This Week in Science

Science  18 Jun 2004:
Vol. 304, Issue 5678, pp. 1713
  1. Echo-Location of Surface Potentials


    Helium scattering has long been used to measure the potential energy landscape of surfaces. The method relies on chopping the beam into short sections and determining energies by time-of-flight mass spectrometry. Jardine et al. (p. 1790) now show that a helium “spin echo” approach can dramatically improve energy resolution. They create a spin-polarized beam of helium-3 and use nuclear spin-precession to determine the energy spectra of scattered atoms. They applied this method in two ways. They measured the surface band structure of LiF(100) by the interaction of helium with attractive parts of the surface potential through selective adsorption resonances. They also used quasi-elastic scattering to probe the dynamics of CO diffusing on the Cu(001) surface.

  2. Nanorods with Gold Stickers

    Combining both semiconducting and metallic materials into a single nanoscale object can still be a synthetic challenge. Mokari et al. (p. 1787) have developed a simple chemical method for anisotropically and controllably attaching gold tips onto cadmium selenide nanorods and tetrapods. The addition of the gold changes the optical properties of the rods because of the strong coupling between the two materials. The addition of the gold tips also increases their conductivity and opens up new methods for connecting the rods into electrical devices.

  3. Close Encounter of Comet Wild 2

    The Stardust spacecraft slowly drifted within 236 kilometers of comet Wild 2 in January 2004. Its primary mission was to capture comet dust and bring it back to Earth for analyses. During the close encounter, the navigation camera snapped 72 images of the comet's nucleus, and Brownlee et al. (p. 1764) found an oddly shaped, pockmarked structure that must have more cohesive strength than had been predicted (see the Perspective by Weaver). The dust-flux monitor counted several bursts of particles, and Tuzzolino et al. (p. 1776) interpret these fluxes as fragmentation of larger aggregates. Kissel et al. (p. 1774) analyzed spectra of some particles and found not only organic-rich material but also nitrogen and sulfur-rich species. Most of these particles are accelerating away from the comet and toward Stardust in gaseous jets (see the Perspective by Levasseur-Regourd). Analysis of these jets by Sekanina et al. (p. 1769) suggests that they originate from small source regions and become narrow sheets of particles as they hit the spacecraft. The close encounter reveals a comet that is more structurally and chemically complex than had been expected.

  4. Making and Breaking Prions


    Chaperones and protein amyloids are involved in prion diseases, protein-based elements of inheritance, and mechanisms of learning and memory. Shorter and Lindquist (p. 1793, published online 20 May 2004) describe in detail the multiple mechanisms by which the chaperone protein Hsp104 governs the inheritance of the yeast prion phenotype [PSI+]. Hsp104 has multiple reaction mechanisms, not just one. The nature of its interactions with the prion depends upon three things: the relative concentrations of the Hsp104 and the prion protein, the physical state of the prion protein when it interacts with Hsp104, and whether adenosine triphosphate (ATP) hydrolysis takes place. Prion oligomers are not only on the amyloidogenesis pathway, but are an obligate form for amyloid nucleation, being regulated stringently by Hsp104.

  5. Game, Set, Match

    Monkeys are extraordinarily adept at playing a matching game, where choices are rewarded in a historically biased stochastic fashion; they approach the performance of an ideal observer and can rapidly alter their ratio of choices when the ratio of rewards changes. Sugrue et al. (p. 1782; see the Perspective by Daw and Dayan) used this behavioral setting to examine the activity of neurons in the lateral intraparietal area and how the coding of fractional value influences choice. Averaged activity tracked behavioral performance and shifted when rewards changed. Continual sampling of the less valued target allowed the monkey to detect precisely when these changes occurred.

  6. Putting Hedgehog to Work

    The secreted protein Hedgehog (Hh) has many roles in development and tumorigenesis. Many components of the Hh signaling pathway are conserved across species, but how their interactions transmit a signal from the cell surface to the cytoplasm and nucleus has not been so clear. Lum and Beachy (p. 1755) review how recent genetic studies in Drosophila suggest a complex Hh response mechanism in which a scaffold protein routes the Hh signal from its sensors at the membrane directly to a transcriptional effector. Additional Hh pathway components recently characterized in Drosophila and mouse are also highlighted.

