Dynactin increases the processivity of the cytoplasmic dynein motor

Cytoplasmic dynein supports long-range intracellular movements of cargo in vivo but does not appear to be a processive motor protein by itself. We sho w here that the dynein activator, dynactin, binds microtubules and increase s the average length of cytoplasmic-dynein-driven movements without affecti ng the velocity or microtubule-stimulated ATPase kinetics of cytoplasmic dy nein. Enhancement of microtubule binding and motility by dynactin are both inhibited by an antibody to dynactin's microtubule-binding domain. These re sults indicate that dynactin acts as a processivity factor for cytoplasmic- dynein-based motility and provide the first evidence that cytoskeletal moto r processivity can be affected by extrinsic factors.