Mechanism of membrane curvature sensing by amphipathic helix containing proteins

There are several examples of membrane-associated protein domains that target curved membranes. This behavior is believed to have functional significance in a number of essential pathways, such as clathrin-mediated endocytosis, which involve dramatic membrane remodeling and require the recruitment of various cofactors at different stages of the process. This work is motivated in part by recent experiments that demonstrated that the amphipathic N-terminal helix of endophilin (H0) targets curved membranes by binding to hydrophobic lipid bilayer packing defects which increase in number with increasing membrane curvature. Here we use state-of-the-art atomistic simulation to explore the packing defect structure of curved membranes, and the effect of this structure on the folding of H0. We find that not only are packing defects increased in number with increasing membrane curvature, but also that their size distribution depends nontrivially on the curvature, falling off exponentially with a decay constant that depends on the curvature, and crucially that even on highly curved membranes defects large enough to accommodate the hydrophobic face of H0 are never observed. We furthermore find that a percolation model for the defects explains the defect size distribution, which implies that larger defects are formed by coalescence of noninteracting smaller defects. We also use the recently developed metadynamics algorithm to study in detail the effect of such defects on H0 folding. It is found that the comparatively larger defects found on a convex membrane promote H0 folding by several kcal/mol, while the smaller defects found on flat and concave membrane surfaces inhibit folding by kinetically trapping the peptide. Together, these observations suggest H0 folding is a cooperative process in which the folding peptide changes the defect structure relative to an unperturbed membrane.     

Radiofrequency Ablation of Liver VX2 Tumor: Experimental Results with MR Diffusion-Weighted Imaging at 3.0T

Purpose

To evaluate the value of DWI in detecting the lesions of pre- and post-radiofrequency ablation (RFA) of the rabbit liver VX2 tumors.

Materials and Methods

Twenty-two New Zealand White rabbits were tested. The protocol was approved by the Committee on the Ethics of Animal Experiments. Twenty separate tumor fragments were implanted into the livers of 20 rabbits, the liver was exposed by performing midline laparotomy. 3.0T MR DWI (b = 0, 200, 400, 600, 800,1000 s/mm²) were performed 14–21 days after tumor implantation (mean, 17 days) in the 18 tumor-bearing animals. Then RFA was performed in the 18 tumor-bearing animals and in the two healthy animals. 3.0T MR DWI was performed 7–10 days after RFA (mean, 8 days). Pathology exam was performed immediately after the completion of post- RFA MR imaging. Analyzing the features of MRI and ADC values in the pre- and post- RFA lesions of the VX2 tumors, and histopathologic results were compared with imaging findings.

Results

The difference of ADC value between viable tumor and normal liver parenchyma was significant (P<.001). After RFA, when b = 200, 400, 600, 800, 1000 s/mm², the differences of ADC values of viable tumor, granulation tissue, necrosis, normal liver parenchyma were significant (P<.001). At the time the animals were sacrificed after RFA and MR imaging, histopathologic results of local viable tumors were found in 9 (50%) of the 18 treated tumors. Macroscopic viable tumors were found at the RFA sites in 3 (17%), all 3 macroscopic viable tumors were visualized at the periphery of the RFA areas.

Conclusions

3.0T MR DWI can be used to follow up the progress of the RFA lesion, it is useful in detecting different tissues after RFA, and it is valuable in the further clinical research.     

Membrane Binding by the Endophilin N-BAR Domain

The structure of the endophilin N-terminal amphipathic helix Bin/Amphiphysin/Rvs-homology (N-BAR) domain is unique because of an additional insert helix under the arch of the N-BAR dimer. The structure of this additional helix has not been fully resolved in crystallographic studies, and thus presents a challenge to molecular-level analysis. Large-scale molecular-dynamics simulations were therefore employed to investigate the interaction of a single endophilin N-BAR with a lipid bilayer. Various possible configurations of the additional insert helix under the top of the arch of the endophilin N-BAR were modeled to examine their effect on membrane bending. A residue-residue and residue-lipid headgroup distance analysis, similar to that performed with electron paramagnetic resonance spectroscopy, revealed that the insert helix remains perpendicular to the long axis of the N-BAR over the duration of the simulations. It was also found that the degree of membrane bending is directly related to the orientation of the additional insert helix, and that the perpendicular configuration generates the largest curvature consistent with mutation experiments. In addition, the angle formed between the two N-BAR monomers at the top of the arch is sensitive to the orientation of the insert helices. A membrane sensing-binding-bending mechanism is proposed to describe the process of an endophilin N-BAR interaction with a membrane.     

