Preserving the membrane barrier for small molecules during bacterial protein translocation

Many proteins are translocated through the SecY channel in bacteria and archaea and through the related Sec61 channel in eukaryotes. The channel has an hourglass shape with a narrow constriction approximately halfway across the membrane, formed by a pore ring of amino acids. While the cytoplasmic cavity of the channel is empty, the extracellular cavity is filled with a short helix called the plug, which moves out of the way during protein translocation. The mechanism by which the channel transports large polypeptides and yet prevents the passage of small molecules, such as ions or metabolites, has been controversial. Here, we have addressed this issue in intact Escherichia coli cells by testing the permeation of small molecules through wild-type and mutant SecY channels, which are either in the resting state or contain a defined translocating polypeptide chain. We show that in the resting state, the channel is sealed by both the pore ring and the plug domain. During translocation, the pore ring forms a 'gasket-like' seal around the polypeptide chain, preventing the permeation of small molecules. The structural conservation of the channel in all organisms indicates that this may be a universal mechanism by which the membrane barrier is maintained during protein translocation.     

Low-dose radiation accelerates aging of the T-cell receptor repertoire in CBA/Ca mice

While the biological effects of high-dose-ionizing radiation on human health are well characterized, the consequences of low-dose radiation exposure remain poorly defined, even though they are of major importance for radiological protection. Lymphocytes are very radiosensitive, and radiation-induced health effects may result from immune cell loss and/or immune system impairment. To decipher the mechanisms of effects of low doses, we analyzed the modulation of the T-cell receptor gene repertoire in mice exposed to a single low (0.1 Gy) or high (1 Gy) dose of radiation. High-throughput T-cell receptor gene profiling was used to visualize T-lymphocyte dynamics over time in control and irradiated mice. Radiation exposure induces “aging-like” effects on the T-cell receptor gene repertoire, detectable as early as 1 month post-exposure and for at least 6 months. Surprisingly, these effects are more pronounced in animals exposed to 0.1 Gy than to 1 Gy, where partial correction occurs over time. Importantly, we found that low-dose radiation effects are partially due to the hematopoietic stem cell impairment. Collectively, our findings show that acute low-dose radiation exposure specifically results in long-term alterations of the T-lymphocyte repertoire.     

Habitat evaluation procedure for Rocky Mountain bighorn sheep in the Intermountain West

Several habitat evaluation procedures have been developed for bighorn sheep. However, none of these procedures specifically addresses the Rocky Mountain subspecies nor analyzes both the quantity and quality of potential bighorn habitat with regard to minimum viable population (MVP) criteria. This bighorn habitat evaluation procedure combines (1) a quantitative assessment of bighorn range to determine if there are adequate quantities of resources to support an MVP of bighorn sheep, and (2) a qualitative assessment of a range to predict the probable density of bighorns the range can support. Extensive literature review, intensive bighorn research, and a modeling tool, pattern recognition (PATREC), facilitate critical analysis of proposed bighorn reintroduction sites. The resultant stepwise approach to bighorn habitat evaluation enhances the ability of wildlife biologists to make timely and accurate bighorn habitat assessments.     

Managing a complex exotic vegetation program in Yellowstone National Park

The number of documented exotic plants in Yellowstone National Park has increased from 85 known in 1986 to over 185 today. Exotic plants are substantially impacting the parks natural and cultural resources and are a high management priority. We have adopted an integrated weed management approach with regard to exotic vegetation, emphasizing prevention, education, early detection and eradication, control, and, to a lesser degree, monitoring. The program involves over 140 staff with program expenditures averaging approximately $190,000 annually. Prevention actions include requiring approved gravel on construction projects; banning hay in the backcountry and allowing transport of only certified weed-seed-free hay through Yellowstone; requiring construction equipment to be pressurecleaned prior to entering the park; and native species revegetation after road, housing, and other construction projects have disturbed ground. Over 4500 acres, primarily along roadsides and in developed areas, are surveyed annually in early detection efforts with emphasis placed on eradicating small, new infestations of highly invasive species such as sulfur cinquefoil ( Potentilla recta L.) and leafy spurge ( Euphorbia esula L.). Control efforts focus on about 30 priority species, such as spotted knapweed ( Centaurea maculosa Lam.), oxeye daisy ( Chrysanthemum leucanthemum L.), and hoary cress ( Cardaria draba [L.] Desv.) using chemical, mechanical, and cultural techniques. A total of 2027 acres were treated during 1998, whereas control efforts for 12 species occurred on 2596 acres during the previous 3-year period, 1995-1997. Strong and expanding partnerships with other federal, state, and local agencies and private companies contribute to management efforts within the park. Future program goals emphasize increases in base funding to ensure continued weed management efforts as well as expanding survey, monitoring, and reclamation efforts. Ultimately, a more rigorous assessment of program effectiveness is desired.     

