Specific anchoring modes of two distinct dystrophin rod sub-domains interacting in phospholipid Langmuir films studied by atomic force microscopy and PM-IRRAS.

International audienceDystrophin rod repeats 1-3 sub-domain binds to acidic phosphatidylserine in a small vesicle binding assay, while the repeats 20-24 sub-domain does not. In the present work, we studied the adsorption behaviour of both sub-domains at the air/liquid interface and at the air/lipid interface in a Langmuir trough in order to highlight differences in interfacial properties. The adsorption behaviour of the two proteins at the air/liquid interface shows that they display surface activity while maintaining their alpha-helical secondary structure as shown by PM-IRRAS. Strikingly, R20-24 needs to be highly hydrated even at the interface, while this is not the case for R1-3, indicating that the surface activity is dramatically higher for R1-3 than R20-24. Surface-pressure measurements, atomic force microscopy and PM-IRRAS are used in a Langmuir experiment with DOPC-DOPS monolayers at two different surface pressures, 20mN/m and 30mN/m. At the lower surface pressure, the proteins are adsorbed at the lipid film interface while maintaining its alpha-helical structure. After an increase of the surface pressure, R1-3 subsequently produces a stable film, while R20-24 induces a reorganization of the lipid film with a subsequent decrease of the surface pressure close to the initial value. AFM and PM-IRRAS show that R1-3 is present in high amounts at the interface, being arranged in clusters representing 3.3% of the surface at low pressure. By contrast, R20-24 is present at the interface in small amounts bound only by a few electrostatic residues to the lipid film while the major part of the molecule remains floating in the sub-phase. Then for R1-3, the electrostatic interaction between the proteins and the film is enhanced by hydrophobic interactions. At higher surface pressure, the number of protein clusters increases and becomes closer in both cases implying the electrostatic character of the binding. These results indicate that even if the repeats exhibit large structural similarities, their interfacial properties are highly contrasted by their differential anchor mode in the membrane. Our work provides strong support for distinct physiological roles for the spectrin-like repeats and may partly explain the effects of therapeutic replacement of dystrophin deficiency by minidystrophins

Hydrodynamic modeling of accretion shocks on a star with radiative transport and a chromospheric model

International audienceAccretion flows on the surface of a star is modeled using a high resolution hydrodynamic 1D ALE code (ASTROLABE) coupled to radiative transfer and line cooling, along with a model for the acoustic heating of the chromospheric plasma

Hydrodynamic modeling of accretion shocks on a star with radiative transport and a chromospheric model

International audienceThe aim of the project (ANR STARSHOCK) is to understand the dynamics and the radiative properties of accretion columns, linking the circumstellar disk to the surface photosphere of Young Stellar Objects. The hydrodynamics is computed first, using a high resolution hydrodynamic 1D ALE code (AS-TROLABE) coupled to radiative transfer and line cooling, along with a model for the acoustic heating of the chromospheric plasma. Spectra are then post-processed with a 1D radiative transfer code (SYNSPEC), using DFE solver and an extended atomic database covering a wavelength range from X rays to visible

Fluid and condensed ApoA-I/phospholipid monolayers provide insights into ApoA-I membrane insertion.

International audienceApolipoprotein A-I (ApoA-I) is a protein implicated in the solubilization of lipids and cholesterol from cellular membranes. The study of ApoA-I in phospholipid (PL) monolayers brings relevant information about ApoA-I/PL interactions. We investigated the influence of PL charge and acyl chain organization on the interaction with ApoA-I using dipalmitoyl-phosphatidylcholine, dioleoyl-phosphatidylcholine and dipalmitoyl-phosphatidylglycerol monolayers coupled to ellipsometric, surface pressure, atomic force microscopy and infrared (polarization modulation infrared reflection-absorption spectroscopy) measurements. We show that monolayer compressibility is the major factor controlling protein insertion into PL monolayers and show evidence of the requirement of a minimal distance between lipid headgroups for insertion to occur, Moreover, we demonstrate that ApoA-I inserts deepest at the highest compressibility of the protein monolayer and that the presence of an anionic headgroup increases the amount of protein inserted in the PL monolayer and prevents the steric constrains imposed by the spacing of the headgroup. We also defined the geometry of protein clusters into the lipid monolayer by atomic force microscopy and show evidence of the geometry dependence upon the lipid charge and the distance between headgroups. Finally, we show that ApoA-I helices have a specific orientation when associated to form clusters and that this is influenced by the character of PL charges. Taken together, our results suggest that the interaction of ApoA-I with the cellular membrane may be driven by a mechanism that resembles that of antimicrobial peptide/lipid interaction

