Enzyme-substrate interactions revealed by the crystal structures of the archaeal Sulfolobus PTP-fold phosphatase and its phosphopeptide complexes

The P-loop-containing protein phos-phatases are important regulators in signal transduction. These enzymes have structural and functional similarity with a conserved sequence of Dx(25-41)HCxxGxxR(T/S) essential for catalysis. The singular protein tyrosine phosphatase (PTP) from archaeal Sulfolobus solfataricus is one of the smallest known PTPs with extreme thermostability. Here, we report the crystal structure of this phosphatase and its complexes with two tyrosyl phosphopeptides A-(p)Y-R and N-K-(p)Y-G-N. The structure suggests the minimal structural motif of the PTP family, having two variable sequences inserted between the beta2-beta3 and beta3-beta4 strands, respectively. The phosphate of both phosphopeptide substrates is bound to the P-loop through several hydrogen bonds. Comparison of several phosphatase-substrate complexes revealed that Gln135 on the Q-loop has different modes of recognition toward phosphopeptides. The substrate specificity of SsoPTP is primarily localized at the phosphotyrosine, suggesting that this phosphatase may be a prototypical PTP.     

Impaired neural development caused by inducible expression of Axin in transgenic mice

Ablations of the Axin family genes demonstrated that they modulate Wnt signaling in key processes of mammalian development. The ubiquitously expressed Axin1 plays an important role in formation of the embryonic neural axis, while Axin2 is essential for craniofacial skeletogenesis. Although Axin2 is also highly expressed during early neural development, including the neural tube and neural crest, it is not essential for these processes, apparently due to functional redundancy with Axin1. To further investigate the role of Wnt signaling during early neural development, and its potential regulation by Axins, we developed a mouse model for conditional gene activation in the Axin2-expressing domains. We show that gene expression can be successfully targeted to the Axin2-expressing cells in a spatially and temporally specific fashion. High levels of Axin in this domain induce a region-specific effect on the patterning of neural tube. In the mutant embryos, only the development of midbrain is severely impaired even though the transgene is expressed throughout the neural tube. Axin apparently regulates beta-catenin in coordinating cell cycle progression, cell adhesion and survival of neuroepithelial precursors during development of ventricles. Our data support the conclusion that the development of embryonic neural axis is highly sensitive to the level of Wnt signaling.     

Growth of Trifolium alpinum: Effects of soil properties, symbionts and pathogens

The lowland cultivation of Trifolium alpinum, a clover species found on acid soils in the Alps and suitable for the restoration of erosion areas at high altitudes, failed repeatedly in previous experiments. Three experiments were carried out in a controlled environment to elucidate the reasons for the failure and to develop possible cultivation strategies. In experiment I, T. alpinum was grown in an autochthonous soil from the Alps (high elevation) and in two allochthonous soils, a grassland soil from the Hercynian mountains (medium elevation), and an arable soil (low elevation), in which the seed propagation of T. alpinum had failed previously. The two allochthonous soils had lower contents of soil organic C and ergosterol, an indicator for fungal biomass, than the autochthonous high-elevation soil, but higher levels of exchangeable Ca and extractable P. Plants grown in the allochthonous soils achieved higher biomass and total N amounts per plant than those from the high elevation soil if inoculated with this autochthonous material to establish rhizobial infection. In the allochthonous high elevation soil, the growth of T. alpinum was P-limited as shown in experiment II. In experiment I, plants grown in the low elevation soil had a lower biomass and smaller number of active leaves at 120 days after emergence than those grown on the medium elevation soil. This difference can be explained by strong colonization with the phytophagous nematode Pratylenchus sp., as demonstrated in experiment III by comparing plant growth either in untreated or in autoclaved low-elevation soil. Successful propagation of T. alpinum at low elevation may be achieved through suitable inoculation with autochthonous soil biota, especially Rhizobia, and avoidance of soils infested by Pratylenchus species by choosing sites with acidic soil and ensuring adequate P-availability.     

Testing the growth rate vs. geochemical hypothesis for latitudinal variation in plant nutrients

Two hypotheses have been proposed to explain increases in plant nitrogen (N) and phosphorus (P) concentrations with latitude: (i) geochemical limitation to P availability in the tropics and (ii) temperature driven variation in growth rate, where greater growth rates (requiring greater nutrient levels) are needed to complete growth and reproduction within shorter growing seasons in temperate than tropical climates. These two hypotheses were assessed in one forest type, intertidal mangroves, using fertilized plots at sites between latitudes 36º S and 27º N. The N and P concentrations in mangrove leaf tissue increased with latitude, but there were no trends in N : P ratios. Growth rates of trees, adjusted for average minimum temperature showed a significant increase with latitude supporting the Growth Rate Hypothesis. However, support for the Geochemical Hypothesis was also strong; both photosynthetic P use efficiency and nutrient resorption efficiency decreased with increasing latitude, indicating that P was less limiting to metabolism at the higher latitudes. Our study supports the hypothesis that historically low P availability in the tropics has been an important selective pressure shaping the evolution of plant traits.     

