Making tropomodulin find the right point Actin filaments are a key component of the cell's cytoskeleton. Actin filaments have a characteristic polarized architecture with a so-called barbed end and a pointed end—so described because of how the filaments look under the electron microscope when they are coated with tropomyosin. Rao et al. describe crystal structures of a protein that uniquely binds and caps the pointed end of actin filaments. This protein, tropomodulin, binds to several actin subunits and to a couple of tropomyosin molecules, which generates a high-affinity specific cap to the filaments by combining low-affinity interactions. Science , this issue p. 463