The CAP-Gly domain of p150(Glued) in axonal transport and neurodegernative disease

Dynactin is a required cofactor for the minus-end directed microtubule motor cytoplasmic dynein. Mutations within the highly conserved CAP-Gly domain of dynactin cause two distinct neurodegenerative diseases: Perry syndrome and distal hereditary motor neuropathy 7B. The mechanism by which the CAP-Gly domain promotes dynein motor function remains unclear and how these distinct point mutations cause two disparate diseases is unknown. Here, I report a novel function for the CAP-Gly domain of dynactin. Using a combination of cell biological and biochemical techniques in primary neurons, I show that the CAP-Gly domain is necessary to enrich dynactin at the distal end of primary neurons. While the CAP-Gly domain is not required for sustained transport along the axon, I find that the distal accumulation facilitates the efficient initiation of retrograde vesicular transport from the neurite tip. Dynactin is delivered to the distal neurite by kinesin-1 and this highly stable pool of dynactin is retained in the distal neurite by end-binding proteins, EBs. The EBs recruit dynactin to the microtubule plus-end and are necessary for the efficient initiation of retrograde transport. Further I show that Lis1, also a proposed dynein initiation factor, acts distinctly and independently from dynactin's role in transport initiation. The neurodegenerative disease mutations in the CAP-Gly domain induce distinct disruptions in intracellular motility. The HMN7B mutation destabilizes the CAP-Gly domain, preventing efficient incorporation of the mutant protein into the dynein-dynactin complex and effectively disrupts axonal transport by acting as a dominant-negative inhibitor of dynein motor function. In contrast, the Perry syndrome mutations cause a loss of CAP-Gly function that results in a failure to localize p150Glued in distal processes, leading to an inhibition of transport initiation. Together, these results indicate that dynactin enhances dynein-mediated transport and the efficient initiation of axonal transport through distinct mechanisms. Thus, I propose a model in which distal dynactin is a key mediator in promoting the interaction among the microtubule, dynein motor, and cargo for the efficient initiation of transport. Mutations in the CAP-Gly domain disrupt the formation of the motor-cargo complex, highlighting the specific defects in axonal transport that may lead to neurodegeneration.