Molecular mechanism for recognition of the cargo adapter Rab6^GTP by the dynein adapter BicD2

Rab6 is a key modulator of protein secretion. The dynein adapter Bicaudal D2 (BicD2) recruits the motors cytoplasmic dynein and kinesin-1 to Rab6^GTP-positive vesicles for transport; however, it is unknown how BicD2 recognizes Rab6. Here, we establish a structural model for recognition of Rab6^GTP by BicD2, using structure prediction and mutagenesis. The binding site of BicD2 spans two regions of Rab6 that undergo structural changes upon the transition from the GDP- to GTP-bound state, and several hydrophobic interface residues are rearranged, explaining the increased affinity of the active GTP-bound state. Mutations of Rab6^GTP that abolish binding to BicD2 also result in reduced co-migration of Rab6^GTP/BicD2 in cells, validating our model. These mutations also severely diminished the motility of Rab6-positive vesicles in cells, highlighting the importance of the Rab6^GTP/BicD2 interaction for overall motility of the multi-motor complex that contains both kinesin-1 and dynein. Our results provide insights into trafficking of secretory and Golgi-derived vesicles and will help devise therapies for diseases caused by BicD2 mutations, which selectively affect the affinity to Rab6 and other cargoes.