title: Role of the lever arm in the processive stepping of myosin V.
abstract: Myosin V is a two-headed molecular motor that binds six light chains per heavy chain, which creates unusually long lever arms. This motor moves processively along its actin track in discrete 36-nm steps. Our model is that one head of the two-headed myosin V tightly binds to actin and swings its long lever arm through a large angle, providing a stroke. We created single-headed constructs with different-size lever arms and show that stroke size is proportional to lever arm length. In a two-headed molecule, the stroke provides the directional bias, after which the unbound head diffuses to find its binding site, 36 nm forward. Our two-headed construct with all six light chains per head reconstitutes the 36-nm processive step seen in tissue-purified myosin V. Two-headed myosin V molecules with only four light chains per head are still processive, but their step size is reduced to 24 nm. A further reduction in the length of the lever arms to one light chain per head results in a motor that is unable to walk processively. This motor produces single small approximately 6-nm strokes, and ATPase and pyrene actin quench measurements show that only one of the heads of this dimer rapidly binds to actin for a given binding event. These data show that for myosin V with its normal proximal tail domain, both heads and a long lever arm are required for large, processive steps.
authors.affiliation: Department of Biochemistry, Stanford University Medical Center, Stanford, CA 94305, USA.
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Purcell
authors.forename: Carl
authors.initials: C
authors.lastname: Morris
authors.forename: James A
authors.initials: JA
authors.lastname: Spudich
authors.forename: H Lee
authors.initials: HL
authors.lastname: Sweeney
mesh_terms: D000199:Actins / Q000737:chemistry / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000737:chemistry / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D002645:Chickens; D008958:Models, Molecular; D008969:Molecular Sequence Data; D024701:Myosin Type V / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011499:Protein Processing, Post-Translational; D017434:Protein Structure, Tertiary; D011721:Pyrenes / Q000737:chemistry / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
chemical_list: D000199:Actins; D011721:Pyrenes; D011993:Recombinant Fusion Proteins; D000251:Adenosine Triphosphatases; D024701:Myosin Type V