  7. Forming Safe Havens Within

    The propagation of invasive bacteria and mycobacteria is tightly linked to their ability to withstand delivery to the degradative compartment of the cell, the lysosome. Walburger et al. (p. 1800, published online 20 May 2004) identify a mycobacterial protein involved in the modulation of phagosome-lysosome trafficking. Protein kinase G, a eukaryotic-like serine/threonine kinase from the pathogenic mycobacteria, M. tuberculosis, is responsible for inhibiting phagosome-lysosome fusion, promoting mycobacterial survival inside macrophages. Hernandez et al. (p. 1805) find that Salmonella, the food-poisoning and typhoid fever bacterium, modulates vesicular trafficking by altering phosphoinositide metabolism through SopB, a phosphoinositide phosphatase. SopB is delivered into host cells by an invasion-associated specialized secretion system, and mediates the formation of a characteristic very spacious phagosome within which Salmonella resides after internalization. In the absence of SopB, invading Salmonella reside within tight phagosomes, and exhibit a membrane trafficking defect and impaired bacterial intracellular growth.

  8. Messenger RNA on the Move


    In order to serve as a template for protein synthesis, messenger RNA (mRNA) must find its way from its site of synthesis in the nucleus to the nuclear pore, the exit point from the nucleus that allows access to the cytoplasm. Whether mRNA-protein complexes (mRNPs) move through the nucleus randomly or by a directed transport mechanism has been a matter of controversy. By combining fluorescent reporter gene technology with sequential imaging analysis, Shav-Tal et al. (p. 1797) could monitor the movement of single mRNPs within the nucleus of living mammalian cells from the moment of transcription. mRNAs were observed to move through the nucleus by simple diffusion.

  9. We're Here to Help

    On the road to committing to antibody production, most B cells must become receptive to supporting signals delivered by antigen-specific helper T cells. Jordan et al. (p. 1808) reveal that induction of this competence—known as priming—requires a third class of cells that provide a generic priming cue. The auxiliary population was myeloid in origin and accumulated within the spleens of mice after injection with alum, a well-known adjuvant. However, B cell priming failed to take place when either this accessory population, or the cytokine signals it produces, were absent. Adjuvants, like the innate immune signals they mimic, directly influence the responsiveness of B cells.

  10. Distinguishing Synaptic Release Sites

    Release of the principal excitatory neurotransmitter glutamate depends on its transport into synaptic vesicles by a family of proteins responsible for vesicular glutamate transport. Vesicular glutamate transporters (VGLUT) 1 and 2 have a mutually exclusive distribution in the adult brain. Fremeau et al. (p. 1815, published online 29 April 2004; see the Perspective by Schuske and Jorgensen) now show that earlier in development, their distributions overlap and that they mediate glutamate release from distinct synaptic sites made by the same neuron. Release mediated by the two isoforms also differs in the response to repetitive stimulation, which suggests that there are functional differences between the two release sites. The loss of VGLUT1 selectively reduces the reserve pool of synaptic vesicles and may play a role in membrane trafficking at the nerve terminal.

  11. DNA Damage and Long-Term Memory

    What are the molecular mechanisms underlying the requirement for protein synthesis in learning? Cohen-Armon et al. (p. 1820) found that polyADP-ribose-polymerase-1 was activated during formation of long-term memories in the sea slug Aplysia. Activation occurred in response to repeated serotonin application in isolated pleural-pedal ganglia, as well as during long-term sensitization of a withdrawal reflex and operant conditioning of feeding behavior. Inhibition of polyADP-ribosylation prevented long-term memory but had no effect on short-term memory. The blockade was effective during training, but not during the subsequent consolidation period. The effect on memory could be mediated through histone H1, which the authors showed to be polyADP-ribosylated during long-term memory formation.

  12. Understanding Noise in Gene Expression

    Phenotypic variability may contribute to the evolution of a species. Raser and O'Shea (p. 1811, published online 27 May 2004) now measure variability in gene expression, sometimes known as noise, in yeast. Noise was gene-specific and controlled by the balance between promoter activation and transcription. Two types of noise were examined: intrinsic noise, which occurs during gene expression, and extrinsic noise, which can be caused by cell heterogeneity or signal transduction events.

  13. Going on Holliday?

    Homologous recombination (HR) is a vital DNA damage-repair pathway. A common intermediate of HR is the Holliday junction complex, a structure that also occurs during meiosis and mitosis. Mus81 is an endonuclease implicated in Holliday junction resolution and/or in the processing of stalled replication forks in yeast. In order to probe the function of Mus81 in mammals, McPherson et al. (p. 1822) generated mice lacking the gene. Surprisingly, the mice are viable and fertile, and demonstrate normal lymphocyte development and activation, ruling out a role for Mus81 in the repair of programmed DNA breaks as well as in meiotic recombination. However, heterozygous and homozygous mice show an elevated incidence of chromosomal aberration and various types of malignancy, revealing that Mus81 is a haploinsufficient tumor suppressor protein.