Gates for soluble and membrane proteins, and two trafficking systems (IFT and LIFT), establish a dynamic ciliary signaling compartment

- Primary cilia are sensory organelles essential for cellular signaling.
- Cilia use gates and transport systems to form regulatable signaling compartments.
- Soluble and membrane protein diffusion barriers act within the transition zone.
- Intraflagellar transport (IFT) uses motors to traffic proteins in and out of cilia.
- Lipidated protein intraflagellar targeting (LIFT) ferries prenylated cargo into cilia.

Primary cilia are microtubule-based organelles found on most mammalian cell surfaces. They possess a soluble matrix and membrane contiguous with the cell body cytosol and plasma membrane, and yet, have distinct compositions that can be modulated to enable dynamic signal transduction. Here, we discuss how specialized ciliary compartments are established using a coordinated network of gating, trafficking and targeting activities. Cilium homeostasis is maintained by a size-selective molecular mesh that limits soluble protein entry, and by a membrane diffusion barrier localized at the transition zone. Bidirectional protein shuttling between the cell body and cilium uses IntraFlagellar Transport (IFT), and prenylated ciliary protein delivery is achieved through Lipidated protein IntraFlagellar Targeting (LIFT). Elucidating how these gates and transport systems function will help reveal the roles that cilia play in ciliary signaling and the growing spectrum of disorders termed ciliopathies.