Formin' new ideas about actin filament generation
- Department of Biomedical Engineering, University of Rochester, 601 Elmwood Avenue, P.O. Box 639, Rochester, NY 14642
While cellular conditions strongly favor the growth of actin filaments, growth occurs only when actin monomers have access to a filament's fast-growing barbed end. This is the key to understanding how the only known de novo generators of actin filaments in cells, the Arp2/3 complex and members of the formin family, can give rise to such structurally different networks (Fig. 1). The Arp2/3 complex binds to the sides of an actin filament where it nucleates a daughter filament with a free barbed end at an acute angle to the mother (1). The new filament grows for only a short period before being blocked by barbed-end capping proteins, and so networks generated by the actions of many Arp2/3 complexes consist of short filaments arranged in a dendritic brush-like structure. By contrast, some formins generate filaments that can grow at their barbed ends while protected from the activity of capping proteins (2). Assemblies of these formins create elongated parallel bundles of filaments like those comprising the actin cables of budding yeast (2).
Actin network geometry depends on how long the barbed end is free during filament nucleation. Starting with the same building blocks, the formins and Arp2/3 complex will create very different networks because of their opposite treatment of barbed ends. Arp2/3 complex (blue) nucleates filaments with free barbed ends from the sides of preexisting filaments. Because these filaments are quickly blocked by capping proteins (black), the resulting network is composed of short filaments arranged in a brush-like structure. Such networks are seen, for example, …
