Trafficking to the ciliary membrane: how to get across the periciliary diffusion barrier?

The primary cilium organizes numerous signal transduction cascades, and an understanding of signaling receptor trafficking to cilia is now emerging. A defining feature of cilia is the periciliary diffusion barrier that separates the ciliary and plasma membranes. Although lateral transport through this barrier may take place, polarized exocytosis to the base of the cilium has been the prevailing model for delivering membrane proteins to cilia. Key players for this polarized exocytosis model include the GTPases Rab8 and Rab11, the exocyst, and possibly the intraflagellar tranport machinery. In turn, the sorting of membrane proteins to cilia critically relies on the recognition of ciliary targeting signals by sorting machines such as the BBSome coat complex or the GTPase Arf4. Finally, some proteins need to exit from cilia, and ubiquitination may regulate this step. The stage is now set to dissect the interplay between signaling and regulated trafficking to and from cilia.