Editors' Choice

Science  23 Jan 2004:
Vol. 303, Issue 5657, pp. 435

    Maintaining Distinctions

    1. Stella M. Hurtley

    During mammalian cell division, single-copy organelles such as the Golgi apparatus must be partitioned, with a portion allocated to each daughter cell. At the onset of mitosis, the Golgi apparatus disassembles from a compact structure located next to the nucleus into small vesicles that appear as a dispersed cloud within the cytosol. It is also possible to induce Golgi disassembly experimentally by adding the drug brefeldin A, but in this case the Golgi membranes actually merge with the distributed network of membranes that constitute the endoplasmic reticulum (ER).

    Pecot and Malhotra address a long-standing debate about whether the contents of Golgi-derived vesicles and of the ER mix during mitosis. The small molecule rapamycin enters into a tightly associated trimeric complex with two proteins, FKBP and FRAP, that do not by themselves interact. After joining FKBP to the Golgi enzyme sialyltransferase and FRAP to an ER-resident protein, the authors observed rapamycin-mediated colocalization of the latter with the former only when brefeldin A was added to dividing cells, demonstrating that (in a drug-free environment) the Golgi and the ER remain distinct compartments throughout mitosis. — SMH

    Cell 116, 99 (2004).


    Rapid Decompression

    1. Brooks Hanson

    Abrupt changes in Earth's past climate are often associated with evidence for an increase in methane, a major greenhouse gas, in the atmosphere. One hypothesis is that these changes reflect sudden destabilization and release of large amounts of methane that are stored as clathrates on the continental shelves. Furthermore, extreme cases of destabilization have been proposed as a possible cause of some mass extinctions.

    One way in which the methane in clathrates could be released abruptly would be as a consequence of large or numerous submarine landslides. Maslin et al. tested this relation by compiling dates of known submarine landslides during the past 45,000 years. Although the number of well-studied submarine slides is limited, 11 out of 27 slides occurred during two time intervals associated with known increases of atmospheric methane: about 15,000 to 13,000 and 11,000 to 8000 years ago. These relations broadly support the methane release hypothesis. Some of the slides also seem to be clustered around areas undergoing rapid postglacial rebound, suggesting that further extensive melting of ice sheets could lead to additional submarine slides. — BH

    Geology 32, 53 (2004)


    Seeing the Deadwood Through the Trees

    1. Andrew M. Sugden

    The death rate of trees is a key parameter for ecologists quantifying and elucidating forest dynamics. Knowledge of this rate, especially in tropical forests, provides important clues about how forests are responding to climate change and whether forests are sources or sinks for carbon. However, data have been in short supply because of the practical obstacles to counting dead trees from the ground. Recent research has shown that high-spatial-resolution satellite images can be used to measure a variety of forest features, such as tree crown size and the frequency of forest gaps across wide areas. Clark et al. show that such techniques can be used to provide reliable measures of tree mortality. In their study area at La Selva Biological Station in Costa Rica, satellite remote sensing at a resolution of <1 m, corroborated by ground-based data from inventory plots, indicated annual mortality rates of 2.8% for canopy trees. Regular collection of similar data from this and other tropical forests will reveal whether mortality rates are stable, increasing, or decreasing. — AMS

    Ecol. Lett. 7, 52 (2004)


    Scavenging Cholesterol

    1. Lisa D. Chong

    The plasma low-and high-density lipoproteins (LDL and HDL) carry cholesterol that cells use for processes such as membrane and steroid synthesis. Cell surface receptors that capture LDL are internalized, and cholesterol is recovered at a late stage in the endocytic pathway. In contrast, the scavenger receptor BI (SR-BI) that binds HDL does not undergo endocytosis, and the traffic of cholesterol from these serum lipoproteins to cells has not been clear.

    Peng et al. observed that SR-BI is present in clusters on microvillar extensions of plasma membrane in a variety of cell types; importantly, SR-BI did not localize to caveolae. Moreover, caveolin-1 was not detected in membrane extensions, and the absence of caveolin-1 in some cell types had no effect on receptor localization.

    Detergent solublization analysis indicated that SR-BI is not associated with lipid rafts, which are membrane microdomains where cholesterol is tightly packed with saturated phospholipids in a highly ordered state. The authors propose that SR-BI resides in a more fluid microdomain populated by unsaturated phospholipids and that this disposition favors efficient movement of cholesterol between HDL and the cell membrane. — LDC

    Mol. Biol. Cell 15, 384 (2004).


    Small Is Everlasting

    1. Gilbert J. Chin

    Self-preservation is a wonderful thing, so much so that psychological defenses can operate without our awareness. Gilbert et al. present a trio of studies that lay bare some of these processes. In personal and impersonal behavioral situations, participants were asked to rate the intensity of their dislike for a set of experiences both in the present and the near future, and then asked at that later time to report how they felt. Remarkably, being insulted (via written evaluations) by someone they expected to meet and greet (a partner) was not as distressing as being treated similarly by someone (a nonpartner) they did not expect to see. The explanation of this unusual turn of events relies on the region-β paradox, which posits the activation of a shielding mechanism when a threshold of distress is reached. Thus, the impersonal slight is not sufficiently hurtful, but the personal critique is so devastating as to trigger healing so effective and imperceptible that an event initially thought to be dire is instead shrugged off. The corollary is that the unattended injury persists and can, in time, create a deep and lasting impression. — GJC

    Psychol. Sci. 15, 14 (2004).


    Attosecond Streak Camera

    1. Ian S. Osborne

    Streak cameras take snapshots of dynamical processes, allowing a process to be broken down and analyzed frame by frame. Pump-probe spectroscopy with pico-and femtosecond laser pulses has become a routine laboratory tool for investigating the dynamics of chemical reactions. With the advent of laser pulses only several hundred attoseconds in duration, some of the fastest physical processes, such as the internal dynamics of electrons within the nucleus, can be studied. However, the required relativistic energy of the laser pulses brings with it prohibitive problems: The electrons are stripped from the nucleus by the high electric and magnetic fields of the laser pulses, and recollision with the parent nucleus is prevented.

    Milosevic et al. describe a laser configuration that circumvents this problem. Their scheme involves the use of two counterpropagating laser pulses, circularly polarized and of equal handedness, which can be used to accelerate electrons to multiple-MeV energies and to refocus them onto their parent atom. Such control of the electron dynamics should lead to the ability to image the dynamics of nuclear processes. — ISO

    Phys. Rev. Lett. 92, 013002 (2004).


    A Faster Turn-Off

    1. Marc S. Lavine

    The rod-shaped molecules of a nematic liquid crystal tend to align with each other to give a local orientational ordering. The direction of the ordering can be biased by the presence of surfaces and can be altered by the application of an electric field; this forms the basis of a simple liquid crystal display. However, even though these basic electro-optic cells switch on very quickly, the return to the ground state is a much slower process.

    By making a number of changes to the design of the cell, Alexe-Ionescu et al. found that they could make the switching times equal. They first made the cell anisotropic by coating one of the indium tin oxide (ITO)-covered glass surfaces with an aromatic polymer film, which creates different anchoring effects at the different surfaces. This well-known trick ensures that the nematic switching is gradual instead of being triggered at a critical switching voltage. The second change they made was to replace the dc voltage with square voltage pulses at frequencies from 50 to 15,000 Hz. When this voltage was applied to the polymer side at frequencies above 100 Hz, and when the cell was switched by toggling the ground state side, fast switching off was observed. Under these conditions, there is a buildup of charge at the polymer-nematic interface, which enhances the anchoring of the nematic and thus the return to the ground state. — MSL

    Appl. Phys. Lett. 84, 40 (2004).