title: Biased Brownian motion mechanism for processivity and directionality of single-headed myosin-VI.
abstract: Conventional form to function as a vesicle transporter is not a 'single molecule' but a coordinated 'two molecules'. The coordinated two molecules make it complicated to reveal its mechanism. To overcome the difficulty, we adopted a single-headed myosin-VI as a model protein. Myosin-VI is an intracellular vesicle and organelle transporter that moves along actin filaments in a direction opposite to most other known myosin classes. The myosin-VI was expected to form a dimer to move processively along actin filaments with a hand-over-hand mechanism like other myosin organelle transporters. However, wild-type myosin-VI was demonstrated to be monomer and single-headed, casting doubt on its processivity. Using single molecule techniques, we show that green fluorescent protein (GFP)-fused single-headed myosin-VI does not move processively. However, when coupled to a 200 nm polystyrene bead (comparable to an intracellular vesicle in size) at a ratio of one head per bead, single-headed myosin-VI moves processively with large (40 nm) steps. Furthermore, we found that a single-headed myosin-VI-bead complex moved more processively in a high-viscous solution (40-fold higher than water) similar to cellular environment. Because diffusion of the bead is 60-fold slower than myosin-VI heads alone in water, we propose a model in which the bead acts as a diffusional anchor for the myosin-VI, enhancing the head's rebinding following detachment and supporting processive movement of the bead-monomer complex. This investigation will help us understand how molecular motors utilize Brownian motion in cells.
authors.affiliation: Soft Biosystem Group, Laboratories for Nanobiology, Graduate School of Frontier Biosciences, Osaka University, 1-3,Yamadaoka, Suita, Osaka 565-871, Japan.
authors.forename: Mitsuhiro
authors.initials: M
authors.lastname: Iwaki
authors.forename: Atsuko Hikikoshi
authors.initials: AH
authors.lastname: Iwane
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Ikebe
authors.forename: Toshio
authors.initials: T
authors.lastname: Yanagida
mesh_terms: D020409:Molecular Motor Proteins / Q000737:chemistry* / Q000378:metabolism*; D009068:Movement; D018995:Myosin Heavy Chains / Q000737:chemistry* / Q000378:metabolism*; D011485:Protein Binding; D012996:Solutions
chemical_list: D020409:Molecular Motor Proteins; D012996:Solutions; C089785:myosin VI; D018995:Myosin Heavy Chains