Accretion shock stability on a dynamically heated YSO atmosphere with radiative transfer

Theory and simulations predict Quasi-Periodic Oscillations of shocks which develop in magnetically driven accretion funnels connecting the stellar disc to the photosphere of Young Stellar Objects (YSO). X-ray observations however do not show evidence of the expected periodicity. We examine here, in a first attempt, the influence of radiative transfer on the evolution of material impinging on a dynamically heated stellar atmosphere, using the 1D ALE-RHD code ASTROLABE. The mechanical shock heating mechanism of the chromosphere only slightly perturbs the flow. We also show that, since the impacting flow, and especially the part which penetrates into the chromosphere, is not treated as a purely radiating transparent medium, a sufficiently efficient coupling between gas and radiation may affect or even suppress the oscillations of the shocked column. This study shows the importance of the description of the radiation effects in the hydrodynamics and of the accuracy of the opacities for an adequate modeling

Difference in lipid packing sensitivity of exchangeable apolipoproteins apoA-I and apoA-II: An important determinant for their distinctive role in lipid metabolism.

International audienceExchangeable apolipoproteins A-I and A-II play distinct roles in reverse cholesterol transport. ApoA-I interacts with phospholipids and cholesterol of the cell membrane to make high density lipoprotein particles whereas apolipoprotein A-II interacts with high density lipoprotein particles to release apolipoprotein A-I. The two proteins show a high activity at the aqueous solution/lipid interface and are characterized by a high content of amphipathic α-helices built upon repetition of the same structural motif. We set out to investigate to what extent the number of α-helix repeats of this structural motif modulates the affinity of the protein for lipids and the sensitivity to lipid packing. To this aim we have compared the insertion of apolipoproteins A-I and A-II in phospholipid monolayers formed on a Langmuir trough in conditions where lipid packing, surface pressure and charge were controlled. We also used atomic force microscopy to obtain high resolution topographic images of the surface at a resolution of several nanometers and performed statistical image analysis to calculate the spatial distribution and geometrical shape of apolipoproteins A-I and A-II clusters. Our data indicate that apolipoprotein A-I is sensitive to packing of zwitterionic lipids but insensitive to the packing of negatively charged lipids. Interestingly, apolipoprotein A-II proved to be insensitive to the packing of zwitterionic lipids. The different sensitivity to lipid packing provides clues as to why apolipoprotein A-II barely forms nascent high density lipoprotein particles while apolipoprotein A-I promotes their formation. We conclude that the different interfacial behaviors of apolipoprotein A-I and apolipoprotein A-II in lipidic monolayers are important determinants of their distinctive roles in lipid metabolism

Dynamical processes in stellar radiation zones: Secular magnetohydrodynamics of rotating stars

info:eu-repo/semantics/publishe

New insight on accretion shocks onto young stellar objects - Chromospheric feedback and radiation transfer

Context. Material accreted onto classical T Tauri stars is expected to form a hot quasi-periodic plasma structure that radiates in X-rays. Simulations of this phenomenon only partly match observations. They all rely on a static model for the chromosphere and on the assumption that radiation and matter are decoupled. Aims. We explore the effects of a shock-heated chromosphere and of the coupling between radiation and hydrodynamics on the structure and dynamics of the accretion flow. Methods. We simulated accretion columns that fall onto a stellar chromosphere using the 1D ALE code AstroLabE. This code solves the hydrodynamics equations along with the first two moment equations for radiation transfer, with the help of a dedicated opacity table for the coupling between matter and radiation. We derive the total electron and ion densities from collisional-radiative model. Results. The chromospheric acoustic heating affects the duration of the cycle and the structure of the heated slab. In addition, the coupling between radiation and hydrodynamics leads to a heating of the accretion flow and of the chromosphere: the whole column is pushed up by the inflating chromosphere over several times the steady chromosphere thickness. These last two conclusions are in agreement with the computed monochromatic intensity. Acoustic heating and radiation coupling affect the amplitude and temporal variations of the net X-ray luminosity, which varies between 30 and 94% of the incoming mechanical energy flux, depending on which model is considered.French ANR StarShock projectFrench National Research Agency (ANR) [ANR-08-BLAN-0263-07, ANR-11-IDEX-0004-02]; French ANR LabEx Plas@Par projectFrench National Research Agency (ANR) [ANR-08-BLAN-0263-07, ANR-11-IDEX-0004-02]; PICS [6838]; Programme National de Physique Stellaire of CNRS/INSU; Observatoire de ParisOpen access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]