Structure of the α-Actinin–Vinculin Head Domain Complex Determined by Cryo-electron Microscopy

The vinculin binding site on α-actinin was determined by cryo-electron microscopy of 2D arrays formed on phospholipid monolayers doped with a nickel chelating lipid. Chicken smooth muscle α-actinin was cocrystallized with the β1-integrin cytoplasmic domain and a vinculin fragment containing residues 1–258 (vinculinD1). VinculinD1 was located at a single site on α-actinin with 60–70% occupancy. In these arrays, α-actinin lacks molecular 2-fold symmetry and the two ends of the molecule, which contain the calmodulin-like and actin binding domains, are held in distinctly different environments. The vinculinD1 difference density has a shape very suggestive of the atomic structure. The atomic model of the complex juxtaposes the α-actinin binding site on vinculinD1 with the N-terminal lobe of the calmodulin-like domain on α-actinin. The results show that the interaction between two species with weak affinity can be visualized in a membrane-like environment.