Figure 2.

Motility depends on ActA-His surface density and particle size. (a) For 0.5-μm-diameter polystyrene beads, the probability of comet tail formation strongly depended on ActA-His surface density. Initiation of movement was most efficient when ≈37.5% of available sites were occupied by ActA-His; higher and lower densities inhibited tail formation. Error bars are ±SD. Between 380 and 630 beads were scored at each surface density. (b) For moving 0.5-μm-diameter polystyrene beads, average velocity was independent of ActA-His surface density. Error bars are ±SD; n = 8–16 beads for each point. Differences between points are not statistically significant by Student’s t test. Average velocity of L. monocytogenes in this extract was 0.126 ± 0.035 μm/sec; n = 23. (c) For nonmoving 0.5-μm-diameter polystyrene beads, the intensity and variability of the actin clouds depended on ActA-His surface density. Error bars are ±SD; n = 57–125 for each concentration. Note that SD is large for low ActA surface densities and decreases substantially at higher densities. (d) At optimal ActA-His surface density (37.5% of available sites), the probability of comet tail formation strongly depended on bead diameter. Beads with diameters of 0.7 μm or greater were never observed to form tails whereas most beads of 0.2-μm-diameter formed tails. Error bars are ±SD; n = 100–600 per point. All results (a–d) were taken at a 1-hr time-point of incubation at room temperature.