BicD2 and Dynactin Convert a Non-Processive Cytoplasmic Dynein to an Ultra-Processive Directional Motor

Abstract

Cytoplasmic dynein is the predominant minus-end directed microtubule motor in metazoan cells. Dynein transports diverse cargoes over long distances in neurons, and the motor is thought to be adapted for a myriad of cellular functions through the use of several accessory protein factors that impinge on its basic biophysical characteristics. One of these accessory factors is the multisubunit dynactin complex, which has been implicated in dynein-based cargo transport and the modulation of dynein processivity and directionality. While isolated dynein from Saccharomyces has been shown to be a strongly processive motor, dynein from other organisms displays weakly processive, bidirectional or diffusive motility. Here we show that, on its own, cytoplasmic dynein from humans and other metazoans is not a processive motor. Previous attempts to study dynein-dynactin co-complexes have found relatively modest effects on dynein processivity and directionality by dynactin. We utilize the evolutionarily conserved coiled-coil adapter protein BicD2 to strongly induce the formation of a stable dynein-dynactin-BicD2 (DDB) supercomplex that is over 2MDa in size. Using multicolor single-molecule microscopy, we have found that, remarkably, the purified DDB supercomplex is unidirectional and ultra-processive, displaying run-lengths that greatly exceed the previously observed enhancement of single dynein run-lengths by dynactin. The DDB supercomplex accumulates at microtubule minus-ends and displays characteristics expected of a processive cargo transport motor. Our data suggest that the dynein motor is more plastic that previously thought, able to transition from a non-processive motor to an ultra-processive mode of motility upon association with external regulatory factors.