IM domains (green) push out filopodia by binding and deforming the membrane, not by bundling actin (red).
These filopodia-inducing IM domains are found in cytoskeletal scaffolding proteins such as missing-in-metastasis and IRSp53. Sites of membrane deformation, including filopodial protrusions, are often associated with bundles of actin filaments. Previous results suggested that the IM domain contributes to filopodium formation by bundling actin, but Mattila and colleagues now dispute this result.
The authors find that the IM domain only bundles actin at unnaturally low ionic strengths, at which point the domain tends to form aggregates. The IM domain also did not colocalize with actin bundles in cells, but instead was observed at the plasma membrane surrounding the bundle.
The membrane association...
