Dynactin
 
Dynactin is a large protein complex that is important for the function of cytoplasmic dynein in intracellular motility. Dynactin was first identified as a protein complex that stimulates dynein movement of membranous organelles along microtubules (1). It has been proposed that dynactin functions to bind cytoplasmic dynein to its membranous organelle cargo. Work in diverse species from fungi to mammalian neurons suggests that an alternative function of dynactin is to link cytoplasmic dynein to the actin cortex for the movement of microtubules (2).
The backbone of the dynactin molecule is a short (~40nm) filament composed of 8 to 12 molecules of actin-related protein 1 (Arp1) (3). Arp1 shares many of the binding domains of actin, including the spectrin, myosin, and villin binding sites. Actin-capping protein is bound to one end of the dynactin filament, and a unique capping protein is located at the other end. The p150Glued subunit projects from the filament. This protein is the product of the Glued gene of Drosophila, and it binds both microtubules and the intermediate-chain subunit of cytoplasmic dynein. Overexpression of another major subunit of dynactin, p50, obstructs assembly of the dynactin complex, and thus it is named dynamitin (4). Overexpression of p50 disrupts localization of both dynein and dynactin to membranous organelles or kinetochores. Genetic studies indicate that dynein and dynactin act in the same pathways (5).
References
1. T. A. Schroer and M. P. Sheetz (1991) J. Cell Biol. 115, 1309–1318. 
2. J. T. Carminati and T. Stearns (1997) J. Cell Biol. 138, 629–641. 
3. D. A. Schafer et al. (1994) J. Cell Biol. 126, 403–412. 
4. C. J. Echeverri et al. (1996) J. Cell Biol. 132, 617–633. 
5. T. A. Schroer (1994) J. Cell Biol. 127, 1–4.