From Big Fish to Big Whales
To Degrade or Not to DegradeRegulating the turnover of proteins within the cell is of fundamental importance to almost every physiological process. Hwang et al. (p. 973, published online 28 January; see the Perspective by Mogk and Bukau) now find that acetylated N-terminal methionine (Met) is a degradation signal. This degron is recognized by Saccharomyces cerevisiae Doa10, a transmembrane E3 ubiquitin ligase that resides in the endoplasmic reticulum and inner nuclear membrane. The removal of N-terminal Met by Met-aminopeptidases generates N-terminal residues that are often N-terminally acetylated. Doa10 selectively binds to the resulting N-degrons, which may represent the most prevalent class of cellular protein degradation signals.
2D Quantum Critical TransitionsQuantum critical transitions occur at near-zero temperatures when the properties of quantum matter are tuned by an external parameter such as the magnetic field or pressure. Heavy fermion materials, which have effective charge carrier masses hundreds of times heavier than the bare electron mass, have emerged as a prototypical system for studying these transitions. Now, Shishido et al. (p. 980; see the Perspective by Coleman) use a heavy fermion compound to experimentally realize a new type of quantum phase transition where the tuning parameter is the dimensionality of the system. They engineer a family of superlattices made up of a fixed number of layers of the conventional metal LaIn3 and varying numbers of layers of the heavy fermion material CeIn3. As the number of layers of CeIn3 is decreased, the system gradually changes character from three- to two-dimensional, with corresponding changes in its transport properties.
Detecting Distant PlanetsMore than 400 planets have been detected outside the solar system, most of which have masses similar to that of the gas giant planet, Jupiter. Borucki et al. (p. 977, published online 7 January) summarize the planetary findings derived from the first six weeks of observations with the Kepler mission whose objective is to search for and determine the frequency of Earth-like planets in the habitable zones of other stars. The results include the detection of five new exoplanets, which confirm the existence of planets with densities substantially lower than those predicted for gas giant planets.
Silicate in the Primordial SoupDirect evidence for how prebiotic synthesis of complex organic molecules paved the way for the origin of life is extremely scarce. Thus, studies are mainly limited to controlled simulations of likely reactions in early Earth conditions. Similarly, chemical reactions in the laboratory may generate the products necessary for biosynthesis, but may nevertheless be geochemically irrelevant. Lambert et al. (p. 984) show that silicate ions, present in Earth's surface waters at relatively high concentrations, catalyze the formation of four- and six-carbon sugars from simple sugars via the formose reaction. The resulting complexes stabilize the sugar molecules, allowing sugars to accumulate in greater abundance. Silicate stabilization also circumvents the need for the formose reaction to proceed at high temperatures, thus extending the range of possible environments in which life could have originated.
Metabolic Regulation Through AcetylationCovalent modification of lysine residues in various proteins in the nucleus is a recognized mechanism for control of transcription. Now two papers suggest that acetylation may represent an important regulatory mechanism controlling the function of metabolic enzymes (see the Perspective by Norvell and McMahon). Zhao et al. (p. 1000) found that a large proportion of enzymes in various metabolic pathways were acetylated in human liver cells. Acetylation regulated various enzymes by distinct mechanisms, directly activating some, inhibiting one, and controlling the stability of another. Control of metabolism by acetylation appears to be evolutionarily conserved: Wang et al. (p. 1004) found that the ability of the bacterium Salmonella entericum to optimize growth on distinct carbon sources required differential acetylation of key metabolic enzymes, thus controlling flux through metabolic pathways.
Cropland Acidification in ChinaChina is experiencing increasing problems with acid rain, groundwater pollution, and nitrous oxide emissions. Rapid development of industry and transportation has accelerated nitrate (N) emissions to the atmosphere. Consequently, soil degradation, water shortage, and pollution, in addition to atmospheric quality decline are becoming major public concerns across China. Since the 1990s, China has become both the largest consumer of chemical N fertilizers and the highest cereal producer in the world, which has consequences for arable soil acidification. Guo et al. (p. 1008, published online 11 February) present a meta-analysis of a regional acidification phenomenon in Chinese arable soils that is largely associated with higher N fertilization and higher crop production. Such large-scale soil acidification is likely to threaten the sustainability of agriculture and affect the biogeochemical cycles of nutrients and also toxic elements in soils.
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