Editors' Choice

Science  27 Feb 2004:
Vol. 303, Issue 5662, pp. 1259

    Making Lead "Snowflakes"

    By changing a number of variables during the electrodeposition of lead, Xiao et al. show that they can obtain a wide range of micrometer-scale shapes and architectures—including snowflake shapes. The deposition occurred from solutions of lead nitrate and lead acetate, with highly oriented pyrolytic graphite as the substrate and at pH above 3.0 to prevent the evolution of large hydrogen bubbles. In general, individual particles of triangular, hexagonal, octahedral, decahedral, and icosahedral shape were obtained when the voltages were close to the thermodynamic equilibrium potential. At higher reduction potentials, nanowires, nanobrushes, monopods, and multipods were obtained. The synthesis is similar to the growth of colloidal particles, where higher concentrations lead to more elongated particles. For colloids, particle growth is often directed by capping layers that prevent certain faces from growing; similar effects are seen in lead electrodeposition, where a snowflake structure can be obtained through the addition of ethanol. — MSL

    J. Am. Chem. Soc. 10.1021/ja0315154 (2004).


    Correcting Mistakes

    During cell division, chromosomes must be partitioned accurately between daughter cells. Each condensed chromosome contains at its center two sister kinetochores that must be attached to opposite poles of the mitotic spindle by microtubules. In this way, as the cell progresses through mitosis, the two chromatids will move to opposite poles, ensuring faithful partitioning. But what happens if chromosomes are attached aberrantly to the spindle? Lampson et al. used reversible small molecule inhibitors to examine the role of an important mitotic kinase—the Aurora kinase—in this process. Inactivation of Aurora kinase can cause chromosomes to fail to align correctly at the metaphase plate. Controlled activation of the Aurora kinase during mitosis corrected chromosome attachment errors by selectively dismantling kinetochore-attached microtubules. Inappropriately attached chromosomes moved to the spindle pole as the kinetochore-attached microtubules shortened. The chromosomes then realigned appropriately at the metaphase plate. This ability to correct mistakes in chromosome alignment is likely to be an important contributor to the maintenance of genome integrity. — SMH

    Nature Cell Biol. 10.1038/ncb1102 (2004


    Probing Free Volume

    Molecular motion in liquids and glasses is often explained in terms of the extent of free volumes that can exist between the molecules. Vallée et al. present single-molecule spectroscopic data that characterize local changes in a free volume space in polymer films. The dye molecule, tetraphenoxy-perylenetetracarboxyl diimide, can exist in two conformations: one in which the core of the molecule lies flat and one in which the core is twisted. The flat conformer has a shorter fluorescence lifetime and a larger van der Waals volume (greater by 0.2 cubic nm). Fluorescence lifetime distributions were measured for this dye in thin films of two polymers of the same family: poly(methyl methacrylate) (PMMA) and poly(n-butyl methacrylate) (PnBMA). The smaller side chains of PMMA result in a lower free volume, and indeed, only longer lifetime traces of the twisted dye conformer were seen. However, for PnBMA, about 30% of the dye molecules could find holes large enough to produce the larger flat conformer. Thus holes as large as 0.2 nm3 can form in PnBMA but not in PMMA. — PDS

    J. Am. Chem. Soc. 10.1021/ja031599g (2004).


    Shape-Shifting Skinks

    In egg-laying reptiles, developmental fate depends on the environmental temperature. Higher temperatures influence the rate of development and the size, shape, gender, and behavior of the hatchlings. But during development, environmental temperatures will not be constant. Spring broods will be subjected to gradually increasing temperatures and summer broods to declining temperatures, even though the mean incubation temperature is similar. Shine investigated the effect of seasonal soil temperature gradients on the development of the Australian soil-nesting lizard, the bold-striped cool-skink Bassiana duperreyi. At constant incubation temperatures, the time to hatching was increased. Under falling temperatures, more deformities occurred, which impaired long-distance running speed, especially for the females. The males produced after a falling incubation temperature regime tended to be shorter, with relatively longer tails than the females. Because seasonal shifts in incubation temperature are widespread, sensitivity to such shifts could influence nest site selection, timing of nesting, and possibly the eventual evolution of viviparity. — CA

    Funct. Ecol. 18, 43 (2004).


    Twinkle, Twinkle, Big Black Hole

    A supermassive black hole about 4 million times the mass of the Sun, Sagitarrius A-star (Sgr A*), is thought to lie at the center of the Milky Way.

    In support of this, Ghez et al. measured a variable infrared emission that comes from a region within 5 astronomical units of Sgr A*. No other emissions show such extreme variability in flux density, and the infrared flares are persistent. If all of the emissions come from the same source, then gravitational lensing, accretion disk illumination, and star disk collisions can be eliminated as possible causes of the infrared flares. The flares probably come from plasma that is either accreting or flowing out from the edges of the black hole. Additional observations of these persistent flares should provide a better picture of black hole energetics. — LR

    Astrophys. J. 601, L159 (2004).


    Anergy Management

    Preventing the unwanted activation of T lymphocytes is crucial to the well-being of an individual. One means by which this is accomplished is by exploiting the context of the signals received to permanently turn a T cell off, rather than on—a process known as anergy. Heissmeyer et al. observed that proteins involved in ubiquitination and protein degradation became more abundant in response to the sustained calcium-dependent signals that induce anergy. At the same time, two proteins that normally mediate downstream signals from the T cell receptor—phospholipase Cγ (PLCγ) and protein kinase C-θ (PKCθ—were degraded when anergic cells were restimulated. PKCθ was monoubiquitinated in anergic T cells, indicating that it might be directly targeted for degradation. Anergy and ubiquitination correlated with instability of the immune synapse: the T cell membrane-associated signaling structure. Furthermore, T cells deficient in either of two ubiquitin ligases were resistant to both anergy induction and synapse breakdown. — SJS

    Nature Immunol. 10.1038/ni1047 (2004).

  7. STKE

    Notching Up Long-Term Memories

    The membrane receptor Notch is an important regulator of development in the central nervous system. Notch may also function in the adult nervous system. Presente et al. used a temperature-sensitive allele of Notch in Drosophila to study the role of Notch in the adult fly. Behavioral tests of short- and long-term memory formation were conducted on wild-type and mutant flies. Short-term memory was not affected by the loss of Notch in adult flies, but long-term memory was impaired. Long-term memory is thought to require physical remodeling of synapses, and such events might require actions of Notch similar to those exerted during development. The presenillin gene associated with familial Alzheimer's disease encodes a protein that functions in Notch signaling. Because memory defects are prominent symptoms of Alzheimer's disease, further exploration of the mechanisms by which Notch signaling affects learning and memory is warranted. — LBR

    Proc. Natl. Acad. Sci. U.S.A., 101, 1764 (2004).