Myo4p is a monomeric myosin with motility uniquely adapted to transport mRNA

The yeast Saccharomyces cerevisiae uses two class V myosins to transport cellular material into the bud: Myo2p moves secretory vesicles and organelles, whereas Myo4p transports mRNA. To understand how Myo2p and Myo4p are adapted to transport physically distinct cargos, we characterize Myo2p and Myo4p in yeast extracts, purify active Myo2p and Myo4p from yeast lysates, and analyze their motility. We find several striking differences between Myo2p and Myo4p. First, Myo2p forms a dimer, whereas Myo4p is a monomer. Second, Myo4p generates higher actin filament velocity at lower motor density. Third, single molecules of Myo2p are weakly processive, whereas individual Myo4p motors are nonprocessive. Finally, Myo4p self-assembles into multi-motor complexes capable of processive motility. We show that the unique motility of Myo4p is not due to its motor domain and that the motor domain of Myo2p can transport ASH1 mRNA in vivo. Our results suggest that the oligomeric state of Myo4p is important for its motility and ability to transport mRNA.