A di-acidic sequence motif enhances the surface expression of the potassium channel TASK-3

We have characterized a sequence motif, EDE, in the proximal C-terminus of the acid-sensitive potassium channel TASK-3. Human TASK-3 channels were expressed in Xenopus oocytes, and the density of the channels at the surface membrane was studied with two complementary techniques: a luminometric surface expression assay of hemagglutinin epitope-tagged TASK-3 channels and voltage-clamp measurements of the acid-sensitive potassium current. Both approaches showed that mutation of the two glutamate residues of the EDE motif to alanine (ADA mutant) markedly reduced the transport of TASK-3 channels to the cell surface. Mutation of the central aspartate of the EDE motif had no effect on surface expression. The functional role of the EDE motif was further characterized in chimaeric constructs consisting of truncated Kir2.1 channels to which the C-terminus of TASK-3 was attached. In these constructs, too, replacement of the EDE motif by ADA strongly reduced surface expression. Live-cell imaging of enhanced green fluorescent protein-tagged channels expressed in COS-7 cells showed that 24 h after transfection wild-type TASK-3 was mainly localized to the cell surface whereas the ADA mutant was largely retained in the endoplasmic reticulum (ER). Mutation of a second di-acidic motif in the C-terminus of TASK-3 (DAE) had no effect on surface expression. Coexpression of TASK-3 with a GTP-restricted mutant of the coat recruitment GTPase Sar1 (Sar1H79G) resulted in ER retention of the channel. Our data suggest that the di-acidic motif, EDE, in human TASK-3 is a major determinant of the rate of ER export and is required for efficient surface expression of the channel.     

Impact of sheep grazing on nutrient budgets of dry heathlands

Questions: What effect does sheep grazing have on the nutrient budgets of heathlands? Can grazing compensate for atmospheric nutrient loads in heathland ecosystems? What are the conclusions for heathland management? Location: Lüneburg Heath, NW Germany. Methods: During a one-year grazing experiment (stocking rate 1.1 sheep/ha) nutrient balances for N, Ca, K, Mg and P were calculated by quantifying input rates (atmospheric deposition, sheep excrement) and output rates (biomass removal, leaching). Results: Atmospheric nutrient deposition amounted to 22.8 kg.ha⁻¹.a⁻¹ for N and < 0.2 kg.ha⁻¹.a⁻¹ for P. Sheep excrement increased the inputs for N and P by ca. 3.5 and 0.2 kg.ha⁻¹.a⁻¹, respectively. Grazing reduced N- and P-stores in the above-ground biomass by 25.6 and 1.9 kg.ha⁻¹.a⁻¹, respectively. N-and P-losses via leaching amounted to 2.2 and < 0.2 kg.ha⁻¹.a⁻¹. Output:input ratios for P were high, indicating that grazing severely affected P-budgets of heaths. Conclusions: Our results suggest that sheep grazing has the potential to compensate for atmospheric nutrient loads (particularly for current N deposition rates). However, in the long term the combination of elevated N-deposition and P-loss due to grazing may cause a shift from N-(co-) limited to more P-(co-) limited plant growth. To counteract an aggravation of P-deficiency in the long term, grazing may be combined with management measures that affect P-budgets to a lesser extent (e.g. prescribed burning).     

Soil Organic Phosphorus Transformations During Pedogenesis

Abstract Long-term changes in soil phosphorus influence ecosystem development and lead to a decline in the productivity of forests in undisturbed landscapes. Much of the soil phosphorus occurs in a series of organic compounds that differ in their availability to organisms, but changes in the relative abundance of these compounds during pedogenesis remain unknown. We used alkaline extraction and solution phosphorus-31 nuclear magnetic resonance spectroscopy to assess the chemical nature of soil organic phosphorus along a 120,000-year post-glacial chronosequence at Franz Josef, New Zealand. Inositol phosphates, DNA, phospholipids, and phosphonates accumulated rapidly during the first 500 years of soil development characterized by nitrogen limitation of biological productivity, but then declined slowly to low concentrations in older soils characterized by intense phosphorus limitation. However, the relative contribution of the various compounds to the total organic phosphorus varied along the sequence in dramatic and surprising ways. The proportion of inositol hexakisphosphate, conventionally considered to be relatively recalcitrant in the environment, declined markedly in older soils, apparently due to a corresponding decline in amorphous metal oxides, which weather to crystalline forms during pedogenesis. In contrast, the proportion of DNA, considered relatively bioavailable in soil, increased continually throughout the sequence, due apparently to incorporation within organic structures that provide protection from biological attack. The changes in soil organic phosphorus coincided with marked shifts in plant and microbial communities, suggesting that differences in the forms and bioavailability of soil organic phosphorus have ecological significance. Overall, the results strengthen our understanding of phosphorus transformations during pedogenesis and provide important insight into factors regulating the composition of soil organic phosphorus.     

