Figure 5. Trafficking systems that cross the transition zone ciliary gate create a dynamic signaling compartment.

Intraflagellar transport (IFT) and other transport systems such as lipidated protein intraflagellar targeting (LIFT) cross the TZ and therefore make the cilium a dynamic compartment. The LIFT system, which includes the lipidated protein‐binding chaperones UNC119 and PDE6D, facilitates the transport of lipid‐modified proteins such as INPP5E and NPHP3 into cilia. Cargo release is mediated by effectors, including ARL3 and RP2. At least for some ciliary cargos, including GPCR proteins, the IFT system (including IFT‐A complex‐associated TULP3) makes use of a PIP₂‐PI₄P lipid gradient established by INPP5E to bind cargo outside the cilium (high PIP₂) and release it inside the cilium (low PIP₂). Functional interactions between IFT and LIFT (shown as question mark) have been documented but require further investigation (see text for details).