Molecular motors each have their favorite tracks. Kinesin tugs cargo along microtubules for long distance transport, and myosin continues the haul on actin at the cell edges. But recently the group found that myosin also diffuses along microtubules. “At the time,” says Trybus, “we supposed that the diffusion might help myosin hook up with kinesin and its cargo, but the idea wasn't very satisfying.” They now find a more gratifying explanation.
Using an in vitro system, the group showed that myosin's interactions with microtubules enhanced kinesin's processivity. When both motors were hooked to a cargo, kinesin took longer trips made up of several short runs linked by pauses. Dual-motor cargo might exist in vivo, given that large cargos such as melanosomes harbor dozens of motors.
The improved kinesin run lengths stemmed from electrostatic interactions between myosin and microtubules. Adding more positively charged residues to myosin further improved kinesin's performance, whereas removing them blunted its effects. The findings suggest that myosin tethers a detached kinesin near the microtubule while it finds a site to rebind. “Without the tether,” says Trybus, “kinesin and the cargo are more likely to simply diffuse away.”
Kinesin returned the favor by increasing myosin's run lengths, again through electrostatic effects. If charge is the decisive characteristic, any positively charged cargo can lend a hand to its own transport.