This Week in Science

Science  18 Dec 2009:
Vol. 326, Issue 5960, pp. 1587
  1. Bio-Diodes

      CREDIT: TAO ET AL.

      Inward rectifier potassium channels conduct K+ ions into the cell at internal negative membrane voltages, but at internal positive membrane voltages they are blocked by intracellular multivalent ions. These channels control the resting membrane voltage and are required for the healthy function of many electrically excitable cells. Mutations can result in transient paralysis causing, for example, heart problems. Tao et al. (p. 1668) now report a 3.1 angstrom resolution structure of the inward rectifier, Kir2.2 from chicken, which has a similar structure to the human equivalent. The combination of observations of conductive and inhibitory ion binding sites with electrophysiological data finally explains the mechanism of action of these long-studied channels and reveals how they maintain their low sensitivity to toxins, as well as provides a basis for the design of therapeutic drugs.

    1. Degrees of Darkness

        Fruit flies in Africa have a tendency to be darker, the higher the altitude at which they live, because melanization offers a selective advantage. The dark pigmentation seen in some populations of Ugandan Drosophila melanogaster is owing to a lack of expression of the ebony gene, and expression results in yellow cuticle. Rebeiz et al. (p. 1663; see the News story by Pennisi) show that mutations in cis regulatory elements, rather than in the coding region, are responsible for the dark color. A series of five mutations in a modular enhancer element influences the level of ebony expression: Three mutations already existed in fly populations with light cuticle color and a further two, more recently acquired dark-specific substitutions, together have created an allele of large effect, which has been swept to high frequency in this population of flies.

      1. Watery Worlds

          Water vapor has been detected in the protoplanetary disks of a variety of stars. Explaining its origin is important because it bears on our understanding of planet formation. Bethell and Bergin (p. 1675) propose that this water originates in photochemistry occurring in the upper layers of the disks. In these regions, the rate at which water is produced exceeds the rate at which water is destroyed by ultraviolet radiation. Water forming in the upper layers of the disks then shields water vapor within the disks' interiors, allowing that vapor to interact with planet-forming materials and possibly be incorporated into young planets.

        1. Home Is Where the Hearth Is

            One aspect of human intelligence is the ability to organize our living and working spaces. It was generally thought that this capability arose with modern humans in the past 100,000 years or so. However, Alperson-Afil et al. (p. 1677) found evidence of domestic organization 800,000 years ago at a Pleistocene hominin campsite in the Jordan Valley. Around patches of burnt debris, the remains of a wide range of plant and animal foodstuffs were found, including fruits and seeds, as well as remnants of turtles, elephants, and small rodents. Specific types of stone tools appear to have been made around the hearths, where there was also evidence of nut roasting and consumption of crabs and fish. In a more distant area there were signs of intensive flint knapping and food chopping.

          1. Seed for Food

              The seeds of grain-producing plants are more difficult to harvest than nuts or fruits as food. It has been unclear when early humans began to rely extensively on grains, but Mercader (p. 1680) has discovered films of starch residues on stone tools at a cave site in Mozambique dating to about 100,000 years ago. The residues are consistent with starch grains from wild sorghum and indicate that early humans relied on cereals much earlier than previously thought. The Mozambican example of sorghum exploitation thus represents the longest known tradition of cereal use in the world.

            1. Moving Boundaries

                CREDIT: RUPERT ET AL.

                Classical models of fine-grained metals view grain boundaries as static objects, but this view has been challenged by recent experimental observations. Drawing on techniques used by the fracture mechanics community, Rupert et al. (p. 1686) present experiments on freestanding aluminum films that show specific geometries cause either stress or strain concentrations on deformation. Confirming recent simulations, shear stresses were found to be a key driver of grain boundary motion.

              1. A Glimpse of Wet Carbonic Acid

                  Both carbon dioxide and bicarbonate play extraordinarily widespread roles in biochemical and geochemical reactions. It is surprising therefore that carbonic acid, the intermediate in the water-coupled interconversion of these two compounds, has never been directly characterized in aqueous solution. Adamczyk et al. (p. 1690, published online 12 November) have succeeded in glimpsing the elusive acid by inducing an aqueous photoacid (a compound rendered transiently more acidic upon light absorption) to react with dissolved bicarbonate. Using infrared spectroscopy, they show that the carbonic acid product persists for nanoseconds. Analysis of its formation kinetics affords a direct pKa value of 3.5, substantially lower than the effective value derived from observations of the net bicarbonate/carbon dioxide equilibrium.

