This Week in Science

Science  26 Nov 1999:
Vol. 286, Issue 5445, pp. 1645
  1. Pollen Preferences

    Self-incompatibility in certain plants allows a plant to distinguish its own pollen from among that of other plants of the same species, and preferentially react to nonself pollen in order to limit inbreeding (see the Perspective by Dickinson). Stone et al. (p. 1729) have analyzed the activities of ARC1, which shows a phosphorylation-dependent interaction with a kinase located in the stigmatic plasma membrane. Their in vivo studies show that ARC1 is a key regulator of self-incompatibility from the female side. Schopfer et al. (p. 1697) have now identified a critical component from the pollen side—a polymorphic protein expressed in the pollen that interacts with the stigmatic proteins to determine whether the outcome will be fertile.

  2. Cranking Out ATP

    Although the common form of energy used in cells is adenosine triphosphate (ATP), energy is stored both in chemical forms such as fat or starch and in electrochemical forms such as ion gradients across cellular membranes. Fat and starch are metabolized to produce small amounts of ATP and large amounts of energy in the form of a gradient of protons, which is then converted into ATP by an enzyme known as ATP synthase. Stock et al. (p. 1700; see the cover) present the crystal structure of the membrane-bound portion (F0) of the synthase and the axle that connects it to the ATP-synthesizing part (F1). F0 transforms the energetically downhill movement of protons into rotation of an internal axle (the γ subunit of F1) as shown by Sambongi et al. (p. 1722). These workings of a nanogenerator are discussed in a Perspective by Fillingame.

  3. The Sun, the Moon, and the Plumes

    There have been several episodes of large-scale volcanism recorded on Earth's surface, and many of these events are thought to have been initiated by the upwelling of a major thermal anomaly (a plume) from the core-mantle boundary. Greff-Lefftz and Legros (p. 1707) have modeled a possible connection between episodes of volcanism and the initiation of a plume by the interplay of the gravitational forces between Earth, the moon, and the sun. They show that fluid outer-core oscillations set up by the lunar-solar tides and core rotation resonated with the solar tidal waves and that these resonances, which could initiate a plume, occurred at about the same time as three episodes of volcanism in the Precambrian.

  4. Oxygen in Action

    Despite the importance of oxygen as an electron acceptor in many enzymatic reactions, the mechanism of oxygen activation by redox enzymes is not well understood. Wilmot et al. (p. 1724) determined x-ray structures of freeze-trapped intermediates related to the oxidative half-reaction of copper amine oxidase. They also determined the oxidation state of the quinone cofactor for each structure by using single-crystal spectrophotometry. Their studies reveal the binding site for dioxygen and the hydrogen peroxide product and show the proton transfer pathways to the oxygen. The product aldehyde inhibits enzyme reoxidation in the crystal, and thus the release of the aldehyde is rate-determining.

  5. The Winds of Change

    The subdecadal variability of climate in the region of the tropical North Atlantic Ocean during the past century is thought to be well understood, but longer term variations that occur over decades to centuries are more difficult to resolve because instrumental records do not extend far enough into the past and most existing sedimentary records do not have sufficient temporal resolution. Black et al. (p. 1709) present a 825-year-long record from a southern Caribbean sediment core that reveals decadal and century-scale trade wind variations which indicates that natural climate changes also occurred abruptly in the preindustrial past. These findings also highlight the link between thermohaline circulation in the Atlantic Ocean and other tropical climatic variations.

  6. Nanoporous Ceramic Films via Polymers

    Block copolymers, in which segments (blocks) of different chemical composition are connected, can phase-separate at the nanometer scale and form intricate structures. Chan et al. (p. 1716) show that copolymers containing two hydrocarbon blocks separated by a block made from a silicon-containing polymer can form an open nanoporous network with a double gyroid morphology. A single-step process that uses ozone and ultraviolet light converts the silicon-containing block into a ceramic framework and removes the remaining hydrocarbon material to create nanoporous ceramic films.

  7. Molecular Shuttle

    The scanning tunneling microscope (STM) allows great control over atomic and molecular manipulation on surfaces. The microscope tip can also be used to record vibrational spectra that allow the identification and characterization of single adsorbed molecules and the bonds they form with the substrate. Lee and Ho (p. 1719) used these combined capabilities of their low-temperature STM to study the bond formation between single iron atoms on a silver surface and carbon monoxide (CO) molecules delivered with the STM tip. Complexes of iron with one or two CO molecules were formed and characterized.

  8. Joint Antigen

    Rheumatoid arthritis is an autoimmune disease in which both T cells and B cells become activated. Most current research has centered upon finding specific antigens of the joint that could be the immunogenic targets for T cell-mediated damage. In a mouse model of arthritis that seems to mimic many of the features of the human condition, Matsumoto et al. (p. 1732) have determined that the antigen with the arthritogenic activity for the B cells as well as the T cells is from the same protein, glucose-6-phosphate isomerase, a ubiquitously expressed enzyme. Thus, local autoimmunity need not be caused by a tissue-specific antigen.

  9. Death Is Role Within a Normal Life

    Proteins of the Bcl-2 family play both roles in apoptosis, either promoting or inhibiting cell death. The smaller proteins that contain only the third Bcl-2 homology domain (BH3-only proteins, such as Bim) are thought to have pro-death properties, but their role in normal apoptotic functions in vivo, such as homeostasis and immune function, has been unclear. Bouillet et al. (p. 1735) now report results for a gene-targeted Bim-deficient mouse which show that this BH3-only protein is critical for normal homeostasis of white blood cells, cytokine withdrawal, and calcium-induced apoptosis and also acts as a barrier against autoimmune disease. Thus, Bim is a part of the physiological apoptosis machinery.

