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

Area of Science:

• Cell Biology
• Structural Biology
• Biochemistry

Background:

• Cilia possess a unique membrane composition maintained by a transition zone diffusion barrier.
• This barrier is disrupted by the BBSome complex, which escorts signaling receptors out of cilia.
• Previous understanding of BBSome-mediated protein removal from cilia was limited by a lack of structural data.

Purpose of the Study:

• To determine the high-resolution structure of the BBSome complex.
• To elucidate the mechanism by which the BBSome facilitates protein exit from cilia.
• To provide a structural basis for understanding ciliary membrane composition regulation.

Main Methods:

• Purification of the BBSome complex from cow retina.
• Single-particle cryo-electron microscopy (cryo-EM) to generate a 4.9-Å resolution density map.
• Rosetta-based computational modeling constrained by crosslinking mass spectrometry (XL-MS) data.

Main Results:

• A near-complete Cα model of the BBSome was constructed.
• The BBSome exhibits a high degree of subunit interconnectivity, explaining its obligate nature.
• The BBSome exists in an autoinhibited state in solution, requiring conformational changes upon membrane recruitment by ARL6/BBS3.

Conclusions:

• The presented structural model offers the first detailed view of the machinery responsible for ciliary exit.
• This work provides critical insights into the regulation of ciliary protein composition.
• Understanding BBSome structure and function is key to deciphering mechanisms of ciliary signaling and transport.