Protein transport in growing and steady-state cilia

Cilia and eukaryotic flagella are threadlike cell extensions with motile and sensory functions. Their assembly requires intraflagellar transport (IFT), a bidirectional motor-driven transport of protein carriers along the axonemal microtubules. IFT moves ample amounts of structural proteins including tubulin into growing cilia likely explaining its critical role for assembly. IFT continues in non-growing cilia contributing to a variety of processes ranging from axonemal maintenance and the export of non-ciliary proteins to cell locomotion and ciliary signaling. Here, we discuss recent data on cues regulating the type, amount and timing of cargo transported by IFT. A regulation of IFT-cargo interactions is critical to establish, maintain and adjust ciliary length, protein composition and function. Cilia and flagella are widely distributed cell organelles with motile and sensory functions. The intraflagellar transport (IFT) pathway moves proteins in and out of cilia and is required for ciliary assembly, maintenance and signaling. Here, we discuss recent data revealing a complex regulation of IFT-cargo interactions. Continuously running IFT may prevent diffusional equilibration between the cell body and the cilium establishing and maintaining the specific protein content of cilia.