Amélioration du comportement sismique des assemblages à cisaillement simple des contreventements concentriques en X en acier de construction conventionnelle (type CC)

RÉSUMÉ Les contreventements en X sont souvent utilisés pour reprendre les charges sismiques d’une structure en acier. Ceux-ci sont généralement constitué de diagonales de profilé HSS, dont une est continue et l’autre discontinue. Leur point d’intersection est constitué d’une plaque de gousset traversant et soudée à la diagonale continue qui est boulonnée à des plaques de transfert, elles-mêmes soudées aux diagonales discontinues. L’assemblage se fait en cisaillement simple ce qui introduit une excentricité locale. Les études expérimentales de Gélinas (2013) ont permis de constater que le comportement de ce type de dimensionnement est caractérisé par l’instabilité de l’assemblage central plutôt que le flambement des diagonales comme prévu par le CNB (2010) et la norme CSA S16 (2009). Aussi, ces essais ont mis en évidence l’influence considérable du dimensionnement de la connexion sur le comportement des contreventements en X.----------ABSTRACT X-bracings are often used to support seismic loads of a steel structure. These are generally made of braces of HSS profile, one of which is continuous and the other is discontinuous. Their intersection is made of a gusset plate crossing and welding to the continuous brace which is bolted to transfer plates, themselves welding to the discontinuous braces. The connection is in single shear which introduces a local eccentricity. The experimental studies of Gélinas (2013) shown that the behavior of this type of design is characterized by the instability of the middle connection instead of the brace as provided by the CNB (2010) and the norm CSA S16 (2009). Also, these tests have demonstrated the considerable influence of the design of the connection on the behavior of the X-bracing

Molecular dynamics of ion transport through the open conformation of a bacterial voltage-gated sodium channel

The crystal structure of the open conformation of a bacterial voltage-gated sodium channel pore from Magnetococcus sp. (NaVMs) has provided the basis for a molecular dynamics study defining the channel’s full ion translocation pathway and conductance process, selectivity, electrophysiological characteristics, and ion-binding sites. Microsecond molecular dynamics simulations permitted a complete time-course characterization of the protein in a membrane system, capturing the plethora of conductance events and revealing a complex mixture of single and multi-ion phenomena with decoupled rapid bidirectional water transport. The simulations suggest specific localization sites for the sodium ions, which correspond with experimentally determined electron density found in the selectivity filter of the crystal structure. These studies have also allowed us to identify the ion conductance mechanism and its relation to water movement for the NavMs channel pore and to make realistic predictions of its conductance properties. The calculated single-channel conductance and selectivity ratio correspond closely with the electrophysiology measurements of the NavMs channel expressed in HEK 293 cells. The ion translocation process seen in this voltage-gated sodium channel is clearly different from that exhibited by members of the closely related family of voltage-gated potassium channels and also differs considerably from existing proposals for the conductance process in sodium channels. These studies simulate sodium channel conductance based on an experimentally determined structure of a sodium channel pore that has a completely open transmembrane pathway and activation gate

Liability of a Surgeon for the Extension of an Authorized Operation

Polyunsaturated fatty acids modulate the voltage dependence of several voltage-gated ion channels, thereby being potent modifiers of cellular excitability. Detailed knowledge of this molecular mechanism can be used in designing a new class of small-molecule compounds against hyperexcitability diseases. Here, we show that arginines on one side of the helical K-channel voltage sensor S4 increased the sensitivity to docosahexaenoic acid (DHA), whereas arginines on the opposing side decreased this sensitivity. Glutamates had opposite effects. In addition, a positively charged DHA-like molecule, arachidonyl amine, had opposite effects to the negatively charged DHA. This suggests that S4 rotates to open the channel and that DHA electrostatically affects this rotation. A channel with arginines in positions 356, 359, and 362 was extremely sensitive to DHA: 70 mu M DHA at pH 9.0 increased the current greater than500 times at negative voltages compared with wild type (WT). The small-molecule compound pimaric acid, a novel Shaker channel opener, opened the WT channel. The 356R/359R/362R channel drastically increased this effect, suggesting it to be instrumental in future drug screening

Direct recording and molecular identification of the calcium channel of primary cilia

Summary A primary cilium is a solitary slender non-motile protuberance of structured microtubules (9+0) enclosed by plasma membrane1. Housing components of the cell division apparatus between cell divisions, they also serve as specialized compartments for calcium signaling2 and Hedgehog (Hh) signaling pathways3. Specialized sensory cilia such as retinal photoreceptors and olfactory cilia employ diverse ion channels4-7. An ion current has been measured from primary cilia of kidney cells8 but the responsible genes have not been identified. The polycystin proteins (PC, PKD), identified in linkage studies of polycystic kidney disease9, are candidate channels divided into two structural classes: 11-transmembrane (TM) proteins (PKD1, PKD1-L1 and PKD1-L2) remarkable for a large extracellular N-terminus of putative cell adhesion domains and a GPCR proteolytic site, and the 6-TM channel proteins (PKD2, PKD2-L1, PKD2-L2; TRPPs). Evidence suggests that the PKD1s associate with the PKD2s via coiled-coil domains10-12. Here, we employ a transgenic mouse in which only cilia express a fluorophore and employ it to directly record from primary cilia and demonstrate that PKD1-L1 and PKD2-L1 form ion channels at high densities in several cell types. In conjunction with the companion manuscript2, we show that the PKD1-L1/PKD2-L1 heteromeric channel establishes the cilia as a unique calcium compartment within cells that modulates established Hedgehog pathways

