Editors' Choice

Science  26 Mar 2004:
Vol. 303, Issue 5666, pp. 1947

    A String of Dots

    1. Ian S. Osborne

    The growth of self-assembled quantum dots (QDs) via a strain-driven mechanism has been shown to result in the formation of high-quality defect-free structures. However, this mode of fabrication yields an essentially random positioning of QDs on the substrate surface, and lithographic patterning is used to specify seeding and nucleation sites for QD growth.

    Wang et al. show that the growth of QDs as self-assembled chains of several micrometers in length can be achieved simply by interrupting the growth sequence. Although the mechanism for the long-range alignment remains unclear, the ability to control the size, composition, and position of the dots within the deposition process should, nevertheless, provide the possibility of engineering some interesting device structures. — ISO

    Appl. Phys. Lett. 84, 1931 (2004).


    Way Back When

    1. H. Jesse Smith

    The El Niño-Southern Oscillation (ENSO) is the primary cause of modern interannual variability of tropical climate. Has ENSO always behaved as it does today? Some studies have indicated that its behavior may depend on the background climate state—particularly whether it is a glacial or warm interval—and that its strength, periodicity, and regularity can change. Unfortunately, annually resolved records are scarce.

    Using a 350,000-year-old fossil coral from the southwestern tropical Pacific Ocean island of Vanuatu, Kilbourne et al. have measured Sr/Ca and oxygen isotopic anomalies, which are proxies for temperature and salinity changes, and compare them to those of a modern coral from the same location. Although annual sea surface temperature variations recorded in the corals were of the same amplitude, the seasonal salinity variations recorded by the fossil coral were smaller than those seen in the modern one. They also calculate that both the average sea surface temperature and salinity then were probably lower than today. They interpret these results as showing that ENSO or ENSO-like climate variations were in fact already present 350,000 years ago, although the South Pacific Convergence Zone did not migrate as far northward during austral winter then as it does today. — HJS

    Paleoceanography 19, 10.1029/2003PA000944 (2004).


    Porin Propinquity

    1. Gilbert J. Chin

    Recent successes in membrane protein stucture determination have advanced our understanding of how small hydrophilic entities (such as K+, glycerol, and H2O) can be transported passively across hydrophobic biological membranes. How larger and less readily movable entities such as proteins are transported is still a bit fuzzy, although as usual, bacterial systems are likely to offer the first glimpses.

    Oomen et al. describe the β-barrel structure of the C-terminal translocator domain of the autotransporter NalP from Neisseria meningitidis; an α-helical linker segment (which joins the N-terminal functional domain to the translocator) neatly fills the pore, which has an inner diameter of approximately 1 nm. Autotransporters encode and secrete virulence-related proteins (for example, a protease that attacks IgA molecules), and thus transit of the functional domain across the bacterial outer membrane via the β barrel would seem a plausible scenario.

    Yet the answer isn't quite so simple. Threading the functional domain, N-terminal end first, into and through the barrel would require both a targeting signal, which has not yet been identified, and an energy source analogous to the ribosome-powered translocation of proteins across the inner membrane. On the other hand, using the energy from folding to drive transport would mean pulling the C-terminal end of the functional domain through first (it is the C-terminal portion that folds first), but there isn't enough room in the pore for a helical hairpin, so the polypeptide would have to be dragged across in a fully unfolded and extended form. A third possibility, akin to the entry mechanism for colicin E3 proposed by Kurisu et al., is that the translocator domain serves primarily to bring the functional domain close to an outer membrane porin, which serves as the actual protein transporter. — GJC

    EMBO J. 10.1038/sj.emboj.7600148 (2004); Nature Struct.Biol. 10, 948 (2003).


    Unloading Dock

    1. Stella M. Hurtley

    The peroxisome is a membrane-bound organelle involved in lipid and drug metabolism. The protein constituents of the peroxisome are synthesized in the cytosol and must be imported into peroxisomes. One group of peroxisomal membrane proteins is known as the class I proteins, and their import into the peroxisome is facilitated by a chaperone protein, PEX19, which binds newly synthesized peroxisomal class I membrane proteins in the cytosol and ferries them to the peroxisome.

