Myosin V : tight grip and big steps

杨柳

Moving a muscle and moving an organelle are two very different propositions. Myosin II in muscle, safely held in place in the highly structured sarcomere, can give an occasional shove between lengthy rests. But myosin V is diffusely distributed on organelles, and therefore must maintain a tight grip on an actin cable to prevent random diffusion of the organelle. On page 625, Moore et al. first provide evidence that a single myosin V molecule maintains that grip at least half of the time (compared with less than 5% of the time for myosin II), and then give a measure of the large power stroke that drives myosin V along the actin filament.The power stroke has been the subject of much contention in the myosin field. Toshio Yanagida (Osaka University, Osaka, Japan) contends that ATP hydrolysis biases myosin to bobble along a variable number of steps (the loose-coupling model), whereas a number of other researchers favor a tight-coupling model in which ATP hydrolysis powers the swinging of a lever arm and a single defined step. Moore et al. adhere to the latter model, and have in the past shown that longer levers result in bigger steps.
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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.
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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 whole structure forward another 18 nm. Thus, the 40-nm stride seen by Spudich may have been broken down into two components.(Myosin V makes a step of 20 nm, but a st)

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