When one assembles actin filaments in a test tube, they form a tangled network. In cells, however, actin filaments are found in a variety of distinct structures, such as the highly ordered filament bundles in microvilli or the network characteristic of the leading edge of a motile cell (see Figure 17-4a). These different structures are determined by the filament assembly mechanisms discussed above and by the presence of actin cross-linking proteins. To be able to organize actin filaments, an actin cross-linking protein must have two F-actin–binding sites (Figure 17-20a).

Cross-linking of F-actin can be achieved by having two actin-binding sites within a single polypeptide, as with fimbrin, a protein found in microvilli that builds bundles of filaments all having the same polarity (Figure 17-20b). Other actin cross-linking proteins have a single actin-binding site in a polypeptide chain, and two chains associate to form a dimer that brings together two actin-binding sites. These dimeric cross-linking proteins can assemble to generate a rigid rod connecting the two binding sites, as happens with α-actinin. Like fimbrin, α-actinin bundles parallel actin filaments, but keeps them farther apart than fimbrin. Another protein, called spectrin, is a tetramer with two actin-binding sites; spectrin spans an even greater distance between actin filaments and makes networks under the plasma membrane (shown in Figure 17-21 and discussed in the next section). Other types of cross-linking proteins, such as filamin, have a highly flexible region between the two actin binding sites that functions like a molecular leaf spring, so they can make stabilizing cross-links between filaments in a network (Figure 17-20c) such as that found in the leading edge of a motile cell. The Arp2/3 complex, which we discussed in terms of its ability to nucleate actin filament assembly, is also an important cross-linking protein, attaching the (−) end of one filament to the side of another filament (see Figure 17-15).