Autoinhibition by an internal nuclear localization signal revealed by the crystal structure of mammalian importin alpha

Importin alpha is the nuclear import receptor that recognizes classical monopartite and bipartite nuclear localization signals (NLSs). The structure of mouse importin alpha has been determined at 2.5 Angstrom resolution. The structure shows a large C-terminal domain containing armadillo repeats, and a less structured N-terminal importin beta-binding domain containing an internal NLS bound to the NLS-binding site. The structure explains the regulatory switch between the cytoplasmic, high-affinity form, and the nuclear, low-affinity form for NLS binding of the nuclear import receptor predicted by the current models of nuclear import. Importin beta conceivably converts the low- to high-affinity form by binding to a site overlapping the autoinhibitory sequence. The structure also has implications for understanding NLS recognition, and the structures of armadillo and HEAT repeats.

Effects of beta-escin and saponin on the transverse-tubular system and sarcoplasmic reticulum membranes of rat and toad skeletal muscle

Mechanically skinned skeletal muscle fibres from rat and toad were exposed to the permeabilizing agents beta-escin and saponin. The effects of these agents on the sealed transverse tubular system (t-system) and sarcoplasmic reticulum (SR) were examined by looking at changes in the magnitude of the force responses to t-system depolarization, the time course of the fluorescence of fura-2 trapped in the sealed t-system, and changes in the magnitude of caffeine-induced contractures following SR loading with Ca2+ under defined conditions. In the presence of 2 mu g ml(-1) beta-escin and saponin, the response to t-system depolarization was not completely abolished, decreasing to a plateau, and a large proportion of fura-2 remained in the sealed t-system. At 10 mu g ml(-1), both agents abolished the ability of both rat and toad preparations to respond to t-system depolarization after 3 min of exposure, but a significant amount of fura-2 remained in sealed t-tubules even after exposure to 100 mu g ml(-1) beta-escin and saponin for 10 min. beta-Escin took longer than saponin to reduce the t-system depolarizations and fura-2 content of the sealed t-system to a similar level. The ability of the SR to load Ca2+ was reduced to a lower level after treatment with beta-escin than saponin. This direct effect on the SR occurred at much lower concentrations for rat (2 mu g ml(-1) beta-escin and 10 mu g ml(-1) saponin) than toad (10 mu g ml(-1) beta-escin and 150 mu g ml(-1) saponin). The reverse order in sensitivities to beta-escin and saponin of t-system and SR membranes indicates that the mechanisms of action of beta-escin and saponin are different in the two types of membrane. In conclusion, this study shows that: (1) beta-escin has a milder action on the surface membrane than saponin; (2) beta-escin is a more potent modifier of SR function; (3) simple permeabilization of membranes is not sufficient to explain the effects of beta-escin and saponin on muscle membranes; and (4) the t-system network within muscle fibres is not a homogeneous compartment.

Flyash as support for Ni catalysts in carbon dioxide reforming of methane

A series of Ni catalysts supported on flyash treated by various chemical methods was tested for carbon dioxide reforming of methane. Ni catalyst on the flyash treated with CaO (Ni/Ash-CaO) shows high conversion and stability, being close to those of the well-reported Ni/Al2O3 and Ni/SiO2 catalysts with conversions approaching thermodynamic equilibrium levels.

Crystal structure of mammalian purple acid phosphatase

Background: Mammalian purple acid phosphatases are highly conserved binuclear metal-containing enzymes produced by osteoclasts, the cells that resorb bone. The enzyme is a target for drug design because there is strong evidence that it is involved in bone resorption. Results: The 1.55 Angstrom resolution structure of pig purple acid phosphatase has been solved by multiple isomorphous replacement. The enzyme comprises two sandwiched beta sheets flanked by or-helical segments. The molecule shows internal symmetry, with the metal ions bound at the interface between the two halves. Conclusions: Despite less than 15% sequence identity, the protein fold resembles that of the catalytic domain of plant purple acid phosphatase and some serine/threonine protein phosphatases. The active-site regions of the mammalian and plant purple acid phosphatases differ significantly, however. The internal symmetry suggests that the binuclear centre evolved as a result of the combination of mononuclear ancestors. The structure of the mammalian enzyme provides a basis for antiosteoporotic drug design.

