This Week in Science

Science  24 Nov 2000:
Vol. 290, Issue 5496, pp. 1461
  1. How Bronze Got Left Behind

    Metal alloys have been the subject of many structural studies, but the dynamics of their formation has been more difficult to study. Schmid et al. (p. 1561; see the Perspective by Besenbacher and Nørskov) have investigated the formation of bronze by placing very low concentrations of tin (hundredths of a monolayer) on an atomically flat copper surface. What they find is an unusual cooperative mechanism. The tin atoms form large islands (hundreds of thousands of atoms) on the surface that exchange with copper atoms to form bronze crystallites. However, the tin islands are repelled by the tin atoms that become trapped in the surface bronze, and thus move rapidly across the surface toward fresh copper and leave trails of smaller bronze islands in their wake. This motion, which is driven by surface free energy, is reminiscent of the “camphor dance” seen on liquid surfaces.

  2. Defects Disappearing in Groups

    The interactions between the distinct chemical species of block copolymers result in the tendency of regular lines of each species to evolve as the material is annealed (coarsening). However, topological defects (disclinations and dislocations) will ultimately limit the potential of those nanometer-scale patterns for lithographic processing or self-assembled structure formation, or both. Harrison et al. (p. 1558) used time-lapse atomic force microscopy to monitor the evolution of the stripe patterns with annealing time. Defect annihilation proceeded by a dominant mechanism involving three or four disclinations, and the density of those defects played a crucial role in the kinetic evolution of the two-dimensional patterns.

  3. A Scaffold for Kinases

    The β-arrestins were so named because they function in the desensitization of β2-adrenergic receptors. However, these proteins play other roles, both in receptor endocytosis and in promoting receptor signaling. McDonald et al. (p. 1574; see the Perspective by Pouysségur) now describe yet another function for these versatile proteins—β-arrestin 2 binds to the mitogen-activated protein kinase family member JNK3 (c-Jun NH2-terminal kinase 3) and to the upstream kinases MKK4 and ASK1 that act in a cascade to activate JNK3. β-Arrestin 2 appears to act as a scaffold that brings the kinases together and helps retain JNK3 in the cytosol. In cells treated with agonists of G protein-coupled receptors, the β-arrestin 2 protein appears to target JNK3 to endosomal vesicles.

  4. The Spin on Carbon Nanotubes

    Metallic single-walled carbon nanotubes (SWNTs) present almost ideal systems in which to study electronic transport in one dimension. Recent interest has now turned to the spin physics of these systems, in which the local interaction between magnetic atoms and the conduction electrons are studied. Odom et al. (p. 1549) used scanning tunneling spectroscopy to show that magnetic clusters produce a peak in the tunneling conductance. The decay with distance from the cluster and with temperature are both consistent with a Kondo resonance. Additionally, discrete energy levels of this resonance emerge when the length of the nanotube available to the clusters is shortened, which is consistent with a “spin-in-a-box” scenario.

  5. Going Against the Grains

    Seismologists use differences in wave speeds in different directions (seismic anisotropy) to estimate the direction of mantle flow. On a microscopic scale, the seismic anisotropy is attributed to strain-induced lattice orientation of olivine and pyroxene grains. Bystricky et al. (p. 1564; see the Perspective by Mackwell and Rubie) performed laboratory experiments at high temperature and very high bulk shear strains (conditions similar to those in the mantle) to determine the microscopic deformation of olivine aggregates. At the highest shear strains, olivine recrystallizes along subgrain rotation boundaries to create a somewhat more random texture than was expected. Thus, the slowest speeds will not always be normal to the shear plane, and olivine microstructure may not be a good indicator of mantle motions.

  6. The Dope on Nanotube Diodes

    Semiconducting carbon nanotubes are naturally p-type conductors—they conduct holes better than electrons. Impurities adsorbed onto the surface can alter the electrical conduction of the tubes and are usually to be avoided. However, Zhou et al. (p. 1552) raise the possibility of using adsorbed atoms to good effect. They demonstrate diode behavior for nanotubes where one half of the tube is exposed to potassium atoms, which dopes that region of the tube n-type, while the other half remains p-type. The subsequent diode behavior may suggest a route toward designer nanoelectronics with carbon nanotubes as the basic building blocks.

  7. Harnessing Biomotors

    The F1-ATPase enzyme undergoes rotary motion when it uses adenosine triphosphate as an energy source, and previous studies have imaged this motion by attaching actin filaments to the enzyme's central subunit. Soong et al. (p. 1555) have now harnessed this enzyme to drive inorganic propellers. Appropriately tagged enzymes were attached to the tops of an array of nanometer-scale nickel posts and, in turn, nickel propellers, 150 nanometers (nm) in diameter and 750 to 1400 nm in length, were attached to the tops of the “motors.” Although such assembly is not foolproof (only a small fraction of motors are viable), some can be seen to drive currents in the surrounding solution for more than 2 hours while being fed ATP.

  8. Receptors Follow a Party Line

    A receptor is normally triggered when, and only when, its ligand binds to the receptor. It's not so simple for the ErbB1 receptor, which responds to stimulation with epidermal growth factor (EGF) by dimerization, cross phosphorylation of the monomers, and triggering of downstream pathways. Verveer et al. (p. 1567) have visualized phosphorylated ErbB1 in MCF7 cells with fluoresence lifetime imaging microscopy in conjunction with fluorescence resonance energy transfer, which was used to monitor the binding of an antibody to phosphotyrosine. Focal stimulation of the cell with EGF immobilized on beads resulted in a rapid propagation of receptor phosphorylation over the entire surface of the cell—even those receptors that had not been exposed to EGF.