  10. Working It Out Downstream

    A cell's response to a particular growth factor or hormone involves a relay of signals between the cell surface and the nucleus. The small GTP-binding protein Ras is activated by numerous growth factor receptors and can regulate more than one signaling pathway in response to a single growth factor, depending on the cell type. Hence, Ras can serve as a branch point for signal propagation, and tight regulation of the pathways it stimulates is needed to ensure the proper biological outcome. Two reports describe the cross communication that occurs between two pathways that lie downstream of Ras, the Raf kinase-MEK (mitogen-activated protein kinase)-ERK (extracellular-regulated kinase) pathway and the PI3K (phosphatidylinositol 3-kinase)-Akt kinase pathway. Zimmermann and Moelling (p. 1741) demonstrate that Akt inhibited Raf activity by phosphorylating a known regulatory site on Raf. In breast cancer cells, Akt directly interacted with Raf, and inhibition of Akt resulted in increased Raf activity. The authors suggest that in these cells, this mechanism may regulate growth response to insulin-like growth factor (IGF). Rommel et al. (p. 1738) also reveal that Akt associated with Raf and inhibited the Raf signaling pathway in differentiated muscle cells (myotubes) but not in undifferentiated precursor cells. Activation of the Akt pathway or inhibition of the Raf pathway resulted in a hypertrophic phenotype (thickening) similar to that induced by IGF on muscle cells. These studies suggest that integration of these two pathways into a particular cellular response may depend on cell type or the stage of cell differentiation.

  11. Force of Habit

    Patterns of behavior can be learned and become so routine as to occur unconsciously. Jog et al. (p. 1745) monitored neural activities within the basal ganglia, a portion of the brain known to be involved in the control of movement. These neural activities changed over time in rats learning a T-maze (the rats associated a tone cue with the correct direction in which to turn at a T-junction toward a food reward). As the rat's accuracy and speed improved, the ensemble of neurons appear to fire most actively at the beginning and at the end of what became a habitual behavior. The representation of entire sequences of motor commands within the basal ganglia may relate to the difficulties that Parkinson's patients experience in beginning or ending movements.

  12. Do You Mind?

    Our awareness that other people have beliefs and that their behavior can be explained on the basis of these beliefs has been called “theory of mind.” Frith and Frith (p. 1692) review data from clinical, neurophysiological, and brain imaging studies that support the description of a network of brain areas underlying such a theory of mind. They then go on to propose that this aspect of social intelligence, which appears to be most highly developed in humans, has arisen from more primitive abilities in apes and monkeys for the representation of actions and goals.

  13. MgO and Mantle Anisotropy

    The lower mantle is thought to be composed primarily of perovskite, (Mg,Fe)SiO3, and magnesiowustite, (Mg,Fe)O, and much work has focused on determining how these phases might explain seismic heterogeneities (anisotropy) at the base of the lower mantle in the D” layer. Unfortunately, the high-pressure and high-temperature properties coincident with D” layer conditions can only be approximated by models or experiments for these complex phases in which the exact amount of Mg or Fe is only estimated. Karki et al. (p. 1705) modeled the bulk properties (density and velocities) of the simpler phase, MgO, using density functional perturbation theory. Their results suggest that the lattice-preferred orientation of MgO remains a dominant property at high pressures and temperatures, so that anisotropy in D” may be explained by crystal structure rather than a changes in composition or a thermal anomaly.

  14. Weak But Decisive

    Reactions between atoms and molecules usually proceed via a transition state, the high-energy state through which the reactants must pass before becoming products. The transition-state structure is crucial in determining the overall reaction dynamics and rates. However, Skouteris et al. (p. 1713) found that when the reactants are still relatively far apart, weak van der Waals interactions can also influence reaction rates. In the reaction of chlorine atoms with HD molecules, these weak interactions influenced the way in which the reactants approach each other, and thus affect the reaction dynamics. However, these effects can only be seen at low rotational excitations of the reactants, and their detection requires sophisticated crossed molecular beam experiments and state-of-the-art ab initio calculations.

  15. Species and Regular and Irregular Areas

    Harte et al. (Reports, 9 Apr., p. 334) argued that the probability distribution of species is self-similar with respect to area and use this notion to reconcile and address several common species-area relations. Maddux and Athreya comment that the model of Harte et al. “implies that species are distributed in one of three trivial ways” and thus conclude that the law is “in general…invalid.” Harte et al. respond that “patch…shape does matter” and that the prediction made by [their] theory about the dependence of species richness on patch shape is reasonable.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full//286/5445/1647a

  16. Polymorphisms Return in the IL-13 Promoter

    Interleukin-13 (IL-13) plays an important role in allergic responses. In a previous Technical Comment and Response (28 May 1999), Anderson et al. and Wills-Karp and Rosenwasser examined part of the IL-13 promoter region, found no polymorphisms there, and concluded that the promoter region was not a susceptibility locus for atopy. Van der Pouw Kraan et al. now report that they find evidence of polymorphism just outside the region examined earlier, a finding that Gillespie et al. confirm. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/286/5445/1647b