Without Arp2/3, fascin builds a hollow comet tail on a bacterium.

Actin elongation is a force to be reckoned with. Results on page 233 by Brieher et al. show that elongating bundles of actin can, by themselves, propel a bacterium, and do so faster than frequent actin nucleation does.

Actin filament nucleation by Arp2/3 is a prerequisite for movement of the intracellular pathogen Listeria monocytogenes. Through repeated nucleations, Arp2/3 builds a comet tail of short branched actin filaments that push the bug from behind. But Brieher and colleagues show that once some nucleation occurs, comet tails can be built without Arp2/3.

Upon Arp2/3 inhibition, the branched filaments elongated into long barrel-shaped tails encircling the bacterium. Bugs building these structures were faster than those using Arp2/3. As the barrels are not attached at the bug's rear, it is not clear how movement is achieved. Fractionation experiments showed that the actin-bundling protein fascin is required, although other bundling proteins also worked. The barrels might work like railroad tracks, with the bundling proteins preventing buckling of the filaments. No motor for these tracks has been found at the actin–bacterium interface however.

If there is no motor, motion may rely on diffusion followed by capture by unbundled filaments. If the more posterior, bundled actin is unable to bind to the bug's surface, net forward movement might occur. At least one other cytoplasmic factor was required in addition to fascin for Arpless motility. Its identification should supply clues regarding the force-generating mechanism.

In infected cells, the bug probably uses both polymerization methods, as both barrels and branched filaments were seen by EM. The combination might be needed to raise enough force to push past the host plasma membrane as the pathogen infects other cells. In other systems, such as fibroblasts, a switch from Arp2/3- to elongation-based assembly might initiate the transition from lamellipodia, which are rich in branched filaments, to filopodia, which contain fascin and parallel actin bundles. ▪