Myosin Va doesn't stand by passively waiting for cargo. New results from M. Yusuf Ali, David Warshaw (University of Vermont, Burlington, VT), and colleagues suggest it may pace along microtubules—not its usual actin track—in search of kinesin-tugged cargo.
Kinesins take cargo toward the cell periphery along microtubules, but the trip is finished by myosin on actin filaments. Myosin and cargo seem to meet up later than kinesin and cargo, but how that later step is accomplished is unclear. Now, myosin's response to microtubules offers a hint.
While tracking single myosin Va motors in an in vitro cytoskeletal jungle, the group saw that myosin “sometimes hopped onto microtubules and started a [back-and-forth] diffusive search, like a person pacing at a bus stop,” says Warshaw. As myosin was thought to be solely actin based, Warshaw says, “no one in their right mind would have guessed that it would interact the way it did.”
Pacing on microtubules was up to six times faster than myosin's travels along actin. “Since it scans over long distances and quite rapidly,” says Warshaw, “[pacing] might increase the chance of encountering a kinesin and cargo.” Yet because thermal energy drives the movement, no ATP is expended.
Myosin might also wait at actin–microtubule intersections, but Warshaw says, “we would have to devise a complex targeting mechanism to make sure that the myosin is there and ready.”