Calcium gets myosin VI ready for work.

The myosins are a large family of actin-binding motor proteins that convert stored chemical energy into work, with important functions in intracellular transport, force generation, and mechanosensation (1). Despite many advances in understanding the mechanical and kinetic properties of purified myosins in vitro, the signals that regulate the functions of these molecular motors in cells are not well characterized. In PNAS, Batters et al. (2) identify a key regulatory function for the universal intracellular signal calcium (Ca) in the control of myosin VI activation. Myosin VI is the only myosin that moves toward the minus end of actin filaments and contains many unusual structural features (3). Batters et al. (2) show that for this myosin Ca links motor activation to cargo binding in a stepwise process by controlling the orientation and interactions of its binding protein calmodulin (CaM).Whereas Ca has been shown to exert structural effects on other myosins, Batters et al. (2) unravel another distinct and unusual mechanism used by this most unconventional myosin. All myosin motors contain essentially three domains: a highly conserved N-terminal actin binding head domain, a lever arm, and a tail domain, which may contain coiled-coils that mediate dimerization and/or specialized cargo-binding domains (CBD). The N-terminal head domain is an actin-activated ATPase that undergoes conformational changes coupled to the hydrolysis of ATP (4). The lever arm, typically containing one or several IQmotifs, amplifies these relatively small (angstrom) movements within the head domain into nanometer displacements of the tail. Association of the Ca-binding protein CaM with the IQ motifs confers structural stability to the lever arm, enabling mechanical coupling between the head and tail domains. Variations in ATPase kinetics and lever arm structure and length across the different myosins translate into different molecular functions such as transport or tethering. In cells, the localization of each myosin is determined by their highly divergent cargo-binding tails. Myosin VI has been shown to associate with a large number of different cargo-adaptor proteins through binding to specific sites on its CBD, giving rise to the concept that a single motor may have a multitude of cargoes and functions (5). Despite much progress, there are still many unanswered questions about the cellular regulation of myosin motors. For example, motor activation may result from an unfolding transition (6), cargo-mediated dimerization (7), or recruitment of CaM (8). However, the spatiotemporal regulation of myosins and the coordination of motor activation and cargo attachment are poorly understood. To investigate how Ca might regulate the lever arm of myosin VI, Batters et al. (2) used single-particle electron microscopy to visualize thousands of individual molecules. They found that the orientation of the single CaM on the lever arm of myosin VI was sensitive to Ca. In the absence of Ca the electron density WWY RRL PIP2 LI 1276 1183 1115 1107 1036 1058