SummaryCytoplasmic dynein-1 binds dynactin and cargo adaptor proteins to form a transport machine capable of long-distance processive movement along microtubules. However, it is unclear why dynein-1 moves poorly on its own or how it is activated by dynactin. Here, we present a cryoelectron microscopy structure of the complete 1.4-megadalton human dynein-1 complex in an inhibited state known as the phi-particle. We reveal the 3D structure of the cargo binding dynein tail and show how self-dimerization of the motor domains locks them in a conformation with low microtubule affinity. Disrupting motor dimerization with structure-based mutagenesis drives dynein-1 into an open form with higher affinity for both microtubules and dynactin. We find the open form is also inhibited for movement and that dynactin relieves this by reorienting the motor domains to interact correctly with microtubules. Our model explains how dynactin binding to the dynein-1 tail directly stimulates its motor activity.

Kinesin and dynein motors transport intracellular cargos bidirectionally by pulling them in opposite directions along microtubules, through a process frequently described as a ‘tug of war’. While kinesin produces a 6 pN force, mammalian dynein was found to be a surprisingly weak motor (0.5–1.5 pN) in vitro, suggesting many dyneins are required to counteract the pull of a single kinesin. Mammalian dynein’s association with dynactin and Bicaudal-D2 (BICD2) activates its processive motility, but how this affects dynein’s force output remained unknown. Here, we show that formation of the dynein-dynactin-BICD2 (DDB) complex increases human dynein’s force production to 4.3 pN. An in vitro tug-of-war assay revealed that a single DDB successfully resists a single kinesin. Contrary to previous reports, the clustering of many dyneins is not required to win the tug-of-war. Our work reveals the key role of dynactin and a cargo adaptor protein in shifting the balance of forces between dynein and kinesin motors during intracellular transport.

The neuronal axon is packed with cytoskeletal filaments, membranes, and organelles, many of which move between the cell body and axon tip. Here, we used cryo-electron tomography to survey the internal components of mammalian sensory axons. We determined the polarity of the axonal microtubules (MTs) by combining subtomogram classification and visual inspection, finding MT plus and minus ends are structurally similar. Subtomogram averaging of globular densities in the MT lumen suggests they have a defined structure, which is surprising given they likely contain the disordered protein MAP6. We found the endoplasmic reticulum in axons is tethered to MTs through multiple short linkers. We surveyed membrane-bound cargos and describe unexpected internal features such as granules and broken membranes. In addition, we detected proteinaceous compartments, including numerous virus-like capsid particles. Our observations outline novel features of axonal cargos and MTs, providing a platform for identification of their constituents.

Anterograde intraflagellar transport trains are essential for cilia assembly and maintenance. These trains are formed of 22 IFTA and IFTB proteins that link structural and signalling cargoes to microtubule motors for import into cilia. It remains unknown how the IFTA/B proteins are arranged into complexes and how these complexes polymerise into functional trains. Here, we use in situ cryo-electron tomography and Alphafold2 protein structure predictions to generate the first molecular model of the entire anterograde train. We show how the conformation of both IFTA and IFTB is dependent on lateral interactions with neighbouring repeats, suggesting that polymerization is required to cooperatively stabilize the complexes. The retrograde dynein motor binding site is a composite surface involving multiple IFTB repeats, ensuring that dynein can only form a strong interaction with IFTB upon train assembly. Finally, we reveal how IFTB extends two flexible tethers to maintain a connection with IFTA that can withstand the mechanical stresses present in actively beating cilia. Overall, our findings provide a framework for understanding the fundamental processes that are involved in cilia assembly.