Editors' Choice

Science  21 Nov 2003:
Vol. 302, Issue 5649, pp. 1295

    Gelling Cells

    Hydrogels, which consist of water-filled, highly cross-linked polymer networks, are under consideration as a means of drug or cell delivery. Usually, the polymer networks incorporate physical or chemical cross-links, and the cells are immobilized within this network. Lee et al. consider the case in which the cells themselves are one of the components cross-linked. They started with alginate, a biopolymer that does not appreciably adsorb proteins or cells, and attached a small number of arginine-glycine-aspartic acid (RGD) peptides. When cells were mixed with these modified polymers, a network formed due to the cell surface expression of RGD-binding integrins. The resulting gel was found to be shear-reversible, meaning that its viscosity decreased significantly when subjected to a shear flow, which disrupted the RGD-integrin interactions, yet the gel reformed within 10 min after shearing. Thus, it would potentially be feasible to inject the gel into a patient with a standard syringe for medical applications. — MSL

    Adv. Mater. 15, 1829 (2003).


    Bunker Building

    When environmental conditions take a turn for the worse, Bacillus species seek shelter by changing into extraordinarily resistant spores, which can then germinate after conditions improve. Using atomic force microscopy, Chada et al. investigated the topography of the spore surface. The spore is a multilayered protective shell consisting of up to 60 different proteins encoded by the cot gene family. Of the three species studied, Bacillus cereus and B. anthracis sport an outermost glycoprotein layer called the exosporium, whereas the B. subtilis spore coat is naked. The spore coats could be differentiated by their distinctive patterns of major ridges and thousands of circular bumps. The ridges may be folds resulting from dehydration of the spore core, and the bumps may encircle minute pores that allow influx of the signals that trigger germination but exclude larger, potentially toxic molecules. Both features were obscured by the exosporium, but this structure was not always present in the B. cereus and B. anthracis samples. Spores of cot mutants exhibited varied surface patterns, indicative of specific roles for these genes in coat assembly and structure. — CA

    J. Bacteriol. 185, 6255 (2003).


    The Great Lisbon Earthquake

    On 1 November 1755, an estimated 8.5 magnitude earthquake occurred along the Africa-Eurasia plate boundary in the Atlantic Ocean southwest of Lisbon. This literally earthshaking event devastated one of the most beautiful cities in Europe, producing ground liquefaction, seiches, a tsunami, and subsequent fires. Shaking was felt throughout the rest of Portugal and in Spain, France, Italy, Switzerland, and northern Africa; the tsunami flooded much of the Atlantic and western Mediterranean coastlines; and seiches were noted as far away as Finland.

    Afterward, the Portuguese government requested that priests provide information about the time and duration of shaking as well as the damage in their parishes. In addition, eyewitness accounts from inhabitants and sailors were recorded. Based on these descriptions, Vilanova et al. suggest that the initial offshore earthquake triggered within minutes another earthquake about 300 km away on the Lower Tagus Valley fault. A dynamic effect at this distance would be unusual but possible if the fault were already close to failure. The second event would account for reports of two periods of intense shaking, the sudden subsidence of the riverbed and impulsive rise of the river water, and the high intensity of the shaking within the valley region. The Lower Tagus Valley is an important component of the plate boundary and suffered ruptures in 1344, 1531, and 1909. Adding another event in 1755 would reduce the recurrence interval and likely increase the estimated seismic hazard. — LR

    Bull. Seismol. Soc. Am. 93, 2056 (2003).


    Splicing in Diversity

    The antigen-binding end of an antibody molecule and green fluorescent protein (GEP) both contain β-strands that are connected by exposed loops. Zeytun et al. have taken advantage of this commonality and engineered libraries of “fluorobodies” by inserting antigen-binding sequences (the complementarity-determining regions) within four of the GFP loops. They isolated fluorobodies that bound specifically to several proteins (ubiquitin, tubulin, and myoglobin) with submicromolar affinities and were used successfully in immunofluorescence microscopy, immunoblots, or flow cytometry. — GJC

    NatureBiotechnol. 10.1038/nbt911 (2003).


    Airing Oneself

    As a means of avoiding autoimmunity, most T cells that are capable of recognizing self-proteins are deleted as they develop within the thymus. However, this presents the conundrum of how these thymocytes become exposed to self-proteins in the first place, because many of these proteins should be restricted to tissues outside of the thymus, such as the pancreas. Part of the solution was provided recently by the finding that the transcriptional regulator Aire drives ectopic expression of tissue-specific genes in the thymic epithelium.

    In looking for how Aire itself might be regulated, Chin et al. explored the observation that mice lacking the cytokine lymphotoxin-α (LTα) or its receptor LTγR manifest autoimmune characteristics similar to those of mice that don't have the Aire gene. Presumably as a consequence of reduced ectopic gene expression, mice deficient in either LTγR or LTα generated autoantibodies and self-reactive lymphocytes that then induced pathology in a variety of tissues. Aire expression was considerably reduced in the thymic epithelium of both LTγR- and LTα-deficient mice but could be enhanced in the latter set of mice by administering an antibody that acted as an LTγR agonist. Restoration of Aire expression via LTγR signaling also enhanced the thymic expression of insulin, which is one of the genes under control of Aire. Because LTα is expressed after thymocyte activation, developing T cells may influence their own survival through modulation of Aire expression. — SJS

    Nature Immunol. 4, 1121 (2003).


    Fullerene Peas in Porphyrin Pods

    The host-guest structures formed by carbon nanotubes containing fullerene molecules have unusual transport and electronic properties, but the production of these assemblies is difficult to control. Yamaguchi et al. have produced an organic supramolecular structure in which fullerenes are held by a claw created by two linked porphyrin groups that are attached to small dendrimers. These units then assemble further into nanotubes via the hydrogen bonding of carboxylic acid groups attached to the porphyrins. The nanotubes are thermally stable and form as discrete unentangled entities. — PDS

    J. Am. Chem. Soc. 10.1021/ja038178j.


    Sibling Conflict Before Birth

    Developing fruits commonly contribute to their own growth by photosynthesis, augmenting the nutritive supplies produced by the parental leaves. The proportion of this contribution varies greatly across plants, reaching at least 50% in some small-fruited species. Zangerl et al. have found that the rate of photosynthesis in fruits is also influenced by the paternity of the seeds. Using fluorescence imaging to quantify the efficiency of photosystem II in the developing fruits of the wild parsnip, Pastinaca sativa, they show that photosynthetic rates vary by up to 18%, depending on the identity of the pollen donor. Increased photosynthesis in the fruit tissues surrounding a seed can be expected to have a positive influence on the seed's development. Hence, seed paternity can influence the outcome of competition between sibling seeds for limited maternal resources. — AMS

    Ecol. Lett. 6, 966 (2003).


    Separating the Haves from the Have-Nots

    Proteins destined for insertion into or across a membrane carry a sequence, usually at the N terminus, that distinguishes them from soluble cytosolic proteins. The signal sequence interacts with the targeting machinery and the protein translocation apparatus, called the translocon, which actively transports the polypeptide chain across the membrane. In Escherichia coli, the SecA protein binds to the signal sequence on nascent secretory proteins, maintains them in an unfolded and translocation-competent state, and carries them to the translocon for export. Eser and Ehrmann describe how SecA plays an additional chaperone-like function by acting as a quality-control sensor: SecA binds to proteins that lack signal sequences and promotes their folding, thus helping to divert them away from the translocon. — SMH

    Proc. Natl. Acad. Sci. U.S.A. 100, 13231 (2003).