Single-molecule imaging of cytoplasmic dynein in cellulo reveals the mechanism of motor activation and cargo movement

Cytoplasmic dynein 1 (dynein) is the primary minus end-directed motor protein in most eukaryotic cells. Dynein remains in an inactive conformation until the formation of a tripartite complex comprising dynein, its regulator dynactin and a cargo adaptor. How this process of dynein activation occurs is unclear, since it entails the formation of a three-protein complex inside the crowded environs of a cell. Here, we employed live-cell, single-molecule imaging to visualise and track fluorescently tagged dynein. First, we observed that only ~30% of dynein molecules that bound to the microtubule (MT) engaged in minus end-directed movement, and that too for a short duration of ~0.6 s. Next, using high-resolution imaging in live and fixed cells, and using correlative light and electron microscopy, we discovered that dynactin and endosomal cargo remained in proximity to each other and to MTs. We then employed two-colour imaging to visualise cargo movement effected by single motor binding. Finally, we performed long-term imaging to show short movements are sufficient to drive cargo to the perinuclear region of the cell. We then used these discoveries as the basis for a stochastic model incorporating dynamic motors binding to cargo located along MTs, and also developed a coarse-grained 3-state run- and-tumble particle (RTP) model for the cargo that quantitatively recapitulates the emergent statistics of cargo movement. Taken together, we discovered a search mechanism that is facilitated by dyneins frequent MT binding-unbinding kinetics: (1) in a futile event when dynein does not encounter cargo anchored in proximity to the MT, dynein dissociates and diffuses into the cytoplasm, (2) when dynein encounters cargo and dynactin upon MT-binding, it moves cargo in a short run. Several of these short runs are undertaken in succession for long-range directed movement. In conclusion, we demonstrate that dynein activation and cargo capture are coupled in a step that relies on the reduction of dimensionality to enable minus end-directed transport in cellulo, and that complex cargo behaviour emerges from stochastic motor-cargo interactions.