This Week in Science

Science  18 Apr 1997:
Vol. 276, Issue 5311, pp. 329
  1. Immunoglobulins and membrane expression

    Immunoglobulins (Igs) exist in both secretory and membrane-bound forms. The functions served by the latter forms are revealed in three reports (see the Perspective by Tarlington, p. 374). Targeted disruption of the transmembrane domain was performed by Kaisho et al. (p. 412), who used IgG1 and by Achatz et al. (p. 409), who used IgE, to show that production of antigen-specific secretory IgG1 and IgE is strictly dependent on prior surface expression of the membrane-bound form. Truncation of the highly conserved cytoplasmic domains of IgG1 and IgE, which allowed membrane expression of the molecules with a minimal cytoplasmic tail, resulted in substantial but incomplete reduction in serum and antigen-specific Ig. Weiser et al. (p. 407) show that the longer tail contains an endosomal targeting motif: Antigen bound to membrane Ig is shuttled to the endosomal compartment, where it may be processed for subsequent presentation to T cells. These new results are consistent with the dependence of IgG1 and IgE responses on help from T cells.

  2. Polar organic films

    In a membrane bilayer, the two lipid layers are arranged “tail to tail”; both sides are hydrophilic, and any inherent molecular polarity, such as a dipole moment, is canceled by another molecule in the opposing layer. Stupp et al. (p. 384; see the cover and the news story by Service, p. 354) have formed organic films that arrange in an inherently polar fashion, that is, heads to tails, so that one side is hydrophobic and the other hydrophilic. Relatively short triblock copolymers self-assemble into 200-kilodalton nanostructures, which then aggregate into layers to form the films. The films are highly adherent to their substrates, and their inherent polarity leads to second-order nonlinear optical activity despite lacking strong chromophores.

  3. Comets and shock chemistry

    Comets may have provided Earth and other planets with organic molecules directly, but the high-shock temperatures of an impact may have also helped synthesize additional molecules in primative planetary atmospheres. McKay and Borucki (p. 390) simulated shock heating of an atmosphere with an overall cometary composition. Their results imply that organic molecules could be produced by shock heating of a methane-rich atmosphere but would not form in one rich in carbon dioxide.

  4. Cold but moving

    Strontium titanate (SrTiO3) is one of the most studied materials in solid-state physics, and Grupp and Goldman (p. 392) have added another phenomenon to its repertoire–it shows large piezoelectric effects at temperatures below 10 kelvin, apparently because of a quantum critical point at zero temperature. The effect is large in a temperature range where other low-temperature piezoelectrics show a diminished response, and it may find use in ultralow-temperature thermometry and scanning probe microscopy.

  5. Ancient ape ancestor

    Fossils were discovered in the early 1960s from Moroto in Uganda that may represent an ancient common ancestor to apes and humans, but their age and interpretation have been controversial. Gebo et al. (p. 401; see the news story by Gibbons, p. 355) present argon-40-argon-39 dates on a younger basalt flow which indicate that the Moroto fossils are at least 20.6 million years old. Evidence from newer fossil samples they collected, together with the old fossils, imply that the ape-like body plan had already developed by this time.

  6. Gaining stability

    Some high-energy solid phases, such as diamond, may persist at ambient conditions if the energy barrier to form the lower energy structure is too high to be overcome at low temperature and pressure, but many others spontaneously transform into the lower energy structure upon the release of pressure or the lowering of temperature. Chen et al. (p. 398; see the Perspective by Brus, p. 373) investigated the kinetics of phase transformations in nanocrystals as a function of crystallite size. They show that metastable phases persist to lower temperatures and pressures than in the bulk phases, indicating that nanocrystals may provide a route to the synthesis and stabilization of high-energy structures of materials.

  7. Treating the effect of genetic disease

    Several diseases, including Tay-Sachs and Gaucher's disease, are associated with defects in the pathway of glycosphingolipid breakdown in lysosomes. One strategy for treating these diseases, regardless of the particular genetic defect involved, would be to block the production of glycosphingolipids. In a mouse model of Tay-Sachs disease, Platt et al. (p. 428) showed that treatment with N-butyldeoxynojirimycin inhibited glycosphingolipid synthesis and prevented abnormal accumulation in the brain.

