Editors' Choice

Science  17 Sep 2004:
Vol. 305, Issue 5691, pp. 1679

    Paradise Lost?

    1. Linda Rowan

    The dry and barren landscape on Mars is often compared to dry and desolate deserts on Earth, but McGovern et al. have chosen a tropical paradise, the Hawaiian islands, for a terrestrial analogy to explain the evolution of Olympus Mons, which is the largest known volcano (about 23 km in height and 600 km in diameter) in the solar system. It is partly bounded by an irregular scarp as high as 10 km, and lobes of hummocky terrain, which are called aureole deposits, funnel outward from this scarp. The aureole deposits contain remnants of formerly continuous volcanic flow units and morphologically resemble landslides around the edges of Hawaiian volcanoes. The authors suggest that, in similar fashion, Olympus Mons may have grown and spread by basal detachment faults. In Hawaii, the landslides are lubricated by high pore fluid pressure on the faults and are mostly submarine, which poses the question: Was Olympus Mons once a fluid paradise, too? - LR

    J. Geophys. Res. 109, 10.1029/2004JE002258 (2004).


    Swifter, Higher, Stronger

    1. Andrew M. Sugden

    Sexual selection, the evolutionary corollary of mate choice, is generally studied in organisms where direct matings (for example, internal fertilization) between individuals take place. The variance in male mating success that results when females choose, in particular, can lead to the evolution of showy and sometimes bizarre signals of male quality. However, the ancestral condition for sexual reproduction in animals is broadcast spawning and external fertilization—that is, the release of sperm and eggs by benthic marine organisms into the water column. Does sexual selection operate under these conditions?

    In an experimental study of reproduction in sea urchins, Levitan finds that sexual selection—as identified by the difference between males and females in the variance for fertilization success—does indeed occur, but only at intermediate population densities of males and females. At low and high densities, the variance in fertilization success did not differ between the sexes, because of sperm limitation at low density and sperm competition at high density. Hence, sexual selection in sea urchins is under control of the adult only in the sense of timing and quantity of gamete release; the rest is mediated by traits of the gametes themselves. — AMS

    Am. Nat. 164, 298 (2004).


    Fast-Flowing Filters

    1. Marc S. Lavine

    Porous membranes are used extensively for separation processes such as water purification. A current challenge is to fabricate membrane materials that can separate objects differing in size by only a few nanometers (which means small pores) and can still operate at a reasonable filtration rate (small pores are prone to blockage).

    Akthakul et al. have enhanced the filtration capabilities of a commercial poly(vinylidene fluoride) (PVDF) membrane by spin coating a thin film of a copolymer consisting of a PVDF backbone, with short polyethylene oxide (PEO) side chains grafted on via a methacrylate linkage. The PEO and PVDF segments do not like to mix with each other, so the chains segregate locally into partially crystalline PVDF regions separated by PEO nanochannels. Water is repelled by the PVDF but is able to move through the PEO regions, thus enhancing the overall transport through the commercial PVDF membrane. The PEO segments interact strongly with the water molecules, which prevents organics from clinging and fouling the membrane. The membranes can also be used for molecular sieving, as demonstrated by the separation of similarly charged dye molecules, and for size-exclusion chromatography, as demonstrated by the separation of vitamins B2 and B12. — MSL

    Macromolecules 10.1021/ma048837s (2004).


    One Size Fits Many

    1. Valda Vinson

    Enzymatic reactions generally demand a precise positioning of catalytic residues; thus, structural disorder in a protein might be expected to be inconsistent with catalytic prowess. However, Vamcava et al. show that a monomeric chorismate mutase (mCM), obtained by redesign of the naturally occurring dimer, displays many of the characteristics of a molten globule yet still possesses one-third of the wild-type catalytic efficiency. Spectroscopic and thermal denaturation experiments all suggest that the monomeric form has high conformational flexibility and only adopts an ordered structure when a transition-state analog (inhibitor) is added. In contrast, dimeric CM is ordered both in the absence and presence of ligand. The polar character of the active site in the interior of mCM, unlike the hydrophobic core of the wild-type enzyme, fails to rigidify the folded state. When the inhibitor binds, it fills the pocket and supplies interactions that propagate and improve global ordering, as in the induced fit model of enzyme catalysis, in which the catalytically active conformation is locked into place as the reaction progresses. The idea that folding and catalysis can be linked implies that modern-day enzymes could have evolved from molten globules. Perhaps, a primordial structural plasticity conferred relaxed substrate specificity enabling a limited set of protein enzymes to catalyze a wide range of reactions. — VV

    Proc. Natl. Acad. Sci. U.S.A. 101, 12860 (2004).


    Thermophilic Parasite

    1. Stella M. Hurtley

    Malaria is responsible for the death of more than 1 million people each year. In the course of cycling between the mosquito vector and the human host, the malarial parasite Plasmodium falciparum is exposed to high temperatures, up to 41°C in febrile patients, which are sufficient to send the microbe into heat shock.

    Pavithra et al. examined the role of heat shock proteins in the development of the parasite within infected red blood cells by periodically incubating them at elevated temperatures, mimicking the recurrent febrile episodes typical of malarial infections. They find that elevated temperatures promote parasite development within the erythrocyte and that an inhibitor of one of the heat shock proteins actually disrupted parasite development. These findings support the idea that the parasite exploits the environmental cues provided by elevated body temperature to stage its development during infection, and it suggests that interventions that affect the malarial heat shock response may be useful in combating the disease. — SMH

    J. Biol. Chem. 10.1074/jbc.M409165200 (2004).


    Rare Frameworks

    1. Phillip D. Szuromi

    Many transition metals have been shown to form solid-state compounds with interpenetrating frameworks, which are of interest as they can provide routes to creating microporous materials. However, for the lanthanides and actinides, progress has been slower, with the only known example being an actinide compound, the thiophosphate UP4S12.

    Aitken and Kanatzidis report that the reaction of ytterbium in a potassium thiophosphate flux yields K6Yb3(PS4)5. X-ray crystallography revealed two interlocked networks with three types of Yb3+ centers linking the PS4 tetrahedra, one with the expected bicapped trigonal prismatic geometry and the other two with a distorted octahedral structure. The small size of Yb relative to other lanthanides appears to be the key factor in allowing it to adopt the octahedral geometry needed to form this type of network. - PDS

    J. Am. Chem. Soc. 10.1021/ja0474648 (2004).

  7. STKE

    Moving TRPs to the Membrane

    1. Nancy Gough

    Singh et al. report that cation channels of the transient receptor potential (TRP) family are dynamically inserted into the plasma membrane in response to ligand stimulation of G protein-coupled receptors, as recently found after stimulation of receptor tyrosine kinases. The authors identified proteins involved in exocytosis—vesicle-associated membrane protein 2 (VAMP2) and α soluble N-ethylmaleimide-sensitive factor attachment protein (αSNAP) as interacting partners for the N-terminal domain of TRPC3 in a yeast two-hybrid screen. The interaction with proteins involved in exocytosis was confirmed with heterologously expressed proteins in transfected cells and endogenously expressed protein in rat brain. Exposure of human embryonic kidney cells expressing TRPC3 to the GPCR ligand carbachol resulted in increased abundance of TRPC3 at the cell surface, and this insertion was inhibited by cleavage of VAMP2 with tetanus toxin. Measurements of calcium influx with fluorescent indicators verified that the channels were functional. Thus, regulated insertion appears to contribute to agonist-stimulated TRP activity and calcium signaling. — NG

    Mol. Cell 15, 635 (2004).