Cytoplasmic Dynein
Cytoplasmic dynein, which has a molecular mass of about 1.5 Megadaltons (MDa), contains approximately twelve polypeptide subunits: two identical "heavy chains," 520 kDa in mass, which contain the ATPase activity and are thus responsible for generating movement along the microtubule; two 74 kDa intermediate chains which are believed to anchor the dynein to its cargo; four 53-59 kDa intermediate chains and several light chains which are less understood.
The force-generating ATPase activity of each dynein heavy chain is located in its large doughnut-shaped "head", which is related to other AAA proteins, while two projections from the head connect it to other cytoplasmic structures. One projection, the coiled-coil stalk, binds to and "walks" along the surface of the microtubule via a repeated cycle of detachment and reattachment. The other projection, the extended tail (also called "stem"), binds to the intermediate and light chain subunits which attach the dynein to its cargo. The alternating activity of the paired heavy chains in the complete cytoplasmic dynein motor enables a single dynein molecule to transport its cargo by "walking" a considerable distance along a microtubule without becoming completely detached.
In eukaryotes, cytoplasmic dynein must be activated by binding of dynactin, another multisubunit protein that is essential for mitosis. Dynactin may regulate the activity of dynein, and possibly facilitates the attachment of dynein to its cargo.