Immigration et espace urbain : Les regroupements de population haïtienne dans la région métropolitaine de Montréal

La population haïtienne compte parmi les groupes immigrés les plus récents de la région métropolitaine de Montréal. Dix fois plus nombreuse en 1981 qu’en 1971, cette population d’environ 30 000 personnes réside en majorité (80%) dans trois des quinze municipalités de la région montréalaise : Montréal-Nord, St-Léonard et Montréal. Afin d’identifier les causes et les mécanismes des regroupements de population haïtienne à l’intérieur de ces municipalités, une enquête a été menée au cours de l’été 1982 auprès de 152 ménages haïtiens résidant dans des zones de concentration. Les besoins de ces ménages en matière de logement diffèrent de ceux de l’ensemble des ménages vivant dans les mêmes zones : les ménages haïtiens se caractérisent par un nombre plus élevé de membres et par l’importance accrue des familles comptant des enfants à la maison, enfants qui sont aussi plus jeunes. Les logements qu’ils occupent se situent pour la plupart dans des immeubles à appartements, le plus souvent au sous-sol ou au second étage des bâtiments résidentiels; ce sont surtout des logements de taille moyenne (quatre ou cinq pièces), ce qui donne une densité d’occupation plus forte pour les ménages haïtiens que pour l’ensemble des ménages. La situation socio-économique de ces ménages leur impose également des contraintes particulières, puisque leurs membres actifs sont davantage concentrés dans un même secteur du marché du travail que ne l’est la population active des mêmes zones : les deux tiers sont ouvriers et ouvrières en usine. Ces caractéristiques situent donc la population haïtienne des zones d’enquête parmi les groupes sociaux désavantagés sur le marché du logement et les plus susceptibles d’être touchés par la crise actuelle.The Haitian population is among the most recent groups of immigrants of the Metropolitan Region of Montreal. Ten times as numerous in 1981 than in 1971, this population of about 30 000 people lives mainly (80%) in three of the fifteen municipalities of the Montreal region: North-Montreal, St-Leonard and Montreal. In order to identify the causes and the mechanisms of the formation of groups of Haitians within these municipalities, a survey of 152 Haitian households living within the areas of concentration was conducted during the summer of 1982. The Haitian households have different housing needs than the whole of the households living in the same areas: Haitian households are characterized by a larger number of members, and by the increased importance of families with children living at home, children who are also younger. The housing units that they occupy are mostly found in apartment buildings, most often in basements or on the second floor of residential buildings; these are mostly medium size flats (four or five rooms) which means a higher occupation density for Haitian households than for the whole of the households. The socio-economic situation of these households also imposes other particular constraints, since their work force is more concentrated in one sector of the labour market than the work force of the same areas: two-thirds are factory workers. Therefore, these characteristics place the Haitian population of the areas surveyed, among the social groups that are at a disadvantage in the housing market and which are most liable to be affected by the present (economic) crisis.La población haitiana se coloca entre los grupos de inmigrantes más recientes del área metropolitana de Montreal. Esta población cuenta con alrededor de 30 000 personas, o sea diez veces más numerosa en 1981 que diez años antes, cuya mayoría (80%) reside en tres de los quince municipios de la zona regiomontana: Montreal-Norte, San Leonardo y Montreal. Con objeto de identificar las causas y los mecanismos del agrupamiento de población haitiana en el seno de estos municipios, se llevó a cabo una encuesta durante el verano de 1982, en 152 hogares haitianos ubicados en las zonas de mayor concentración. Las necesidades de estos hogares en cuestión de vivienda difieren de las del conjunto de hogares que se encuentran en las mismas zonas: los hogares haitianos se distinguen por un mayor número de miembros, así como por el número relevante de familias que cuentan con niños dentro de la casa, los cuales son incluso más pequeños. La mayor parte de las viviendas que ocupan los haitianos se ubican en edificios de departaaentos, situandose a ienudo en el sôtano o en el priaer piso de estos conjuntos residenciales; generalmente son viviendas de tamaño mediano (de cuatro o de cinco piezas) Io que produce un índice de hacinamiento mucho mayor entre los hogares haitianos que en el resto de los hogares. Asimismo la situación socioéconomica de estos hogares los somete a presiones particulares, ya que sus miembros activos están principalmente incorporados en el mercado de trabajo, a diferencia de la población activa de las mismas zonas: dos terceras partes son obreros u obreras de fábricas. Estas peculiaridades colocan así a la población haitiana de las zonas estudiadas, dentro de los grupos sociales menos privilegiados dentro del mercado de la vivienda, además de ser los más susceptibles en cuanto a ser afectados por la crisis actual

