Editors' Choice

Science  08 Aug 2008:
Vol. 321, Issue 5890, pp. 746

    Tolerating Chlorine

    Supplies of fresh water are steadily dwindling, but salt water remains plentiful, and desalination is increasingly being used for purification. Membrane-based desalination methods require less energy than do distillation-based approaches and are now the dominant technology. However, a complex pretreatment protocol is necessary. Feed waters must be treated with chlorine to eliminate microorganisms that would deposit biofilms onto the membranes, but the chlorine must subsequently be removed to prevent chemical damage to the membranes. After passing through the membranes, the water is then rechlorinated before it is distributed for use. These dechlorination and rechlorination steps increase water purification costs. Park et al. have now developed membranes that can tolerate chlorine over a wide pH range. The membranes consist of disulfonated copolymers, which retain the desirable properties of polysulfone—a tough and stable thermoplastic—but are less hydrophobic. The membranes have the potential to be tailored for particular uses and should not require dechlorination of feeds. Further work is required to optimize water transport rates through, and salt retention by, these membranes. — JFU

    Angew. Chem. Int. Ed. 47, 6019 (2008).


    Unfolding the Power of Solar

    Thin-film photovoltaics, such as those based on amorphous silicon or organic films, can be deposited over large areas and so offer the potential to provide a cheap power source that harnesses the free energy from the Sun in which we bask. Combined with the ease of deposition onto flexible substrates, these films also offer the possibility of a lightweight portable power source suited to installation in remote areas. However, the conversion efficiency of such solar cells is relatively low compared to that of their single- and polycrystalline silicon cousins, the workhorses of the present “renewables” technology base for electricity generation. A larger-area film-based cell would thus be required to produce the same amount of power, which could hamper the above-mentioned keenly sought-after applications. Zhou et al. now show that the flexibility of thin-film polymer solar cells can get around this problem. They demonstrate a polymer solar cell that can be unfolded like a map. The V-shaped corrugations of the unfolded cell not only enhance practicality but also serve to optimize the collection of light (by multiple reflection) so that the overall efficiency of the cell increases. The future prospects for these thin-film solar cells have just gotten a little bit brighter. — ISO

    Appl. Phys. Lett. 93, 33302 (2008).


    Ready, Set, Go

    Synaptotagmin 1 (Syt1) is a Ca2+-binding protein in synaptic vesicles and triggers rapid exocytosis (vesicle fusion with the plasma membrane) in response to Ca2+ influx. Fusion mediated by Syt1 and SNAREs (a family of membrane fusion proteins) can be studied in vitro by mixing two populations of vesicles that have been reconstituted with SNAREs: one population with target membrane-associated SNAREs and one with the synaptic vesicle SNARE synaptobrevin 2. Addition of the soluble cytoplasmic domain of Syt1 in the presence of Ca2+ triggers fusion, whereas addition of the soluble cytoplasmic domain of synaptobrevin (cd-syb) immediately blocks fusion through competitive inhibition. Chicka et al. use this assay to show that both the rate and extent of Ca2+-triggered fusion are increased when the vesicle mixture is pre-incubated with Syt1 for 20 min before addition of Ca2+. Furthermore, fusion is no longer immediately inhibited by cd-syb when vesicles have been pre-incubated with Syt1, demonstrating the presence of a population of docked vesicles in which SNAREs from opposing membranes are already paired. Syt1, therefore, acts in the absence of Ca2+ to increase the number of docked fusion-ready vesicles, possibly by stalling partially assembled SNARE complexes. In vivo, this function of Syt1 may contribute to the buildup of docked vesicles, which is essential for the rapid and coordinated release of neurotransmitter. — NM*

    Nat. Struct. Mol. Biol. 10.1038/nsmb.1463 (2008).

    • * Nilah Monnier is a summer intern in Science's editorial department.


