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SummaryDefects in primary cilia lead to devastating disease because of their roles in sensation and developmental signaling but much is unknown about ciliary structure and mechanisms of their formation and maintenance. We used cryo-electron tomography to obtain 3D maps of the connecting cilium and adjacent cellular structures of a modified primary cilium, the rod outer segment, from wild-type and genetically defective mice. The results reveal the molecular architecture of the cilium and provide insights into protein functions. They suggest that the ciliary rootlet is involved in cellular transport and stabilizes the axoneme. A defect in the BBSome membrane coat caused defects in vesicle targeting near the base of the cilium. Loss of the proteins encoded by the Cngb1 gene disrupted links between the disk and plasma membranes. The structures of the outer segment membranes support a model for disk morphogenesis in which basal disks are enveloped by the plasma membrane.PaperFlick

SummaryPrimary cilium dysfunction underlies the pathogenesis of Bardet-Biedl syndrome (BBS), a genetic disorder whose symptoms include obesity, retinal degeneration, and nephropathy. However, despite the identification of 12 BBS genes, the molecular basis of BBS remains elusive. Here we identify a complex composed of seven highly conserved BBS proteins. This complex, the BBSome, localizes to nonmembranous centriolar satellites in the cytoplasm but also to the membrane of the cilium. Interestingly, the BBSome is required for ciliogenesis but is dispensable for centriolar satellite function. This ciliogenic function is mediated in part by the Rab8 GDP/GTP exchange factor, which localizes to the basal body and contacts the BBSome. Strikingly, Rab8GTP enters the primary cilium and promotes extension of the ciliary membrane. Conversely, preventing Rab8GTP production blocks ciliation in cells and yields characteristic BBS phenotypes in zebrafish. Our data reveal that BBS may be caused by defects in vesicular transport to the cilium.

The BBSome is a complex of Bardet-Biedl Syndrome (BBS) proteins that shares common structural elements with COPI, COPII and clathrin coats. Here we show that the BBSome constitutes a coat complex that sorts membrane proteins to primary cilia. Biochemically, the BBSome is the major effector of the Arf-like GTPase Arl6/ BBS3. In vivo, the BBSome and Arl6 localize to ciliary punctae and Arl6GTP is required to target the BBSome to cilia. Congruently, GTP-bound Arl6 and acidic phospholipids are sufficient to efficiently recruit the BBSome to chemically defined liposomes. Finally, ultrastructural analyses demonstrate that BBSome binding to liposomes produces distinct patches of polymerized coat. Since we establish that the ciliary targeting signal of somatostatin receptor 3 needs to be directly recognized by the BBSome to mediate targeting to cilia, we propose that trafficking to cilia entails the coupling of BBSome coat polymerization to the recognition of sorting signals.