EDITOR'S CHOICE IN DEVELOPMENTAL BIOLOGY When a cell hits an
obstacle, the actin filaments driving the membrane protrusion
must reorganize and create additional branches to resist the
pressure. Dan Fletcher at the University of California,
Berkeley, and colleagues wanted to understand what effect that
force has on the branching of actin filaments. They first glued
unbranched actin filaments to a surface, some curved, some
straight, and then added the raw materials necessary for the
branching: the branch-initiating complex Arp2/3, the nucleation-
promoting factor that activates it, and raw actin monomers,
which polymerize into two tightly wound strands under the right
conditions. The researchers found that new actin branches were
more likely to form on the convex side, or outer side, of curved
segments. The questions were why and how the branching was
occurring mostly on that side, says Fletcher. He recalled early
studies on actin in which researchers noticed that isolated
filaments would wiggle and squirm with