscosity and thus the dynamics of magma ascent and mixing. Ochs and Lange (p. 1314; see the Perspective by Spera) measured the density of three hydrous silicate melts and, using previous measurements on anhydrous melts, they determined that the density of a water-enriched melt is independent of the melt composition, water speciation, and concentration of water. This independence will allow geologists to model the magmatic processes of hydrothermal systems without specific information on the composition of the magma.

  9. Smad3 Signal Junction

    The peptide growth factor, transforming growth factor-β (TGF-β), and the steroid hormone vitamin D can have cooperative effects on gene transcription. Yanagisawa et al. (p. 1317) provide evidence for a molecular basis for such effects. The Smad3 protein binds to and is phosphorylated by the activated TGF-β receptor. Smad3 then forms a complex with Smad4, which is translocated to the nucleus. In the nucleus, SMADs act as transcription factors or coactivators of transcription. The vitamin D receptor is a nuclear receptor that acts, along with coactivators, as a transcription factor when the vitamin is bound. Smad3 (but not other SMADs) was shown to interact with a member of the steroid receptor coactivator-1 family. This interaction may account for the observed effects of the TGF-β pathway in the stimulation of transcriptional activation initiated by vitamin D.

  10. Recognizing Threonine and Serine Phosphorylation

    Various proteins that participate in intracellular signaling pathways contain WW domains, which are characterized by the presence of two tryptophan (W) residues and bind to proline-rich sequences in some protein binding partners. Lu et al. (p. 1325; see the news story by Barinaga) analyzed the function of WW domains in the peptidyl-prolyl isomerase Pin1, which binds specifically to certain proteins that are phosphorylated during mitosis, and the ubiquitin ligase Nedd4, which binds and ubiquitinates substrates in a phosphorylation-dependent manner. They find that the WW domains of these proteins bind to certain proteins and peptides phosphorylated on serine or threonine residues but not their dephosphorylated counterparts. In a yeast assay, the function of human Pin1 was lost if mutations were made in the WW domain. Thus, WW domains apparently can specifically recognize proteins phosphorylated on serine or threonine residues, much like SH2 (Src-homology 2) domains recognize proteins phosphorylated on tyrosine.

  11. Smaller Signals for G Proteins?

    Binding of ligands to receptors on the cell surface leads to activation of signals within the cell that can be transmitted through protein-protein interactions. In the case of receptors coupled to G proteins (heterotrimeric guanine nucleotide binding proteins), G protein βγ subunits can interact with and activate effectors such as phospholipase C-β2 (PLC-β2). Buck et al. (p. 1332) analyzed the effects of peptides from the Gβ subunit on activity of PLC-β2 and showed that distinct regions could be identified that contributed primarily to signal transfer or to general protein binding. The demonstration that a small region of the Gβ protein can activate PLC-β2 with efficacy similar to that of the complete protein suggests that it may be possible to synthesize small agonists or antagonists that would modulate such signaling through G proteins.

  12. Antigen Mimicry in Chlamydia Infections

    Heart disease is the major killer in Western society, and Chlamydia infections often correlate with the disease. Myocarditis can also be caused experimentally by injecting mice with the myosin heavy chain from the heart. Bachmaier et al. (p. 1335; see the news story by Gura) found that the “autoaggressive” peptide of myosin resembled a peptide found in many types of Chlamydia. The Chlamydia peptide or whole organisms induced cardiac inflammation similar to the myosin peptide and stimulated the generation of CD4+ T cells and antibodies that were specific for heart myosin. T cells from mice with heart disease induced by the Chlamydia peptides could transfer heart disease to fresh mice. Thus, a local Chlamydia infection can lead to systemic activation of an autoimmune response through molecular mimicry that can trigger organ-specific inflammation of the heart.

  13. Yeast Amyloid Analogs

    Several human diseases are thought to be propagated by infectious proteins known as prions. In yeast, similar prion-like phenomena have also been found. Taylor et al. (p. 1339) now confirm and extend the prion-like character of the Ure2 protein by showing that it can form amyloid-like structures in vitro. Such amyloid structures are a hallmark of prion diseases in animals.

  14. Glial Connections

    Glial cells can exert direct effects on neuronal activity, but factors that exert such influence have been hard to pinpoint. Yuan and Ganetzky (p. 1343) report that, in Drosophila, the axo gene encodes a member of the neurexin superfamily of neuronal cell surface proteins that is secreted by glial cells. Mutants lacking the axo gene undergo paralysis at higher temperatures that is associated with defective axonal signal conduction.

  15. Fighting Bioterrorism

    Many analysts looking at the potential for bioterrorist attacks have commented that it is not a matter of “if” but of “when.” In a review article, Henderson (p. 1279) discusses the probable agents that could be used and the need to focus on the differences between biological agents and the chemical or nuclear agents that have been more commonly considered. Such considerations need to be made in order to set up an effective “first-response” team and infrastructure to deal with future threats.

  16. Aerosol Effects

    Aerosol particles in the troposphere, which are produced both naturally (for example, by volcanoes and sea salt) and by pollution, tend to cool Earth, and thus counteract the greenhouse effect. It has been difficult to predict their global effects because aerosols tend to have short atmospheric lifetimes, many sources, and thus a complex distribution globally. To evaluate their global effect, Haywood et al. (p. 1299) compare a series of global computer simulations that include progressively different types and effects of aerosols with satellite observations of the reflected solar radiation of Earth. The results allow a global quantification of the effect of aerosols and their distribution. Sea salt is particularly important in windy areas of the oceans and comparable to aerosols associated with pollution. [See the Perspective by Kiehl]

  17. A Marbled Mantle

    Basalt is the most ubiquitous form of lava erupted on Earth's surface, and compositional variations between different basaltic rocks can be used to trace the composition of the upper mantle because basalts are formed from the melting of mantle rocks. Current geochemical evidence favors polybaric melting of a relatively homogeneous peridotite-rich mantle versus isobaric melting of a heterogeneous peridotite- and pyroxenite-rich mantle. Blichert-Toft et al. (p. 1303) measured the hafnium isotopic composition of garnet-pyroxenites from Beni Bousera, Morocco, and found a wider range of radiogenic hafnium-176 than is found for oceanic basalts. Based on these measurements and other compositional data, the authors suggest that the mantle is a mixture of pyroxenite residues left by melting of subducted oceanic crust and subsequently folded into the fertile mantle peridotite. Thus, many basalts may not represent primary melts from a pristine mantle, but rather melting of an altered, compositionally layered mantle.

  18. Helping Turn Signals On and Off

    Adenylyl cyclase (AC), the enzyme that produces the second messenger cyclic AMP (adenosine 3′,5′-monophosphate), is activated by interaction with the α subunit of the G protein (heterotrimeric guanine nucleotide binding protein) G. Scholich et al. (p. 1328) have found that interaction with AC (Type V) enhanced the guanosine triphosphatase activity of G and thus led to its inactivation. However, interaction of AC with G enhanced guanine nucleotide exchange and activation of G induced by interaction with a peptide that mimics stimulation by the β2-adrenergic receptor. Thus, interaction of AC with G may both sensitize receptor signaling through the complex and also contribute to rapid termination of signaling by G once the stimulus from the receptor is removed.

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