Primary cilia are specialized calcium signaling organelles

Summary Primary cilia are solitary nonmotile extensions of the centriole found on nearly all nucleated eukaryotic cells between cell divisions. Only ∼200-300 nm in diameter and a few microns long, they are separated from the cytoplasm by the ciliary neck and basal body. Often called sensory cilia, they are hypothesized to receive chemical and mechanical stimuli and initiate specific cellular signal transduction pathways. When activated by a ligand, Hedgehog (Hh) pathway proteins, such as Gli2 and Smoothened (Smo), translocate from the cell into the cilium1,2. Mutations in primary ciliary proteins are associated with severe developmental defects3. The ionic conditions, permeability of the primary cilia membrane, and effectiveness of the diffusion barriers between the cilia and cell body are unknown. Here we show that cilia are a unique calcium compartment regulated by a heteromeric TRP channel, PKD1-L1/PKD2-L1. In contrast to the hypothesis that polycystin (PKD) channels initiate changes in ciliary calcium that are conducted into the cytoplasm4, we show that changes in ciliary calcium concentration ([Ca2+]cilia) occur without substantially altering global cytoplasmic calcium ([Ca2+]cyto). PKD1-L1/PKD2-L1 acts as a ciliary calcium channel controlling [Ca2+]cilia and thereby modifying Smo-activated Gli2 translocation and Gli1 expression

Ionic selectivity and thermal adaptations within the voltage-gated sodium channel family of alkaliphilic Bacillus

Entry and extrusion of cations are essential processes in living cells. In alkaliphilic prokaryotes, high external pH activates voltage-gated sodium channels (Nav), which allows Na+ to enter and be used as substrate for cation/proton antiporters responsible for cytoplasmic pH homeostasis. Here, we describe a new member of the prokaryotic voltage-gated Na+ channel family (NsvBa; Non-selective voltage-gated, Bacillus alcalophilus) that is nonselective among Na+, Ca2+ and K+ ions. Mutations in NsvBa can convert the nonselective filter into one that discriminates for Na+ or divalent cations. Gain-of-function experiments demonstrate the portability of ion selectivity with filter mutations to other Bacillus Nav channels. Increasing pH and temperature shifts their activation threshold towards their native resting membrane potential. Furthermore, we find drugs that target Bacillus Nav channels also block the growth of the bacteria. This work identifies some of the adaptations to achieve ion discrimination and gating in Bacillus Nav channels. DOI: http://dx.doi.org/10.7554/eLife.04387.00

The complete structure of an activated open sodium channel

Voltage-gated sodium channels (Navs) play essential roles in excitable tissues, with their activation and opening resulting in the initial phase of the action potential. The cycling of Navs through open, closed and inactivated states, and their closely choreographed relationships with the activities of other ion channels lead to exquisite control of intracellular ion concentrations in both prokaryotes and eukaryotes. Here we present the 2.45 Å resolution crystal structure of the complete NavMs prokaryotic sodium channel in a fully open conformation. A canonical activated conformation of the voltage sensor S4 helix, an open selectivity filter leading to an open activation gate at the intracellular membrane surface and the intracellular C-terminal domain are visible in the structure. It includes a heretofore unseen interaction motif between W77 of S3, the S4–S5 interdomain linker, and the C-terminus, which is associated with regulation of opening and closing of the intracellular gate

A tamoxifen receptor within a voltage-gated sodium channel

Voltage-gated sodium channels are targets for many analgesic and antiepileptic drugs whose therapeutic mechanisms and binding sites have been well characterized. We describe the identification of a previously unidentified receptor site within the NavMs voltage-gated sodium channel. Tamoxifen, an estrogen receptor modulator, and its primary and secondary metabolic products bind at the intracellular exit of the channel, which is a site that is distinct from other previously characterized sodium channel drug sites. These compounds inhibit NavMs and human sodium channels with similar potencies and prevent sodium conductance by delaying channel recovery from the inactivated state. This study therefore not only describes the structure and pharmacology of a site that could be leveraged for the development of new drugs for the treatment of sodium channelopathies but may also have important implications for off-target health effects of this widely used therapeutic drug

Interpreting the functional role of a novel interaction motif in prokaryotic sodium channels

Voltage-gated sodium channels enable the translocation of sodium ions across cell membranes and play crucial roles in electrical signaling by initiating the action potential. In humans, mutations in sodium channels give rise to several neurological and cardiovascular diseases, and hence they are targets for pharmaceutical drug developments. Prokaryotic sodium channel crystal structures have provided detailed views of sodium channels, which by homology have suggested potentially important functionally related structural features in human sodium channels. A new crystal structure of a full-length prokaryotic channel, NavMs, in a conformation we proposed to represent the open, activated state, has revealed a novel interaction motif associated with channel opening. This motif is associated with disease when mutated in human sodium channels and plays an important and dynamic role in our new model for channel activation