Shape Changes of Self-Assembled Actin Bilayer Composite Membranes

We report the self-assembly of thin actin shells beneath the membranes of giant vesicles. Ion-carrier mediated influx of Mg2+ induces actin polymerization in the initially spherical vesicles. Buckling of the vesicles and the formation of blisters after thermally induced bilayer expansion is demonstrated. Bilayer flickering is dominated by tension generated by its coupling to the actin cortex. Quantitative flicker analysis suggests the bilayer and the actin cortex are separated by 0.4 \mum to 0.5 \mum due to undulation forces.Comment: pdf-file, has been accepted by PR

CPT\cal CPT-invariant two-fermion Dirac equation with extended hyperfine operator

For the S-states of muonium and positronium, the hyperfine shifts to the order $\alpha^6$ of a recently derived two-fermion equation with explicit $\cal CPT$-invariance are checked against the results of a nonrelativistic reduction, and the leading $\alpha^8$ shifts are calculated. An additional hyperfine operator is discovered which can milden the singularity for $r\to 0$ of the Dirac hyperfine operator, such that the resulting extended operator can be used nonperturbatively. The binding correction to magnetic moments is mentioned

Eight-component two-fermion equations

An eight-component formalism is proposed for the relativistic two-fermion problem. In QED, it extends the applicability of the Dirac equation with hyperfine interaction to the positronium case. The use of exact relativistic two-body kinematics entails a CP-invariant spectrum which is symmetric in the total cms energy. It allows the extension of recent \alpha^6 recoil corrections to the positronium case, and implies new recoil corrections to the fine and hyperfine structures and to the Bethe logarithm.Comment: Revtex, accepted for publication in Phys. Rev.

Effects of Fully and Partially Solubilized Amphiphiles on Bilayer Bending Stiffness and Temperature Dependence of the Effective Tension of Giant Vesicles

We report the modification of the bending elastic modulus $k_{\rm c}$ of lipid bilayers (here DMPC) by small amounts ($c \leq 5$ mol %) of (i) small amphiphiles which exchange between the bilayer and the aqueous phase (e.g. the ion carrier valinomycin and the Ca$^{++}$ carrier A23187) and (ii) amphiphiles solubilized in the membrane (cholanic acid). Large reductions of the bending stiffness may be induced by a few percent of the solutes, e.g. 1 mol % of valinomycin reduce $k_{\rm c}$ by a factor of two. The effect is rationalised in terms of local thinning of the bilayer. The strong effect of solutes on $k_{\rm c}$ contrasts with its weak dependence on the lipid structure since the C18:0/C18:1-lipid stearoyl-oleoyl-phosphatidyl-choline (SOPC) exhibits only a 15% higher value of $k_{\rm c}$ than DMPC. The effect of temperature on the flicker behaviour was analysed in order to establish correlations between the effective tension and the excess area of the quasi-spherical vesicles. The temperature dependence of the bilayer excess area for a DMPC vesicle leads to the thermal expansion coefficient, $\beta$, for which a value of $\beta = 10.4 \times 10^{-3}$ K$^{-1}$ is obtained. A much stronger tendency for budding on $\mu$m-scale (micro budding) during thermal area expansion of POPC and SOPC compared to DMPC was observed.

Energy filtered electron tomography of ice-embedded actin and vesicles.

Semiautomatic single-axis tilt electron tomography has been used to visualize the three-dimensional organization of actin filaments in "phantom cells," i.e. lipid vesicles. The instrumentation consisted of a 120-kV electron microscope equipped with a postcolumn energy filter, which was used in the zero-loss imaging mode. Apart from changing the tilt angle, all steps required for automated tomography, such as recentering the image area, refocusing, and centering the energy-selecting slit, were performed by external computer control. This setup permitted imaging of ice-embedded samples up to a thickness of 800 nm with improved image contrast compared with that produced by tomography with a conventional electron microscope. In spite of the missing-wedge effect that is especially obvious in the study of membrane-filament interaction, single-axis tilt tomography was found to be an appropriate (in fact the only available) method for this kind of investigation. In contrast to random actin networks found in actin gels, actin filaments in and on vesicles with a bending radius of less than approximately 2 microns tend to be arranged in single layers of parallel filaments and often induce an elongated shape of the vesicles. Actin filaments located on the outside usually associate with the vesicle membrane