This Week in Science

Science  21 May 2010:
Vol. 328, Issue 5981, pp. 949
  1. Wet Twists and Turns


    When salts dissolve in water, their constituent positively and negatively charged ions are pulled apart and surrounded by shells of H2O molecules (see the Perspective by Skinner). Ji et al. (p. 1003) looked closely at the motion in these shells, using a type of vibrational spectroscopy sensitive to both the orientation and to the neighbors of the targeted molecules. In agreement with recent theoretical predictions, the individual water molecules shifted orientation between an anion and the surrounding liquid in sudden discrete steps, rather than by making smooth incremental rotations. Tielrooij et al. (p. 1006) compared the relative impacts of cations and anions on the rigidity of the wider water network, using spectroscopic techniques sensitive to the role of each ion. Certain cation/anion combinations, such as magnesium sulfate, appeared to act together to restrict water motion beyond the boundaries of individual shells.

  2. Astronomical Inflation

    In astronomy, according to the theory of inflation, the universe underwent a period of extremely accelerated expansion when it was only a fraction of a second old. This process made the universe flat, isotropic, and homogeneous, and it explains how quantum mechanical seeds developed into the large-scale structure we observe today. The theory also predicts that gravitational waves were produced during the early inflationary phase. Alternative theories either predict no gravitational waves or waves with different properties. Krauss et al. (p. 989) review how primordial gravitational waves could provide information on the physics shortly after the Big Bang and how they could be detected indirectly through their imprint on the cosmic microwave background radiation—relic radiation from the Big Bang released when the universe became transparent to electromagnetic radiation.

  3. All Together Now

    In the social amoeba Dictyostelium discoideum, periodic synthesis and release of cyclic AMP (cAMP) guides the cellular aggregation required to form fruiting bodies. It has been unclear whether the initiation of this behavior is owing to synchronization of autonomously oscillating cells or whether individual cells remain nonoscillatory unless the entire population becomes oscillatory. Gregor et al. (p. 1021, published online 22 April; see the Perspective by Prindle and Hasty) used live-cell imaging to show that cAMP pulses originate from a specific location in space and that individual cells move in and out of these signaling centers. The observations suggest that oscillations do not originate from autonomous activities of specialized cells. However, individual cells do display stochastic cAMP-pulsing below a threshold external concentration of cAMP, and the generation of synchronized oscillations could only be modeled accurately when this random pulsing was taken into account.

  4. Clearing Up the Inner Core


    The behavior of Earth's core controls the planet's heat budget and magnetic field, yet its structure remains enigmatic. For instance, the seismic properties of the solid inner core suggest hemispherical structural asymmetry, but questions remain as to how these variations arose (see the Perspective by Buffett). Monnereau et al. (p. 1014, published online 15 April) modeled grain sizes of crystalline iron—the predicted dominant mineral phase in the core—and found that a slow translational motion eastward may trigger melting in the Eastern Hemisphere and solidification in the Western Hemisphere, creating a lopsided core. Deuss et al. (p. 1018, published online 15 April) examined the normal-mode seismic structure of the inner core, collected from 90 large earthquakes, which reveal not just simple hemispherical variations, but more nuanced regional structures. The overlap of the seismic data with Earth's magnetic field suggests that directionally dependent crystal alignment in the inner core formed during the solidification of the core or as a consequence of strong forces exerted by magnetism.

  5. More Crossings for Graphene

    Graphene, which consists of single sheets of graphite, has a number of distinctive electronic properties, including a conical structure that leads to a “Dirac point” where the valence and conduction band intersect at a zero-energy point. Bostwick et al. (p. 999) used angle-resolved photoemission spectroscopy to study graphene that was doped with alkali atoms and suspended from its substrate. They observed features associated with plasmarons, which arise from the interaction of the charge carriers with plasmons, the density oscillations of the electron gas. The Dirac crossing now becomes three crossings: one that involves charge bands, one involving plasmarons, and one involving the interaction between the two.

  6. Janus Drug Delivery Vehicle

    Efficient drug delivery vehicles need to be produced in a limited size range and with uniform size distribution. The self-assembly of traditional small-molecule and polymeric amphiphiles has led to the production of micelles, liposomes, polymeric micelles, and polymersomes for use in drug delivery applications. Now, Percec et al. (p. 1009) describe the self-assembly of Janus-type (i.e., two-headed) dendrimers to produce monodisperse supramolecular constructs, termed “dendrimersomes,” and other complex architectures. The structures, which showed long-term stability as well as very narrow size distributions, were easily produced by the injection of an ethanolic solution of the dendrimer into water. The dendrimersomes could be loaded with the anticancer drug doxorubicin and exhibited controlled drug release with changing pH.

