Dynamique de l’occupation des sols dans le parc national de Bouhedma (Tunisie)

La présente étude a été effectuée dans le Parc national de Bouhedma, dans les zones arides tunisiennes. Il constitue une zone protégée avec des caractéristiques écologiques particulières et une dynamique importante de l’occupation des espaces. Le Parc abrite l’unique steppe arborée à Acacia tortilis en Tunisie. Dans un but de préservation et de gestion du Parc, une meilleure connaissance de la répartition et de la dynamique de la végétation de cette zone permet d’envisager des programmes de restauration et de gestion durable du milieu naturel. L’étude a été consacrée, grâce à l’interprétation des données de la télédétection et l’utilisation des SIG, à l’élaboration et à la comparaison de deux cartes, l’une relative aux systèmes écologiques présents en 2008 et l’autre qui concerne l’occupation des terres de la même zone en 1985

Structure of the C-terminal domain of the Prokaryotic Sodium Channel Orthologue NsvBa

Crystallographic and electrophysiological studies have recently provided insight into the structure, function and drug binding of prokaryotic sodium channels. These channels exhibit significant sequence identities, especially in their transmembrane regions, with human voltage-gated sodium channels. However, rather than being single polypeptides with four homologous domains, they are tetramers of single domain polypeptides, with a C-terminal domain (CTD) composed of an inter-subunit four helix coiled-coil. The structures of the CTDs differ between orthologues. In NavBh and NavMs, the C-termini form a disordered region adjacent to the final transmembrane helix, followed by a coiled-coil region, as demonstrated by synchrotron radiation circular dichroism (SRCD) and double electron-electron resonance electron paramagnetic resonance spectroscopic measurements. In contrast, in the crystal structure of the NavAe orthologue, the entire C-terminus is comprised of a helical region followed by a coiled-coil. In this study we have examined the CTD of the NsvBa from Bacillus alcalophilus, which unlike other orthologues is predicted by different methods to have different types of structures: either a disordered adjacent to the transmembrane region, followed by a helical coiled-coil, or a fully helical CTD. To discriminate between the two possible structures we have used SRCD spectroscopy to experimentally determine the secondary structure of the C-terminus of this orthologue and used the results as the basis for modelling the transition between open and closed conformations of the channel

Efficient binary fuzzy measure representation and Choquet integral learning

The Choquet integral (ChI), a parametric function for information aggregation, is parameterized by the fuzzy measure (FM), which has 2N real-valued variables for N inputs. However, the ChI incurs huge storage and computational burden due to its exponential complexity relative to N and, as a result, its calculation, storage, and learning becomes intractable for even modest sizes (e.g., N = 15). Inspired by empirical observations in multi-sensor fusion and the more general need to mitigate the storage, computational, and learning limitations, we previously explored the binary ChI (BChI) relative to the binary fuzzy measure (BFM). The BChI is a natural _t for many applications and can be used to approximate others. Previously, we investigated different properties of the BChI and we provided an initial representation. In this article, we propose a new efficient learning algorithm for the BChI, called EBChI, by utilizing the BFM properties that add at most one variable per training instance. Furthermore, we provide an efficient representation of the BFM (EBFM) scheme that further reduces the number of variables required for storage and computation, thus enabling the use of the BChI for \big N". Finally, we conduct experiments on synthetic data that demonstrate the efficiency of our proposed techniques

Differential lipid dependence of the function of bacterial sodium channels

The lipid bilayer is important for maintaining the integrity of cellular compartments and plays a vital role in providing the hydrophobic and charged interactions necessary for membrane protein structure, conformational flexibility and function. To directly assess the lipid dependence of activity for voltage-gated sodium channels, we compared the activity of three bacterial sodium channel homologues (NaChBac, NavMs, and NavSp) by cumulative 22Na+ uptake into proteoliposomes containing a 3:1 ratio of 1-palmitoyl 2-oleoyl phosphatidylethanolamine and different “guest” glycerophospholipids. We observed a unique lipid profile for each channel tested. NavMs and NavSp showed strong preference for different negatively-charged lipids (phosphatidylinositol and phosphatidylglycerol, respectively), whilst NaChBac exhibited a more modest variation with lipid type. To investigate the molecular bases of these differences we used synchrotron radiation circular dichroism spectroscopy to compare structures in liposomes of different composition, and molecular modeling and electrostatics calculations to rationalize the functional differences seen. We then examined pore-only constructs (with voltage sensor subdomains removed) and found that in these channels the lipid specificity was drastically reduced, suggesting that the specific lipid influences on voltage-gated sodium channels arise primarily from their abilities to interact with the voltage-sensing subdomains

Mutagenesis of the NaChBac sodium channel discloses a functional role for a conserved S6 asparagine

Asparagine is conserved in the S6 transmembrane segments of all voltage-gated sodium, calcium, and TRP channels identified to date. A broad spectrum of channelopathies including cardiac arrhythmias, epilepsy, muscle diseases, and pain disorders is associated with its mutation. To investigate its effects on sodium channel functional properties, we mutated the simple prokaryotic sodium channel NaChBac. Electrophysiological characterization of the N225D mutant reveals that this conservative substitution shifts the voltage-dependence of inactivation by 25 mV to more hyperpolarized potentials. The mutant also displays greater thermostability, as determined by synchrotron radiation circular dichroism spectroscopy studies of purified channels. Based on our analyses of high-resolution structures of NaChBac homologues, we suggest that the side-chain amine group of asparagine 225 forms one or more hydrogen bonds with different channel elements and that these interactions are important for normal channel function. The N225D mutation eliminates these hydrogen bonds and the structural consequences involve an enhanced channel inactivation

NaChBac: The Long Lost Sodium Channel Ancestor

In excitable cells, the main mediators of sodium conductance across membranes are voltage-gated sodium channels (Na(V)s). Eukaryotic Na(V)s are essential elements in neuronal signaling and muscular contraction and in humans have been causally related to a variety of neurological and cardiovascular channelopathies. They are complex heavily glycosylated intrinsic membrane proteins present in only trace quantities that have proven to be challenging objects of study. However, in recent years, a number of simpler prokaryotic sodium channels have been identified, with NaChBac from Bacillus halodurans being the most well-characterized to date. The availability of a bacterial Na(V) that is amenable to heterologous expression and functional characterization in both bacterial and mammalian systems has provided new opportunities for structure--function studies. This review describes features of NaChBac as an exemplar of this class of bacterial channels, compares prokaryotic and eukaryotic Na(V)s with respect to their structural organization, pharmacological profiling, and functional kinetics, and discusses how voltage-gated ion channels may have evolved to deal with the complex functional demands of higher organisms

Dynamique de l’occupation des sols dans le parc national de Bouhedma (Tunisie)

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2. INTRODUCTION................................................................................................................................................................... 2 3. ENERGY FOCUSING GROUND PENETRATING RADAR............................................................................................ 2 4. SYSTEM OVERVIEW...........................................................................................................................................................