Editors' Choice

Science  20 Aug 2004:
Vol. 305, Issue 5687, pp. 1079
  1. BIOCHEMISTRY

    Air Tight

    1. Gilbert J. Chin

    A recent theme in explanations of the catalytic efficacy of protein enzymes is the harnessing of correlated fluctuations (at the atomic scale) of the polypeptide backbone and amino acid side chains. In some cases, these motions would serve to align and reposition chemically powerful groups within the active site, whereas in other cases, they would act to control the entry of reactants and exit of products.

    Salomonsson et al. assess just how snug the entryway into the cytochrome c oxidase catalytic center is; this enzyme uses four electrons and four protons to transform gaseous O2 into liquid H2O. Using CO as an inactive analog of O2, they find that adding a three-carbon unit (by swapping valine for glycine) obstructs access, implying that fluctuations, at least in this portion of the enzyme, are constrained to the extent that a diatomic gas cannot squeeze through. — GJC

    Proc. Natl. Acad. Sci. U.S.A. 101, 11617 (2004).

  2. CHEMISTRY

    Synthetic GPIs

    1. Julia Fahrenkamp-Uppenbrink

    An effective malaria vaccine could potentially save millions of lives each year but is not yet available. Conjugated carbohydrates that mimic the structures of surfaced-exposed moieties such as glycosylphosphatidylinositol (GPI) would be useful for vaccine development (see also Verez-Bencomo et al., Reports, 23 July, p. 522). Seeberger et al. report a synthesis strategy for two such conjugates that resemble the structure of a malarial GPI toxin. Compared to their earlier synthesis of one of the compounds by automated assembly, the new solution-phase synthesis is slower but can be scaled up more easily. It may thus provide access to large enough quantities for preclinical and clinical trials. The GPI toxin is a hexasaccharide with fatty acid chains attached in two places, but the exact nature of these lipid chains is not known. In a related paper, Liu and Seeberger report a synthesis route that allows lipid chains to be attached to a disaccharide subunit using a protection group strategy that involves substituted benzyl ethers and may also be applied to the synthesis of other complex molecules. — JFU

    Chem. Commun. 2004, 1706; 1708 (2004).

  3. IMMUNOLOGY

    Intercellular Traffic

    1. Stephen J. Simpson

    The cells of the immune system are dispersed and highly mobile elements that employ diverse means of intercellular communication, such as direct cell-to-cell contact and soluble cytokines. Studies of other cell types (for instance, see Rustom et al., Reports, 13 Feb, p. 1007) have suggested that intercellular communication may also involve actin-driven protrusions that elaborate into tubular networks between cells, which have been dubbed nanotubular highways.

    Using single-photon confocal microscopy, Önfelt et al. describe the formation in cell culture of similar nanotube-like entities between Epstein-Barr virus-transformed B cells, macrophages, and natural killer (NK) cells. These transient structures varied considerably in length, but were sometimes observed to extend over 100 μm. Unlike previously described cellular nanotubes, which form via extensions that reach out to neighboring cells, these structures appeared to form as cells moved apart after cell contact had taken place. The detection of membrane proteins and lipids along the nanotubes and their transfer between interacting cells raises the possibility that nanotubular networks might facilitate exchange of cell surface components during an immune response. — SJS

    J. Immunol. 173, 1511 (2004).

  4. APPLIED PHYSICS

    Watching Crystals Grow

    1. Marc S. Lavine

    Solvents are commonly used to process polymers because they make it possible to cast thin films or to spin strong fibers that cannot be fabricated directly from the melt. Solvents can also change the stability of certain morphologies or crystal forms, as is the case for syndiotactic polystyrene (s-PS); solvent treatment leads to less stable helical crystal forms that cannot be reached via heat treatment alone.

