Three-dimensional structure of the myosin V inhibited state by cryoelectron tomography

Myosin V: special delivery There is growing interest in the mechanisms that cells use to deliver specific components to correct sites. Myosin motor proteins perform many of these transport roles. Now Liu et al . have determined the three-dimensional structure of an inhibited state of myosin V: the structure suggests a novel mechanism for solving the problem of returning a molecular motor from its destination to its starting position. When myosin V has no cargo it has a compact structure that binds to rapidly treadmilling actin filaments. In a separate paper, Thirumurugan et al . show that, in the absence of cargo, the cargo-binding domain of myosin V binds to a specific target on its own motor domain to inhibit its own movement along the actin track and weaken its binding to actin. These two papers reveal the elegant method used by cells to keep cargo transport under control. Cryoelectron tomography of two-dimensional arrays of myosin V reveals that it downregulates its activity by folding, positioning the cargo binding domain on the motor domain's active site. Unconventional myosin V (myoV) is an actin-based molecular motor that has a key function in organelle and mRNA transport, as well as in membrane trafficking 1 . MyoV was the first member of the myosin superfamily shown to be processive, meaning that a single motor protein can ‘walk’ hand-over-hand along an actin filament for many steps before detaching 2 , 3 , 4 . Full-length myoV has a low actin-activated MgATPase activity at low [Ca 2+ ], whereas expressed constructs lacking the cargo-binding domain have a high activity regardless of [Ca 2+ ] (refs 5–7 ). Hydrodynamic data and electron micrographs indicate that the active state is extended, whereas the inactive state is compact 8 , 9 , 10 . Here we show the first three-dimensional structure of the myoV inactive state. Each myoV molecule consists of two heads that contain an amino-terminal motor domain followed by a lever arm that binds six calmodulins. The heads are followed by a coiled-coil dimerization domain (S2) and a carboxy-terminal globular cargo-binding domain. In the inactive structure, bending of myoV at the head–S2 junction places the cargo-binding domain near the motor domain's ATP-binding pocket, indicating that ATPase inhibition might occur through decreased rates of nucleotide exchange. The actin-binding interfaces are unobstructed, and the lever arm is oriented in a position typical of strong actin-binding states. This structure in