This Week in Science

Science  12 Dec 1997:
Vol. 278, Issue 5345, pp. 1857
  1. Signaling structures

    The heterotrimeric guanine nucleotide-binding proteins (G proteins) couple receptors for hormones and other molecules at the cell surface to intracellular signaling pathways that control various cellular responses to their environment. For example, the alpha subunit of the G protein Gsα interacts with and activates the enzyme adenylyl cyclase. Adenylyl cyclase passes along a signal by converting adenosine triphosphate to adenosine 3,5-monophosphate, which, in turn, stimulates the activity of the protein kinase PKA. Tesmer et al. (p. 1907) and Sunahara et al. (p. 1943) present crystal structures of the activated form of Gsα alone and in a complex with the catalytic domains of adenylate cyclase (see the Perspective by Bourne, p. 1898). Their data provide insights into the structural basis for the specificity of interaction of various Gα proteins with particular receptors and targets and a potential mechanism by which Gsα may activate its primary target, adenylylcyclase.

  2. Cooler films

    Silicon dioxide is the main insulating material used in silicon devices, but more complex architectures will require lower temperature routes for its formation to avoid damaging previously fabricated parts of devices. Klaus et al. (p. 1934) show that a two-step reaction that forms single layers of silicon dioxide at temperatures greater than 600 kelvin proceeds readily at room temperature if an organic base, pyridine, is included as a catalyst.

  3. A middle road for protein folding

    Two extreme views have been invoked for protein folding, one in which the protein folds along a well-defined specific pathway, and one where there is a general bias in the energy surface that guides the protein to the native (folded) state through a variety of routes. Lazaridis and Karplus (p. 1928) have performed extensive simulations of folding pathways and show that the two views can be reconciled; they suggest that while there is considerable diversity between the different trajectories, there are common structural features that define an average folding pathway.

  4. Planning and execution

    Neurons in the motor cortex change their firing rates during the planning and execution of movements such that a vector summation of the neurons' preferred directions corresponds to the desired movement. Riehle et al. (p. 1950; see the Perspective by Fetz, p. 1901) recorded simultaneously from motor cortical neurons in monkeys during preparatory stages and actual movement. They find that the temporal pattern of neuronal spikes becomes synchronized both when the monkey is planning to make a movement and when the movement is made, but that there is no change in overall firing rates during the former stages, only during execution of the plan.

  5. Chemical kinetics by counting

    Chemical kinetics reflects the sum of enormous numbers of individual reactions, so the thought of examining single reaction events to understand overall kinetics is quite daunting. With the aim of the scanning tunneling microscope, Wintterlin et al. (p. 1931) were able to perform an atomic level kinetic study of a simple bimolecular reaction (oxidation of carbon monoxide on platinum). Their kinetic parameters derived from observations of individual reaction events, which take place predominantly at the boundary between domains of CO and O2, agree with those determined macroscopically.

  6. A cell cycle twist

    Pin1 is a peptidyl-prolyl isomerase that plays a critical but poorly understood role in regulation of the cell division cycle. Yaffe et al. (p. 1957) examined the peptide-binding specificity of Pin1 and found that it preferentially binds and isomerizes proline (Pro) residues preceded by phosphorylated serine (Ser) or threonine (Thr). The Pro-directed Ser-Thr kinases are important regulators of the cell cycle, and Pin1 binds to a number of proteins that undergo phosphorylation in mitotic cells. Pin1 apparently recognizes phosphorylated Ser-Pro or Thr-Pro sites in proteins phosphorylated at mitosis and facilitates conformational changes in such proteins through its isomerase activity.

  7. Grainy echoes in Hyakutake

    Comet Hyakutake passed within 0.1 astronomical units of Earth in March 1996 allowing Harmon et al. (p. 1921) to image the nucleus and coma by bouncing radio waves off the comet and thus see the cometary structure. They estimate that the nucleus is 2 to 3 kilometers in diameter, consistent with previous estimates and the fact that Hyakutake's brillant activity is not well correlated with its small size. The signature of the echoes also indicates that the high gas production in the coma is the result of porous icy grains in the coma; a cometary mechanism that has not been fully appreciated until Hyakutake's visit.

  8. Cluster excesses

    The Virgo and Coma galaxy clusters emit an excess radiation in the extreme ultraviolet that has not been explained. Theories have suggested exotic mechanisms to produce the excess, such as unusually hot gases or turbulent mixing. Hwang (p. 1917) finds that a more standard mechanism, inverse Compton scattering, can explain the observations. He shows that inverse Compton scattering of photons in the cosmic microwave background produce excess extreme ultraviolet emissions and he supports this model by showing that the same mechanism also explains the diffuse radio emissions observed for the Virgo and Coma clusters.