                1. Fanconi Cross-Links

                    Fanconi anemia is a rare genetic disease characterized by bone marrow failure, developmental abnormalities, and dramatically increased cancer susceptibility. Cells derived from Fanconi anemia patients are sensitive to agents that cause DNA interstrand cross-links, indicating that under normal circumstances the Fanconi pathway controls the repair of these DNA lesions. Knipscheer et al. (p. 1698, published online 12 November) found that two Fanconi anemia proteins, FANCI and FANCD2, promoted the DNA replication–coupled repair of interstrand cross-links in cell extracts. The FANCI-FANCD2 complex was required for the incisions that unhook the cross-link and for the insertion of a nucleotide across from the damaged template base during lesion bypass.

                  1. Growing on You

                      CREDIT: COSTELLO ET AL.

                      The human gut and skin harbor diverse microbial communities that are known to vary strikingly among individuals. Here, Costello et al. (p. 1694, published online 5 November) analyzed microbial diversity among several distinct body habitats (including the gut, mouth, inside the ears and nose, and skin) of the same person at different times. They found that body habitat had more influence on microbial community composition than temporal differences and variation among people. Some skin locations, such as the index finger, back of the knee, and sole of the foot, on occasion harbored higher microbial diversity than the gut or oral cavity.

                    1. Cheaper Cooperation

                        In the context of public goods games in which optimal benefit is achieved when all participants contribute, bad behavior cannot always be deterred by direct punishment, and has the added disadvantage that the punisher may suffer a cost. Alternatively, instead of punishment, rewarding those who contribute can be effective in encouraging and maintaining widespread cooperation, with the added plus that group benefits are not diminished by the costs of punishment. But Ule et al. (p. 1701) discovered experimentally that if someone is treated depending on how they have behaved in previous interactions, retaining the option to occasionally apply punishment shifts the payouts to favor cooperators more than defectors.

                      1. Local Selection of Magic Traits

                          Ecological interactions can favor specialization, and sexual selection can induce reproductive isolation; however, these processes are insufficient by themselves to create new species. They must act in concert and on the same set of genes. Van Doorn et al. (p. 1704, published online 26 November; see the Perspective by Mank) present a theoretical model that shows that within a larger population the evolution of mating preferences will favor sexual ornaments that indicate the degree of adaptation to the local ecological conditions, for example, the abundant song of a male bird that can obtain food easily because it has the right bill size for the seeds in that locale. Once mate choice evolves on the basis of a signal of local adaptation, natural and sexual selection will mutually enforce each other, ultimately leading to speciation.

                        1. Silent Hate

                            A great deal of information can be communicated nonverbally. Weisbuch et al. (p. 1711; see the Perspective by Dovidio) have used experimental, archival, and survey studies to find that the nonverbal communication of racial bias in popular television shows perpetuates implicit racism in viewers. A subsequent field data analysis yielded a correlation between U.S. viewer ratings and a Federal Bureau of Investigation tally of anti-black hate crimes in the same localities.

                          1. More Is Different, Few Is Exotic

                              As powerful as theoretical physics can be, when it comes to describing the dynamics of several interacting particles, it stumbles at three. This few-body physics is interesting in many contexts, particularly interactions within the nucleus and between atoms and molecules. Although predicted 40 years ago for the special case of resonant interaction, which occurs at the edge of where two-particle bound states start to form, Efimov trimers were only recently observed in ultracold Bose gases. More work followed, revealing evidence of multiple trimer, and even tetramer, states. Now, Pollack et al. (p. 1683, published online 19 November; see the Perspective by Modugno) observe as many as 11 features near a lithium resonance that can be directly related to different few-body processes. The positions of the features with respect to each other are in excellent agreement with the theoretical prediction, although some deviations, attributed to short-range interactions, present a challenge for a future, more detailed theory.

                            1. Solving Pseudokinases

                                Mutations of the protein kinase LKB1 are associated with cancer in humans. Many kinases are activated by phosphorylation, but LKB1 is activated by STRADα, a pseudokinase that is similar to protein kinases and binds ATP, but does not phosphorylate substrates. By solving the crystal structure of an activating complex containing LKB, Zeqiraj et al. (p. 1707, published online 5 November) show that STRADα works with another protein, MO25α, to hold LKB1 in an active conformation. The results may help explain the evolutionary origin of pseudokinases, the biological roles of other pseudokinases, and the mechanisms of disease-causing mutations in LKB1.