We investigated the role of full-length Drosophila Bicaudal D (BicD) binding partners in dynein-dynactin activation for mRNA transport on microtubules. Full-length BicD robustly activated dynein-dynactin motility only when both the mRNA binding protein Egalitarian (Egl) and K10 mRNA cargo were present, and electron microscopy showed that both Egl and mRNA were needed to disrupt a looped, auto-inhibited BicD conformation. BicD can recruit two dimeric dyneins, resulting in faster speeds and longer runs than with one dynein. Moving complexes predominantly contained two Egl molecules and one K10 mRNA. This mRNA-bound configuration makes Egl bivalent, likely enhancing its avidity for BicD and thus its ability to disrupt BicD auto-inhibition. Consistent with this idea, artificially dimerized Egl activates dynein-dynactin-BicD in the absence of mRNA. The ability of mRNA cargo to orchestrate the activation of the mRNP (messenger ribonucleotide protein) complex is an elegant way to ensure that only cargo-bound motors are motile.Cytoplasmic dynein is a motor-like protein that uses energy to transport cargo where it is needed within cells. It moves along protein filaments called microtubules, which act like miniature tracks. Once dynein engages with microtubules, it then picks up cargo using adaptor proteins. In fruit flies, this cargo includes a messenger RNA molecule known as K10, which attaches to dynein via adaptors called Egalitarian and BicD (short for “Bicaudal D”). Egalitarian grabs hold of K10, and BicD links Egalitarian to the dynein motor.In the absence of a cargo, full-length BicD does not bind to dynein. But, shortening BicD to remove its link to Egalitarian allows it to bind and activate the motor for transport. Much of our current understanding of dynein comes from studies that use shortened adaptor proteins like these. These proteins cannot bind cargo, so we know little about how the cargo and the adaptors control dynein activity.To address this, Sladewski, Billington, Ali et al. purified the components of the K10 transport system and then recreated it in the laboratory. This revealed that it is BicD that decides when dynein is ready to go. First, imaging techniques showed that empty BicD forms a looped shape that hides the part of its structure that binds to dynein. This essentially switches it “off”, preventing empty dynein motors from moving. When the K10 cargo is ready for transport, it binds to two Egalitarian molecules, which work together to uncurl the BicD loop. This frees up the end of the BicD molecule, allowing it to link up with the dynein motor.The key to uncurling the BicD protein was the presence of two Egalitarian molecules. And it was the cargo, K10, that brought them together, ensuring that the motors only moved when the cargo was ready. What is more, the uncurled BicD could bind not one but two dyneins. This allowed the cargo to move faster, and over longer distances than cargo with one dynein motor.Recreating molecular machines and imaging their molecules provides a way to understand how they work. Studying how dynein moves cargo is key to understanding how molecules are transported within cells. This, in turn, could reveal what happens when the system goes wrong. Transport defects can cause diseases in humans, including neurodegenerative diseases. As such, a better understanding of how the transport system works may one day open new avenues for health research.
Sladewski, T. E., Billington, N., Ali, M. Y., Bookwalter, C. S., Lu, H., Krementsova, E. B., … Trybus, K. M. (2018). Recruitment of two dyneins to an mRNA-dependent Bicaudal D transport complex. ELife, 7. https://doi.org/10.7554/elife.36306