(A) α-actinin, which is a homodimer, cross-links actin filaments into loose bundles, which allow the motor protein myosin-II (not shown) to participate in the assembly. Fimbrin cross-links actin filaments into tight bundles, which exclude this motor protein. Fimbrin and α-actinin tend to exclude each other because of the very different spacing of the actin filament bundles that they form. (B) Electron micrograph of purified α-actinin molecules. (B, courtesy of John Heuser.)