Aquaglyceroporins,one channel for two molecules

In the light of the recently published structure of GlpF and AQP1, we have analysed the nature of the residues which could be involved in the formation of the selectivity filter of aquaporins, glycerol facilitators and aquaglyceroporins. We demonstrate that the functional specificity for major intrinsic protein (MIP) channels can be explained on one side by analysing the polar environment of the residues that form the selective filter. On the other side, we show that the channel selectivity could be associated with the oligomeric state of the membrane protein. We conclude that a non-polar environment in the vicinity of the top of helix 5 could allow aquaglyceroporins and GlpF to exist as monomers within the hydrophobic environment of the membrane.     

Contrasting the effects of environment,dispersal and biotic interactions to explain the distribution of invasive plants in alpine communities

Despite considerable efforts devoted to investigate the community assembly processes driving plant invasions, few general conclusions have been drawn so far. Three main processes, generally acting as successive filters, are thought to be of prime importance. The invader has to disperse (1st filter) into a suitable environment (2nd filter) and succeed in establishing in recipient communities through competitive interactions (3rd filter) using two strategies: competition avoidance by the use of different resources (resource opportunity), or competitive exclusion of native species. Surprisingly, despite the general consensus on the importance of investigating these three processes and their interplay, they are usually studied independently. Here we aim to analyse these three filters together, by including them all: abiotic environment, dispersal and biotic interactions, into models of invasive species distributions. We first propose a suite of indices (based on species functional dissimilarities) supposed to reflect the two competitive strategies (resource opportunity and competition exclusion). Then, we use a set of generalised linear models to explain the distribution of seven herbaceous invaders in natural communities (using a large vegetation database for the French Alps containing 5,000 community-plots). Finally, we measure the relative importance of competitive interaction indices, identify the type of coexistence mechanism involved and how this varies along environmental gradients. Adding competition indices significantly improved model’s performance, but neither resource opportunity nor competitive exclusion were common strategies among the seven species. Overall, we show that combining environmental, dispersal and biotic information to model invasions has excellent potential for improving our understanding of invader success.     

Ecological requirements of Abies alba in the French Alps derived from dendro-ecological analysis

Abstract. We used dendro-ecological techniques to investigate fundamental relationships between climate and growth of Abies alba (silver fir) in eastern France. Seven Abies forests in the Trièves region of the French Alps were chosen to represent a wide range of ecological conditions based on the results of previous forest vegetation surveys. In each forest, four trees were sampled in each of five different stands with two cores per tree. These 280 cores were studied using two separate dendro-ecological methods: the pointer years method (based on extreme growth events), and correlation functions between tree ring-widths and monthly climatic data. Data from 11 meteorological stations were combined to provide a regional analysis of precipitation and minimum and maximum temperatures. The two dendro-ecological methods appear to be complementary, as the first technique emphasizes common and low intensity linear correlations between ring-widths and climatic variations, and the second method emphasizes extreme and unusual climatic events such as exceptionally cold or dry years. Across all sites, drought in the previous year was consistently correlated with a low growth rate; however, other climatic variables varied substantially among sites. For example, drought in the current year reduced growth more in the low elevation sites than in the high elevation sites and severe winter frost reduced growth the most in the high altitude sites and the driest site. Moreover, certain growth responses are better correlated with the age of the stands, the canopy closure and the floristic composition of the community than the abiotic factors, emphasizing the value of dendro-ecological sampling based on phytosociological units.     

Toc159- and Toc75-independent import of a transit sequence-less precursor into the inner envelope of chloroplasts

Chloroplast envelope quinone oxidoreductase (ceQORH) is an inner plastid envelope protein that is synthesized without cleavable chloroplast transit sequence for import. In the present work, we studied the in vitro-import characteristics of Arabidopsis ceQORH. We demonstrate that ceQORH import requires ATP and is dependent on proteinaceous receptor components exposed at the outer plastid surface. Competition experiments using small subunit precursor of ribulose-bisphosphate carboxylase/oxygenase and precursor of ferredoxin, as well as antibody blocking experiments, revealed that ceQORH import does not involve the main receptor and translocation channel proteins Toc159 and Toc75, respectively, which operate in import of proteins into the chloroplast. Molecular dissection of the ceQORH amino acid sequence by site-directed mutagenesis and subsequent import experiments in planta and in vitro highlighted that ceQORH consists of different domains that act concertedly in regulating import. Collectively, our results provide unprecedented evidence for the existence of a specific import pathway for transit sequence-less inner plastid envelope membrane proteins into chloroplasts.     

