Vertebrate myosin Va is a dimeric processive motor that walks on actin filaments to deliver cargo. In contrast, the two class V myosins in budding yeast, Myo2p and Myo4p, are non-processive (Reck-Peterson, S. L., Tyska, M. J., Novick, P. J., and Mooseker, M. S. (2001) J. Cell Biol. 153, 1121-1126). We previously showed that a chimera with the motor domain of Myo4p on the backbone of vertebrate myosin Va was processive, demonstrating that the Myo4p motor domain has a high duty ratio. Here we examine the properties of a chimera containing the rod and globular tail of Myo4p joined to the motor domain and neck of mouse myosin Va. Surprisingly, the adaptor protein She3p binds to the rod region of Myo4p and forms a homogeneous single-headed myosin-She3p complex, based on sedimentation equilibrium and velocity data. We propose that She3p forms a heterocoiled-coil with Myo4p and is a subunit of the motor. She3p does not affect the maximal actin-activated ATPase in solution or the velocity of movement in an ensemble in vitro motility assay. At the single molecule level, the monomeric myosin-She3p complex showed no processivity. When this construct was dimerized with a leucine zipper, short processive runs were obtained. Robust continuous movement was observed when multiple monomeric myosin-She3p motors were bound to a quantum dot "cargo." We propose that continuous transport of mRNA by Myo4p-She3p in yeast is accomplished either by multiple high duty cycle monomers or by molecules that may be dimerized by She2p, the homodimeric downstream binding partner of She3p.
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