隔离降雨对马尾松针叶非结构性碳水化合物和碳、氮、磷的影响

植物叶片中的非结构性碳水化合物（NSC）和碳（C）、氮（N）、磷（P）含量可反映植物和生态系统对水分亏缺环境的响应及适应程度。以福建省长汀县25a马尾松（Pinus massoniana）为对象,原位观测分析了3年持续100%隔离降雨对成年马尾松针叶NSC与C、N、P的影响。结果表明：（1）持续隔离降雨导致马尾松针叶NSC含量先显著增加后减少,最终导致针叶NSC含量季节变化消失;且针叶可溶性糖含量、可溶性糖/淀粉比值在后期显著增加;（2）持续隔离降雨使马尾松针叶N含量、P含量、N ： P均表现为前期与对照组差异不显著,后期显著高于对照组;（3）马尾松针叶NSC含量、可溶性糖含量、P含量、N ： P与土壤水分含量呈显著或极显著相关,且均与P含量显著或极显著相关,与N素无相关性。以上研究结果表明,随着土壤水分含量的持续减少马尾松可通过调整针叶中NSC含量的积累及分配和提高针叶N、P含量来适应缺水环境,P含量的增加对NSC含量波动及可溶性糖和淀粉的相互转化起促进作用。     

重引入对麋鹿种群分娩定时及同步化的影响

种群持续繁殖是重引入物种对原灭绝地气候成功恢复适应的重要标志,研究重引入前后种群的繁殖波动规律,可为重引入管理者提供精准的繁殖预测信息和科学依据。本文整理北京重引入麋鹿种群后的1987年、1997年、2007年和2017年的总分娩数据,以每年最早分娩日作参照基准,统计个体分娩距此的天数,计算同步化率,并用ANOVA分析气温、降水、空气湿度、光照等变量对麋鹿分娩定时格局的影响。结果表明：(1) 引入后麋鹿种群年分娩节律呈“钟摆样”前后摆动,后逐渐回调,并最终处于相对稳定状态：时隔85年 (从灭绝至重引入) 后,北京麋鹿种群首次分娩时间较乌邦寺种群推迟35 ~ 42 d;重引入定植阶段：分娩节律逐年提前;种群扩繁阶段：分娩节律逐年向后推迟;种群复壮阶段：分娩节律又逐年微回调并最终处于稳定状态。(2) 北京麋鹿种群分娩有较强光周期定时和同步化：在重引入的第一年,分娩同步化率达到0 ~ 25%、25% ~ 50%、50% ~ 75%,分别用时18 d、14 d、5 d;重引入定植阶段分别用时41 d、19 d、11 d;种群扩繁阶段分别用时45 d、10 d、9 d;种群复壮阶段分别用时34 d、20 d和11 d。(3) 累积光照和积温,对北京麋鹿种群正常分娩启动影响差异极显著,对同步化分娩没有影响：妊娠期积温达到 (2748.34 ± 157.69)℃,累计光照达到 (3684.77 ± 514.26) h可启动正常分娩。(4) 北京麋鹿种群的分娩峰期与地上生物量峰期相关;从分娩时间来看,北京麋鹿种群已经恢复了对原灭绝地气候的适应。(5) 随时间增长,北京麋鹿种群非同步化分娩的个体数量逐年增加,分娩期总跨度并未延长。1997年以来,平均每年有30%左右的非同步化分娩现象。因此,基于分娩数据分析表明,麋鹿引入北京37年后,种群已经恢复对原灭绝地环境的重适应。     

Interplay between vitamin E and phosphorus availability in the control of longevity in Arabidopsis thaliana

Background and Aims Vitamin E helps to control the cellular redox state by reacting with singlet oxygen and preventing the propagation of lipid peroxidation in thylakoid membranes. Both plant ageing and phosphorus deficiency can trigger accumulation of reactive oxygen species, leading to damage to the photosynthetic apparatus. This study investigates how phosphorus availability and vitamin E interact in the control of plant longevity in the short-lived annual Arabidopsis thaliana.Methods The responses of tocopherol cyclase (VTE1)- and γ-tocopherol methyltransferase (VTE4)-null mutants to various levels of phosphorus availability was compared with that of wild-type plants. Longevity (time from germination to rosette death) and the time taken to pass through different developmental stages were determined, and measurements were taken of photosynthetic efficiency, pigment concentration, lipid peroxidation, vitamin E content and jasmonate content.Key Results The vte1 mutant showed accelerated senescence under control conditions, excess phosphorus and mild phosphorus deficiency, suggesting a delaying, protective effect of α-tocopherol during plant senescence. However, under severe phosphorus deficiency the lack of α-tocopherol paradoxically increased longevity in the vte1 mutant, while senescence was accelerated in wild-type plants. Reduced photoprotection in vitamin E-deficient mutants led to increased levels of defence chemicals (as indicated by jasmonate levels) under severe phosphorus starvation in the vte4 mutant and under excess phosphorus and mild phosphorus starvation in the vte1 mutant, indicating a trade-off between the capacity for photoprotection and the activation of chemical defences (jasmonate accumulation).Conclusions Vitamin E increases plant longevity under control conditions and mild phosphorus starvation, but accelerates senescence under severe phosphorus limitation. Complex interactions are revealed between phosphorus availability, vitamin E and the potential to synthesize jasmonates, suggesting a trade-off between photoprotection and the activation of chemical defences in the plants.