Vesicular transport to the primary cilium: basic principles
The membrane of the primary cilium is continuous with the plasma membrane. Despite this continuity, ciliary and plasma membranes contain distinct complements of lipids and proteins, thus making the cilium a bona fide compartment exposed to the extracellular milieu. Consequently, the cilium has been compared to a "cellular antenna" that capture and transduces developmental signals to the inside of the cell. But how do the relevant signaling receptors reach the ciliary membrane instead of the plasma membrane? This question of vesicular targeting to the cilium has recently been illuminated by our discovery of two key ciliogenic factors, the BBSome and Rab8. Consistent with the general paradigm of the small GTPases of the Rab family, Rab8 likely mediates the docking and fusion of transport vesicles with the base of the primary cilium. But where and when does Rab8 precisely function in this transport process? What effectors does Rab8 utilize to specifically target vesicles to the primary cilium? How is the subsequent step of entry into the cilium proper linked to this fusion step? These are the questions we aim to study. Conversely to Rab8, the molecular activity of the BBSome is still very poorly understood. The BBSome is a complex of 7 proteins encoded by genes defective in Bardet-Biedl Syndrome (BBS), an obesity/retinopathy ciliopathy. We plan to develop in vitro and in vivo assays to pinpoint the role of the BBSome in transport to the cilium and to define what type of molecular machine the BBSome represents. Finally, a structural analysis of the BBSome has been initiated in collaboration with the lab of Chris Garcia.