    Gupper et al. have applied Fourier transform infrared (FT-IR) imaging to track the motion of toluene through s-PS and simultaneously to observe local changes in crystallinity. Toluene was introduced orthogonally to the IR beam, and specific IR bands were monitored over time. The diffusion of toluene was found to obey Fick's Law. However, the crystallization process at the surface occurred much faster than previous studies had indicated, which suggests that the notion that there is a delay period during which critical sequence lengths organize may not be true. They also found that there is a minimum solvent concentration needed for crystallization to occur, but that longer exposure times do not increase the overall crystal fraction. — MSL

    Macromolecules 10.1021/ma049313v (2004).

  5. MOLECULAR BIOLOGY

    Secondary Metabolites

    1. Lisa D. Chong

    Bacteria need to be able to change gene expression rapidly when conditions change. A sudden decrease in amino acid nutrients will trigger the synthesis of guanosine 5'-diphosphate 3'-diphosphate (ppGpp), a molecule that halts continued growth by blocking ribosomal RNA (rRNA) synthesis and that turns on the expression of amino acid biosynthetic genes. Paul et al. and Perederina et al. provide complementary evidence that the bacterial protein DksA binds to RNA polymerase (RNAP) and mediates the inhibitory effect of ppGpp by destabilizing the open RNAP-promoter complex and thus blocking initiation of transcription at rRNA promoters. DksA is structurally similar to GreA, which regulates the elongation phase of transcription. Both proteins possess a coiled-coil domain that appears to fit into the RNAP secondary channel, whose likely function is to allow nucleotides access to the active site. By accommodating a variety of transcriptional regulators, the secondary channel of RNAP may also facilitate bacterial adjustments to a broad range of environmental conditions, and the development of occluders of the secondary channel may prove therapeutically useful against harmful bacteria. — LDC

    Cell 118, 311; 297 (2004)

  6. CHEMISTRY

    Stamping Out Peptides

    1. Jake S. Yeston

    To run a bimolecular reaction, a chemist typically mixes the two reagents in a flask and relies on collisions to bring them together. Sullivan et al. have taken a more forceful approach to inducing molecular encounters. They are able to create peptide (amide) linkages by first preparing a self-assembled monolayer (SAM) of thioamines on a gold surface, and then pressing down on it with a stamp that has been coated with a carboxylate ink. After the stamp is withdrawn and the surface rinsed, infrared spectroscopy and ellipsometry confirm the formation of C-N bonds. The stamp is a surface-oxidized poly(dimethylsiloxane) (PDMS) elastomer used in microcontact printing, and the ink contains any of a variety of N-protected amino acids. When reactions of this sort are run in solution, they generally require a coupling catalyst; none is used in the stamping process. Three successive stamp, rinse, and deprotection cycles produced a layer of Arg-Gly-Asp capable of binding mouse fibroblasts. — JSY

    Angew. Chem. Int. Ed. 43, 2 (2004).

  7. STKE

    A Scaffold for Addiction?

    1. Elizabeth M. Adler

    Behavioral sensitization to cocaine, which occurs following repeated administration, involves long-lasting changes in glutamatergic transmission in the nucleus accumbens. Cocaine elicits changes in the expression of genes encoding Homer scaffolding proteins [which exist in complexes with metabotropic glutamate receptors (mGluRs)], and Szumlinski et al. have now investigated the effects of Homer deletion. Mice lacking Homer1 or Homer2 showed enhanced sensitivity to cocaine-induced place conditioning (preference for environments associated with cocaine administration), as well as an enhanced locomotor response to cocaine. Furthermore, these mice had decreased basal extracellular glutamate concentration in the nucleus accumbens compared to wild-type mice (as occurs with repeated cocaine administration) and, like rats in withdrawal from repeated cocaine administration, showed increased extracellular glutamate following cocaine exposure. Restoring Homer2 reversed the effects on conditioning and on the behavioral and neurochemical response to cocaine. Homer2 knockout also led to more rapid development of cocaine self-administration and, as in cocaine-sensitized animals, attenuated the increase in extracellular glutamate produced by a mGluR1 agonist. Thus, lack of Homer2 mimics many aspects of the sensitization seen with withdrawal from repeated cocaine administration, leading the authors to propose that Homer proteins may play a key role in cocaine addiction. — EMA

    Neuron 43, 401 (2004).

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