  9. Foldings FADs

    The endoplasmic reticulum (ER) presents an oxidative folding environment to newly synthesized proteins that have been translocated from the cytosol. The formation of disulfide bonds within and between proteins is one of the important reactions that is promoted within the ER, whose faithful performance is crucial for the production of correctly folded secretory and membrane proteins. Tu et al. (p. 1571) describe the successful reconstitution of ER-like folding in a cell-free system and find that the cofactor for successful folding is flavin adenine dinucleotide (FAD)—rather than ubiquinone or heme, as may have been expected.

  10. A Delible Mark

    Cloning of mammals by nuclear transfer has been thought to require epigenetic reprogramming of the donor cell from a differentiated to an undifferentiated state, but this process has not been demonstrated at the cellular level. One example of an epigenetic mark imposed during differentiation of female somatic cells is X chromosome inactivation. Eggan et al. (p. 1578; see the Perspective by Clerc and Avner) used an X-linked reporter gene encoding a fluorescent marker to study X chromosome inactivation in cloned mouse embryos derived from female fibroblast nuclei. Somatically inactivated X chromosomes can be reactivated during early cleavage of the cloned embryos and are then subject to normal random inactivation in embryonic lineages. Thus, the marks that distinguish active and inactive X chromosomes can be erased and re-established during the cloning process. The cloned embryos may be useful tools for studying the precise nature of these epigenetic marks.

  11. Perceptive Maturation

    The visual process whereby images are decomposed into their component attributes (a line, its orientation, its color, and so forth) imposes the challenge of constructing the perceived world by properly assorting and recombining these characteristics. In particular, the illusory square known as a Kanizsa figure (formed by 90° wedge cutouts in four circles placed at corners of the square) is perceived by 8-month-old infants but not by 6-month-olds, and thus tests the capability for visually “binding” the four isolated objects. Csibra et al. (p. 1582) examined the electrophysiological responses in these two groups of infants and found a concomitant appearance of stimulus-induced oscillations in the gamma frequency range (40 hertz) of neural activity. These findings support the idea that this activity represents the correlate of perceptual binding.

  12. How Green Was My Protein

    Green fluorescent protein (GFP) is widely used as a non-invasive probe to monitor protein localization or to trace expression. The recently discovered red fluorescent protein is complementary to GFP and allows applications such as fluorescence resonance energy transfer. Terskikh et al. (p. 1585; see the news story by Chichurel) have generated a mutant of red fluorescent protein (E5) that changes its fluorescence from green to red over time. Because the rate of color conversion is independent of protein concentration, E5 can be used as a fluorescent clock to monitor the activation and down-regulation of gene expression in whole organisms.

  13. Killing Signaling

    Yersinia species are bacterial pathogens responsible for a variety of diseases in animals and plants, including Y. pestis, which was responsible for the Black Death. Orth et al. (p. 1594; see the news story by Brown) examined the activities of one of the Yersinia virulence factors, YopJ. The protein showed homology to cysteine proteases and appeared to act specifically to degrade signaling proteins that had been tagged by the ubiquitin-like molecule known as SUMO-1.

  14. The Advantages of Taking Breaks

    Reports on AIDS patients undergoing highly active antiretroviral therapies have begun to suggest that structured treatment interruptions (STI) can lead to improved immune response, reduced toxicity, and can favor the appearance of wild-type virus over resistant mutants. If proven, these protocols might be easier for patients to tolerate, and less costly. A randomized, controlled test of the hypothesis that STI is beneficial has been performed by Lori et al. (p. 1591), who studied the effects of alternating therapy (3 weeks on and 3 weeks off) on rhesus macaques infected with simian immunodeficiency virus. The protocol resulted in virus-specific immune responses and control of virus replication relative to animals that were treated continuously and then had treatment stopped.

  15. Mixing Up a Magnetic Stew

    Metallic ferromagnetism is a familiar phenomenon, but a clear understanding of the mechanism underlying the complex interactions of electronic charge and magnetic spin remains unclear. Unbalanced populations of spin-up and spin-down states are thought to play a major role, but studies on bulk materials prevent the isolation of the contribution of the different spin states. De Poortere et al. (p. 1546) present results on a dilute two-dimensional electron gas (2DEG) placed in a magnetic field. The electrons condense into a ladder of energy level, or Landau level, with each level occupied by either spin-up or spin-down electrons. By tilting the magnetic field with respect to the plane of the 2DEG and measuring the response of the 2DEG resistance, information on the mixing of the different spin-states can be observed.

  16. A Well-Designed Pump

    During respiration, the enzyme cytochrome oxidase reduces O2 to water and uses the released energy to drive proton translocation and adenosine 5'-triphosphate synthesis. A tyrosine acidified by a covalently linked imidazole has been proposed as a possible proton donor for O2 reduction by the enzyme. Proshlyakov et al. (p. 1588) show that in the intermediate that lies at the junction of O2 reduction and proton pumping, this tyrosine exists as a radical. Thus, when oxygen is reduced to water, the oxidizing equivalents generated are transferred to the enzyme to avoid the formation of toxic oxygen species. The oxidizing equivalents can then be processed by the enzyme to achieve proton pumping.

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