Regulation of melanosome movement in the cell cycle by reversible association with myosin V

Previously, we have shown that melanosomes of Xenopus laevis melanophores a re transported along both microtubules and actin filaments in a coordinated manner, and that myosin V is bound to purified melanosomes (Rogers, S., an d V.I. Gelfand, 1998. Curr. Biol, 8:161-164), In the present study, we have demonstrated that myosin V is the actin-based motor responsible for melano some transport. To examine whether myosin V was regulated in a cell cycle-d ependent manner, purified melanosomes were treated with interphase- or meta phase-arrested Xenopus egg extracts and assayed for in vitro motility along Nitella actin filaments. Motility of organelles treated with mitotic extra ct was found to decrease dramatically, as compared with untreated interphas e extract-treated melanosomes, This mitotic inhibition of motility correlat ed with the dissociation of myosin V from melanosomes, but the activity of soluble motor remained unaffected, Furthermore, we find that myosin V heavy chain is highly phosphorylated in metaphase extracts versus interphase ext racts. We conclude that organelle transport by myosin V is controlled by a cell cycle-regulated association of this motor to organelles, and that this binding is likely regulated by phosphorylation of myosin V during mitosis.