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Theymediate the sliding of ftlaments relatiue to one another and the transport of cargoalongfilament tracks.All known motor proteinsthat moueon actin filaments are members of the myosin superfamily. The motor proteins that moue onmicrotubules are either membersof the kinesin superfamily or the dynein family. Themyosin and kinesin superfamiliesare diuerse,with about 40 genesencodingeachtypeofprotein in humans. The only structural element shared among all membersof eachsuperfamily is the motor "head"domain.

Theseheadsarefused to a wide uarieryof different "tails,"which attach to different typesof cargoand enablethe uariousfamily membersto perform dffirent functions in the cell. Thesefunctions include the transportationand localization of specificproteins,membrane-enclosedorganelles,and mRNAs.Although myosin and kinesin walk along different tracks and usedifferent mech'anisms to produceforce and mouementby ATP hydrolysis,they sharea common structural core,suggestingthat they are deriuedfrom a common ancestor.Thedynein motorprotein has independentlyeuolued,and it has a distinct structureand mechanismofaction.THECYTOSKELETONANDCELLBEH IORA central challenge in all areasof cell biology is to understand how the functionsof many individual molecular components combine to produce complex cellbehaviors.

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