This Week in Science

Science  21 Nov 1997:
Vol. 278, Issue 5342, pp. 1377
  1. Protective immune responses to HIV

    In order to rationally design a vaccine against AIDS, it will be necessary to understand the correlates of protective immunity. Rosenberg et al.(p. 1447; see the related news story by Balter, p. 1399) have studied individuals infected with the human immunodeficiency virus (HIV) who managed to control HIV proliferation in the absence of antiviral drug therapy. These individuals had specific proliferative responses of CD4+ T helper cells that resulted in the production of interferon-γ and antiviral β chemokines. Virus load was inversely related to the strength of the proliferative response to the viral protein p24.

  2. At tree line

    It has generally been thought that changes in temperature were primarily responsible for shifts in tree lines in the past, although some recent models have suggested that the large changes in atmospheric CO2 levels at the end of the last glaciation also had an effect. Street-Perrott et al. (p. 1422; see the Perspective by Farquhar, p. 1411) analyzed carbon isotopes in fossil leaf waxes and algal biomarkers to show that the changes in atmospheric CO2 had a significant effect on high-altitude tropical ecosystems.

  3. Mind the pseudogap

    The parent compounds of the high-temperature superconductors are antiferromagnetic insulators. Blumberg et al. (p. 1427) performed electronic Raman scattering studies of the superconducting bismuth cuprate compound to see how such ordering may evolve into the superconducting state. At temperatures well above the superconducting transition temperature, possible precursors to superconductivity develop, such as long-lived hole states with dx2-y2 symmetry and a binding energy of 75 millielectron volts. As the temperature is decreased, greater coherence is seen in the electronic system and a pseudogap opens up in the spectrum.

  4. Current events

    The flow of currents between macroscopic quantum systems, such as superfluids or superconductors, that are weakly connected will depend on the relative quantum phase describing each condensate. Such effects, which govern superconducting tunnel junctions, are much more difficult to observe directly in superfluids such as liquid helium-3. Backhaus et al. (p. 1435) determined the current-phase relation directly for superfluid helium by using a membrane array of 4225 apertures to connect two reservoirs, and they observed the transition with increasing temperature from linear to sinusoidal behavior.

  5. Repeated deliveries

    Placental mammals have been thought to have dispersed from South America through Antarctica to Australia in the Cenozoic, sometime after about 60 million years ago. Rich et al. (p. 1438; see the news story by Wuethrich, p. 1401) have now found a jaw of an early placental mammal (or early ancestor of one) from the Early Cretaceous in Australia, about 115 million years ago. This fossil thus implies that Australia and the southern continents were not faunally isolated from northern continents in the Cretaceous. These early placentals may have become extinct in Australia, only to be reintroduced later.

  6. Two by two

    The atomistic growth pathway from single adsorbed atoms (adatoms) of silicon or germanium adatoms to larger rows and islands is complex. Qin and Lagally (p. 1444) now provide a possible missing link between monomer absorption and the formation of two-dimensional islands. Scanning tunneling microscope images show that prenucleation structures, consisting of paired adatoms that are distinctly different electronically from previously characterized dimers, play a crucial role in the formation of larger rows and islands on this surface.

  7. Coral connections

    The eggs and larvae of marine organisms can be transported over great distances. Based on a detailed analysis of the surface currents and assuming a larval lifespan of 1 or 2 months, Roberts (p. 1454; see the Perspective by Ogden, p. 1414) mapped out potential “transport envelopes” that link different reef areas around the Caribbean. Such studies could help in assigning priority for preservation by identifying “upstream” supplier sites.

  8. Dissolved partnership

    Many features of the stepwise mechanism by which HIV binds to CD4 and its coreceptors are currently unclear. Binding of the HIV viral envelope protein gp120 to T cells likely exposes a previously concealed coreceptor binding site. Although binding of HIV gp120 to the coreceptor CCR5 depends on CD4, Martin et al. (p. 1470) show that this is not so for the simian counterpart SIVmac239. The difference can be attributed to a one amino acid change in the CCR5 amino terminus, which may be in the region that makes direct contact with gp120. The finding of the critical role of a single amino acid rather than an entire domain may make it possible to design targeted assay systems for vaccines and therapeutics.