Crystal and molecular structure of 2-hydroxy-1-naphthaldehyde isonicotinoyl hydrazone (NIH) and its iron(III) complex: an iron chelator with anti-tumour activity

Previous studies have demonstrated that 2-hydroxy-1-naphthaldehyde isonicotinoyl hydrazone (NIH) and several other aroylhydrazone chelators possess anti-neoplastic activity due to their ability to bind intracellular iron. In this study we have examined the structure and properties of NIH and its Fe-III complex in order to obtain further insight into its anti-tumour activity. Two tridentate NIH ligands deprotonate upon coordination to Fe-III in a meridional fashion to form a distorted octahedral, high-spin complex. Solution electrochemistry of [Fe(NIH-H)(2)](+) shows that the trivalent oxidation state is dominant over a wide potential range and that the Fe-II analogue is not a stable form of this complex. The fact that [Fe(NIH-H)(2)](+) cannot-cycle between the Fe-II and Fe-III states suggests that the production of toxic free- radical species, e.g. OH. or O2(.-),is not part of this ligand's cytotoxic action. This suggestion is supported by cell culture experiments demonstrating that the addition of Fe-III to NIH prevents its anti-proliferative effect. The chemistry of this chelator and its Fe-III complex are discussed in the context of understanding its anti-tumour activity.

A comparative study of N2O conversion to N-2 over Co/AC and Cu/AC catalysts

Catalytic conversion of N2O to N-2 over Cu- and Co-impregnated activated carbon catalysts (Cu/AC and Co/AC) was investigated. Catalytic activity measurements were carried out in a fixed-bed flow reactor at atmospheric pressure. The catalysts were characterized by N-2 adsorption, X-ray diffraction (XRD) and thermogravimetric analysis (TGA). This study aimed to provide insights into the following aspects: the metal dispersion, changes in pore structure, influence of catalyst loading on reaction, and reaction mechanism. Increasing loading of Co or Cu led to decreasing dispersion, but 20 wt % loading was an upper limit for optimal activities in both cases, with too high loading causing sintering of metal. Co exhibited a relatively better dispersion than Cu. Impregnation of metal led to a large decrease in surface area and pore volume, especially for 30 wt % of loading. 20 wt % of loading has proved to be the optimum for both Cu and Co, which shows the highest activity. Both N2O-Co/AC and -Cu/AC reactions are based upon a redox mechanism, but the former is limited by the oxygen transfer from catalysts to carbon, while N2O chemisorption on the surface of Cu catalyst controls the latter. The removal of oxygen from cobalt promotes the activity of Co/AC, but it is beneficial for Cu/AC to keep plenty of oxygen to maintain the intermediate oxidation of copper-Cu1+. The different nature of the two catalysts and their catalytic reaction mechanisms are closely related to their different electronegativities.

Adaptation of sorghum: characterisation of genotypic flowering responses to temperature and photoperiod

Sorghum [Sorghum bicolor (L.) Moench] is an important cereal crop grown in a wide range of tropical and temperate environments. This study was conducted to characterise the photothermal flowering responses of sorghum genotypes and to examine relationships between photothermal characteristics and environment of origin in order to better understand the phenological basis of adaptation to environment in sorghum. Twenty-four germplasm accessions and one hybrid from 24 major sorghum-growing areas were grown in a wide range of environments varying in temperature and photoperiod in India, Kenya and Mall between 1992 and 1995. Times from sowing to flowering (f) were recorded, and the responsiveness of 1/f to temperature and photoperiod was quantified using photothermal models. Times from sowing to flowering were accurately predicted in a wide range of environments using a multiplicative rate photothermal model. Significant variation in the minimum time to flower (F-m) and photoperiod sensitivity (critical photoperiod, P-c, and photoperiod-sensitivity slope, P-s) was observed among the genotypes; in contrast there was little variation in base temperature (Tb) Adaptation of sorghum to the diverse environments in which it is grown was largely determined by photoperiod sensitivity and minimum time to flower; photoperiod sensitivity determines bread adaptation to latitude (daylength), while variation in the minimum time to flower determines specific adaptation within smaller ranges of latitude, e.g. within the humid and sub-humid tropics.

