Dissection of GTPase activating proteins reveals functional asymmetry in the COPI coat of budding yeast.

The Arf GTPase controls formation of the COPI vesicle coat. Recent structural models of COPI revealed the positioning of two Arf1 molecules in contrasting molecular environments. Each of these pockets for Arf1 is expected to also accommodate an Arf GTPase-activating protein (ArfGAP). Structural evidence and protein interactions observed between isolated domains indirectly suggests that each niche may preferentially recruit one of the two ArfGAPs known to affect COPI, Gcs1/ArfGAP1 and Glo3/ArfGAP2/3, although only partial structures are available. The functional role of the unique non-catalytic domain of either ArfGAP has not been integrated into the current COPI structural model. Here, we delineate key differences in the consequences of triggering GTP hydrolysis via the activity of one versus the other ArfGAP. We demonstrate that Glo3/ArfGAP2/3 specifically triggers Arf1 GTP hydrolysis impinging on the stability of the COPI coat. We show that the yeast homologue of AMP kinase, Snf1, phosphorylates the region of Glo3 that is critical for this effect and thereby regulates its function in the COPI-vesicle cycle. Our results revise the model of ArfGAP function in the molecular context of COPI

Rôle de la perturbation par le vent dans les forêts tropicales via un modèle dynamique de végétation et l'observation satellitaire

Les perturbations naturelles ont une influence importante sur la structure, la composition et le fonctionnement des forêts tropicales et un rôle dans la régulation des cycles biogéochimiques. La fréquence et l'intensité des perturbations naturelles sont modifiés par les changements climatiques : une meilleure connaissance de leur mécanisme d'action est nécessaire pour prédire les conséquences de cette modification. La modélisation permet d'évaluer le rôle de chacun des processus écologiques et leur lien avec les facteurs environnementaux. Les outils de la télédétection nous informent sur la structure et le fonctionnement des forêts à large échelle, et peuvent être utiles à la calibration et la validation des modèles de végétation. Dans cette thèse, j'ai employé ces deux approches pour examiner comment les forêts tropicales sont façonnées par les perturbations naturelles, notamment le vent, qui est un facteur majeur de perturbation dans de nombreuses régions tropicales. Dans un premier temps, j'ai évalué la transférabilité d'un modèle individu-centré et spatialement explicite via un test de sensibilité et la calibration des paramètres globaux. Le modèle prédit correctement la structure de la forêt sur deux sites contrastés, et sa réponse est cohérente avec les variations du forçage climatique. La calibration d'un petit nombre de paramètres clés a été nécessaire, dont notamment celui qui contrôle la mortalité. Pour étudier la sensibilité du modèle à la mortalité, j'ai mis en œuvre un module de dégâts de vents fondé sur les principes biophysiques et couplé avec la vitesse de vent, afin de modéliser les réponses de la forêt aux évènements de vent extrême. Avec l'augmentation du niveau de perturbation, la hauteur de la canopée diminue de manière constante mais la biomasse montre une réponse non-linéaire. L'intensité du vent a un fort impact sur la hauteur de la canopée et la biomasse, mais pas la fréquence des évènements de vent extrême. Finalement, j'ai testé si les données radar des satellites Sentinel-1 pourraient servir à détecter les trouées dues aux perturbations naturelles en Guyane française. Les données Sentinel-1 détectent plus de trouées naturelles au-dessus de 0.2 ha que les données satellitaires optiques, et elles présentent un patron spatial cohérent avec les images optiques. Le niveau de perturbation ne varie pas en fonction de l'altitude. Nous avons trouvé plus de perturbations pendant les saisons sèches, ce qui pourrait être dû à la réponse tardive des précipitations plutôt qu'à la réponse directe de la sècheresse. En conclusion, cette thèse démontre que l'intégration entre la modélisation et la télédétection éclairent les effets des perturbations naturelles sur les forêts tropicales. Les résultats qui en découlent peuvent servir à étudier d'autres types de perturbations et leurs interactions sur une large échelle.Natural disturbances have an important influence on the structure, composition and functioning of tropical forests and a role in the regulation of biogeochemical cycles. The frequency and intensity of natural disturbances are modified by climate change: a better knowledge of their mechanism of action is necessary to predict the consequences of this modification. Modeling allows us to evaluate the role of each of the ecological processes and their link with environmental factors. Remote sensing tools inform us about the structure and functioning of forests at large scales, and can be useful for the calibration and validation of vegetation models. In this thesis, I employed both approaches to examine how tropical forests are shaped by natural disturbances, particularly wind, which is a major disturbance factor in many tropical regions. First, I evaluated the transferability of a spatially explicit, individual-based model via sensitivity testing and calibration of global parameters. The model correctly predicts forest structure at two contrasting sites, and its response is consistent with variations in climate forcing. Calibration of a small number of key parameters was required, including the parameter controlling mortality and crown allometry. To investigate the sensitivity of the model to mortality, I implemented a wind damage module based on biophysical principles and coupled with wind speed to model forest responses to extreme wind events. With increasing disturbance level, canopy height decreased steadily but biomass showed a non-linear response. Wind intensity had a strong impact on canopy height and biomass, but not the frequency of extreme wind events. Finally, I tested whether radar data from Sentinel-1 satellites could be used to detect gaps due to natural disturbances in French Guiana. The Sentinel-1 data detected more natural gaps above 0.2 ha than the optical satellite data, and they showed a spatial pattern consistent with the optical images. The level of disturbance did not vary with altitude. We found more disturbance during dry seasons, which could be due to the delayed response of precipitation rather than the direct response of drought. In conclusion, this thesis demonstrates that the integration between modeling and remote sensing sheds light on the effects of natural disturbances on tropical forests. The resulting results can be used to study other types of disturbances and their interactions on a large scale

