Structural basis of actin filament capping at the barbed-end:: a cryo-electron microscopy study

The intracellular distribution and migration of many protein complexes and organelles is regulated by the dynamics of the actin filament. Many actin filament end-binding proteins play crucial roles in actin dynamics, since polymerization and depolymerization of actin protomers occur only at the filament ends. We present here an EM structure of the complex of the actin filament and hetero-dimeric capping protein (CP) bound to the barbed-end at 23 angstrom resolution, by applying a newly developed methods of image analysis to cryo-electron micrographs. This structure was fitted by the crystal structure of CP and the proposed actin filament structure, allowing us to construct a model that depicts two major binding regions between CP and the barbed-end. This binding scheme accounted for the results of newly performed and previously published mutation experiments, and led us to propose a two-step binding model. This is the first determination of an actin filament end structure.