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41.
Donatella Zona Peter M. Lafleur Koen Hufkens Beniamino Gioli Barbara Bailey George Burba Eugénie S. Euskirchen Jennifer D. Watts Kyle A. Arndt Mary Farina John S. Kimball Martin Heimann Mathias Göckede Martijn Pallandt Torben R. Christensen Mikhail Mastepanov Efrén López-Blanco Albertus J. Dolman Roisin Commane Charles E. Miller Josh Hashemi Lars Kutzbach David Holl Julia Boike Christian Wille Torsten Sachs Aram Kalhori Elyn R. Humphreys Oliver Sonnentag Gesa Meyer Gabriel H. Gosselin Philip Marsh Walter C. Oechel 《Global Change Biology》2023,29(5):1267-1281
Long-term atmospheric CO2 concentration records have suggested a reduction in the positive effect of warming on high-latitude carbon uptake since the 1990s. A variety of mechanisms have been proposed to explain the reduced net carbon sink of northern ecosystems with increased air temperature, including water stress on vegetation and increased respiration over recent decades. However, the lack of consistent long-term carbon flux and in situ soil moisture data has severely limited our ability to identify the mechanisms responsible for the recent reduced carbon sink strength. In this study, we used a record of nearly 100 site-years of eddy covariance data from 11 continuous permafrost tundra sites distributed across the circumpolar Arctic to test the temperature (expressed as growing degree days, GDD) responses of gross primary production (GPP), net ecosystem exchange (NEE), and ecosystem respiration (ER) at different periods of the summer (early, peak, and late summer) including dominant tundra vegetation classes (graminoids and mosses, and shrubs). We further tested GPP, NEE, and ER relationships with soil moisture and vapor pressure deficit to identify potential moisture limitations on plant productivity and net carbon exchange. Our results show a decrease in GPP with rising GDD during the peak summer (July) for both vegetation classes, and a significant relationship between the peak summer GPP and soil moisture after statistically controlling for GDD in a partial correlation analysis. These results suggest that tundra ecosystems might not benefit from increased temperature as much as suggested by several terrestrial biosphere models, if decreased soil moisture limits the peak summer plant productivity, reducing the ability of these ecosystems to sequester carbon during the summer. 相似文献
42.
David P. Watts 《Ethology : formerly Zeitschrift fur Tierpsychologie》1995,100(2):139-157
Researchers have documented elevated rates of affinitive interaction between opponents shortly after aggressive conflicts, or reconciliation, in many primate species. Reconciliation may ameliorate the immediate negative effects of aggression by reducing the chance of further aggression between opponents and thus reducing tension, and may avert or repair damage to long-term social relationships important to the animals' fitness. Data on post-conflict interactions between opponents in two groups of wild mountain gorillas (Gorilla gorilla beringei) fail to show reconciliation after conflicts between females, between males, or between immatures, but females seem to commonly reconcile themselves with males after receipt of male aggression. Females and subordinate males often avoid same-sex opponents after conflicts. Females commonly retaliate against female aggressors, and post-conflict rates of aggression between females are higher than baseline levels. Females may not need to achieve reconciliation with each other because relationships between co-resident relatives are resiliant, while those between non-relatives are mostly neutral to antagonistic. Males are, however, important social partners and protectors of females, and female transfer is common. Thus, the results strongly support the ‘important relationships’ hypothesis for the function of reconciliation. 相似文献
43.
Summary Opioid peptides are thought to interact with the cell membrane in their biological journey to the membrane-bound receptor. Both organic solvents and model membranes have been used previously to determine the stable solution conformations of peptide hormones. Leucine enkephalin has been studied in a number of different environments, but with limited resolution. Here it is shown that leucine enkephalin forms a stable type IV -turn structure in dodecylphosphocholine micelles. We have observed a highly solvent-shielded amide proton with no evidence for a complementary hydrogen bond acceptor. The structural details of the peptide as determined by NMR spectroscopy in solution are described. 相似文献
44.
The plasma membrane of the nucleated pigeon erythrocyte was isolated by a method that is simple, reproducible and minimally disruptive, the final preparation consisting of whole cell 'ghosts', recovered at over 40% yield. Alternative methods, which yield membrane fragments, were also tested and some of their possible disadvantages demonstrated. Analysis of the protein components of the isolated membranes by gel elctrophoresis in the presence of sodium dodecyl sulphate revealed that their composition is very similar to that of the proteins of human erythrocyte membranes. However, two major proteins are unique to the nucleated cell membrane; these have apparent mol.wts. of 97000 and 57000. Also, the bands designated 4.2 (74500 mol.wt.) and 6 (35000 mol wt.) by Steck [(1974) J. Cell Biol. 62, 1-19] for the human cell membrane are absent from pigon cell membrane. Glycosylated membrane proteins could not be detected in gels stained with the periodate-Schiff-base procedure. Analysis of membrane phospholipids revealed the same components known to be present in mammalian erythrocytes, though in different proportions. These findings are discussed in the light of known physiological and biochemical differences between avian and mature mammalian erythrocytes. 相似文献
45.