新疆库尔勒市土地利用变化对土壤性状的影响研究

土地利用与土地覆被变化是全球变化研究的热点问题。在新疆库尔勒市选择土地利用变化后已持续利用15－20a的9种典型利用方式,11个剖面,与荒漠、原始胡杨林2各参照利用方式3个标准剖面进行对比分析。结果显示土地利用变化对土壤养分、土壤盐分、土壤侵蚀、土壤水分和土地生产力有明显的影响,荒漠开垦后土壤养分呈下降趋势,土壤侵蚀强度也呈下降趋势,土地生产力与土壤有水分含量呈上升趋势。同时发现土壤侵蚀强度与生产力呈负相关关系,土地生产力与土壤水分呈正相关关系。林、草有利于保护干旱区生态环境,调整土地利用结构,合理开垦,加之预防和治理措施在一定程度上可防止或减弱土地退化（荒漠化）进程。     

Understanding the Role of Amphipathic Helices in N-BAR Domain Driven Membrane Remodeling

Endophilin N-BAR (N-terminal helix and Bin/amphiphysin/Rvs) domain tubulates and vesiculates lipid membranes in vitro via its crescent-shaped dimer and four amphipathic helices that penetrate into membranes as wedges. Like F-BAR domains, endophilin N-BAR also forms a scaffold on membrane tubes. Unlike F-BARs, endophilin N-BARs have N-terminal H0 amphipathic helices that are proposed to interact with other N-BARs in oligomer lattices. Recent cryo-electron microscopy reconstructions shed light on the organization of the N-BAR lattice coats on a nanometer scale. However, because of the resolution of the reconstructions, the precise positioning of the amphipathic helices is still ambiguous. In this work, we applied a coarse-grained model to study various membrane remodeling scenarios induced by endophilin N-BARs. We found that H0 helices of N-BARs prefer to align in an antiparallel manner at two ends of the protein to form a stable lattice. The deletion of H0 helices causes disruption of the lattice. In addition, we analyzed the persistence lengths of the protein-coated tubes and found that the stiffness of endophilin N-BAR-coated tubules qualitatively agrees with previous experimental work studying N-BAR-coated tubules. Large-scale simulations on membrane liposomes revealed a systematic relation between H0 helix density and local membrane curvature fluctuations. The data also suggest that the H0 helix is required for BARs to form organized structures on the liposome, further illustrating its important function.     

Poly‐γ‐glutamic acid produced from Bacillus licheniformis CGMCC 2876 as a potential substitute for polyacrylamide in the sugarcane industry

As an environmentally friendly and industrially useful biopolymer, poly‐γ‐glutamic acid (γ‐PGA) from Bacillus licheniformis CGMCC 2876 was characterized by the high‐resolution mass spectrometry and ¹H NMR. A flocculating activity of 11,474.47 U mL^?1 obtained with γ‐PGA, and the effects of carbon sources, ions, and chemical properties (D‐/L‐composition and molecular weight) on the production and flocculating activity of γ‐PGA were discussed. Being a bioflocculant in the sugar refinery process, the color and turbidity of the sugarcane juice was IU 1,877.36 and IU 341.41 with 0.8 ppm of γ‐PGA, respectively, which was as good as the most widely used chemically synthesized flocculant in the sugarcane industry—polyacrylamide with 1 ppm. The γ‐PGA produced from B. licheniformis CGMCC 2876 could be a promising alternate of chemically synthesized flocculants in the sugarcane industry. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:1287–1294, 2015     

