This Week in Science

Science  23 Jan 2004:
Vol. 303, Issue 5657, pp. 429
  1. On Multivesicular Liposomes and Endosomes

    After endocytosis, membrane proteins are sorted to their final destinations by the formation of multiple membrane vesicles within late endosomes. Matsuo et al. (p. 531) discovered that an unconventional phospholipid found in these compartments can act to form multivesicular liposomes in the presence of a transmembrane pH gradient in vitro. Addition of cytosolic components to the reaction revealed that a protein known as Alix appears to participate in the formation of multivesicular endosomes inside cells.

  2. Red Supergiant Is the Culprit

    Type II Plateau supernovae are thought to be produced by the explosive collapse of a red supergiant, a star that has a mass between 8 to 25 solar masses and has reached the end of its nuclear burning life. Unfortunately, a red supergiant progenitor for a type II-P supernova has never been observed; in fact, only two unambiguous progenitors have been found for other type II supernovae and neither was a red supergiant. Smartt et al. (p. 499), using the Hubble Space Telescope and the Gemini Telescope, have identified the progenitor for the type II-P supernova 2003gd, which exploded in the nearby spiral galaxy M74. The progenitor is a red supergiant, with a spectral type between M0 to M3Ib and an initial mass on the main sequence of about 8 solar masses. This relatively low mass progenitor is barely capable of undergoing core collapse, based on theory and supernova models; nonetheless, it provides confirmation and critical new details about stellar evolution for the most common type of supernovae observed in the universe.

  3. Diffusion Charges Ahead

    The partial reduction of transition metal oxides creates vacancies on the surface that act as donor sites for the materials' conduction band. Wahlström et al. (p. 511) followed the diffusion of O2 on reduced TiO2(110) surfaces, which have a corrugated “row and trough” appearance, with time-resolved scanning tunneling microscopy. The hopping rate for O2 along the rows was higher for surfaces that were more reduced and had more defects. The authors argue that diffusion is facilitated by charge transfer from the surface to the O2 molecule, rather than by the defects causing local strain of the surface. The existence of such charge-transfer excited states of adsorbates could be important for better understanding chemical reactions and photoprocesses on these oxides.

  4. Less Silica, More Insulation

    Future generations of integrated circuits will place wires closer together, and the dielectric constants of the insulating layers will need to drop. One way to reduce the dielectric constant of silica is to introduce porosity into the layers. Pai et al. (p. 507; see the Perspective by Johnston and Shah) used the nanoscale phase separation of block copolymers and supercritical CO2 processing conditions to create microporous silica films containing spherical holes with dielectric constants as low as 1.8. The separation of the copolymer organization and metal oxide formation steps led to films with a high degree of long-range order. These films survived the chemical-mechanical planarization step needed for device manufacturing.

  5. High-Level Delta-Icing

    Whether low-temperature cirrus clouds form from the surrounding air depends on the relative humidity with respect to ice, since it is of ice particles that they are composed. Gao et al. (p. 516) report observations of increased relative humidity in upper tropospheric cirrus clouds, which they speculate is caused by a new class of HNO3-containing ice particles that they call “Δ-ice.” They suggest that these particles depress the freezing point of ice and are responsible for the measured increases of relative humidity of nearly a third. The resulting decrease in cirrus cloud extent could have significant implications for climate modeling, and is particularly important to understand because nitrogen oxide emissions are one of the major products of anthropogenic pollution by combustion.

  6. Degenerate Excitons

    Bound pairs of electrons and holes, or excitons, are bosonic particles and as such should, according to theory, condense into the lowest quantum state and form a Bose-Einstein condensate (BEC) when cooled to sufficiently low temperature. However, there are a number of barriers and obstacles to forming a BEC in an excitonic system—the main and fundamental one being getting enough excitons to low enough temperatures. Lai et al. (p. 503) report on a system of indirect excitons in which the coupled electrons and holes are formed in spatially separate double-quantum well structures. They present a systematic study of how the excitons trap and behave collectively and provide important information for the eventual goal of achieving a semiconductor-based BEC.

  7. Developing Multifunctional Materials

    As consumer appetite for larger storage capacity and faster electronics grows, it is widely thought that downsizing conventional electronics will not be sufficient, and new technologies will be required. Ahn et al. (p. 488) review the recent developments in the fabrication, manipulation, and theoretical understanding of ferroelectric materials with a view toward exploiting their properties for nanometer-scale applications. Adding to their attractiveness is the possibility of combining the ferroelectric properties with the ferromagnetic and piezoelectric properties of similar and compatible materials, thus creating the opportunity to develop multifunctional materials and devices.