Genetic variation for sexual dimorphism in flower size within and between populations of gynodioecious Thymus vulgaris

There has been very little empirical study of quantitative genetic variation in flower size in sexually dimorphic plant species, despite the frequent occurrence of flower size differences between sexual phenotypes. In this study we quantify the nature of quantitative flower size variation in females and hermaphrodites of gynodioecious Thymus vulgaris. In a field study, females had significantly smaller flowers than hermaphrodites, and the degree of flower size dimorphism varied significantly among populations. To quantify the genetic basis of flower size variation we sampled maternal progeny from 10 F₀ females in three populations (across the range of variation in flower size in the field), performed controlled crosses on F₁ offspring in the glasshouse and grew F₂ progeny to flowering in uniform field conditions. A significant population * sex interaction was again observed, hence the degree of sexual dimorphism shows genetic variation among populations. A significant family * sex interaction was also observed, indicating that the degree of sexual dimorphism shows genetic variation among families. Females showed significantly greater variation among populations and among families than hermaphrodites. Female flower size varied significantly depending on the degree of stamen abortion, with morphologically intermediate females having flowers more similar to hermaphrodites than to other females. The frequency of female types that differ in the degree of stamen abortion varied among populations and families and mean family female flower size increased as the proportion of intermediate female types increased across families. Variation in the degree of flower size dimorphism thus appears to be a result of variation in the degree of stamen abortion in females, the potential causes of which are discussed.     

SNX12 role in endosome membrane transport

In this paper, we investigated the role of sorting nexin 12 (SNX12) in the endocytic pathway. SNX12 is a member of the PX domain-containing sorting nexin family and shares high homology with SNX3, which plays a central role in the formation of intralumenal vesicles within multivesicular endosomes. We found that SNX12 is expressed at very low levels compared to SNX3. SNX12 is primarily associated with early endosomes and this endosomal localization depends on the binding to 3-phosphoinositides. We find that overexpression of SNX12 prevents the detachment (or maturation) of multivesicular endosomes from early endosomes. This in turn inhibits the degradative pathway from early to late endosomes/lysosomes, much like SNX3 overexpression, without affecting endocytosis, recycling and retrograde transport. In addition, while previous studies showed that Hrs knockdown prevents EGF receptor sorting into multivesicular endosomes, we find that overexpression of SNX12 restores the sorting process in an Hrs knockdown background. Altogether, our data show that despite lower expression level, SNX12 shares redundant functions with SNX3 in the biogenesis of multivesicular endosomes.     

Climatic drivers of pinyon mouse Peromyscus truei population dynamics in a resource-restricted environment

Highly variable patterns in temperature and rainfall events can have pronounced consequences for small mammals in resource-restricted environments. Climatic factors can therefore play a crucial role in determining the fates of small mammal populations. We applied Pradel's temporal symmetry model to a 21-year capture–recapture dataset to study population dynamics of the pinyon mouse (Peromyscus truei) in a semi-arid mixed oak woodland in California, USA. We examined time-, season- and sex-specific variation in realized population growth rate (λ) and its constituent vital rates, apparent survival and recruitment. We also tested the influence of climatic factors on these rates. Overall monthly apparent survival was 0.81 ± 0.004 (estimate ± SE). Survival was generally higher during wetter months (October–May) but varied over time. Monthly recruitment rate was 0.18 ± 0.01, ranging from 0.07 ± 0.01 to 0.63 ± 0.07. Although population growth rate (λ) was highly variable, overall monthly growth rate was close to 1.0, indicating a stable population during the study period (λ ± SE = 0.99 ± 0.01). Average temperature and its variability negatively affected survival, whereas rainfall positively influenced survival and recruitment rates, and thus the population growth rate. Our results suggest that seasonal rainfall and variation in temperature at the local scale, rather than regional climatic patterns, more strongly affected vital rates in this population. Discerning such linkages between species' population dynamics and environmental variability are critical for understanding local and regional impacts of global climate change, and for gauging viability and resilience of populations in resource-restricted environments.     