    Let's Move It

    Animal morphogenesis requires that both individual cells and groups of them migrate in a concerted fashion. The mechanisms involved in migration and coordination have been the focus of many studies. Nakao et al. reveal how a protein with a role in cell-cell contact actually promotes the joint migration of interacting cells. Generally, cell-cell contact is a signal for cells to stop moving. However, the interaction between OL-protocadherin molecules on neighboring cells stimulates the Nap1-dependent and WAVE-dependent rearrangement of the actin cytoskeleton and encourages the cells to move in tandem. Using astrocytoma cells that expressed OL-protocadherin, or Nap1, or both, the authors defined a pathway through which OL-protocadherin specifically stimulates joint cell migration, while having no effect on an individual cell's capacity to migrate. — SMH

    J. Cell Biol. 182, 395 (2008).


    Stringing Large Rings

    Polymerization reactions that yield products with large cyclic side chains tend to be rare and often exhibit high product polydispersity (a measure of the spread in the length of chains created). However, large cyclic side chains can act as sites for trapping small molecules, which make the polymers useful as absorbents or stationary supports in chromatography, Ochiai et al. report that a bis-methacrylate monomer, in which a central cyclohexane linkage and terminal urethane groups act as structure-directing agents, can be used to form polymers with 19-atom cyclic side chains; the macrocycles close during the chain-propagation process. Low polydispersity was observed for RAFT synthesis (reversible addition-fragmentation chain-transfer polymerization, a form of living chain polymerization) in dioxane solvent with cumyl dithiobenzoate as a RAFT agent. The controlled nature of the polymerization should allow further modifications through the formation of block copolymers. — PDS

    J. Am. Chem. Soc. 130, 10.1021/ja801491m (2008).


    Taking the Long View

    It can be difficult to establish the phylogeny of microorganisms because they are composed of genes that have moved vertically (via inheritance) or horizontally (via lateral transfer mechanisms such as conjugation) or both. Dagan et al. have applied a network analysis approach to estimate the cumulative impact of lateral gene transfer in the genomes of 181 fully sequenced prokaryotes. By examining the presence or absence of all genes and by tracing the evolutionary history of these genes on the basis of genome size, they were able to calculate the rate of lateral gene transfer and have concluded that approximately 80% of the genes in each genome appear to have been involved in lateral transfer at some point in their history. Hence, well-defined phylogenetic trees, which describe genetic relationships accurately on short-term evolutionary time scales, become rather less clearly delineated when looked at over very long time periods. — LMZ

    Proc. Natl. Acad. Sci. U.S.A. 105, 10039 (2008).

  7. Science Signaling

    Just Looking or Settling In?

    Natural killer (NK) cells navigate to transformed or virus-infected cells and bind to them through integrins and NK receptors to form a lytic synapse. Both steps depend on the actin cytoskeleton, leading Butler et al. to investigate the role of HS1 (a homolog of the actin-binding protein cortactin) in NK cell-mediated cytolysis. When NK cells were exposed to target cells or to beads coated with ICAM-1 (a ligand of the β2 integrin LFA-1) and the NK receptor ligand ULBP, HS1 localized to the contact site and became phosphorylated on tyrosine. Experiments in which HS1 was knocked down and cells were transfected with HS1 mutants where one or both of two tyrosine residues were substituted with phenylalanine implicated HS1 phosphorylation in NK cell cytolytic activity. Adhesion to ICAM-1 stimulated phosphorylation of HS1 on Tyr397; further, Tyr397 was required for chemokine-dependent conversion of LFA-1 into a high-affinity state and for downstream recruitment of actin, the actin regulator WASp, and the guanine nucleotide exchange factor Vav1 to the lytic synapse. Although HS1 Tyr397 was not required for recruitment of the adaptor DAP10 to the NKG2D receptor, it was implicated in downstream signaling. In contrast, phosphorylation of HS1 Tyr378 was required for chemotaxis. Thus, HS1 appears to be critical to NK cell chemotaxis, formation of the lytic synapse, and cytolysis, and may act as a switch to enable NK cells to convert from a migratory mode to one in which they form a stable contact with a target cell. — EMA

    Nat. Immunol. 9, 887 (2008).