  8. Imprinting Through Generations in Plants

    Genetic imprinting, the silencing in the embryo of one of the two copies of each gene received from the mother and father, is best understood in animals, where the epigenetic state is erased and re-established in each generation. Imprinting also occurs in plants, but epigenetic states can be inherited over many generations. DNA methylation plays a critical role in determining the imprinted state in both animals and plants. Kinoshita et al. (p. 521; see the Perspective by Berger) show that the FWA gene, involved in flowering in Arabidopsis, is imprinted in the endosperm of the seed. The imprint is established by the DNA glycosylase DEMETER and, as is the case in animals, requires maintenance DNA methylation. Unlike animals, FWA does not require de novo DNA methylation for re-establishing the imprint, which explains the difference in heritability of epigenetic states across generations.

  9. Bending the BAR to Induce Membrane Curvature

    Many proteins involved in membrane trafficking possess a conserved BAR domain. Peter et al. (p. 495; see the cover and the Perspective by Lee and Schekman) describe the structure of this protein domain, and find that it can sense and induce curvature in associated membranes. A variety of different proteins possessing the BAR domain acted in a similar fashion and could bind to lipid bilayers and induce their curvature in vitro and in vivo.

  10. Lipids in T Cell Presentation

    Immune-detection by T cells can involve the recognition of specific lipids (see the Perspective by De Libero by De Libero), as well as protein-derived antigens. Unlike the well-characterized intracellular pathways that load and present peptides, scant detail has been available about how lipid antigens come to be processed and presented. Zhou et al. (p. 523) find that members of a family of proteins known to transfer lipids during synthesis and degradation within the endosome also assist in extracting and loading lipids onto CD1d molecules, which present the lipids to T cells. In the absence of such lipid transfer protein genes, mice failed to present particular lipids and displayed defects in development of a major subset of CD1d-specific T cells. Moody et al. (p. 527) describe a new species of naturally occurring ligands presented by CD1a with an unusual antigenic structure in which peptides are chemically complexed with lipid. These lipopeptides resemble essential iron-scavenging siderophores from mycobacteria. Thus, CD1-based microbial immune detection may have evolved to recognize molecular species specifically generated during obligate survival pathways of bacteria.

  11. Disulfide Formation Caught in the Act

    Disulfide bonds are an important structural feature in many proteins exported from the cytoplasm. In Escherichia coli, substrate proteins in the periplasm are rapidly oxidized by the enzyme, DsbA, a member of the thioredoxin family of proteins. Kadokura et al. (p. 534) have found a mutant of DsbA that traps the mixed disulfide intermediary complex and have used it to identify a set of substrate proteins. This type of approach might also prove useful in detecting substrates of other thioredoxin family proteins.

  12. Activity on the X

    Functional genes can be produced during evolution by the reverse copying of RNA into DNA (retroposition). Emerson et al. (p. 537) have demonstrated that for humans and mice, the generation and recruitment of functional retroposed genes on the X chromosome is higher than for the autosomes. Gene expression studies indicated that many of the genes that leave the X for the autosomes have male-specific expression, which is likely to be the result of natural selection.

  13. All Singing, All Dancing

    How do birds that simultaneously sing and dance coordinate their body movements and sound production, and how do the two activities constrain one another? Cooper and Goller (p. 544) now show that in the cowbird, wing display and song respiratory patterns are coordinated, and that the most pronounced wing movements coincide with the silent periods in the song. This behavior avoids the most severe, negative biomechanical effects of body movement on respiration.

  14. Fluid Viscosity

    Magmas formed in subduction zones that supply volcanic arcs with fresh lava tend to be rich in silica, alkali elements, and fluids. Audétat and Keppler (p. 513) have developed a technique to measure the viscosity of aqueous fluids enriched in silicate melts at high pressures and temperatures. The silicate-laden fluids have low viscosities, close to that of water, and this suggests that subduction zone fluids should be mobile.

  15. Interactions Within Worms

    Interaction maps for the whole proteome of an organism will lend insight into multiple metabolic pathways. The generation of such a map for a simple metazoan should provide a model for generating such a map for humans. Li et al. (p. 540) describe more than 5000 protein-protein interactions in the nematode worm, Caenorhabditis elegans. Analysis of interactions that have been conserved from unicellular yeast to the multicellular nematode revealed that evolutionarily recent proteins and ancient proteins do not segregate into distinct interaction clusters. Thus, evolution of new cellular functions has proceeded by combining novel elements with those from pre-existing architectures.