The cargo-binding domain regulates structure and activity of myosin 5

Abstract

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. Myosin 5 is a two-headed motor protein that moves cargoes along actin filaments1,2. Its tail ends in paired globular tail domains (GTDs) thought to bind cargo3. At nanomolar calcium levels, actin-activated ATPase is low and the molecule is folded. Micromolar calcium concentrations activate ATPase and the molecule unfolds3,4,5,6. Here we describe the structure of folded myosin and the GTD's role in regulating activity. Electron microscopy shows that the two heads lie either side of the tail, contacting the GTDs at a lobe of the motor domain (∼Pro 117–Pro 137) that contains conserved acidic side chains, suggesting ionic interactions between motor domain and GTD. Myosin 5 heavy meromyosin, a constitutively active fragment lacking the GTDs, is inhibited and folded by a dimeric GST–GTD fusion protein. Motility assays reveal that at nanomolar calcium levels heavy meromyosin moves robustly on actin filaments whereas few myosins bind or move. These results combine to show that with no cargo, the GTDs bind in an intramolecular manner to the motor domains, producing an inhibited and compact structure that binds weakly to actin and allows the molecule to recycle towards new cargoes.