Grinding Nanotubes into Gels
Although single-walled carbon nanotubes (SWNTs) have excellent electrical and mechanical properties, the inherent difficulties in processing and blending them with other materials have limited their applicability. Fukushima et al. (p. 2072) show that they can make a rather stiff gel by physically grinding up SWNTs with ionic liquids. They argue that the gel results from a physical cross-linking of the tubes with the ionic liquid. The gels showed good thermal and dimensional stability and could be shaped into conductive sheets that also had enhanced mechanical properties.
The Changing Surface of Mars
The Thermal Emission Imaging System (THEMIS) on Mars Odyssey has been mapping the different rock structures and compositions on the surface of Mars at 100 to 18 meters per pixel resolution for about 1 year. Christensen et al. (p. 2056; see the Perspective by Golombek) synthesize and summarize these images to create a more comprehensive view of the evolution and erosion of the surface of Mars over time. In particular, the detailed mapping shows that the rate of exposure of bedrock is greater than the rate of burial, contrary to previous models, and that olivine-rich basaltic flows can be distinguished from other formations. The mapping will help to delineate impact processes from wind and water erosion as well as differentiate sedimentary-rock formation processes from tectonic processes such as volcanism. The seasonal variation of carbon dioxide ice at the martian poles has made it difficult for the orbiting spacecrafts, Mars Odyssey and Mars Global Surveyor, to find and measure the amount of water in the icy caps or in the adjacent permafrost. Using a combination of data from the high-energy neutron detector (HEND) on Odyssey and the Mars Orbiter Laser Altimeter (MOLA) on Surveyor, Mitrofanov et al. (p. 2081) have determined that as much as 50 weight % of water is contained in the shallow subsurface at northern polar latitudes. This amount is about one third more water than estimated for the southern polar latitudes.
Water on the Double
The greenhouse warming caused by atmospheric water vapor is normally calculated based on the free molecule, but weakly bound water dimers may also form that have somewhat greater warming potential. However, the concentration of the dimer species has been difficult to determine because many of its bands overlap those of the monomer. Pfeilsticker et al. (p. 2078) obtained long-path (18 kilometer) atmospheric near-infrared spectra and used the long-wavelength end of an overtone of an OH stretching mode in order to minimize the monomer contribution to the dimer signal. They estimate that in saturated air at 20°C, there is about one dimer for every thousand monomers. They discuss the implications of this finding for global warming.
Clues to Outer-Core Motion
Fluid motions in the outer core induce electrical currents that produce Earth's magnetic field, but the details of how the geodynamo works are still poorly understood. Finlay and Jackson (p. 2084; see the Perspective by Johnson et al.) used direct observations made during the past 400 years by mariners, observatories, and satellites to determine the spatial and temporal variations of the radial nonaxisymmetric field at the core surface. They found a high-intensity field feature at the equator that has been drifting westward during this period. Such a feature could be generated by a wavelike propagation of the field rather than convective motions of the fluid core. Modeling of this unusual but seemingly systematic recent variation in the field could help to refine possible mechanisms for the geodynamo.
Producing Hydrogen Without Precious Metals
Hydrogen fuel will have a large impact on the problem of reducing greenhouse gas emissions only if it can be generated from non-fossil fuel sources. Huber et al. (p. 2075) now show that biomass compounds (oxygenated hydrocarbons in water) can be used to generate hydrogen at moderate temperatures (from 225° to 250°C) without the need for expensive precious metals like platinum. A porous nickel catalyst activated with tin helps promote the needed C-C bond cleavage reaction and avoids methane formation.
Wandering Water Masses
Many atmospheric and oceanic processes display variations on decadal or centennial time scales that are comparable to the entire instrumental record available for their study. Thus, it is important to develop longer time series in order to determine if short-term trends are natural variations or responses to anthropogenic activity. Bryden et al. (p. 2086) present a record of upper thermocline waters from the Indian Ocean expeditions dating from 1936. These waters have become saltier and colder since 1987, reversing a freshening trend that had been observed during the previous three decades. Combined with their observations of trends in the lower thermocline and intermediate waters, the authors argue that previous suggestions that these changes have been unidirectional and are evidence of climate change will have to be re-evaluated.
