Conserved Roles for the Dynein Intermediate Chain and Ndel1 in Assembly and Activation of Dynein.

Cytoplasmic dynein, the primary retrograde microtubule transport motor within cells, must be activated for processive motility through the regulated assembly of a dynein-dynactin-adapter (DDA) complex. The interaction between dynein and dynactin was initially ascribed to the N-terminus of the dynein intermediate chain (IC) and a coiled-coil of the dynactin subunit p150 (Glued) . However, cryo-EM structures of DDA complexes have not resolve these regions of the IC and p150 (Glued) , raising questions about the importance of this interaction. The IC N-terminus (ICN) also interacts with the dynein regulators Nde1/Ndel1, which compete with p150 (Glued) for binding to ICN. Using a combination of approaches, we reveal that the ICN plays critical, evolutionarily conserved roles in DDA assembly by interacting with dynactin and Ndel1, the latter of which recruits the DDA assembly factor LIS1 to the dynein complex. In contrast to prior models, we find that LIS1 cannot simultaneously bind to Ndel1 and dynein, indicating that LIS1 must be handed off from Ndel1 to dynein in temporally discrete steps. Whereas exogenous Ndel1 or p150 (Glued) disrupts DDA complex assembly in vitro , neither perturbs preassembled DDA complexes, indicating that the IC is stably bound to p150 (Glued) within activated DDA complexes. Our study reveals previously unknown regulatory steps in the dynein activation pathway, and provides a more complete model for how the activities of LIS1/Ndel1 and dynactin/cargo-adapters are integrated to regulate dynein motor activity.