First record of the genus Leptodactylus (Anura: Leptodactylidae) in Cuba: Leptodactylus fragilis,a biological invasion?

The Neotropical genus Leptodactylus is currently represented by three species in the West Indies (Leptodactylus albilabris, Leptodactylus fallax and Leptodactylus validus). Based on morphological, acoustic and molecular evidence, we document the presence of a fourth species in the Caribbean region, Leptodactylus fragilis (Brocchi, 1877). The species was found at two localities in western Cuba, and molecular data suggest a northern South American origin, possibly Venezuela, for these populations. We discuss the potential invasive status of L. fragilis, based on its known distribution, relative abundance, behaviour and possible impacts on native species of Cuban amphibians.     

CD4+ T cells are required for secondary expansion and memory in CD8+ T lymphocytes

A long-standing paradox in cellular immunology concerns the conditional requirement for CD4+ T-helper (T(H)) cells in the priming of cytotoxic CD8+ T lymphocyte (CTL) responses in vivo. Whereas CTL responses against certain viruses can be primed in the absence of CD4+ T cells, others, such as those mediated through 'cross-priming' by host antigen-presenting cells, are dependent on T(H) cells. A clearer understanding of the contribution of T(H) cells to CTL development has been hampered by the fact that most T(H)-independent responses have been demonstrated ex vivo as primary cytotoxic effectors, whereas T(H)-dependent responses generally require secondary in vitro re-stimulation for their detection. Here, we have monitored the primary and secondary responses of T(H)-dependent and T(H)-independent CTLs and find in both cases that CD4+ T cells are dispensable for primary expansion of CD8+ T cells and their differentiation into cytotoxic effectors. However, secondary CTL expansion (that is, a secondary response upon re-encounter with antigen) is wholly dependent on the presence of T(H) cells during, but not after, priming. Our results demonstrate that T-cell help is 'programmed' into CD8+ T cells during priming, conferring on these cells a hallmark of immune response memory: the capacity for functional expansion on re-encounter with antigen.     

Monitoring crustal deformation by GPS and InSAR in the three gorge area

The on-going Three Gorges Project (TGP) aims to build one of the largest hydropower stations in the world. Previous investigations have concluded that the reservoir is likely to induce medium-to-intense earthquakes. 23 GPS stations have been established to maintain a geodetic and seismological monitoring network around the reservoir to monitor seismicity and crustal deformation, and to facilitate mitigation activities in case an intense earthquake occurs. The velocity of GPS site was derived from 3 repeated GPS surveys. Meanwhile, another effort has been made to investigate the feasibility of application of satellite interferometric synthetic aperture radar (InSAR) techniques in detecting crustal deformation possibly caused by impoundment of the Three Gorges Reservoir. It has been made to derive digital elevation models (DEM) using European Remote Sensing (ERS) synthetic aperture radar (SAR) imagery. Pairs of ERS SAR images in raw format are processed to generate digital elevation models. This effort will help us evaluate the accuracy of the InSAR techniques for further derivation of background deformation patterns and future detection of reservoir-induced crustal deformation in the area. Foundation item: Supported by the Seismological Union Foundation of China (602015), NASA grant of US(NAG5-9065) and Key Project of China Seismological Bureau(95-12-02) Biography: Qiao Xue-jun(1968-), male, Associated professor, research direction: crustal deformation.