Lipid-lipid and lipid-protein interactions in chromaffin granule membranes. A spin label ESR study 总被引:1,自引:0,他引:1
The ESR spectra of six different positional isomers of a stearic acid and three of a phosphatidylcholine spin label have been studied as a function of temperature in chromaffin granule membranes from the bovine adrenal medulla, and in bilayers formed by aqueous dispersion of the extracted membrane lipids. Only minor differences were found between the spectra of the membranes and the extracted lipid, indicating that the major portion of the membrane lipid is organized in a bilayer arrangement which is relatively unperturbed by the presence of the membrane protein. The order parameter profile of the spin label lipid chain motion is less steep over the first half of the chain than over the section toward the terminal methyl end of the chain. This 'stiffening' effect is attributed to the high proportion of cholesterol in the membrane and becomes less marked as the temperature is raised. The isotropic hyperfine splitting factors of the various positional isomers display a profile of decreasing polarity as one penetrates further into the interior of the membrane. No marked differences are observed between the effective polarities in the intact membranes and in bilayers of the extracted membrane lipids. The previously observed temperature-induced structural change occurring in the membranes at approx. 35 degrees C was found also in the extracted lipid bilayers, showing this to be a result of lipid-lipid interactions and not lipid-protein interactions in the membrane. A steroid spin label indicated a second temperature-dependent structural change occurring in the lipid bilayers at lower temperatures. This correspond to the onset of a more rapid rotation about the long axis of the lipid molecules at a temperature of approx. 10 degrees C. The lipid bilayer regions probed by the spin labels used in this study may be involved in the fusion of the chromaffin granule membrane leading to hormone release by exocytosis. 相似文献
46.
47.
Glycogen-storage disease in rats, a genetically determined deficiency of liver phosphorylase kinase. 总被引:13,自引:8,他引:5 下载免费PDF全文
Rats from an inbred strain (NZR/Mh) were found to have high concentrations of glycogen in their livers, even after 24 h of starvation. Despite this, blood glucose concentrations were well maintained on starvation for up to 72 h. The primary defect is a deficiency of liver phosphorylase kinase, causing a lack of active glycogen phosphorylase, although total phosphorylase is normal. The intravenous injection of glucagon caused a rapid activation of cyclic AMP-dependent protein kinase in the liver, but no increase in either phosphorylase kinase or phosphorylase a activity. Although total glycogen synthase activity in the livers of affected rats was higher than normal, glycogen synthase in the active form was very low, presumably as a result of the high liver glycogen content. The condition is transmitted as autosomal recessive and, apart from hepatomegaly, the affected rats appear healthy. 相似文献
48.
Acetylcholinesterase was studied in human red cells that had been fractionated on Ficoll/Triosil density gradients into classes representing different ages in vivo. Reticulocytes have negligible acetylcholinesterase activity; this is rapidly acquired on maturation to the erythrocyte. The activity per cell reaches a maximum and then, after a constant period, declines again towards the end of cell life. The maximum activity and the rates of activity gain and loss per cell are quantitatively different in adults and children. Kinetic studies showed that Vmax. follows the same age/activity profile but Km is unaffected by cell age. The acetylcholinesterase protein content, determined by quantitative crossed immunoelectrophoresis, also shows a profile of increase and then decrease with cell age but the specific activity calculated from the protein estimate shows a reverse picture in which there is a slight decrease from young to mid-age cells followed by an increase again in older cells. These results are interpreted to indicate a complex developmental picture in which the overall cell age against enzyme activity profile is determined partly by the amount of enzyme protein present and partly from the modifying effect on the enzyme activity, of interactions with an aging cell membrane. 相似文献
49.
50.
MHC proteins are polymorphic cell surface glycoproteins involved in the binding of peptide Ag and their presentation to T lymphocytes. The polymorphic amino acids of MHC proteins are primarily located in the N-terminal domains and are thought to influence T cell recognition both by influencing the binding of peptide Ag and by direct contact with the T cell receptor. In order to determine the relative importance of individual polymorphic amino acids in Ag presentation, a number of groups have taken the approach of interchanging polymorphic amino acids between different alleles of MHC protein in an attempt to define which of the polymorphisms influence peptide binding and which influence T cell recognition by direct contact with the TCR. The peptide OVA323-339 has been previously shown to bind to the MHC class II protein Ad and to have a much lower affinity for Ak, whereas the peptide hen egg lysozyme 46-61 binds well to Ak and poorly to Ad. In the present report, we have analyzed the ability of purified wild-type MHC class II proteins as well as the ability of three different hybrid molecules between Ad and Ak to bind and present these peptides. We find that the alpha-chain of the MHC class II protein plays a critical role in the binding of HEL46-61 and confers the specificity for binding OVA323-339, regardless of which beta-chain is present. We also find that the beta-chain region 65-67 does not control the specificity of peptide binding to the MHC protein, but is important in T cell responses to preformed MHC-peptide complexes, suggesting a role for this region in contacting the TCR. 相似文献