The novel proteins Rng8 and Rng9 regulate the myosin-V Myo51 during fission yeast cytokinesis

The myosin-V family of molecular motors is known to be under sophisticated regulation, but our knowledge of the roles and regulation of myosin-Vs in cytokinesis is limited. Here, we report that the myosin-V Myo51 affects contractile ring assembly and stability during fission yeast cytokinesis, and is regulated by two novel coiled-coil proteins, Rng8 and Rng9. Both rng8Δ and rng9Δ cells display similar defects as myo51Δ in cytokinesis. Rng8 and Rng9 are required for Myo51's localizations to cytoplasmic puncta, actin cables, and the contractile ring. Myo51 puncta contain multiple Myo51 molecules and walk continuously on actin filaments in rng8(+) cells, whereas Myo51 forms speckles containing only one dimer and does not move efficiently on actin tracks in rng8Δ. Consistently, Myo51 transports artificial cargos efficiently in vivo, and this activity is regulated by Rng8. Purified Rng8 and Rng9 form stable higher-order complexes. Collectively, we propose that Rng8 and Rng9 form oligomers and cluster multiple Myo51 dimers to regulate Myo51 localization and functions.