The ciliary targeting of membrane proteins by a ternary complex comprising transportin1, Rab8 and the ciliary targeting signal

The sensory functions of cilia are dependent on the enrichment of ciliary resident proteins. While it is known that ciliary targeting signals (CTSs) specifically target ciliary proteins to cilia, it is still unclear how CTSs facilitate the entry and retention of ciliary residents at the molecular level. We found that non-ciliary membrane reporters can passively diffuse to cilia via the lateral transport pathway and the translocation of membrane reporters through the ciliary diffusion barrier is facilitated by importin binding motifs/domains. Screening known CTSs of ciliary membrane residents uncovered that fibrocystin, photoreceptor retinol dehydrogenase, rhodopsin and retinitis pigmentosa 2 interact with transportin1 (TNPO1) via previously identified CTSs. We further discovered that a novel ternary complex, comprising TNPO1, Rab8 and CTS, can assemble or disassemble under the guanine nucleotide exchange of Rab8. Our study suggests a novel mechanism in which TNPO1/Rab8/CTS complex mediates selective entry and retention of cargos within cilia.