HIV-1 Protease and Reverse Transcriptase Control the Architecture of Their Nucleocapsid Partner

The HIV-1 nucleocapsid is formed during protease (PR)-directed viral maturation, and is transformed into pre-integration complexes following reverse transcription in the cytoplasm of the infected cell. Here, we report a detailed transmission electron microscopy analysis of the impact of HIV-1 PR and reverse transcriptase (RT) on nucleocapsid plasticity, using in vitro reconstitutions. After binding to nucleic acids, NCp15, a proteolytic intermediate of nucleocapsid protein (NC), was processed at its C-terminus by PR, yielding premature NC (NCp9) followed by mature NC (NCp7), through the consecutive removal of p6 and p1. This allowed NC co-aggregation with its single-stranded nucleic-acid substrate. Examination of these co-aggregates for the ability of RT to catalyse reverse transcription showed an effective synthesis of double-stranded DNA that, remarkably, escaped from the aggregates more efficiently with NCp7 than with NCp9. These data offer a compelling explanation for results from previous virological studies that focused on i) Gag processing leading to nucleocapsid condensation, and ii) the disappearance of NCp7 from the HIV-1 pre-integration complexes. We propose that HIV-1 PR and RT, by controlling the nucleocapsid architecture during the steps of condensation and dismantling, engage in a successive nucleoprotein-remodelling process that spatiotemporally coordinates the pre-integration steps of HIV-1. Finally we suggest that nucleoprotein remodelling mechanisms are common features developed by mobile genetic elements to ensure successful replication

Taylor instabilities in colloidal crystals

Recently we have developed a Couette viscosimeter for studying the shearing of colloidal crystals. We report here that with increasing shear rate the colloidal crystal melts (for ωcrit) and above a critical shear rate, ωT0, Taylor instabilities develop. After stopping the shearing flow the suspension recrystallizes in a special texture which preserves the pattern of the Taylor rolls. This texture determines the configuration of Taylor instabilities in subsequent flow experiments.Nous avons développé récemment un viscosimètre de Couette pour l'étude des cristaux colloidaux sous cisaillement. Nous avons observé que, dans un premier temps, lors de l'accroissement du cisaillement, le cristal fond, puisque au-dessus d'un seuil critique se développent les instabilités de Taylor. Après l'arrêt de l'écoulement, la suspension recristallise en gardant mémoire des rouleaux de Taylor et ceci détermine la configuration de l'instabilité de Taylor lors de l'expérience suivante

Water-dispersed lamellar phases of symmetric poly(styrene)-block-poly(acrylic acid) diblock copolymers: Model systems for flat dense polyelectrolyte brushes

We investigate the static properties of a water-dispersed lamellar (L) phase formed in the melt state with a nearly symmetric poly(styrene)-block-poly(acrylic acid) (PS-b-PAA) diblock copolymer. The PAA brush is considered as a model flat polyelectrolyte (PE) brush of controlled surface density. Thanks to small-angle X-ray scattering, its behavior in water is studied as a function of (i) its ionization, through the pH of the dispersions which is increased by an addition of a known amount of a base, i.e. sodium hydroxyde NaOH, and (ii) in the presence of a monovalent salt, i.e. sodium chloride NaCl, of concentration C S . At low pH, we find that the brush effectively behaves as a neutral brush. At high pH, the brush is in the so-called “osmotic regime”, in which all sodium counterions are trapped within the brush volume and stretch the chains via an osmotic effect. The properties of such a brush in the presence of a monovalent salt, confirm this result, showing a C S -1/3 dependence in the brush height L O , in agreement with mean-field predictions. The L O -C S profiles at different ionizations give access to the actual brush internal charge fraction f. The results are found to be in very good quantitative agreement with experimental measures found in the literature, and can be completely and quantitatively described by Oosawa’s approach to counterion condensation in a semi-dilute to concentrated solution of charged, rod-like chains

Pearling instabilities in water-dispersed copolymer cylinders with charged brushes

We investigate the structural behavior of a poly(styrene)-block-poly(acrylic acid) diblock copolymer which forms hexagonally-packed PS cylinders (C-phase) in the melt state. The water dispersion of this structure provides hairy cylinders which comprise a PAA swollen cylindrical brush with a height h tunable via its degree of ionization and the ionic strength in the solution, and a water-free, PS cylindrical core of constant radius R C. Such system constitutes an “out-of-equilibrium” frustrated model system: the selective swelling of the PAA brush results in a frustration of the interface curvature, which the ratio h/R C allows to quantify. Upon heating at a temperature higher than the glass transition temperature of the PS core, the glassiness of the core is relieved and the mechanical constraints arising from the selective swelling of the structure can be relaxed: the cylinders undergo a cylinder-to-sphere transition upon annealing at high temperature, when above a frustration threshold h/R C ≈ 1.8. Thanks to a careful mapping of the transition diagram, an undulating cylindrical morphology (UC) is identified between unchanged cylinders ( h/R C $\leqslant$ 1.8) and spheres ( h/R C ⩾ 2.0), which appears to result from a Rayleigh-like pearling instability of the copolymer cylinders

Processing of temperature field in chemical microreactors with infrared thermography

This work is devoted to the first analysis of temperature fields related to chemical microfluidic reactors. The heat transport around and inside a microchannel is both convective and diffusive with spatial distribution of source terms and strong conductive effects in the channel surrounding. With simplified assumptions, it is shown that Infrared thermography and processing methods of the temperature frames allow to estimate important fields for the chemical engineers, such as the heating source distribution of the chemical reaction along the channel. A validation experiment of a temperature field processing method is proposed with Joule effect as calibrated source term and non reactive fluids. From such previous experiment, a Peclet field is estimated and used in a further step in order to study an acid-base flow configuratio

Shear waves in colloidal crystals : II. Effects of finite height in cylindrical samples

The vibration spectra of a cylindrical sample of colloidal crystals are studied We indicate the proper choice of the aspect ratio α = R/H which optimizes the quality of resonances. A new experimental set-up allows us to compare the theory with the experimental results and to deduce a precise value of the elastic modulus and of the viscosity for several latex suspensions.On étudie les vibrations de symétrie axiale, excitées par des moyens mécaniques, dans un échantillon de cristal colloïdal ayant la forme d'un cylindre de rayon R et de hauteur H. On démontre que, à volume constant, le rapport α = R/H = 0,5 minimise les fréquences des modes propres et optimise la précision des mesures du module élastique. Un nouveau montage expérimental nous a permis de comparer la théorie à l'expérience et d'en déduire le module élastique et la viscosité moyenne de plusieurs échantillons de cristal colloidal