subunits. According to one model, gelsolin binds the side of an actin filament until a thermal fluctuation creates a small gap between neighboring subunits, at which point gelsolin inserts itself into the gap to break the filament. After the severing event, gelsolin remains attached to the actin filament and caps the new plus end.
Another important actin-filament destabilizing protein, found in all eukaryotic cells, is cofilin. Also called actin depolymerizing factor, cofilin binds along the length of the actin filament, forcing the filament to twist a little more tightly (Figure 16–20). This mechanical stress weakens the contacts between actin subunits in the filament, making the filament brittle and more easily severed by thermal motions, generating filament ends that undergo rapid disassembly. As a result, most of the actin filaments inside cells are shorter lived than are filaments formed from pure actin in a test tube.
Cofilin binds preferentially to ADP-containing actin filaments rather than to ATP-containing filaments. Since ATP hydrolysis is usually slower than filament assembly, the newest actin filaments in the cell still contain mostly ATP and are resistant to depolymerization by cofilin. Cofilin therefore tends to dismantle the older filaments in the cell. As we will discuss later, the cofilin-mediated disassembly of old but not new actin filaments is critical for the polarized, directed growth of the actin network that is responsible for unidirectional cell crawling and the intracellular motility of pathogens. Actin filaments can be protected from cofilin by tropomyosin binding. Thus, the dynamics of actin in different subcellular locations depends on the balance of stabilizing and destabilizing accessory proteins.
Figure 16–20 Twisting of an actin filament induced by cofilin. (A) Three-dimensional reconstruction from cryoelectron micrographs of filaments made of pure actin. The bracket shows the span of two twists of the actin helix. (B) Reconstruction of an actin filament coated with cofilin, which binds in a 1:1 stoichiometry to actin subunits all along the filament. Cofilin is a small protein  (14 kD) compared to actin (43 kD), and so the filament appears only slightly thicker. The energy of cofilin binding serves to deform the actin filament, twisting it more tightly and reducing the distance spanned by each twist of the helix. (From A. McGough et al., J. Cell Biol. 138:771–781, 1997. With permission from the authors.)
Actin filaments in animal cells are organized into several types of arrays: dendritic networks, bundles, and weblike (gel-like) networks (Figure 16–21). Different structures are initiated by the action of distinct nucleating proteins: the actin filaments of dendritic networks are nucleated by the Arp 2/3 complex, while bundles are made of the long, straight filaments produced by formins. The proteins nucleating the filaments in the gel-like networks are not yet well defined.
The structural organization of different actin networks depends on specialized accessory proteins. As explained earlier, Arp 2/3 organizes filaments into dendritic networks by attaching filament minus ends to the side of other filaments. Other actin filament structures are assembled and maintained by two classes of proteins: bundling proteins, which cross-link actin filaments into a parallel array, and gel-forming proteins, which hold two actin filaments together at a large angle to each other, thereby creating a looser meshwork. Both bundling and gel-forming proteins generally have two similar actin-filament-binding sites, which can either be part of a single polypeptide chain or contributed by each of two polypeptide chains held together in a dimer (Figure 16–22). The spacing and arrangement of these two filament-binding domains determine the type of actin structure that a given cross-linking protein forms.