Structure and activation mechanism of the BBSome membrane protein trafficking complex
- Harvard Medical School, United States
Abstract
Bardet-Biedl syndrome (BBS) is a currently incurable ciliopathy caused by the failure to correctly establish or maintain cilia-dependent signaling pathways. Eight proteins associated with BBS assemble into the BBSome, a key regulator of the ciliary membrane proteome. We report the electron cryomicroscopy (cryo-EM) structures of the native bovine BBSome in inactive and active states at 3.1 and 3.5 Å resolution, respectively. In the active state, the BBSome is bound to an Arf-family GTPase (ARL6/BBS3) that recruits the BBSome to ciliary membranes. ARL6 recognizes a composite binding site formed by BBS1 and BBS7 that is occluded in the inactive state. Activation requires an unexpected swiveling of the b-propeller domain of BBS1, the subunit most frequently implicated in substrate recognition, which widens a central cavity of the BBSome. Structural mapping of disease-causing mutations suggests that pathogenesis results from folding defects and the disruption of autoinhibition and activation.
Data availability
The EM density map for the BBSome has been deposited under accession code EMD-21144 and the EM density map for the BBSome:ARL6:GTP complex has been deposited under accession code EMD-21145. Masks and maps from multibody refinement are included as additional maps in these depositions. The corresponding atomic models have been deposited under accession codes 6VBU and 6VBV.
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Structure of the bovine BBSome (map)Electron Microscopy Data Bank, EMD-21144.
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Structure of the bovine BBSome:ARL6:GTP complex (map)Electron Microscopy Data Bank, EMD-21145.
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Structure of the bovine BBSome:ARL6:GTP complex (model)Protein Data Bank, 6VBV.
Article and author information
Author details
Funding
Pew Charitable Trusts
- Alan Brown
Internal Retinal Research Foundation
- Alan Brown
E. Matilda Ziegler Foundation for the Blind
- Alan Brown
Richard and Susan Smith Family Foundation
- Alan Brown
The funders had no role in study design, data collection and interpretation, or the decision to submit the work for publication.
Copyright
© 2020, Singh et al.
This article is distributed under the terms of the Creative Commons Attribution License permitting unrestricted use and redistribution provided that the original author and source are credited.
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Further reading
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- Structural Biology and Molecular Biophysics
The structures of the bovine and human BBSome reveal that a conformational change is required to recruit the complex to the ciliary membrane.
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- Immunology and Inflammation
- Structural Biology and Molecular Biophysics
A potent class of HIV-1 broadly neutralizing antibodies (bnAbs) targets the envelope glycoprotein’s membrane proximal exposed region (MPER) through a proposed mechanism where hypervariable loops embed into lipid bilayers and engage headgroup moieties alongside the epitope. We address the feasibility and determinant molecular features of this mechanism using multi-scale modeling. All-atom simulations of 4E10, PGZL1, 10E8, and LN01 docked onto HIV-like membranes consistently form phospholipid complexes at key complementarity-determining region loop sites, solidifying that stable and specific lipid interactions anchor bnAbs to membrane surfaces. Ancillary protein-lipid contacts reveal surprising contributions from antibody framework regions. Coarse-grained simulations effectively capture antibodies embedding into membranes. Simulations estimating protein-membrane interaction strength for PGZL1 variants along an inferred maturation pathway show bilayer affinity is evolved and correlates with neutralization potency. The modeling demonstrated here uncovers insights into lipid participation in antibodies’ recognition of membrane proteins and highlights antibody features to prioritize in vaccine design.