  8. Inside an isomerase

    Numerous biological reactions, enzymes involve the cleavage of a carbon-hydrogen bond that is next to a carbonyl (CO) group or a carboxyl (COOH) group to form an enol or enolate intermediate, which may in turn be protonated at the same or an adjacent carbon atom. A paradigm for this class of compounds is 3-oxo-Δ5-steroid isomerase. The solution structure was determined by Wu et al.(p. 415) using multidimensional heteronuclear magnetic resonance spectroscopy. From this structure and additional kinetics and mutagenesis studies, they suggest that the dienolate is stabilized by two hydrogen bonds, rather than by a low-barrier hydrogen bond as had been suggested previously.

  9. Thin films, atom by atom

    The growth of thin films of metals and semiconductors by depositing atoms from the gas phase occurs far from equilibrium and proceeds on flat terraces, at the steps and kinks that join them, and on “islands” that form as several adsorbing atoms come together. How kinetic processes compete with thermodynamic constraints is discussed in an article by Zhang and Lagally. Controlled growth involves effects such as atom diffusion, island nucleation, and atom transfer between the growing layers.

  10. Growing cold

    Recent studies have shown that some of the biological primary production in the southern oceans occurs within Antarctic sea ice. Arrigo et al. developed a numerical model of sea ice production around Antarctica calibrated to observations. The model results predict that most of the production is in first-year ice and in areas of low snow cover and that annual production in the ice may total 40 teragrams of carbon. [See the news story by Brown.]

  11. Multiple endocrine neoplasia gene

    Multiple endocrine neoplasia-type 1 is a cancer syndrome characterized by tumors in parathyroids, pancreatic islets, duodenal endocrine cells, and the anterior pituitary. After a decade-long search, the gene associated with this disease has been found by Chandrasekharappa et al., who identified mutations in 15 families with the inherited form of MEN1. This gene, which in its normal form may act as a tumor suppressor, does not resemble any currently known proteins.

  12. Managing mast cells

    The Cbl protein is a proto-oncogene product that participates in the signaling mechanisms by which hematopoietic cells respond to extracellular signals. Ota and Samelson studied the function of Cbl in mast cells that were stimulated to release mediators of the allergic response. Not only does Cbl serve as a complex adaptor protein that binds to other proteins of the signaling pathways, but it also directly binds to and inhibits the Syk nonreceptor protein tyrosine kinase, an enzyme that is required for release of histamine from mast cells.

  13. Mosquitoes versus malaria

    Understanding how mosquitoes can resist malaria parasites could aid the design of malaria control strategies. One strain of the mosquito vector, Anopheles gambiae, resists by coating the mature parasites with a melanin-rich capsule. Zheng et al. found that encapsulation is associated with one major and two minor quantitative trait loci in the Anopheles genome. These findings will help in the isolation of the genes involved in this form of resistance.

  14. AraC structure

    Genes that control the uptake and catabolism of the monosaccharide L-arabinose are regulated in Escherichia coli by the AraC protein. The DNA binding properties of this dimeric transcription regulator change when it forms a complex with L-arabinose. Soisson et al. present the crystal structures of AraC in its free form and in complex with L-arabinose, which shows that, in the free form, the amino-terminal arm is disordered and exposes the sugar-binding pocket, allowing it to serve as a oligomerization interface. In the bound form, the amino-terminal arm completely covers the L-arabinose ligand.

  15. Roles in release

    Members of heat shock protein 70 (Hsp70) and Hsp90 families act as molecular chaperones that help proteins avoid misfolding. The binding and release of peptides involves conformational changes induced by adenosine triphosphate binding and hydrolysis that are assisted by cofactors, such as the adenine nucleotide exchange factor GrpE, but the mechanism is not well understood. Harrison et al. present the crystal structure of a truncated form of GrpE in complex with the adenosine triphosphatase (ATPase) domain of DnaK, the Escherichia coli homolog of Hsp70. Two Grp molecules bind to the ATPase domain, and GrpE appears to open the nucleotide binding cleft of DnaK to allow the release of adenosine diphosphate from the ATPase. Two long a-helices of GrpE extend beyond the ATPase domain, which suggests that GrpE also plays a role in peptide release.