Actin comets, which drive bead or bacteria movement, are thought to depend strongly on Arp2/3–built, branched actin networks. Now, Plastino et al. show that another actin polymerizer, VASP, builds comet tails that are less dense overall and hollow in the center, but nevertheless led to bead speeds that were seven times that of Arp2/3–built tails. “It's not about having as much polymer as possible,” says Plastino. “It's how the geometry of filaments affects movement.”
VASP is thought to weaken interactions between actin filaments and the membrane or bead. The first filaments to be detached would be those at the center back of the bead, as these bead-filament attachments produce the strongest pulling force on the moving bead. This hollowing out of the comet reduces friction between the bead and the comet and speeds the bead on its way. This model also supports the idea that actin squeezes the sides of beads to move them forward rather than pushing beads from behind.
VASP-built tails were aligned in the direction of movement, not angled like the branched Arp2/3 networks. The aligned arrangement resembles that of actin in filopodia, where VASP is prevalent. One speculation proposes that the inside surface of membrane invaginations at the leading edge of fibroblasts may be somewhat rounded like beads and similarly squeezed forward by actin filaments. Others have shown that VASP slows overall cell movement but quickens the protrusion of small membrane fluctuations. These bursts of speed are perfect for the exploratory nature of filopodia.