PROJECT SUMMARY The contents of eukaryotic cells are highly dynamic, yet organized spatially and temporally. This is achieved primarily by the microtubule cytoskeleton and associated transport machinery, whose fundamental nature is highlighted by the many neurological diseases caused by mutations in them. The overarching goal of my research program is to understand how this system works at the molecular, cellular, and organismal scales. My team is highly interdisciplinary and we use in vitro biochemical reconstitution, protein engineering, single-molecule imaging, proteomics, live-cell imaging, and fungal genetics to achieve our goals. Through collaborative projects we use cryo-electron microscopy (cryo-EM) and cryo-electron tomography (cryo-ET) to incorporate a structure-guided approach to understanding intracellular transport, and we develop testable quantitative physical models of transport. We have made major contributions to determining how the dynein motor works and is regulated, to developing tools and screening strategies to study bi-directional movement of cargos on microtubules, and to understanding the regulation of intracellular transport in cells. Fundamental questions that we will address here include: (1) How does the dynein motor work? Our earlier work revealed how Lis1, a protein mutated in the neurodevelopmental disease lissencephaly, interacts with dynein and regulates its mechanochemical cycle. Here, we will focus on determining the mechanistic underpinnings for how Lis1 promotes the formation of activated dynein/dynactin complexes. We will also explore a new direction—the role of RNA editing—as a previously undescribed mechanism to regulate dynein and kinesin motors. Microtubule-based motors move dozens if not hundreds of cargos. (2) How is cargo-specificity achieved? Our past work used two complementary discovery-based approaches—genetics and proteomics—to identify molecules responsible for specifying dynein’s many functions. One mechanism revealed by our past work is organelle hitchhiking, where cargos link to motors indirectly, by attaching themselves to other cargos that are directly bound to the motors. A second strategy for achieving cargo specificity is the expansion of dynein activating adaptor genes in vertebrates. However, the molecular connections between most activating adaptors and dynein’s cargo are unknown. Here, we will determine the mechanisms underlying hitchhiking and the linkages between the Hook and Ninein families of activating adaptors and their cargos. As an additional approach to understand how dynein and kinesin link to their cargos, we will visualize these connections in cells in three dimensions using cryo-electron tomography of endosomes in Aspergillus nidulans and melanosomes in Xenopus laevis melanophores, two systems where we can use exquisite genetics or chemical tools to control microtubule- based motility.