PMC

Structure of the primary cilium. The core structure of primary cilia is composed of microtubule bundles (ciliary axoneme) extending from the basal body, a microtubule-based structure derived from the mother centriole. The ciliary membrane is continuous with the plasma membrane, but contains a unique protein composition, such as channels and receptors. Thus, primary cilia can function as a sensory organelle for receiving and transducing extracellular stimuli into cells, such as fluid flow or via signaling molecules. The inset image is of ciliated murine inner medullary collecting duct (IMCD3) cells in which the basal body is labeled with a γ-tubulin antibody (red) and the primary cilium is marked by an Ift88 antibody (green).

Polarized vesicle trafficking mediates the formation and function of primary cilia. Enriched expression of receptors and ion channels on the ciliary membrane make the primary cilium a specialized organelle for receiving and transducing extracellular stimuli into cells. Proteins synthesized from the Golgi move to the primary cilium through polarized vesicle trafficking utilizing microtubule networks (not shown). For instance, vesicles carrying ciliary proteins leave the Golgi and move toward the basal body of the primary cilium. These vesicles can either be delivered to the surface plasma membrane and then the protein cargo moves to the ciliary membrane or they can be trafficked toward the basal body through Rab proteins, IFT20, or exocysts. Entry of protein cargo to the cilium is regulated by active forms of Rab8, a master modulator for the ciliary protein trafficking. Rab8 is recruited to the basal body of primary cilium, possibly mediated by Ahi1. The activities of Rab8 are then regulated by Rabin8, and its activity and basal body localization is modulated by the BBSome and Rab11. Once proteins are transported into the primary cilium, the IFT system continues the trafficking of these proteins or membrane receptors up and down along the ciliary axoneme. IFT-B (anterograde) and IFT-A (retrograde) are protein complexes associated with the molecular motors, kinesin-2 and cytoplasmic dynein, respectively.