Vesicular restriction of synaptobrevin suggests a role for calcium in membrane fusion

Release of neurotransmitter occurs when synaptic vesicles fuse with the plasma membrane. This neuronal exocytosis is triggered by calcium and requires three SNARE (soluble-N-ethylmaleimide-sensitive factor attachment protein receptors) proteins: synaptobrevin (also known as VAMP) on the synaptic vesicle, and syntaxin and SNAP-25 on the plasma membrane. Neuronal SNARE proteins form a parallel four-helix bundle that is thought to drive the fusion of opposing membranes. As formation of this SNARE complex in solution does not require calcium, it is not clear what function calcium has in triggering SNARE-mediated membrane fusion. We now demonstrate that whereas syntaxin and SNAP-25 in target membranes are freely available for SNARE complex formation, availability of synaptobrevin on synaptic vesicles is very limited. Calcium at micromolar concentrations triggers SNARE complex formation and fusion between synaptic vesicles and reconstituted target membranes. Although calcium does promote interaction of SNARE proteins between opposing membranes, it does not act by releasing synaptobrevin from synaptic vesicle restriction. Rather, our data suggest a mechanism in which calcium-triggered membrane apposition enables syntaxin and SNAP-25 to engage synaptobrevin, leading to membrane fusion.     

An ordered mesoporous organosilica hybrid material with a crystal-like wall structure

Surfactant-mediated synthesis strategies are widely used to fabricate ordered mesoporous solids in the form of metal oxides, metals, carbon and hybrid organosilicas. These materials have amorphous pore walls, which could limit their practical utility. In the case of mesoporous metal oxides, efforts to crystallize the framework structure by thermal and hydrothermal treatments have resulted in crystallization of only a fraction of the pore walls. Here we report the surfactant-mediated synthesis of an ordered benzene-silica hybrid material; this material has an hexagonal array of mesopores with a lattice constant of 52.5 A, and crystal-like pore walls that exhibit structural periodicity with a spacing of 7.6 A along the channel direction. The periodic pore surface structure results from alternating hydrophilic and hydrophobic layers, composed of silica and benzene, respectively. We believe that this material is formed as a result of structure-directing interactions between the benzene-silica precursor molecules, and between the precursor molecules and the surfactants. We expect that other organosilicas and organo-metal oxides can be produced in a similar fashion, to yield a range of hierarchically ordered mesoporous solids with molecular-scale pore surface periodicity.     

SWAP-70 is a guanine-nucleotide-exchange factor that mediates signalling of membrane ruffling

Phosphoinositide-3-OH kinase (PI(3)K), activated through growth factor stimulation, generates a lipid second messenger, phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P3). PtdIns(3,4,5)P3 is instrumental in signalling pathways that trigger cell activation, cytoskeletal rearrangement, survival and other reactions. However, some targets of PtdIns(3,4,5)P3 are yet to be discovered. We demonstrate that SWAP-70, a unique signalling protein, specifically binds PtdIns(3,4,5)P3. On stimulation by growth factors, cytoplasmic SWAP-70, which is dependent on PI(3)K but independent of Ras, moved to cell membrane rearrangements known as ruffles. However, mutant SWAP-70 lacking the ability to bind PtdIns(3,4,5)P3 blocked membrane ruffling induced by epidermal growth factor or platelet-derived growth factor. SWAP-70 shows low homology with Rac-guanine nucleotide exchange factors (GEFs), and catalyses PtdIns(3,4,5)P3-dependent guanine nucleotide exchange to Rac. SWAP-70-deficient fibroblasts showed impaired membrane ruffling after stimulation with epidermal growth factor, and failed to activate Rac fully. We conclude that SWAP-70 is a new type of Rac-GEF which, independently of Ras, transduces signals from tyrosine kinase receptors to Rac.     

Geobacter metallireducens accesses insoluble Fe(III) oxide by chemotaxis

Microorganisms that use insoluble Fe(III) oxide as an electron acceptor can have an important function in the carbon and nutrient cycles of aquatic sediments and in the bioremediation of organic and metal contaminants in groundwater. Although Fe(III) oxides are often abundant, Fe(III)-reducing microbes are faced with the problem of how to access effectively an electron acceptor that can not diffuse to the cell. Fe(III)-reducing microorganisms in the genus Shewanella have resolved this problem by releasing soluble quinones that can carry electrons from the cell surface to Fe(III) oxide that is at a distance from the cell. Here we report that another Fe(III)-reducer, Geobacter metallireducens, has an alternative strategy for accessing Fe(III) oxides. Geobacter metallireducens specifically expresses flagella and pili only when grown on insoluble Fe(III) or Mn(IV) oxide, and is chemotactic towards Fe(II) and Mn(II) under these conditions. These results suggest that G. metallireducens senses when soluble electron acceptors are depleted and then synthesizes the appropriate appendages to permit it to search for, and establish contact with, insoluble Fe(III) or Mn(IV) oxide. This approach to the use of an insoluble electron acceptor may explain why Geobacter species predominate over other Fe(III) oxide-reducing microorganisms in a wide variety of sedimentary environments.     

Coprophagy: an unusual source of essential carotenoids

The rare Egyptian vulture (Neophron percnopterus) stands out among the Old World vultures (Family Accipitridae) because of its brightly ornamented head, which is coloured yellow by carotenoid pigments, and its practice of feeding on faeces. Here we show that Egyptian vultures obtain these pigments from the excrement of ungulates. To our knowledge, this is the first demonstration that faeces can be used as a source of carotenoids by a vertebrate.     

The signature of supernova ejecta in the X-ray afterglow of the gamma-ray burst 011211

Now that gamma-ray bursts (GRBs) have been determined to lie at cosmological distances, their isotropic burst energies are estimated to be as high as 1054 erg (ref. 2), making them the most energetic phenomena in the Universe. The nature of the progenitors responsible for the bursts remains, however, elusive. The favoured models range from the merger of two neutron stars in a binary system to the collapse of a massive star. Spectroscopic studies of the afterglow emission could reveal details of the environment of the burst, by indicating the elements present, the speed of the outflow and an estimate of the temperature. Here we report an X-ray spectrum of the afterglow of GRB011211, which shows emission lines of magnesium, silicon, sulphur, argon, calcium and possibly nickel, arising in metal-enriched material with an outflow velocity of the order of one-tenth the speed of light. These observations strongly favour models where a supernova explosion from a massive stellar progenitor precedes the burst event and is responsible for the outflowing matter.