  9. Forming front and back growth

    Early in embryonic growth, signaling pathways specify the development pattern. The Xenopus signaling proteins Chordin and Noggin function in dorsalization while the bone morphogenic proteins (BMP) function in ventralization. Blader et al. (p. 1937) isolated zebrafish tolloid, which encodes a protein similar to BMP-1 and is involved in ventralization. Tolloid acts as antagonist toward Chordin. Dorsalization of Xenopus embryos by the addition of lithium has been thought to function through the polyphosphoinositide cycle, but recent work has implicated glycogen synthase kinase. Kume et al. (p. 1940) inhibited the receptor IP3 by adding an IP3 antibody to Xenopus embryos and found incomplete dorsal differentiation. Thus, the IP3 signaling system transduces ventral signals, but other factors may also be involved.

  10. Solar, not extrasolar

    The pulsar PSR B1257+12 shows a variation in its emission with a period of about 25 days that was attributed to an orbiting planet. Scherer et al. found that Doppler data from the Pioneer 10 spacecraft radio carrier wave also has a period of about 25 days, but this period correlates with plasma wave data recorded from low and mid-latitude solar wind. The correlation indicates that Pioneer 10, which is now drifting at about 65 astronomical units (Earth-sun distance) near the ecliptic plane, is being periodically perturbed by the oscillations in the solar wind created by the sun's rotation. Given that the pulsar is also aligned along the ecliptic plane and shows the same period, the fluctuations are probably solar artifacts.

  11. Chiral liquid crystals from achiral molecules

    Molecules that lack right- or left-handednessxs (achiral molecules) usually form crystals with no handedness, but in rare cases they can pack as right- and left-handed crystals. This avoidance of a more symmetrical packing is said to break achiral symmetry. Link et al. now show that such symmetry breaking can occur in a liquid crystal. A molecule with a “bent” core undergoes two types of symmetry breaking to spontaneously form smectic (layered) liquid crystals with chiral domains.

  12. COUP in patterning

    Drosophila Sonic hedgehog (Shh) and its vertebrate homolog Hedgehog (Hh) function in dorsoventral patterning and mediate the development of axial midline structures. What is the mechanism through which these factors act? Krishnanet al. identified a novel target gene, COUP-TFII, for the Shh signaling pathway. In response to Shh stimulation, a factor binds to the COUP-TFII promoter. This binding is distinct from regulation by the Gli protein, another factor downstream of Shh signaling. Binding of the novel protein is shown to result from a dephosphorylation event in response to Shh signaling.

  13. Supply and demand

    Certain proteins of the chloroplast photosynthetic complex are translated according to need. Kim and Mayfield have cloned the nuclear-encoded protein that, in response to changes in redox potential, in turn modifies an RNA binding protein in the chloroplast. These interactions provide a mechanism by which the translation of messenger RNA in the chloroplast can be regulated according to the amount of photosynthetic activity. The newly identified protein is homologous to known protein disulfide isomerases.

  14. Room to work

    During transcription, how does the large eukaryotic polymerase molecule, RNA polymerase III (Pol III), progress through DNA that is packaged into nucleosomes. Studitsky et al. report that Pol III transcribes through nucleosomes without displacing the histone octamer from the DNA to which it is bound. As Pol III enters nucleosome-bound DNA, the nucleosome is transferred up to a position about 80 base pairs toward the promoter through a looping mechanism, thus freeing the previously bond DNA for transcription. See the Perspective by Widom.

  15. A point of resistance

    Recent progress in understanding how plants respond to pathogens has come from cloning of disease resistance genes. Although each gene is specific for an individual plant-pathogen interaction, these genes nonetheless share certain structural similarities. Century et al. cloned a gene, NDR1, from Arabidopsis, that interacts with several of these resistance genes and may represent a point at which these response pathways converge. Mutations in the NDR1 gene affect responses to several pathogens, both bacterial and fungal, as mediated through several resistance genes. The specific molecular function of the NDR1 gene remains unclear

  16. Switching sides

    When a cell is induced to die, a cascade of proteases, called caspases, become active and their protein targets are cleaved. Cheng et al. have found that a cellular protein that normally protects a cell from apoptosis, Bcl-2, is cleaved by caspases. The carboxyl-terminal region of this protein then can trigger cell death and accelerate apoptosis. The truncated Bcl-2 seems to work by further amplification of the downstream caspase cascade. Thus, to ensure apoptosis, the cell not only inactivates the inhibitors, but converts them into additional death tools.