EAEC噬菌体PNJ1809-11和PNJ1809-13作为环境消毒剂的杀菌效果评估

[目的] 分析2株肠聚集性大肠杆菌（EAEC） CVCC232噬菌体PNJ1809-11、PNJ1809-13的生物学特性,并对其作为环境消毒剂的杀菌效果进行评估。[方法] 透射电镜下观察PNJ1809-11、PNJ1809-13的形态;通过宿主谱、最佳感染复数（MOI）、一步生长曲线、对pH和温度耐受性的测定分析PNJ1809-11、PNJ1809-13的生物学特性;比较2株噬菌体的体外杀菌效果和喷雾、雾化处理后的杀菌效果;检测噬菌体在模拟养殖环境下的耐受性,及喷雾处理的噬菌体制剂在模拟养殖环境下的杀菌效果和对宿主菌生物被膜的清除效果;分析宿主菌的抗性突变率。[结果] 电镜下观察PNJ1809-11、PNJ1809-13均为肌尾病毒科噬菌体;PNJ1809-11可裂解155株大肠杆菌,PNJ1809-13可裂解46株大肠杆菌;噬菌体PNJ1809-11、PNJ1809-13的最佳感染复数（MOI）均为10,最适pH均为7;与PNJ1809-13相比,PNJ1809-11具有较好的热稳定性;在模拟的封闭装置中,将两株噬菌体分别经喷雾、雾化处理后,二者对宿主菌的杀灭效果均可达99%以上;对人工污染的粪便表面细菌的杀灭效果均达到99%以上。噬菌体PNJ1809-11、PNJ1809-13及两者的混合制剂（噬菌体鸡尾酒）对宿主菌CVCC232形成的生物被膜的裂解效率分别为78%、30%和83%。噬菌体PNJ1809-11在养殖温度、粪便pH以及阳光照射下,其耐受性均强于PNJ1809-13。宿主菌对PNJ1809-11、PNJ1809-13的抗性突变率分别为2.5×10^–3和1.0×10^–3。[结论] 综上,噬菌体PNJ1809-11的环境耐受力更强,噬菌体鸡尾酒对生物被膜的裂解效果更好,提示噬菌体PNJ1809-13或噬菌体鸡尾酒经喷雾处理后具有作为环境杀菌剂的潜力。     

Nitrogen Modulates the Effects of Heat,Drought, and Combined Stresses on Photosynthesis,Antioxidant Capacity,Cell Osmoregulation,and Grain Yield in Winter Wheat

Longer and more intense heat and drought stresses will occur in terrestrial ecosystems in the future. Although the effects of individual heat or drought stress on wheat plants have been largely explored, the regulatory effect of nitrogen (N) on winter wheat under heat, drought, and combined stresses and whether N alleviates damage to wheat plants caused by these stresses remain unclear. Therefore, the objective of the present study was to investigate the growth, photosynthesis, antioxidant enzyme and N metabolism-related enzyme activity, cell membrane system, osmoregulatory substance, and yield responses to heat, drought, and combined stresses in wheat plants and to clarify the regulatory effects of N on the growth, physiological and biochemical characteristics, and yield of wheat plants under stress conditions. The results showed that wheat plant exposure to individual heat or drought stress reduced photosynthesis and N metabolism-related enzyme activities and increased antioxidant enzyme activities, electrolyte leakage (EL), and the contents of MDA (malondialdehyde) and O₂^? (superoxide anion). The above parameters showed typical superposition effects under combined stress. Under individual heat or drought stress, wheat plants treated with a medium (N₂) or high (N₃) N supply maintained higher photosynthesis and N metabolism-related enzyme activities than did those treated with a low N supply (N₁). Enhanced heat and drought tolerance in wheat plants under an appropriate N supply may be attributed to improved antioxidant capacity, as exemplified by increased activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), glutathione reductase (GR) and ascorbate peroxidase (APX), and to enhanced osmoregulation capacity, as signified by increased contents of soluble sugar (SS), soluble protein (SP), and proline (Pro). Variable importance in projection (VIP) analysis indicated that efficient SOD, POD, CAT, and GR activities and an increased Pro content had superior potential to alleviate heat, drought, and combined stress stresses in wheat plants, and the improvements in growth and grain yield in wheat plants further confirmed the oxidative stress alleviation and stress tolerance enhancement. However, positive effects of N on wheat growth and grain yield under combined stress were usually observed under a low N supply. These results may facilitate future research on the effects of N fertilizer on the stress resistance of winter wheat.

Graphical Abstract