Editors' Choice

Science  10 Jan 2014:
Vol. 343, Issue 6167, pp. 119
  1. Geophysics

    Tremor Sweet Spots

    1. Nicholas S. Wigginton

    Seismic tremor is thought to be indicative of the slow release of small amounts of stress along plate boundaries, but it can also be triggered by large-amplitude seismic waves generated during large earthquakes. Gomberg and Prejean determined the distribution of tremor in Alaska after 11 of the largest earthquakes (M < 7.2) around the globe between 2006 and 2012. As in previous observations, triggered tremor in the Aleutian Islands is related to the transition of friction along the subducting plate boundary from a locked state to a creeping state. However, tremor was also triggered in central mainland Alaska—a region far away from the subduction zone and devoid of any major crustal faults or appreciable seismic activity. Because there was no single characteristic of the triggering wave source or tectonic environment associated with the two regions, transient frictional processes at the plate interface may be responsible for tremor triggering. However, according to GPS data, neither zone shows any clear evidence of other concurrent related seismic processes such as slow slip events.

    J. Geophys. Res. 118, 10.1002/2013JB010273 (2013).

  2. Biochemistry

    Chirality Check

    1. Valda Vinson

    Most biological macromolecules are homochiral, and enzymes help to maintain this state of affairs; for example, checkpoints ensure that only l-amino acids are incorporated into proteins during translation. Among these enzymes is d-aminoacyl-tRNA deacylase (DTD), which removes d-amino acids mischarged onto tRNAs. Three types of DTDs have been identified, with the most common form being present in many bacteria and all eukaryotes. DTD faces the mechanistic challenge of acting on diverse d-aminoacyl-tRNAs (d-aa-tRNAs) while not harming l-aminoacyl-tRNAs (l-aa-tRNAs) that are present at much higher concentrations. Although crystal structures have been determined for DTD in the apo form and bound to free d-amino acids, the structural basis of enantioselectivity remained unclear. Ahmad et al. report the crystal structure of dimeric DTD from Plasmodium falciparum in complex with a substrate analog that mimics d-tyrosine attached to the 3′-OH of the terminal adenosine of tRNA. A critical role in shaping the active site for enantioselectivity is played by a Gly-cisPro motif that is inserted from one DTD monomer into the active site of the other monomer. Mainly main-chain atoms from DTD interact with the substrate, facilitating interaction with a range of d-aa-tRNAs. On the basis of mutational studies of active site residues, the authors suggest an RNA-assisted catalytic mechanism in which the RNA 2′-OH activates a water molecule.

    eLife 2, e01519 (2013).

  3. Cell Biology

    Sugar Sabotage

    1. L. Bryan Ray

    In patients with diabetes, too much of a good thing—glucose—in the bloodstream causes the debilitating loss of biological functions and can eventually lead to death. Scientists continue to home in on the precise mechanisms by which this occurs, in hope of mitigating the damage. Warren et al. have found a mechanism by which excess glucose can alter the functions of vascular endothelial cells, one of the main sites of complications in diabetes. In mouse endothelial cells, too much glucose leads to the overproduction of reactive oxygen species (ROS) in the mitochondria. This excess of ROS causes the phosphorylation of the receptor for vascular endothelial growth factor (VEGF) within the Golgi, rendering the receptor vulnerable to proteolysis. This reduces the levels of VEGF receptor at the cell surface, where it would be able to detect circulating VEGF. Thus, cells chronically exposed to excess glucose become less responsive to VEGF, which is necessary for the proper growth, function, and survival of endothelial cells.

    Sci. Signal. 7, ra1 (2014).

  4. Chemistry

    As Thin As Clay Gets?

    1. Phil Szuromi

    Thin films of aluminosilicates can mimic the reactivity of zeolites but avoid the kinetic limitations of diffusion through pore networks. Włodarczyk et al. build on their recent work on creating monolayers of aluminosilicates on the surfaces of single crystals of Ru to create thin films of Fe-containing silicates similar to the layers in smectite clays. Analysis of Si-O-Si stretching bands from infrared reflection-absorption spectroscopy revealed that the addition of Fe led to the formation of two-component films containing pure silica and an iron silicate. X-ray photoelectron spectroscopy (XPS) of an oxide with a 1:1 ratio of Fe to Si revealed a high coordination of O to Fe, and low-energy electron diffraction revealed greatly increased ordering even for small amounts of Fe incorporation. Density functional theory confirmed that uniform mixing of Fe is unfavorable thermodynamically as compared to phase separation, and favored a structure in which Fe atoms substitute for Si in the layer adjacent to the substrate and the formation of bridge Fe-O-Ru bonds. Although the Fe oxidation state could not be assigned from the XPS data, assuming that the Fe is in the 3÷ oxidation state, the bilayer formed would represent a dehydroxylated form of nontronite, an Fe-rich smectite.

    J. Am. Soc. Chem. 135, 19222 (2013).