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DTSTART;TZID=Europe/Paris:20260622T133000
DTEND;TZID=Europe/Paris:20260622T153000
DTSTAMP:20260618T132946
CREATED:20260604T143528Z
LAST-MODIFIED:20260604T143528Z
UID:10000188-1782135000-1782142200@sfp-alpes.fr
SUMMARY:Soutenance de Thèse de Khadeeja MUBASHIRA (CEA-Irig/IBS)
DESCRIPTION:Étude de la séparation de phase de la phosphoprotéine du virus de la rage et de sa régulation par LC8\nRésumé : \nRabies virus (RABV) replication occurs in cytoplasmic\, membrane-less compartments known as Negri bodies (NBs)\, formed through liquid-liquid phase separation (LLPS) of viral components. The phosphoprotein (RABV P) is a central\, intrinsically disordered scaf fold of the viral replication machinery. This thesis investigates the structural\, biophysical\, and dynamic properties of RABV P\, with emphasis on its phase separation behavior and interactions with molecular partners. To enable this\, recombinant expression and purification protocols were optimized to produce stable\, high-quality protein samples for reproducible analyses. \nWe first characterized the intrinsic phase behavior of RABV P in vitro. The protein undergoes thermoresponsive LLPS with a lower critical solution temperature (LCST)\, forming reversible condensates within a narrow range of protein and salt concentrations. This process is driven by multivalent interactions within a heterogeneous ensemble of conformations\, where dimers assemble into higher-order oligomers prior to phase separation. The resulting phase diagram reveals a complex\, reentrant system governed by a balance between electrostatic repulsion and attractive dipole-dipole interactions. \nThe role of ionic conditions was further examined. While NaCl induced reentrant phase separation\, LLPS strongly depended on ion identity rather than ionic strength alone. Chloride salts promoted condensate formation\, whereas bromide salts did not\, indicating ion-specific (Hofmeister-type) effects. Systematic trends showed that fluoride enhances phase separation\, while cation effects are weaker. Divalent ions also promoted LLPS\, highlighting valency contributions. Chemical perturbations confirmed that condensates are stabilized by weak interactions: 1\,6-hexanediol partially disrupted droplets\, whereas ATP fully dissolved them. Notably\, RABV P intrinsically phase separates even in water\, modulated by pH\, protein concentration\, and ionic conditions. \nTime-resolved small-angle X-ray scattering (SAXS) revealed the structural evolution underlying LLPS. Following a temperature jump\, RABV P undergoes a hierarchical assembly process\, transitioning from dispersed species to larger structures. Early conformational rearrangements precede the formation of intermediate clusters\, followed by growth into larger assemblies. These structures remain disordered and liquid-like\, supporting a multistep nucleation-and-growth mechanism. \nThe host protein LC8 was investigated as a regulator of RABV P condensation. LC8 binds a conserved motif in RABV P with high affinity\, forming a defined complex and partitioning into condensates. Functionally\, LC8 enhances phase separation by increasing condensate size\, enriching RABV P in the dense phase\, and broadening the phase-separation window. It shifts phase boundaries toward lower concentrations and temperatures while preserving liquid-like properties. These results indicate that LC8 actively promotes condensation by stabilizing interaction-competent conformations and enhancing intermolecular connectivity. \nTo assess whether LC8 can compensate for intrinsic multivalency\, a truncated RABV P lacking the dimerization domain was analyzed. Although LC8 bound this construct\, the interaction was weaker and failed to restore robust phase separation. Only weak condensation was observed under crowding conditions\, demonstrating that LC8 cannot substitute for the native dimerization-driven multivalency.\nOverall\, this work establishes RABV P as a finely tuned multivalent scaffold whose phase behavior arises from the interplay of intrinsic disorder\, ion-specific effects\, and hierarchical assembly. LLPS emerges as a multistep\, non-ideal process rather than a simple binary transition. LC8 acts as a key host regulator that enhances phase separation without altering condensate dynamics\, while intrinsic multivalency remains essential. These findings provide a mechanistic framework for understanding viral condensate formation and highlight potential avenues for antiviral intervention. \n_ \nContact : alain.farchi@cea.fr
