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.