Myosin-V makes a random twist as it traverses hand over hand along its actin filament, according to a study by Yasunori Komori, Atsuko Iwane, and Toshio Yanagida (Osaka University, Osaka, Japan).
The transport molecule myosin-V, which bears two actin-binding heads each linked by an arm to a central stalk, carries cargo along the cytoskeletal network. The hydrolysis of ATP drives the lagging head off of actin, but how that head swings around the leading head to rejoin the actin was unknown.
In the new experiments, fluorescently labeled actin filaments bound to an immobilized myosin-V were seen to twist randomly clockwise or counterclockwise during each clamp-release-reclamp cycle. The finding indicates that in the cell, where actin is fixed and myosin is free to twist, the trailing myosin head can swing in either direction as it searches out its next forward binding site.
Yanagida thinks the ability to twist in either direction gives the motor better mobility. “The cell is very crowded by the many kinds of proteins within,” he says. “It's probably not easy to transport cargo along the complex actin meshwork.” In addition, unlike a twist in a fixed direction, a random twist allows multiple myosin-V molecules on a single cargo to avoid twirling each other around as they move.