Submarine groundwater discharge and associated chemical input to a coastal sea

This paper presents a theoretical model of flow and chemical transport processes in subterranean estuaries (unconfined brackish groundwater aquifers at the ocean-land interface). The model shows that groundwater circulation and oscillating flow, caused by wave setup and tide, may constitute up to 96% of submarine groundwater discharge (SGWD) compared with 4% due to the net groundwater discharge. While these local flow processes do not change the total amount of land-derived chemical input to the ocean over a long period (e.g., yearly), they induce fluctuations of the chemical transfer rate as the aquifer undergoes saltwater intrusion. This may result in a substantial increase in chemical fluxes to the ocean over a short period (e.g., monthly and by a factor of 20 above the averaged level), imposing a possible threat to the marine environment. These results are essentially consistent with the experimental findings of Moore [1996] and have important implications for coastal resources management.

Life tables of the processionary caterpillar Ochrogaster lunifer Herrich-Schaffer (Lepidoptera : Thaumetopoeidae) at local and regional scales

A major ongoing debate in population ecology has surrounded the causative factors underlying the abundance of phytophagous insects and whether or not these factors limit or regulate herbivore populations. However, it is often difficult to identify mortality agents in census data, and their distribution and relative importance across large spatial scales are rarely understood. Were, we present life tables for egg batches and larval cohorts of the processionary caterpillar Ochrogaster lunifer Herrich-Schaffer, using intensive local sampling combined with extensive regional monitoring to ascertain the relative importance of different mortality factors at different localities. Extinction of entire cohorts (representing the entire reproductive output of one female) at natural localities was high, with 82% of the initial 492 cohorts going extinct. Mortality was highest in the egg and early instar stages due to predation from dermestid beetles, and while different mortality factors (e.g. hatching failure, egg parasitism and failure to establish on the host) were present at many localities, dermestid predation, either directly observed or inferred from indirect evidence, was the dominant mortality factor at 89% of localities surveyed. Predation was significantly higher in plantations than in natural habitats. The second most important mortality factor was resource depletion, with 14 cohorts defoliating their hosts. Egg and larval parasitism were not major mortality agents. A combination of predation and resource depletion consistently accounted for the majority of mortality across localities, suggesting that both factors are important in limiting population abundance. This evidence shows that O. lunifer is not regulated by natural enemies alone, but that resource patches (Acacia trees) ultimately, and frequently, act together to limit population growth.

The last glacial cycle from the humid tropics of northeastern Australia: comparison of a terrestrial and a marine record

A detailed pollen record from the Ocean Drilling Program Site 820 core, located on the upper part of the continental slope off the coast of northeast Queensland, was constructed to compare with the existing pollen record from Lynch's Crater on the adjacent Atherton Tableland and allow the production of a regional picture of vegetation and environmental change through the last glacial cycle. Some broad similarities in patterns of vegetation change are revealed, despite the differences between sites and their pollen catchments, which can be related largely to global climate and sea-level changes. The original estimated time scale of the Lynch's Crater record is largely confirmed from comparison with the more thoroughly dated ODP record. Conversely, the Lynch's Crater pollen record has assisted in dating problematic parts of the ODP record. In contrast to Lynch's Crater, which reveals a sharp and sustained reduction in drier araucarian forest around 38,000 yrs BP, considered to have been the result of burning by Aboriginal people, the ODP record indicates, most likely, a stepwise reduction, dating from 140,000 yrs BP or beyond. The earliest reduction shows lack of a clear connection between Araucaria decline and increased burning and suggests that people may not have been involved at this stage. However, a further decline in araucarian forest, possibly around 45,000 yrs BP, which has a more substantial environmental impact and is not related to a time of major climate change, is likely, at least partially, the result of human burning. The suggestion, from the ODP core oxygen isotope record, of a regional sea-surface temperature increase of around 4 degrees C between about 400,000 and 250,000 yrs BP, may have had some influence on the overall decline in Araucaria and its replacement by sclerophyll vegetation. (C) 2000 Elsevier Science B.V. All rights reserved.