    Fang et al. have identified a protein, PEX3, which plays an essential role in this process by acting as a docking factor on the peroxisomal membrane for PEX19-carried import substrates. In the absence of PEX3, class I peroxisomal membrane proteins cannot enter peroxisomes, whereas class II peroxisomal membrane proteins and matrix proteins are still imported efficiently. — SMH

    J. Cell Biol. 164, 863 (2004).


    Two Ways to Trap Water

    1. Phil D. Szuromi

    Clusters and networks of water molecules in highly hydrated organic crystals can provide clues into the structures that make up bulk water. Ma et al. have grown crystals of bpedo [trans-bis(4-pyridyl)ethylene dioxide], a molecule that readily forms hydrogen bonds, which contain one-dimensional (1D) chains (when grown from methanol solution) or 2D hydrogen-bonded water layers (when grown from aqueous solution). In both forms, the water structures interact with the bpedo molecules to extend the structure in three dimensions. The 2D arrays contain (H2O)12 rings that are similar to those found in ice Ih, but that exhibit an unusual pseudo C3v symmetry. These rings are stable from 90 K up to room temperature. — PDS

    Angew. Chem. Int. Ed. 43, 1374 (2004).


    The Demographics of Leaves

    1. Andrew M. Sugden

    Leaf life-span (LLS) is a key functional trait of plants, yet co-occurring plant species often display a wide range of LLS. Documenting these patterns and relating them to variation in other traits and external ecological factors help to elucidate the range of ecological strategies exhibited by plants. Two studies suggest that LLS has a close association with factors other than water availability.

    Ackerly measured LLS in 20 co-occurring shrub species in the Californian chaparral—a habitat with a pronounced dry season—and assessed its relation to maximum water deficit and a suite of physiological, functional, and morphological traits. LLS ranged from 2 to 4.5 months in deciduous species and from 7.2 to 22.4 months in evergreens. It was not significantly correlated with water deficit, and shorter LLS was generally associated with an ability to assimilate carbon rapidly and a shorter plant stature. Reich et al. censused the timing of birth and death of 40,000 leaves of 23 tree species in the northern Amazonia rain forest—where the dry season is short and unpronounced—recording a range of LLS from 0.7 to 4.2 years. In this case, LLS was related to environmental factors such as light availability, disturbance regime, and soil fertility, but bore little relation to seasonal patterns of rainfall. — AMS

    Ecol.Monogr. 74, 25; 3 (2004).

  7. STKE

    An Antiangiogenic Network

    Endostatin is an endogenous peptide that inhibits endothelial cell proliferation, migration, and tube formation. Abdollahi et al. performed DNA and antibody microarray analysis along with selected Western blotting and immunocytochemistry to show that endostatin regulates multiple pathways in human dermal microvascular endothelial cells. Endostatin displayed a broad effect on gene expression, with genes associated with the promotion of angiogenesis generally being down-regulated and genes associated with stress responses being up-regulated. The antibody arrays showed that for many of the pathways, the phosphorylation status of individual proteins was altered by endostatin treatment. Eight signaling cascades that were down-regulated by endostatin treatment were investigated in more detail: Id signaling and activator protein 1 (AP-1) signaling (cell proliferation), hypoxia-inducible factor 1α (HIF-1α) signaling (low oxygen and metabolic adaptation), ephrin and tumor necrosis factor-α (TNF-α) signaling (cell migration), nuclear factor κB (NF-κB) (proliferation and antiapoptosis), signal transducer and activator of transcription (STAT) signaling (regulator of proliferation and migration), Ets (migration and antiapoptosis), coagulation cascades, and adhesion molecule pathways. These results identify several new pathways regulated by endostatin and present a picture of a complex network of responses to a single ligand. — NG

    Mol. Cell 13, 649 (2004).