Water and Ice
The water at the base of glacial ice sheets lubricates the ice-earth interface, allowing the ice sheet to slide more quickly and easily over the ground below, but its role in how glaciers lose mass is unclear. How will ice-sheet mass loss be affected by climate warming? Das et al. (p. 778, published online 17 April) describe the disappearance of a large supraglacial lake in Greenland, which occurred with startling speed in the summer of 2006, when a torrent of water emptied into the kilometer-thick ice sheet below it at an average rate of flow greater than that of Niagara Falls. Taking a wider perspective, Joughin et al. (p.781, published online 17 April) synthesize a wide array of data on ice motion from across Greenland to show how the ice sheet responds to surface melting and suggest that surface melting will probably cause a clear, but not catastrophic, increase in mass loss from the Greenland Ice Sheet in the foreseeable future.
A Not-So-Happy Anniversary
This year marks a quarter-century since the discovery of the human immunodeficiency virus (HIV) as the cause of AIDS. Over this time, enormous effort has been expended in developing a vaccine that will help tackle this global epidemic, but success has not been forthcoming: Last year saw the failure of what many viewed as the most promising vaccine trial to date, coordinated by the National Institutes of Health and Merck. Walker and Burton (p. 760) review the principles that have guided HIV vaccine research to date, and discuss the major hurdles that now face the scientific community in generating effective protective immunity to the virus.
The Making of the Sahara
Analysis of mid- and late Holocene environmental and climate change in northern Africa has been hampered by the extreme aridity of the Sahara during recent millennia, which has eradicated most sites that might have provided a depositional record. In an attempt to fill this gap, several expeditions have been undertaken to exploit the paleoclimatic potential of the largest Saharan lakes at Ounianga in Northeast Chad. Kröpelin et al. (p. 765; see the Perspective by Holmes; cover) provide a continuous and well-dated record of climate and ecosystem change over the past 6000 years from one of these lakes, Lake Yoa. The analysis suggests that the present desert became established about 2700 years ago and, contrary to expectations, the pattern of terrestrial ecosystem change from humid to dry was gradual rather than abrupt.
Nanoscale Thermal Motors
Powering very small devices—nanoelectromechanical systems—will require compact ways of converting electrical energy into directed motion. Barreiro et al. (p. 775, published online 10 April) describe a linear motor in which a multiwall carbon nanotube (MWNT), attached to electrodes at each end, serves as a track. A small section of the MWNT was decorated with a metal plate, and upon application of a large current, the remaining outer walls were ablated to leave a mobile sleeve. Passage of a smaller current through this assembly caused it to move toward one electrode, driven by the creation of a thermal gradient. The motion consisted of translation and rotational steps, and its overall direction was independent of the bias of the applied current.
Quantum Dot Optoelectronics
There is great interest in developing architectures that allow the quantum state of one part of a system to be controlled by the quantum state of another part. The ability to manipulate and control the optical properties and/or the charge and spin states of a single quantum dot have marked them as ideal candidates for further development. Robledo et al. (p. 772) demonstrate controlled interaction between a pair of self-assembled quantum dots, whereby the optical response of one of the dots is conditioned on the state of its neighbor. In a separate study, Fushman et al. (p. 769) show that a single quantum dot coupled to a photonic crystal cavity can be used to induce optical phase shift up to π4 radians down at the single photon level. The ability to control and manipulate the optoelectronic properties of quantum dots is an essential ingredient for quantum logic applications.
Monte Verde, in Chile, provides some of the best evidence for the early inhabitation of the Americas prior to the widespread Paleo-Indian Clovis culture. Dillehay et al. (p. 784) now provide some direct radiocarbon dates on the antiquity of the site and show that the early inhabitants used a wide variety of marine resources. A variety of different seaweeds were recovered from the site, which was about 15 km from the nearest bay 14,000 years ago. Early tools containing seaweed flakes were also found. This evidence helps confirm the importance of Monte Verde and supports the idea that early settlers relied on marine resources to move rapidly down the coast.
...and Pre-Clovis North
Evidence for a human presence in North America significantly before the appearance of the widespread Clovis complex about 13,000 years ago has been enigmatic. Gilbert et al. (p. 786, published online 3 April) now report the recovery and genetic typing of human coprolites from a cave deposit in Oregon that predate the Clovis culture by about 1000 years. The coprolites were confirmed as human by several techniques and directly dated and yielded mitochondrial DNA fragments consistent with haplogroups of Native Americans that are thought to have arisen about 18,000 years ago.
