Processive Cargo Movement by Multiple Non-Processive Motors Bound to a Tetrameric Adapter Protein

Class V myosins can be processive or non-processive, but both support cargo transport. Here we investigate the mechanism by which Myo4p, the single-headed non-processive class V myosin of Saccharomyces cerevisiae, can transport mRNA cargo from the mother to the bud tip. The adapter protein that couples the Myo4p/She3p motor complex to mRNA (She2p) is tetrameric, and thus can in principle recruit multiple motors. Total internal reflection fluorescence (TIRF) microscopy was used to show that one She2p tetramer recruits enough motors to support processive runs. Metal-shadowed images show two motors attached to a She2p tetramer. Deletion of a prominent α-helix that protrudes from the middle of She2p abolishes correct cellular localization of ASH1 mRNA, suggesting that it is a binding site for She3p. These results highlight that one strategy used by non-processive motors is to work together in small groups, which functionally allows them to support transport that is as robust as a single processive motor.