  7. Network for Recovery

    A long-standing theory suggests that social diversity leads to economic development. By combining the United Kingdom's telephone communication records (both landline and mobile) with information on regional economic conditions, Eagle et al. (p. 1029) demonstrate that network diversity alone accounts for over three-quarters of the variance of a region's economic status. Although the data cannot be used to show causality, the association suggests that economic development and recovery may depend not solely on monetary stimulus but also on the development of a nation's social infrastructure.

  8. Complex SAGA


    The SAGA (Spt-Ada-Gcn5-Acetyltransferase) complex, which is conserved in eukaryotes, plays a key role in regulating gene expression. It is comprised of 21 proteins, and its functions include histone acetylation and deubiquitination. Samara et al. (p. 1025, published online 15 April) now report the structure of the SAGA deubiquitinating module (DUBm), a four-protein subcomplex, both on its own and bound to ubiquitin aldehyde. The domains are interconnected and stabilized by eight zinc atoms. The organization gives insight into why DUBm complex formation is required to activate the catalytic domain of the enzyme and suggests how DUBm might bind to monoubiquitinated histones.

  9. Penetrating Attack on Tumors

    While considerable research effort in oncology is focused on the design of new cancer drugs, an important but relatively understudied research area is the development of methods that optimize the delivery and tumor penetration of existing cancer drugs. Previous work has characterized a peptide (iRGD) that selectively targets and penetrates tumor tissue by virtue of its specific interaction with tumor blood vessels. Now, studying mouse models, Sugahara et al. (p. 1031, see the cover) show that coinjection of the iRGD peptide increases the tumor penetration and antitumor activity of several cancer drugs, including the cytotoxic agent doxorubicin and the therapeutic antibody trastuzumab (Herceptin), without increasing their harmful effects on healthy tissue. Importantly, these effects did not require chemical conjugation of the cancer drugs to the peptide.

  10. Cytosol, Salmonella, and pH

    Salmonella and other bacterial pathogens grow inside animal host cells within intracellular vacuoles. The bacteria secrete effector proteins across the vacuole membrane, altering the host-cell physiology to the pathogen's advantage. The secretion process involves a specialized secretory apparatus, the type III secretion system, whose assembly is triggered by the low pH within the host-cell vacuole. Now, Yu et al. (p. 1040, published online 15 April; see the Perspective by Collier) have identified neutral pH as a physiological signal for effector translocation by intracellular Salmonella. The process involves the disassembly of a membrane-bound regulatory complex that is also found in other animal pathogens. Thus, Salmonella exploits the low pH of the vacuole as a signal to induce assembly of the secretion system, and then the neutral pH of the cytoplasm to trigger effector translocation.

  11. Budding Yeast Kinome Revealed

    Covalent modification of proteins by phosphorylation is a primary means by which cells control the biochemical activities and functions of proteins. To better understand the full spectrum of cellular control mechanisms mediated by phosphorylation, Breitkreutz et al. (p. 1043; see the Perspective by Levy et al.) used mass spectrometry to identify proteins that interacted with the complete set of protein kinases from budding yeast and with other molecules, including phosphatases, which influence phosphorylation reactions. The results reveal a network of interacting protein kinases and phosphatases, and analysis of other interacting proteins suggests previously undiscovered roles for many of these enzymes.

  12. News from the Inner Tube of Life

    A major initiative by the U.S. National Institutes of Health to sequence 900 genomes of microorganisms that live on the surfaces and orifices of the human body has established standardized protocols and methods for such large-scale reference sequencing. By combining previously accumulated data with new data, Nelson et al. (p. 994) present an initial analysis of 178 bacterial genomes. The sampling so far barely scratches the surface of the microbial diversity found on humans, but the work provides an important baseline for future analyses.

  13. Subtle Variation

    Despite vast phenotypic differences, vertebrates have many readily recognizable specific cell types, like liver hepatocytes. The gene expression that defines specific cells depends on evolutionarily conserved orthologous transcription factors. Schmidt et al. (p. 1036, published online 8 April) studied the conservation and divergence in the genome-wide binding of two such transcription factors, CEBPA and HNF4A, in livers from human, dog, mouse, short-tailed opossum, and chicken. Although the sequence bound by orthologous transcription factors was similar, the vast majority of binding events were unique to each species.