Watching Myosin Walk
Myosin V is a processive cellular motor that carries cargo along actin filaments. This protein is comprised of two heads held together by a coiled-coil stalk, and the interpretation of experimental studies of how this movement occurs has been controversial. Myosin V could move by an inchworm mechanism, in which the same head always leads, or by a hand-over-hand mechanism, in which the leading head alternates. Yildiz et al. (p. 2061; see the cover and the Perspective by Molloy and Veigel) labeled the light-chain domain of myosin V and used a fluorescence imaging technique to measure the step size of single molecules to within 1.5 nanometers. The step size alternates and depends on the location of the dye, which is consistent with a hand-over-hand mechanism but not the inchworm mechanism.
Doubling Up to Recognize HIV-1 Glycoprotein
In the battle against the human immunodeficiency virus type 1 (HIV-1), there is much interest in understanding the interactions of the few broadly neutralizing antibodies that have been isolated from patients. One of these, 2G12, binds to a particular carbohydrate epitope on the outer-envelope glycoprotein, gp120, even though several carbohydrate molecules screen this face from antibody interactions. Calarese et al. (p. 2065) have determined the structures of the Fab 2G12 alone and bound to either an oligosaccharide or a disaccharide and show that the Fabs form an unusual domain-swapped dimer with an extended surface that allows high-affinity recognition of the carbohydrate epitope on gp120.
Engineering a Bacterial Source for Bixin
Bixin, which is sold as the colorant annatto, is one of the most highly consumed red colorants in the United States' food supply. Currently, bixin is extracted from the brilliantly red-colored seeds of the plant Bixa orellana. Bouvier et al. (p. 2089) now elucidate the natural biosynthetic pathway for bixin and genetically engineer Escherichia coli to express the relevant genes and synthesize bixin. The results may improve supplies of bixin while reducing pressure on natural harvests.
How to Concentrate Sodium Channels
Neurons target and concentrate voltage-dependent sodium channels at the axonal initial segment, where action potentials are generated, and in nodes of Ranvier of myelinated axons. How are these components selectively sorted to these domains? Garrido et al. (p. 2091) show that a 27-amino acid motif contained in one of the intracellular linkers of the pore-forming subunit can target and concentrate the sodium channel within the initial segment of the axon. This peptide motif was sufficient to direct and to concentrate different proteins to the initial segment. In addition, the overexpression of a cytosolic chimeric protein containing the motif disorganized endogenous sodium channels.
Spillover and Cross-Talk
The inhibitory neurotransmitter glycine also plays an inportant role as coagonist at excitatory NMDA-type glutamate receptors. Despite high glycine concentrations in the cerebrospinal fluid and in nervous tissue, the amount of glycine that normally reaches NMDA receptors is reduced to subsaturing levels by perisynaptic glycine transporters. Ahmadi et al. (p. 2094) show that in the dorsal horn of the spinal cord, synaptically released glycine enhances NMDA receptor-mediated currents. During high levels of neuronal activity, glycine released from glycinergic inhibitory interneurons can spill over and reach NMDA receptors at neighboring glutamatergic synapses. Blockade of this phenomenon by the neuropeptide nocistatin can selectively inhibit NMDA receptor-mediated excitation in the dorsal horn and could thus be used therapeutically to treat inflammatory pain.
Quantitating the Ties That Bind
The ability of leucine zipper-containing proteins (bZIP proteins) to form homo- and heterodimers increases their selectivity and diversity. To help determine the basis for their interactions, Newman and Keating (p. 2097) used protein arrays to analyze the ability of 49 of 51 human bZIP proteins to dimerize in all possible pairwise combinations. Reciprocal binding analyses and biophysical studies in solution were used to confirm the validity of the interactions. New associations were observed that could be related to the functioning of the circadian clock and an intracellular signaling pathway that responds to unfolded proteins.
Following the Assembly of Receptor Complex
Gp130 is a shared signal-transducing receptor for a family of four-helix bundle cytokines including the pro-inflammatory cytokine interleukin-6 (IL-6). Signaling through the gp130 receptor is critical to the normal growth and differentiation of numerous tissue types. Boulanger et al. (p. 2101) have determined a 3.65 angstrom-resolution structure of a multicomponent receptor complex comprising human IL-6, the extracellular binding domains of human IL-6 α-receptor, and the extracellular activation and binding domains of gp130. The hexamer assembles sequentially; a binary complex between IL-6Rα and IL-6 binds to gp130, and this step facilitates a cooperative transition to the signaling-competent hexamer.