Structures of the free and capped ends of the actin filament.

The barbed and pointed ends of the actin filament (F-actin) are the sites of growth/shrinkage and the targets of capping proteins that block subunit exchange, including CapZ at the barbed end and tropomodulin at the pointed end. We describe cryo-electron microscopy structures of the free and capped ends of F-actin. Terminal subunits at the free barbed end adopt a "flat" F-actin conformation. CapZ binds with minor changes to the barbed end but major changes to itself. In contrast, subunits at the free pointed end adopt a "twisted" G-actin conformation. Tropomodulin binding forces the second subunit into an F-actin conformation. The structures reveal how the ends differ from the middle in F-actin and how these differences control subunit addition/dissociation, capping, and interactions with end-binding proteins.