Summary
Bardet-Biedl syndrome (BBS) is a genetic disease caused by mutations that disrupt the function of the BBSome, an eight-subunit complex that plays an important role in transport of proteins in primary cilia. To better understand the molecular basis of the disease, we analyzed the structure of a BBSome subcomplex consisting of three homologous BBS proteins (BBS2, BBS7, and BBS9) by an integrative structural modeling approach using electron microscopy and chemical crosslinking coupled with mass spectrometry. The resulting molecular model revealed an overall structure that resembles a flattened triangle. Within the structure, BBS2 and BBS7 form a tight dimer based on a coiled-coil interaction, and BBS9 associates with the dimer via an interaction with the α-helical domain of BBS2. Interestingly, a BBS-linked mutation of BBS2 (R632P) is located in the α-helical domain at the interface between BBS2 and BBS9, and binding experiments showed that this mutation disrupted the interaction of BBS2 with BBS9. This finding suggests that BBSome assembly is disrupted by the R632P substitution, providing a molecular explanation for BBS in patients harboring this mutation.
Footnotes
Abbreviations: BBS, Bardet-Biedl Syndrome; BBSome, BBS protein ciliary transport complex; BBS2-7-9, BBSome subcomplex containing BBS2, 7 and 9 proteins; IFT, intraflagellar transport; ARL6, ADP-ribosylation factor-like protein 6; LZTFL1, Leucine zipper transcription factor-like protein 1; CCT, cytosolic chaperonin containing tailless polypeptide 1; EM, electron microscopy; XL-MS, chemical crosslinking coupled with mass spectrometry; HEK-293T, human embryonic kidney 293T cells; DMEM, Dulbecco modified eagle media; FBS, fetal bovine serum; HPC4, protein C peptide tag; PEI, polyethylenimine; LC, liquid chromatography; MS/MS, tandem mass spectrometry; DSS, disuccinimidyl suberate; DSG, disuccinimidyl glutarate; IMP, integrative modeling platform; GAE, γ-adaptin ear domain; RMSD, root mean square deviation.