Mechanisms Required to Detach Myosin V Motors from Cargoes.

Abstract

Establishing the accurate subcellular distribution of organelles is essential for cell function and homeostasis. Transport of organelles to their correct locations by molecular motors is critical for the proper distribution of organelles. Recent studies suggest an unexpected requirement for the accurate detachment of organelles from their associated motors and for the proper deposition of organelles at their correct destinations. In Saccharomyces cerevisiae, in coordination with the cell cycle, organelles are transported from the mother cell to the bud along actin cables. Most cytoplasmic organelles are transported by the myosin V motor, Myo2. Myo2 attaches to organelles via cargo-specific adaptor proteins. For example, Inp2, Mmr1, Vac17 and Ypt31/32, Sec4/Sec15 attach Myo2 to peroxisomes, mitochondria, the vacuole and secretory vesicles respectively. Each cargo has a distinct itinerary. Myo2 orchestrates the transport of diverse organelles in part through the regulated attachment to and detachment from cargoes. Studies of vacuole transport demonstrate that cargo adaptors play key roles in regulating the transport of organelles. At the start of the cell cycle, the vacuole-specific adaptor, Vac17, is phosphorylated by Cdk1 at four sites. Cdk1 dependent phosphorylation promotes the interaction of Vac17 with Myo2 which attaches Myo2 to the vacuole and initiates vacuole transport in coordination with the start of the cell cycle. Subsequently, the degradation of Vac17 detaches the vacuole from Myo2 and deposits the vacuole in the bud. The mechanisms which regulate cargo detachment from Myo2 remain poorly understood. Studies reported here demonstrate that vacuole detachment from Myo2 occurs in multiple regulated steps along the entire pathway of vacuole transport. Detachment initiates in the mother cell with the phosphorylation of Vac17 which recruits the E3 ligase, Dma1, to the vacuole. However, Dma1 recruitment also requires the assembly of the vacuole transport complex and is first observed after the vacuole enters the bud. Dma1 remains on the vacuole until the bud and mother vacuoles separate. Subsequently, Dma1 targets Vac17 for proteasomal degradation. Notably, we find that the termination of peroxisome transport also requires Dma1. We predict that this is a general mechanism which detaches myosin V from select cargoes.