Paramagnetic limiting of the upper critical field of the layered organic superconductor kappa-(BEDT-TTF)(2)Cu(SCN)(2)

We report detailed measurements of the interlayer magnetoresistance of the layered organic superconductor kappa-(BEDT-TTF)(2)Cu(SCN)(2) for temperatures down to 0.5 K and fields up to 30 T. The upper critical field is determined from the resistive transition for a wide range of temperatures and field directions. For magnetic fields parallel to the layers, the upper critical field increases approximately linearly with decreasing temperature. The upper critical field at low temperatures is compared to the Pauli paramagnetic limit, at which singlet superconductivity should be destroyed by the Zeeman splitting of the electron spins. The measured value is comparable to a value for the paramagnetic limit calculated from thermodynamic quantities but exceeds the limit calculated from BCS theory. The angular dependence of the upper critical field shows a cusplike feature for fields close to the layers, consistent with decoupled layers.

A new approach to current and noise in double quantum dot systems

We use a quantum master equation to describe transport in double-dot devices. The coherent dot-to-dot coupling affects the noise spectra strongly. For phonon-assisted tunneling, the calculated current spectra are consistent with those of experiments. The model shows that quantum stochastic theory may he applied to some advantage in mesoscopic electronic systems. (C) 2000 Elsevier Science B.V. All rights reserved.

Leu(10) of alpha-conotoxin PnIB confers potency for neuronal nicotinic responses in bovine chromaffin cells

Two alpha-conotoxins PnIA and PnIB (previously reported as being mollusc specific) which differ in only two amino acid residues (AN versus LS at residues 10 and 11, respectively), show markedly different inhibition of the neuronal nicotinic acetylcholine receptor response in bovine chromaffin cells, a mammalian preparation. Whereas alpha-conotoxin PnIB completely inhibits the nicotine-evoked catecholamine release at 10 mu M, with IC50 = 0.7 mu M, alpha-conotoxin PnIA is some 30-40 times less potent. Two peptide analogues, [A10L]PnIA and [N11S]PnIA were synthesized to investigate the extent to which each residue contributes to activity. [A10L]PnIA (IC50 = 2.0 mu M) completely inhibits catecholamine release at 10 mu M whereas [N11S]PnIA shows Little inhibition. In contrast, none of the peptides inhibit muscle-type nicotinic responses in the rat hemi-diaphragm preparation. We conclude that the enhanced potency of alpha-conotoxin PnIB over alpha-conotoxin PnIA in the neuronal-type nicotinic response is principally determined by the larger, more hydrophobic leucine residue at position 10 in alpha-conotoxin PnIB. (C) 2000 Elsevier Science B.V. All rights reserved.

Grazers and diggers: Exploitation competition and coexistence among foragers with different feeding strategies on a single resource

A mathematical model is presented that describes a system where two consumer species compete exploitatively for a single renewable resource. The resource is distributed in a patchy but homogeneous environment; that is, all patches are intrinsically identical. The two consumer species are referred to as diggers and grazers, where diggers deplete the resource within a patch to lower densities than grazers. We show that the two distinct feeding strategies can produce a heterogeneous resource distribution that enables their coexistence. Coexistence requires that grazers must either move faster than diggers between patches or convert the resources to population growth much more efficiently than diggers. The model shows that the functional form of resource renewal within a patch is also important for coexistence. These results contrast with theory that considers exploitation competition for a single resource when the resource is assumed to be well mixed throughout the system.

Structural basis of recognition of monopartite and bipartite nuclear localization sequences by mammalian importin-alpha

Importin-alpha is the nuclear import receptor that recognizes cargo proteins which contain classical monopartite and bipartite nuclear localization sequences (NLSs), and facilitates their transport into the nucleus. To determine the structural basis of the recognition of the two classes of NLSs by mammalian importin-alpha, we co-crystallized an N-terminally truncated mouse receptor protein with peptides corresponding to the monopartite NLS from the simian virus 40 (SV40) large T-antigen, and the bipartite NLS from nucleoplasmin. We show that the monopartite SV40 large T-antigen NLS binds to two binding sites on the receptor, similar to what was observed in yeast importin-alpha. The nucleoplasmin NLS-importin-alpha complex shows, for the first time, the mode of binding of bipartite NLSs to the receptor. The two basic clusters in the NLS occupy the two binding sites used by the monopartite NLS, while the sequence linking the two basic clusters is poorly ordered, consistent with its tolerance to mutations. The structures explain the structural basis for binding of diverse NLSs to the sole receptor protein. (C) 2000 Academic Press.