The picture represents a serial image sequence, captured every thirty seconds of .5-micron bead coated with the ActA protein of Listeria monocytogenes. The bead has been superimposed over a photo of the fluorescent actin "comet tail" which propels the bead forward, just as it would propel Listeria. To see this movie (140k), click here.  More can be found by clicking here (1 MB).

This motility is highly dependent on the surface density of ActA on the beads; however, average velocity is not. Larger beads are less likely to break their symmetry and start to move than smaller ones, but by coating the beads asymmetrically with a low-angle shadowing technique developed in this lab, larger particles can move. Breaking of symmetry from a uniformly-coated sphere to a moving bead with a comet tail has been studied and modeled, revealing heretofore unpredicted cooperative properties of actin-based motility.