Dynactin and cytoplasmic dynein-based motility

Dynactin is a large multiprotein complex that contains at least nine different polypeptide subunits. Ultrastructural analysis reveals two domains, a short filament of the actin-related protein Arpl (aka centractin) and a projecting sidearm thought to participate in dynein binding Previous work (Echeverri et al, J Cell Biol, 1996) indicates that overexpression of the 50kD dynactin subunit (dynamitin) causes the sidearm to dissociate from the Arp 1 filament which leads to disruptions in mitosis and membranous organelle localization, specifically Golgi and endosomes. We have found that, although the overall organization of the microtubule network in interphase cells is maintained, the structure and function of centrosomes are also disrupted by dynamitin overexpression. Real time observation of the dynamics of early and late endosomal markers reveal defects in endosomal motility while virus infection studies indicate that endocytic trafficking is perturbed. We have also analyzed the effects of overexpressing other dynactin subunits (p24, p62, p!50Glued or its subdomains) on mitotic progression, centrosome function and membranous organelle localization. These results suggest a novel model of the dynactin-cargo interaction.