  9. Making methane

    Methyl-coenzyme M reductase catalyzes the formation of methane. Ermler et al. (p. 1457; see the Perspective by Ferry, p. 1413) present the high-resolution structure of this enzyme. It contains an unusual Ni-porphinoid cofactor that accepts the methyl group from methyl-coenzyme M and combines it with a hydrogen atom obtained from coenzyme B to yield methane and the mixed disulfide of coenzyme M-coenzyme B. A long hydrophobic channel helps to order and orient the two linearly shaped coenzymes.

  10. Targeting cells with receptors

    Traditionally, retroviral vectors have targeted cells through the cell-surface receptors. Recent studies have indicated that it is possible to reverse the process, by putting the receptor on the vector to target ligand-expressing cells. Endres et al. (p. 1462) show that the interaction of the envelope glycoprotein of HIV and SIV with the cellular receptor is not unidirectional; vectors containing a functional virus receptor complex can target chronically infected cells and infected macrophages.

  11. Tokamak alternative

    Fusion power is usually associated with the Tokamak design for plasma confinement. Recent Tokamak studies on plasma confinement suggest that an alternative, the colliding beam fusion reactor, is feasible as a small reactor. Rostoker et al. outline this idea for the reaction of protons with boron-11, which would have the advantages of producing all charged particles that could be directly converted to electricity and low fluxes of neutrons that normally lead to materials fatigue.

  12. Not so normal

    Although high-temperature superconductivity is the hallmark of layered cuprate compounds, their normal insulating state exhibits unusual charge behavior. Aeppli et al. have used inelastic magnetic neutron scattering to show that the normal state of the superconductor La1.86Sr0.14CuO4 is also unusual in terms of spin behavior. Magnetic fluctuations are observed that indicate the presence of a quantum critical point at zero temperature. [See the Perspective by Chakravarty.]

  13. Orbital forcing

    Variations in ocean productivity can be influenced by many factors, and separating these different causes in past and present climates represents a major challenge. Beaufort et al. analyze a continuous sedimentary record from the Maldives that spans 900,000 years, and investigate how insolation (the amount of radiation received by the Earth depending on its orbital cycles around the sun) and global ice volume (glaciation cycles) affect the primary production in the equatorial Indian Ocean. The observations imply that productivity in the equatorial system is directly controlled by insolation but is independent from global ice volume variations. It is suggested that the precession of the Earth's orbit forced changes in the Southern Oscillation, which in turn affected paleoproductivity.

  14. Washed downstream

    Urbanization is known to have deleterious effects on stream channels, but there have only been a few long-term studies. Trimble shows that during a 10-year period of urbanization in San Diego Creek, California, erosion within the stream channel produced about two-thirds of the sediment carried by the stream.

  15. Halting cells

    Through their effects on cytoskeletal proteins, the small guanine nucleotide binding proteins of the Rho family can influence cell motility. One such protein, Rac, and its activating exchange factor, Tiam1, induce invasion by cells of lymphoid tumors. Hordijk et al. report that in epithelial cells, Tiam1 and Rac can also produce the opposite effect, inhibition of motility, by enhancing E-cadherin-mediated cell-cell adhesion. Expression of Tiam1 and Rac reduced invasiveness of Ras-transformed Madin-Darby canine kidney cells. The results indicate a role for Tiam1 and Rac in receptor-mediated control of cell adhesion.

  16. Still shared with bacteria

    Of the several molecular mechanisms by which proteins get sorted and targeted to their specific destinations within a cell, two have been known to be conserved from bacteria to plants, but a third had been thought to be a new feature developed with the evolution of higher plants. Settles et al., with the cloning of the gene encoding the maize mutation hcf106, now show that this third pathway also is found in bacteria. Perhaps the substrates transported have changed, but the proteins that make up the machinery of the transport system find related genes among open reading frames of unknown function from several fully sequenced bacterial genomes. Investigation of the specific functions of those bacterial proteins may yield further surprises. [See the cover.]