The Molecular Architecture of Native BBSome Obtained by an Integrated Structural Approach

• Published in: Structure (London) 2019-09, Vol.27 (9), p.1384-1394.e4
• Main Authors: Chou, Hui-Ting, Apelt, Luise, Farrell, Daniel P., White, Susan Roehl, Woodsmith, Jonathan, Svetlov, Vladimir, Goldstein, Jaclyn S., Nager, Andrew R., Li, Zixuan, Muller, Jean, Dollfus, Hélène, Nudler, Evgeny, Stelzl, Ulrich, DiMaio, Frank, Nachury, Maxence V., Walz, Thomas
• Format: Article
• Language: English
• Subjects: ADP-Ribosylation Factors - metabolism; Animals; BBSome; Cattle; cilia; Cryoelectron Microscopy; Homeostasis; Humans; Mass Spectrometry; Microtubule-Associated Proteins - chemistry; Microtubule-Associated Proteins - metabolism; Models, Molecular; Protein Conformation; Protein Multimerization; Retina - metabolism; trafficking
• ISSN: 0969-2126; 1878-4186
• DOI: 10.1016/j.str.2019.06.006

The unique membrane composition of cilia is maintained by a diffusion barrier at the transition zone that is breached when the BBSome escorts signaling receptors out of cilia. Understanding how the BBSome removes proteins from cilia has been hampered by a lack of structural information. Here, we present a nearly complete Cα model of BBSome purified from cow retina. The model is based on a single-particle cryo-electron microscopy density map at 4.9-Å resolution that was interpreted with the help of comprehensive Rosetta-based structural modeling constrained by crosslinking mass spectrometry data. We find that BBSome subunits have a very high degree of interconnectivity, explaining the obligate nature of the complex. Furthermore, like other coat adaptors, the BBSome exists in an autoinhibited state in solution and must thus undergo a conformational change upon recruitment to membranes by the small GTPase ARL6/BBS3. Our model provides the first detailed view of the machinery enabling ciliary exit. [Display omitted] •4.9-Å density map of the intact native BBSome obtained by single-particle cryo-EM•Cα model generated using Rosetta establishes the subunit organization•BBSome subunits have a very high degree of interconnectivity•In solution, the BBSome exists predominantly in an autoinhibited state The BBSome transports activated signaling receptors across the transition zone, a diffusion barrier that separates the ciliary from the plasma membrane. Chou et al. combined cryo-electron microscopy with structural modeling to deduce an almost complete Cα model for the BBSome and show that it is autoinhibited in solution.