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Here processivity wouldactually inhibit biological function, since efficient muscle contraction requiresthat each myosin head perform its power stroke and then quickly get out of theway-in order to avoid interfering with the actions of the other heads attachedto the same actin filament.There are two reasonsfor the high degree of processivity of kinesin-1 movement. The first is that the mechanochemical cycles of the two motor heads in akinesin- I dimer are coordinated with each other, so that one kinesin head doesnot let go until the other is poised to bind. This coordination allows the motorprotein to operate in a hand-over-hand fashion, never allowing the organellecargo to diffuse away from the microtubule track.
In contrast, there is no apparent coordination between the myosin heads in a myosin II dimer. The secondreason for the high processivity of kinesin- I movement is that kinesin- 1 spendsa relatively large fraction of its ATPasecycle tightly bound to the microtubule.For both kinesin- I and myosin II, the conformational change that produces theforce-generating working stroke must occur while the motor protein is tightlybound to its polymer, and the recovery stroke in preparation for the next stepmust occur while the motor is unbound. But myosin II spends only about 5% ofits AIPase cycle in the tightly bound state, and it is unbound the rest of the time.\.Vhat myosin loses in processivity it gains in speed; in an array in whichmany motor heads are interacting with the same actin filament, a set of linkedmyosins can move its filament a total distance equivalent to 20 steps during asingle cycle time, while kinesins can move only two.
Thus, myosin II can typically drive filament sliding much more rapidly than kinesin-1, even though thetwo different motor proteins hydrolyze NIP at comparable rates and take molecular steps of comparable length. This property is particularly important in therapid contraction of skeletal muscle, as we will discuss later.Within each motor protein class,movement speedsvary widely, from about0.2 to 60 pm/sec for myosins, and from about 0.02 to 2 pm/sec for kinesins.These differences arise from a fine-tuning of the mechanochemical cycle. Thenumber of steps that an individual motor molecule can take in a given time, andthereby the velocity, can be decreasedby either decreasing the motor protein'sintrinsic ATPaserate or by increasing the proportion of cycle time spent boundto the filament track. For example, myosin v (which acts as a processivevesiclemotor) spends up to 907oof its nucleotide cycle tightly bound to the actin filament, in contrast to 5% for myosin IL Moreover, a motor protein can evolve tochange the size of each step by either changing the length of the lever arm (forexample, the lever arm of myosin V is about three times longer than the leverarm of myosin II) or the angle through which the helix swings (Figure f 6-65).Each of these parameters varies slightly among different members of the myosinand kinesin families, corresponding to slightly different protein sequencesandstructures.M y o s i nl l5 to 10nm swingof lever armMyosinV30to 40 nm swingo f l e v e ra r mneao---mrnuSendp lu sendFigure 16-65 The effect of lever armlength on the step sizefor a motorprotein.The leverarm of myosinll ismuchshorterthan the leverarm ofmyosinV.The power strokein the headswingstheir leverarmsthroughthe sameangle,so myosinV is ableto takea biggerstepthan myosinll.('eurqSpr^e6lo ^sallnof) 'urseldol^)sr1;araqt tnoq6norqlpas.redsrppue s1uau6e.r;snleredde1610gaqi ,(7-g1alqel aas)azuau,{;odapo} salnqn}ol)il.usasne)q)tqM'alozepo)ouo1atnsodxarel;V (g)'snal)nueql,o saprsalrsoddool pe^oura^eq saulosoltua)pale)tldnpaql pue'aseqdordur sttlal aql uo lla) aql'aurosorlua)a16urse qlran'aseqdrelulu; 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