Each of the proteins shown has two actin-binding sites ( red) that are related in sequence. Fimbrin has two directly adjacent actin-binding sites, so that it holds its two actin filaments very close together (14 nm apart), aligned with the same polarity (see Figure 16-66). The two actin-binding sites in α-actinin are more widely separated and are linked by a somewhat flexible spacer 30 nm long, so that it forms actin filament bundles with a greater separation between the filaments (40 nm apart) than does fimbrin. Filamin has two actin-binding sites that are very widely spaced, with a V-shaped linkage between them, so that it cross-links actin filaments into a network with the filaments oriented almost at right angles to one another (see Figure 16-67). Spectrin is a tetramer of two α and two β subunits, and the tetramer has two actin-binding sites spaced about 200 nm apart. The spacer regions of these various proteins are built in a modular fashion from repeating units that include α-helical motifs ( light green), β-sheet motifs ( dark green), and Ca²⁺-binding domains ( blue ovals).