Actin as stairmaster

Simultaneously holding onto and building something is no mean feat. Yingwu Xu, Michael Eck (Dana-Farber Cancer Institute and Harvard Medical School, Boston, MA), and colleagues find that the formin homology-2 (FH2) domain is ideally suited to the task. The apparently flexible structure of an FH2 dimer is consistent with a stair-stepping model of actin filament growth in which each half of the dimer alternately dissociates to make room for an incoming actin monomer even as the other half of the dimer maintains its grip on the actin filament.

The model did not come to Eck immediately. “We stared at the structure for about a year,” he says. But then the group tried to reconcile the perfect twofold symmetry of the FH2 crystal...