Dunn et al. show. The finding explains how the molecules, which previous work suggested couldn't move forward, work as haulers.
When a yeast cell sprouts a bud, two myosin proteins help furnish the new structure with necessities. Myo2p trucks in organelles and vesicles essential for growth, whereas Myo4p hauls mRNA that helps the bud differentiate from the mother cell. Running on tracks of actin, the proteins seem to keep their cargos moving continuously. That presents a mystery, however, because evidence suggests that the individual myosins are nonprocessive—they let go of the tracks after every power stroke, instead of remaining attached and sliding along.
To resolve that apparent contradiction, Dunn et al. isolated the two myosins and determined that they move differently. Molecules of Myo2p didn't always detach after the power stroke, the researchers found. They were “weakly processive.” Several Myo2p molecules latched onto each cargo. At any time, most of them might not be attached to the tracks, but one Myo2p probably will be and can nudge the cargo along. The method makes sense for what Myo2p transports. The vesicles and organelles it totes are large and have room for multiple myosins to hook on.
An individual Myo4p was nonprocessive, but the molecules formed clusters that were processive. A single cluster was able to ferry one molecule of mRNA. Again, the strategy matches the cargo: mRNA has few attachment points for myosins.
The researchers also wanted to determine what accounts for the differences between the two myosins. They created hybrid molecules that carried the tail end of one protein—which attaches to the cargo—fused to the motor portion of the other. A hybrid with the motor of Myo4p and the tail of Myo2p worked like Myo2p, and vice versa. The tail thus dictates myosin's behavior. The researchers now want to investigate how cells integrate pulling by separate motors.