The Voltage-Dependent Deactivation of the KvAP Channel Involves the Breakage of Its S4 Helix

Voltage-gated potassium channels (Kv) allow ion permeation upon changes of the membrane electrostatic potential (Vm). Each subunit of these tetrameric channels is composed of six transmembrane helices, of which the anti-parallel helix bundle S1-S4 constitutes the voltage-sensor domain (VSD) and S5-S6 forms the pore domain. Here, using 82 molecular dynamics (MD) simulations involving 266 replicated VSDs, we report novel responses of the archaebacterial potassium channel KvAP to membrane polarization. We show that the S4 α-helix, which is straight in the experimental crystal structure solved under depolarized conditions (Vm ∼ 0), breaks into two segments when the cell membrane is hyperpolarized (Vm < 0), and reversibly forms a single straight helix following depolarization (Vm = 0). The outermost segment of S4 translates along the normal to the membrane, bringing new perspective to previously paradoxical accessibility experiments that were initially thought to imply the displacement of the whole VSD across the membrane. The novel model is applied through steered and unbiased MD simulations to the recently solved whole structure of KvAP. The simulations show that the resting state involves a re-orientation of the S5 α-helix by ∼ 5-6 degrees in respect to the normal of the bilayer, which could result in the constriction and closure of the selectivity filter. Our findings support the idea that the breakage of S4 under (hyper)polarization is a general feature of Kv channels with a non-swapped topology

Mechanism of Ambipolar Field-Effect Carrier Injections in One-Dimensional Mott Insulators

To clarify the mechanism of recently reported, ambipolar carrier injections into quasi-one-dimensional Mott insulators on which field-effect transistors are fabricated, we employ the one-dimensional Hubbard model attached to a tight-binding model for source and drain electrodes. To take account of the formation of Schottky barriers, we add scalar and vector potentials, which satisfy the Poisson equation with boundary values depending on the drain voltage, the gate bias, and the work-function difference. The current-voltage characteristics are obtained by solving the time-dependent Schr\"odinger equation in the unrestricted Hartree-Fock approximation. Its validity is discussed with the help of the Lanczos method applied to small systems. We find generally ambipolar carrier injections in Mott insulators even if the work function of the crystal is quite different from that of the electrodes. They result from balancing the correlation effect with the barrier effect. For the gate-bias polarity with higher Schottky barriers, the correlation effect is weakened accordingly, owing to collective transport in the one-dimensional correlated electron systems.Comment: 21 pages, 10 figures, to appear in J. Phys. Soc. Jp

The Voltage-Dependent Deactivation of the KvAP Channel Involves the Breakage of Its S4 Helix

We show that the response of the KvAP voltage-sensor domain to membrane polarization consists in two co-occurring conformational changes: the rupture of the Asp62-Arg133 salt-bridge and the breakage of the S4 helix. This finding is consistent with the hypothesis that the breakage of S4 is a general feature of non-swapped Kv channels. Upon polarization, the N-ter segment of the S4 helix translates toward the intracellular side of the membrane, which allows to solve previously paradoxical avidin accessibility measurements. Simulations involving the recently solved whole structure of KvAP show how these conformational changes induce a reorientation of the S5 and pore helices, putatively leading to the constriction and closure of the selectivity filter. This study also links mechanistic insights in prokaryotic potassium channel membrane potential sensing with the perspective of finding new antibiotic targets

