In the new work, Moore et al. use a laser trap to show that the double-headed myosin V undergoes an initial, tightly tethered step of 19 nm, and a second, looser step of another 18 nm. The initial 19-nm step fits well with the previous correlation of lever length and step size—in the case of myosin V, both parameters are large. But, the 19 nm contrasts with the 40-nm “stride” detected by Jim Spudich (Stanford University, Stanford, CA) and colleagues using tissue-isolated myosin V.

Moore et al. suggest that their two, smaller steps arise as follows. The myosin V starts with one head tethered and the other free. As the second head collapses toward its destination on the actin filament, this gives rise to the first 19-nm step. This leading head then goes through a power stroke, yanking the lagging head free, and bringing the...

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