This Week in Science

Science  14 Mar 2014:
Vol. 343, Issue 6176, pp. 1175
  1. Hexing Complement

    CREDIT: JOOST BAKKER

    Complement activation is an immediate and potent immune defense mechanism, but how immunoglobulin G (IgG) antibodies activate complement at the molecular level is poorly understood. Using high-resolution crystallography, Diebolder et al. (p. 1260) show that human IgGs form hexameric structures by interacting with neighboring IgG molecules, and the complex then activates complement. Thus, IgG molecules and the complement system can coexist in the blood because complement activation will only be triggered after IgG senses a surface antigen and starts to aggregate.

  2. Light Alkanes, Heavy Metals

    Hydraulic fracturing, or fracking, has rapidly increased the supply of natural gas and has motivated methods to convert its constituents into commodity chemicals. Hashiguchi et al. (p. 1232) have found that lead and thallium salts are both efficient and selective oxidants, not only for methane, but for ethane and propane as well. In trifluoroacetic acid solvent, the alkanes are cleanly oxidized to the trifluoroacetate esters of their respective alcohols and 1,2-diols. Building on earlier discoveries, this work paves the way to developing methods that reduce our dependence on petroleum for industrial feedstocks.

  3. Cas9 Solved

    Clustered regularly interspaced short palindromic repeats (CRISPR)–associated (Cas) loci allow prokaryotes to identify and destroy invading DNA. Not only important to bacteria, the universal value of Cas endonuclease specificity has also resulted in Cas9 being exploited as a tool for genome editing. Jinek et al. (10.1126/science.1247997, published online 6 February) determined the 2.6 and 2.2 angstrom resolution crystal structures of two Cas9 enzymes to reveal a common structural core with distinct peripheral elaborations. The enzymes are autoinhibited, undergo large conformational changes on binding RNA, and have channels lined with basic residues that are candidates for an RNA-DNA binding groove. Based on these and other insights from the structures, this work provides important revelations both for the CRISPR mechanism and for genome editing.

  4. A Palladium 1–2 Punch

    Methods to replace carbon-hydrogen bonds directly with carbon-carbon bonds offer enticing prospects for streamlining the synthesis of organic compounds. The trouble is that it is hard to select any particular C-H bond and to avoid making complex mixtures of products. He et al. (p. 1216) report that a pair of powerful pyrimidine ligands induces a palladium catalyst to add aryl groups selectively to amino acid derivatives. One ligand promotes addition of a single aryl group to the β-carbon center; the other appends a second, potentially different aryl group to the same carbon—all in the same flask.

  5. Iron in Graphene

    CREDIT: ZHAO ET AL.

    Carbon or other covalently bonded materials, like boron nitride, can form two-dimensional sheets because of the strong bonding between the atoms. In contrast, metals share electrons in a three-dimensional delocalized way, and this could preclude the formation of thin stable sheets. Nevertheless, Zhao et al. (p. 1228) observed pure iron membranes suspended across the pores in a graphene sheet. This phenomenon was discovered when an iron chloride solution, used to process the graphene, decomposed to form pure iron films across the pores.

  6. G Below Sea

    Rheological differences between Earth's lithosphere and asthenosphere help drive plate tectonics. Geophysical analyses repeatedly reveal a seismic Gutenberg (G) discontinuity at 40- to 100-kilometer depth in oceanic plates, although the origin of this boundary remains enigmatic. Beghein et al. (p. 1237, published online 27 February) found that vertical stratification of anisotropy aligned with the depths of the G discontinuity, but not with the lithosphere-asthenosphere boundary. It appears that the G discontinuity forms when there are geophysical changes in the mantle, such as dehydration beneath mid-ocean ridges.

  7. Where Do You Want Your Leg?

    Initiation of limb formation is the key to understanding limb-cell specification and patterning during development. Gros and Tabin (p. 1253) show that a localized epithelial-to-mesenchymal transition triggers the formation of the limb. Finding that limbs initiate through cell-state change, and not through differential proliferation, redefines questions such as how limb buds are placed on the body plan.

  8. Touchy-Feely Genes

    All animals need accurate proprioceptive sensation to control motor function. Desai et al. (p. 1256) identified a Drosophila mutant with impaired walking coordination. The affected gene, stumble (stum), is conserved throughout the animal kingdom and expressed in a subpopulation of multidendritic neurons. stum-expressing neurons are found proximal to leg joints, and if the angle of the joint shifts, dendritic stretching occurs that in turn elevates cellular calcium. Therefore, it looks as if stum is a transducer for mechanical stimuli.

  9. Quick, Quick, Slow

    The slow muscles of postural stability and the fast muscles of running and jumping are driven by motor neurons that are differentiated by fast and slow biophysical properties. By retrograde labeling of mouse and chick muscle fibers, Müller et al. (p. 1264) characterized the developmental distinctions between fast and slow motor neurons. A transmembrane protein, when over- or underexpressed, was discovered to drive specification of the motor neurons and a downstream effector specified some, but not all, of the biophysical attributes.

  10. Clearance of Chronic Virus

    The family of mRNA-editing enzymes, APOBEC, restricts hepatitis B virus (HBV) replication. Lucifora et al. (p. 1221, published online 20 February; see the Perspective by Shlomai and Rice) provide evidence that specific APOBECs mediate the anti-HBV effects of host cytokines, which in turn apparently induce nuclear deaminase activity without damaging host cells. Thus, there may be potential in these findings for developing a therapeutic route to curing chronic HBV infection.

  11. Hidden Diversity

    CREDIT: TAINA LITWAK/SYSTEMATIC ENTOMOLOGY LABORATORY, USDA

    Why are there so many species in the tropics? Niche partitioning by highly specialized plant species seems to be the main generator of high diversity. Condon et al. (p. 1240; see the Perspective by Godfray) show that niche partitioning can also be generated by interactions between plant resources and parasites, resulting in hyperdiverse communities. The cryptic diversity of 14 neotropical fly pollinators and 18 of their highly specific wasp parasites induced mortality partitions between multiple narrow niches. The extreme specificity of the wasp-fly relationships was initially only revealed by molecular analysis.

  12. Crossed Homodimer

    Our cells respond to infection by releasing interferons, which protect neighboring cells, in part through the cleavage of intracellular RNA by a protein kinase family receptor, RNase L. RNase L is activated by 2′,5″-linked oligoadenylates (the second messenger 2-5 A), sensors of pathogen- and damage-associated RNA. Han et al. (p. 1244, published online 27 February) report crystal structures of human RNase L in complexes with 2-5 A, nucleotides, and an 18-nucleotide oligomer RNA target.

  13. Making a Histone Mark

    The covalent marks on histones (the principal components of chromatin) play a critical role in the regulation of gene expression. Somehow these marks are preserved when a cell in a tissue divides so that the daughter cells maintain the gene expression program and tissue identity of the parent cell. Jacob et al. (p. 1249) show that the Arabidopsis histone methylase ATXR5 is specific for the replication-dependent histone variant H3.1 and maintains the repressive histone H3 lysine-27 methyl mark on the H3.1 variant during genome replication, thus, preserving cell-type–specific regions of heterochromatin and gene repression through cell division and beyond.

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