Divide and Conquer
During division, the bacterial cell division machinery selects the center of the rod-shaped cell as the division site and uses a protein array to physically separate daughter cells (see the Perspective by Lutkenhaus). Several protein components, including MinE and MinD, are known to play a role in selecting the division site. Loose et al. (p. 789) describe the emergence of self-organized behavior and dynamic pattern formation in a reconstituted system, which includes only the bacterial Min proteins MinE and MinD, a supported lipid bilayer, and energy in the form of ATP. The proteins formed dynamic patterns by a reaction-diffusion mechanism in vitro, which could account for the behavior of these same proteins in selecting the bacterial cell division site. FtsZ, a tubulin homolog, is the primary cell division protein in most bacterial and archaeal species. It is tethered to the membrane by FtsA, and, together with a dozen other proteins, it assembles into the Z ring which constricts to divide the cell. Whether constriction requires proteins other than FtsZ has been unclear. Now Osawa et al. (p. 792, published online 17 April 2008), in a system that may recapitulate the primordial division machine, have reconstituted Z rings comprised only of a membrane-targeted FtsZ in liposomes and shown that the rings generate a constriction force.
Birds of a Feather Adapt Together
Recent studies of biological responses to climate change have mostly focused on changes in abundance and distribution of species and adaptive evolutionary change. Charmantier et al. (p. 800) use data from almost five decades of continuous study of a wild bird species—great tits in a UK woodland—to show that phenotypic plasticity can be another effective means for populations to respond to rapidly changing climates. Linking information about individual variation, population-level responses, natural selection, climate, and the timing of the food supply, they show that close tracking of the environment is adaptive, and that the UK great tit population has tracked the changing environment almost perfectly over a very wide range of conditions.
Home Is Where the Hearth Is
The nematode Caenorhabditis elegans can “remember” the temperature at which it was grown and will seek out that temperature if later exposed to a gradient of temperatures. In an effort to define the neuronal circuitry that allows this behavior, Kuhara et al. (p. 803, published online 10 April) found that a neuron known as AWC has an important role in sensing temperature. This came as a surprise because the AWC cell is also known to function as an olfactory neuron. The cell could respond independently to odorants and temperature, even though when sensing temperature the neuron used a similar G protein-mediated intracellular signaling system to that used in olfactory responses.
Identifying Receptor-Ligand Pairs
Secreted proteins that interact with receptors on the cell surface control all sorts of biological processes and provide targets for therapeutic intervention. Lin et al. (p. 807) used a systematic approach to search for previously unknown ligand-receptor pairs. They produced recombinant proteins from genes that appeared to encode secreted proteins and then tested the proteins individually for effects in a range of in vitro assays. Having identified a new cytokine, interleukin 34 (IL-34), the authors then looked for binding to recombinant extracellular domains of about 1600 transmembrane proteins, which led to the detection of IL-34 binding to the colony-stimulating factor 1 (CSF-1) receptor. Biological effects of IL-34 mediated through the CSF-1 receptor were confirmed in further studies. Thus, the authors' approach can identify important regulatory proteins and their receptors.
Plants have evolved an exquisitely tunable mechanism for harmlessly dissipating excess light energy in bright conditions without otherwise sacrificing the efficiency of photosynthesis. Ahn et al. (p. 794) make progress in uncovering the molecular basis of this quenching process using ultrafast transient absorption measurements of selectively mutated light-harvesting antennas in photosystem II. Quenching can occur in all three individual minor antenna complexes (CP24, CP26, CP29) through charge transfer from a pair of electronically coupled chlorophyll chromophores to a zeaxanthin molecule. Such shifts in the relative conformation of the two chlorophylls could thus underlie the tunability of the light-harvesting machinery.
Viral Subversion Strategy
Viruses have numerous clever strategies for commandeering control of cellular functions. Hume et al. (p. 797) found that the human cytomegalovirus UL97 had effects on progression through the cell division cycle that were similar to those of cyclin-cyclin-dependent kinase complexes. UL97 is itself a protein kinase that phosphorylates and inactivates the retinoblastoma tumor suppressor protein. UL97 may thus act like a cyclin-dependent kinase to promote proliferation, but lacks the feedback control that cells normally use to control endogenous cyclin-dependent kinases.
Wholesale Heat Shock
Exposure to increased temperature can damage cells, and individual cells respond to heat shock by synthesizing protective chaperone proteins that help maintain proper protein folding. Prahlad et al. (p. 811) report experiments in the nematode Caenorhabditis elegans that show an organism-wide, rather than cell autonomous, response to heat shock. Synthesis of heat shock proteins was inhibited in animals carrying mutations that disrupt the function of a sensory neuron previously known to respond to increased temperature and influence behavior. The heat shock response was also altered in response to dauer pheromone, which regulates growth and metabolism. Thus, C. elegans appears to possess a regulatory system that coordinates the organismal heat shock response with multiple environmental cues.