Unidirectional Transport Mechanism in an ATP Dependent Exporter

ATP-binding cassette (ABC) transporters use the energy of ATP binding and hydrolysis to move a large variety of compounds across biological membranes. P-glycoprotein, involved in multidrug resistance, is the most investigated eukaryotic family member. Although a large number of biochemical and structural approaches have provided important information, the conformational dynamics underlying the coupling between ATP binding/hydrolysis and allocrite transport remains elusive. To tackle this issue, we performed molecular dynamic simulations for different nucleotide occupancy states of Sav1866, a prokaryotic P-glycoprotein homologue. The simulations reveal an outward-closed conformation of the transmembrane domain that is stabilized by the binding of two ATP molecules. The hydrolysis of a single ATP leads the X-loop, a key motif of the ATP binding cassette, to interfere with the transmembrane domain and favor its outward-open conformation. Our findings provide a structural basis for the unidirectionality of transport in ABC exporters and suggest a ratio of one ATP hydrolyzed per transport cycle

Essai d’estimation de la population des métis et indiens sans statut du Québec

Il n’est pas facile d’estimer l’effectif des Indiens sans statut et Métis du Québec. Le caractère distinctif de cette sous-population dans l’ensemble des Amérindiens étant de ne pas jouir d’un statut légal, il serait nécessaire, pour la délimiter, de tenir compte à la fois de l’origine indienne, des effets de la loi des Indiens et de l’intensité du sentiment d’appartenance de ses membres a la collectivité indienne. Il n’est donc pas surprenant de constater l’absence de données satisfaisantes en ce qui les concerne dans les statistiques officielles.L’estimation proposée dans ce travail s’appuie sur l’exploitation de sources indirectes et sur l’utilisation d’un modèle simple, décrivant les effets de mariages mixtes d’Indiennes et de non-Indiens qui sont, pour l’essentiel, à l’origine de la population étudiée. Une autre voie consiste à comparer l’évolution observée de l’effectif des Indiens inscrits à ce qu’il aurait été en l’absence des règles privatives du statut contenues dans la loi des Indiens. On aboutit dans les deux cas, à l’aide d’hypothèse volontairement minimales, à estimer entre 23 000 et 50 000 le nombre d’Indiens sans statut et Métis au Québec en 1976

Dynamics of ions in the selectivity filter of the KcsA channel

The statistical and dynamical properties of ions in the selectivity filter of the KcsA ion channel are considered on the basis of molecular dynamics (MD) simulations of the KcsA protein embedded in a lipid membrane surrounded by an ionic solution. A new approach to the derivation of a Brownian dynamics (BD) model of ion permeation through the filter is discussed, based on unbiased MD simulations. It is shown that depending on additional assumptions, ion’s dynamics can be described either by under-damped Langevin equation with constant damping and white noise or by Langevin equation with a fractional memory kernel. A comparison of the potential of the mean force derived from unbiased MD simulations with the potential produced by the umbrella sampling method demonstrates significant differences in these potentials. The origin of these differences is an open question that requires further clarifications

Conformational dynamics and role of the acidic pocket in ASIC pH-dependent gating.

Acid-sensing ion channels (ASICs) are proton-activated Na &lt;sup&gt;+&lt;/sup&gt; channels expressed in the nervous system, where they are involved in learning, fear behaviors, neurodegeneration, and pain sensation. In this work, we study the role in pH sensing of two regions of the ectodomain enriched in acidic residues: the acidic pocket, which faces the outside of the protein and is the binding site of several animal toxins, and the palm, a central channel domain. Using voltage clamp fluorometry, we find that the acidic pocket undergoes conformational changes during both activation and desensitization. Concurrently, we find that, although proton sensing in the acidic pocket is not required for channel function, it does contribute to both activation and desensitization. Furthermore, protonation-mimicking mutations of acidic residues in the palm induce a dramatic acceleration of desensitization followed by the appearance of a sustained current. In summary, this work describes the roles of potential pH sensors in two extracellular domains, and it proposes a model of acidification-induced conformational changes occurring in the acidic pocket of ASIC1a