Addition of filamin (right) results in a more three-dimensional actin organization.

Thomas Stossel is a fervent promoter of his favorite molecule, filamin. “For 25 years I've being saying that this protein is important for making orthogonal actin networks at the leading edge of the cell,” he says. “However, recent work has focused attention on a similar structural role for the Arp2/3 complex, which can nucleate actin filaments to form branched structures.”

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. ▪