English in the Amazon: Unhomeliness in Evelyn Waugh’s “The Man Who Liked Dickens”

In the short story “The Man Who Liked Dickens,” Evelyn Waugh describes a cultural collision deep in the jungles of Brazil. The story’s narrative centers around two men, one of whom is an Englishman taking what he believes to be a temporary exploratory expedition to Brazil. The other, Mr. McMaster, is a half-Brazilian, half-white landowner who loves the Dickens books he cannot read for himself. Henty, the Englishman, leaves home to escape his wife, who loves another man, and goes on an ill-fated mission to explore the unmapped regions of Brazil. Along the way, he loses his companions and ends up sick and alone in a jungle surrounded by natives with whom he cannot communicate. Mr. McMaster however, is an exception because he speaks English and is drawn to Henty because he too speaks English. He nurses Henty back to health and, as payment, asks him to read aloud every day from one of his Dickens novels. Through the character of McMaster, Waugh demonstrates how unhomeliness, resulting from cultural colonization and mimicry, can cause desperation and loneliness when the colonized are alienated from both the colonized and colonizing groups

Variational methods with coupled Gaussian functions for Bose-Einstein condensates with long-range interactions. II. Applications

Bose-Einstein condensates with an attractive 1/r interaction and with dipole-dipole interaction are investigated in the framework of the Gaussian variational ansatz introduced by S. Rau, J. Main, and G. Wunner [Phys. Rev. A, submitted]. We demonstrate that the method of coupled Gaussian wave packets is a full-fledged alternative to direct numerical solutions of the Gross-Pitaevskii equation, or even superior in that coupled Gaussians are capable of producing both, stable and unstable states of the Gross-Pitaevskii equation, and thus of giving access to yet unexplored regions of the space of solutions of the Gross-Pitaevskii equation. As an alternative to numerical solutions of the Bogoliubov-de Gennes equations, the stability of the stationary condensate wave functions is investigated by analyzing the stability properties of the dynamical equations of motion for the Gaussian variational parameters in the local vicinity of the stationary fixed points. For blood-cell-shaped dipolar condensates it is shown that on the route to collapse the condensate passes through a pitchfork bifurcation, where the ground state itself turns unstable, before it finally vanishes in a tangent bifurcation.Comment: 14 pages, 14 figures, submitted to Phys. Rev. A, some equations correcte

The temperature dependent bandstructure of a ferromagnetic semiconductor film

The electronic quasiparticle spectrum of a ferromagnetic film is investigated within the framework of the s-f model. Starting from the exact solvable case of a single electron in an otherwise empty conduction band being exchange coupled to a ferromagnetically saturated localized spin system we extend the theory to finite temperatures. Our approach is a moment-conserving decoupling procedure for suitable defined Green functions. The theory for finite temperatures evolves continuously from the exact limiting case. The restriction to zero conduction band occupation may be regarded as a proper model description for ferromagnetic semiconductors like EuO and EuS. Evaluating the theory for a simple cubic film cut parallel to the (100) crystal plane, we find some marked correlation effects which depend on the spin of the test electron, on the exchange coupling, and on the temperature of the local-moment system.Comment: 11 pages, 9 figure

Discovery of SXP265, a Be/X-ray binary pulsar in the Wing of the Small Magellanic Cloud

We identify a new candidate for a Be/X-ray binary in the XMM-Newton slew survey and archival Swift observations that is located in the transition region of the Wing of the Small Magellanic Cloud and the Magellanic Bridge. We investigated and classified this source with follow-up XMM-Newton and optical observations. We model the X-ray spectra and search for periodicities and variability in the X-ray observations and the OGLE I-band light curve. The optical counterpart has been classified spectroscopically, with data obtained at the SAAO 1.9 m telescope, and photometrically, with data obtained using GROND at the MPG 2.2 m telescope. The X-ray spectrum is typical of a high-mass X-ray binary with an accreting neutron star. We detect X-ray pulsations, which reveal a neutron-star spin period of P = (264.516+-0.014) s. The source likely shows a persistent X-ray luminosity of a few 10^35 erg/s and in addition type-I outbursts that indicate an orbital period of ~146 d. A periodicity of 0.867 d, found in the optical light curve, can be explained by non-radial pulsations of the Be star. We identify the optical counterpart and classify it as a B1-2II-IVe star. This confirms SXP 265 as a new Be/X-ray binary pulsar originating in the tidal structure between the Magellanic Clouds.Comment: 11 pages, 12 figures, accepted for publication in MNRA

Pathway to a Compact SASE FEL Device

Newly developed high peak power lasers have opened the possibilities of driving coherent light sources operating with laser plasma accelerated beams and wave undulators. We speculate on the combination of these two concepts and show that the merging of the underlying technologies could lead to new and interesting possibilities to achieve truly compact, coherent radiator devices

PKS 0537-286, carrying the information of the environment of SMBHs in the early Universe

We present the results of a multifrequency campaign on the high-redshift (z = 3.1) blazar PKS 0537-286. The source was observed at different epochs from 2006 to 2008 with INTEGRAL and Swift, and nearly simultaneously with ground-based near-IR/optical telescopes. The SEDs are compatible with a model based on synchrotron radiation and external inverse Compton scattering. The campaign gives an insight into the physical environment of the blazar.Comment: 8 pages, 5 figures, accepted for publication in A&