Formin-1 entered the story as a binding partner of α-catenin, a component of cadherin adhesion complexes. Cells lacking α-catenin fail to form actin cables at adherens junctions, and the Rockefeller group found that the same was true when the formin-1–α-catenin interaction was disrupted in vivo. In vitro, formin-1 was shown to polymerize actin into linear filaments. Finally, fusion of a β-catenin-binding domain to the actin-polymerization domains of formin-1 restored adhesion ability to cells lacking α-catenin.
Actin polymerization is important in two steps of adhesion. First, branched polymerization of actin by Arp2/3 pushes out both filopodia and broad areas of membrane as lamellipodia. Many of the resultant contacts are not productive, and the processes retract. But any surviving contact prompts the formation of an actin cable, which stabilizes the contact. It also pushes on a specific area of membrane so that more adherens junctions form nearby, thus zippering cells together. Just how formin-1 is regulated during this process remains to be determined. ▪