URL:https://sfp-alpes.fr/event/soutenance-de-these-de-khadeeja-mubashira-cea-irig-ibs/
LOCATION:Amphi A de Biologie\, Rue de la Piscine\, Saint-Martin-d'Hères\, 38400\, France
CATEGORIES:Soutenance,Soutenance de Thèse
ORGANIZER;CN="IRIG - CEA":MAILTO:odile.rossignol@cea.fr
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BEGIN:VEVENT
DTSTART;TZID=Europe/Paris:20260622T140000
DTEND;TZID=Europe/Paris:20260622T150000
DTSTAMP:20260618T132946
CREATED:20260522T124820Z
LAST-MODIFIED:20260522T124835Z
UID:10000160-1782136800-1782140400@sfp-alpes.fr
SUMMARY:Maurizio SACCHI (CNRS - Institut des NanoSciences de Paris and Synchrotron SOLEIL)
DESCRIPTION:X-rays with Orbital Angular Momentum for spectroscopy and imaging\nRésumé : \nIn addition to the spin angular momentum (SAM) associated to the light polarization\, Laguerre-Gaussian light beams carry also an orbital angular momentum (OAM) of ℓ /photon [1]ℏ associated to an azimuthal dependence exp(iℓϕ) of the electric field phase. Over the last thirty years\, OAM beams at vis-IR wavelengths found applications in fields as different as biology\, telecommunication and imaging [2]. The azimuthal phase dependence\, with a singularity on the propagation axis\, is accompanied by a radial modulation of the intensity (ring-shaped beams)\, properties that have been used to modify local magnetic ordering\, to improve the spatial resolution in microscopy\, and to enhance the edge sharpness in phase-contrast imaging. \nOver the last decade\, several approaches to the generation of OAM beams at shorter wavelengths\, from XUV to hard x-rays\, were proposed. Potential applications are often based on the extrapolation of previous work carried out in the vis-IR range. For instance\, as for the SAM\, the handedness imposed by the OAM has been exploited to study magnetic materials [3] and chiral molecules [4]. The interest of extending the use of OAM beams from the vis-IR to the x-ray range has been growing steadily over the last few years. Nonetheless\, the offer of user accessible beamlines and endstations remains limited\, especially when one aims at independently varying both SAM and OAM in a controlled way. At the SEXTANTS beamline of the SOLEIL synchrotron\, we have implemented and commissioned a new setup for soft x-ray spectroscopy (absorption and resonant scattering experiments) with OAM beams [5]. \n1. L. Allen at al.\, Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes\, Phys. Rev. A 45\, 8185 (1992).\n2. Y. Shen et al.\, Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities\, Light: Science & Applications 8\, 90 (2019).\n3. M. Fanciulli et al.\, Electromagnetic theory of Helicoidal Dichroism in reflection from magnetic structures\, Phys. Rev. A 103\, 013501 (2021); Observation of magnetic helicoidal dichroism with extreme ultraviolet light vortices\, Phys. Rev. Lett. 128\, 077401 (2022); Magnetic vortex dynamics probed by time-resolved magnetic helicoidal\ndichroism\, Phys. Rev.Lett. (2025).\n4. J. R. Rouxel et al.\, Hard X-ray helical dichroism of disordered molecular media\, Nature Phot. 16\, 570 (2022).\n5. P. Carrara et al.\, Soft x-rays with Orbital Angular Momentum for resonant scattering experiments at the SOLEIL synchrotron\, J. Synchr. Rad. 33\, 858 (2026). \nContact : matteo.dastuto@neel.cnrs.fr
URL:https://sfp-alpes.fr/event/maurizio-sacchi-cnrs-institut-des-nanosciences-de-paris-and-synchrotron-soleil/
LOCATION:CNRS – Salle Louis Weil (E424)\, CNRS - Institut Néel 25 avenue des Martyrs\, Grenoble\, 38042\, France
CATEGORIES:Séminaire
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