Role of coiled-coil registry shifts in activation of human Bicaudal D2 for dynein recruitment upon cargo-binding

Abstract

Highlights• Stable, bona fide BicD2 coiled-coils with distinct registries can be formed.• We provide evidence that a human disease mutation causes a coiled-coil registry shift.• A coiled-coil registry shift could relieve BicD2-autoinhibition upon cargo-binding.• The ability to undergo registry shifts may be an inherent property of coiled-coils. In BriefOur results support that stable coiled-coils of BicD2 with distinct registries can be formed, and suggest a molecular mechanism for such registry switches. We provide evidence that disease-causing mutations in coiled-coils may alter the equilibrium between registry-shifted conformers, which we propose as a general mechanism of pathogenesis for coiled-coils. Graphical Abstract3 SUMMARY Dynein adaptors such as Bicaudal D2 (BicD2) recognize cargo for dynein-dependent transport. BicD2-dependent transport pathways are important for brain and muscle development. Cargo-bound adaptors are required to activate dynein for processive transport, but the mechanism of action is elusive. Here, we report the structure of the cargo-binding domain of human BicD2 that forms a dimeric coiled-coil with homotypic registry, in which both helices are aligned. To investigate if BicD2 can switch to an asymmetric registry, where a portion of one helix is vertically shifted, we performed molecular dynamics simulations. Both registry types are stabilized by distinct conformations of F743. For the F743I variant, which increases dynein recruitment in the Drosophila homolog, and for the human R747C variant, which causes spinal muscular atrophy, spontaneous coiled-coil registry shifts are observed, which may cause the BicD2-hyperactivation phenotype and disease. We propose that a registry shift upon cargo-binding activates auto-inhibited BicD2 for dynein recruitment.

Coil-to-Helix Transition at the Nup358-BicD2 Interface Activates BicD2 for Dynein Recruitment

“…Notably, the minima at 208 and 222 nm were shifted towards more negative values in the complex compared to the sum of the individual spectra of Nup358-min and BicD2-CTD, suggesting that the α-helical content increases upon complex formation. S8), as well as the assumption, that native (0% unfolded) BicD2-CTD has an α-helical content of 96% as observed in the crystal structure 30 , the Nup358/BicD2 complex has a 14 % increase in α-helical content compared to the sum of the spectra of BicD2 and Nup358.…”

“…1b) 30 . A minimal complex was reconstituted with BicD2-CTD and a minimal fragment of human Nup358 containing residues 2148-2240, which is called Nup358min 30,47 (Fig. 1c).…”

“…Previously, we have determined an X-ray structure of the C-terminal domain of human BicD2 (BicD2-CTD, residues 715-804), which contains the binding sites for cargoes, including human Nup358 (Fig. 1b) 30 . A minimal complex was reconstituted with BicD2-CTD and a minimal fragment of human Nup358 containing residues 2148-2240, which is called Nup358min 30,47 (Fig.…”

“…To provide further evidence for the random coil-to-α-helix transition, circular dichroism (CD) spectroscopy was carried out. The CD wavelength scans of the Nup358-min/BicD2-CTD complex had minima at 208 nm and 222 nm, characteristic for α-helical proteins 30 (Fig. 5a).…”

“…Binding of dynein adaptors/cargo to the CTD releases auto-inhibition, likely resulting in an extended conformation that recruits dynein and dynactin [17][18][19][20][21][22][23][24][25][26][27] . The crystal structures of the C-terminal cargo recognition domains of three BicD2 homologs have been determined 26,29,30 ; However, the structural mechanisms of BicD2-mediated cargo recognition and dynein activation still remain poorly understood.…”