Expanding horizons: ciliary proteins reach beyond cilia.

Once obscure, the cilium has come into the spotlight during the past decade. It is now clear that aside from generating locomotion by motile cilia, both motile and immotile cilia serve as signaling platforms for the cell. Through both motility and sensory functions, cilia play critical roles in development, homeostasis, and disease. To date, the cilium proteome contains more than 1,000 different proteins, and human genetics is identifying new ciliopathy genes at an increasing pace. Although assigning a function to immotile cilia was a challenge not so long ago, the myriad of signaling pathways, proteins, and biological processes associated with the cilium have now created a new obstacle: how to distill all these interactions into specific themes and mechanisms that may explain how the organelle serves to maintain organism homeostasis. Here, we review the basics of cilia biology, novel functions associated with cilia, and recent advances in cilia genetics, and on the basis of this framework, we further discuss the meaning and significance of ciliary connections.