Estimated calf and perinatal mortality in western North Atlantic right whales (Eubalaena glacialis)

Reduced reproductive success has contributed to lack of recovery of the endangered western North Atlantic right whale (Eubalaena glacialis). Here we examined the specific life history period from just before birth through the first year to estimate calf and perinatal losses between 1989 and 2003. The lower bound estimate (17 mortalities from 208 calving events) included documented calf mortalities and presumed deaths from serious injury or disappearance from the sighting record. The upper bound estimated potential calf losses from females with delayed first parturition (>10 yr) and shortened (2 yr) or lengthened (≥4 yr) calving intervals, if the female migrated to the calving ground during these intervals. Because cows were sighted in the calving ground predominantly in years when they were available to calve, adult females sighted there in a possible calving year without a calf were assumed to have experienced a perinatal loss. Twenty-eight potential perinatal losses were detected, bringing the upper bound of calf and perinatal mortality to 45 (3.0 calves/yr). The high frequency of lengthened calving intervals in E. glacialis suggests that abortion and neonatal losses are contributing to lower reproductive success compared to Southern Hemisphere right whales (Eubalaena australis).     

Structural Insights into the Nucleotide-Binding Domains of the P1B-type ATPases HMA6 and HMA8 from Arabidopsis thaliana

Copper is a crucial ion in cells, but needs to be closely controlled due to its toxic potential and ability to catalyse the formation of radicals. In chloroplasts, an important step for the proper functioning of the photosynthetic electron transfer chain is the delivery of copper to plastocyanin in the thylakoid lumen. The main route for copper transport to the thylakoid lumen is driven by two P_IB-type ATPases, Heavy Metal ATPase 6 (HMA6) and HMA8, located in the inner membrane of the chloroplast envelope and in the thylakoid membrane, respectively. Here, the crystal structures of the nucleotide binding domain of HMA6 and HMA8 from Arabidopsis thaliana are reported at 1.5Å and 1.75Å resolution, respectively, providing the first structural information on plants Cu⁺-ATPases. The structures reveal a compact domain, with two short helices on both sides of a twisted beta-sheet. A double mutant, aiding in the crystallization, provides a new crystal contact, but also avoids an internal clash highlighting the benefits of construct modifications. Finally, the histidine in the HP motif of the isolated domains, unable to bind ATP, shows a side chain conformation distinct from nucleotide bound structures.     

Photoaffinity labeling of human sex hormone-binding globulin using 17alpha-alkylamine derivatives of 3beta-androstanediol substituted with azidonitrophenylamido, azidonitrophenylamino, or trifluoroazidonitrophenylamino chromophores. Localization of Trp-84 in the vicinity of the steroid-binding site.

Purified human SHBG was photoaffinity labeled with 17alpha-aminomethyl (M), 17alpha-aminoethyl (E), and 17alpha-aminopropyl (P) derivatives of [3alpha-(3)H]-5alpha-androstane-3beta,17beta-diol coupled to 5-azido-2-nitrobenzoylamido (ANB), 4-azido-2-nitrophenylamino (ANP), and 5-azido-2-nitro-3,4,6-trifluorophenylamino (ANTFP) chromophores. Successful labeling was achieved in all cases except for the two photoreagents with the shortest side chains, namely, ANP-M and ANTFP-M derivatives. Edman sequencing and mass spectrometry of immunopurified photolabeled tryptic fragments revealed that radioactivity was present either on the sequence of residues 73-94, uniquely at the level of Trp-84 (stable covalent labeling), or on one of the two overlapping sequences of residues 126-134 and 126-135, at the level of Pro-130 (labile labeling) and Lys-134 (either stable or partially labile labeling), respectively. The same Trp-84 was photolabeled with the three ANB derivatives of increasing lengths, and by the ANP-P photoreagent. This residue was the exclusive target for the shortest [(3)H]ANB-M photoreagent but was a minor site for the longest [(3)H]ANB-P photoreagent, essentially recovered at the level of Pro-130. The [(3)H]ANB-E photoreagent of intermediate size also labeled exclusively Trp-84, except in some experiments in which photolabeling was recovered predominantly at the level of Pro-130. The [(3)H]ANP-P photoreagent with an overall length similar to that of the ANB-P photoreagent labeled simultaneously Trp-84 (minor site) and Lys-134. The other [(3)H]ANP-E, [(3)H]ANTFP-E, and [(3)H]ANTFP-P derivatives labeled in all cases Lys-134. These findings indicate that the conserved Trp-84 and the two Pro-130 and Lys-134 residues are all located in the vicinity of the D ring of steroid ligands and remain freely accessible from the C17alpha position, thus providing biochemical data delineating the corresponding region of the steroid-binding site.