Stossel claims that branching is not sufficient for the formation of a strong actin network capable of pushing out the front of a migrating cell. A highly branched structure can still give way, like a bush that cannot support any significant weight. The cell needs to cross-link the branches together so that they no longer bend under pressure. This, says Stossel, is where filamin comes into the picture.
On page 511, Stossel and colleagues take a closer look at actin filament structure in cells lacking filamin. These cells cannot migrate, and the authors find that they have a dense mat of actin filaments that are almost parallel to each other. The addition of filamin to these cells results in a more open, delicate, and three-dimensional actin network. By immunogold microscopy, many junctions between actin filaments contain filamin, some have both filamin and Arp2/3, and a few have only Arp2/3.With filamin back in the spotlight, Stossel is hoping that he can determine how filamin cross-linking and Arp2/3 nucleation might be coordinated. ▪