The dynein adapter BicD2 recognizes its cargo through short cargo-recognition alpha-helices.

Abstract

Nup358, a protein of the nuclear pore complex, facilitates a nuclear positioning pathway that is essential for brain development. Nup358 interacts with the dynein adaptor Bicaudal D2 (BicD2), which in turn recruits the dynein machinery to position the nucleus. However, the molecular mechanisms of the Nup358/BicD2 interaction and the activation of transport remain poorly understood. Using nuclear magnetic resonance, mutagenesis and circular dichroism spectroscopy, a short Nup358 α-helix was identified, which transitioned from a random coil to an α-helical conformation upon BicD2-binding and formed the core of the Nup358-BicD2 interface. Mutations in this region of Nup358 decreased the Nup358/BicD2 interaction, resulting in decreased dynein recruitment and impaired motility. BicD2 thus recognizes Nup358 though a “cargo recognition α-helix”, a structural feature that may stabilize BicD2 in its activated state and promote processive dynein motility. To test the hypothesis that the other BicD2 cargoes are recognized by similar α-helical motifs, we identified the BicD2-binding site on Rab6GTP, which recruits BicD2 to Golgi-derived vesicles for transport. The BicD2 binding site is located in a region of Rab6 that undergoes conformational changes in the GTP- compared to the GDP-bound state, explaining why GTP-bound Rab6 has a higher affinity. This region is also intrinsically disordered, and we propose that it transitions to an alpha-helical conformation upon binding to BicD2, similar as observed for Nup358. Our results provide insights into the molecular mechanism of cargo selection of BicD2, which facilitates transport pathways that are important for brain development and the transport of Golgi-derived vesicles. Our results also pave the way for studies that establish the function of BicD2/dynein-dependent transport pathways and their regulatory mechanisms in the developing brain and their contributions to neuromuscular diseases.