ICK (also known as CILK1) is a MAPK-like kinase localized at the ciliary tip. Its deficiency is known to result in the elongation of cilia, and causes ciliopathies in humans. However, little is known about how ICK is transported to the ciliary tip. We here show that the C-terminal noncatalytic region of ICK interacts with the intraflagellar transport (IFT)-B complex of the IFT machinery, and participates in its transport to the ciliary tip. Furthermore, total internal reflection fluorescence microscopy demonstrated that ICK undergoes bidirectional movement within cilia, similarly to IFT particles. Analysis of ICK-knockout cells demonstrated that ICK deficiency severely impairs the retrograde trafficking of IFT particles and ciliary GPCRs. In addition, we found that in ICK-knockout cells, ciliary proteins are accumulated at the bulged ciliary tip, which appeared to be torn off and release into the environment as an extracellular vesicle. The exogenous expression of various ICK constructs in ICK-knockout cells indicated that the IFT-dependent transport of ICK, as well as its kinase activity and phosphorylation at the canonical TDY motif, is essential for ICK function. Thus, we unequivocally show that ICK transported to the ciliary tip is required for retrograde ciliary protein trafficking and consequently for normal ciliary function.

CCRK/CDK20 was reported to interact with BROMI/TBC1D32 and regulate ciliary Hedgehog signaling. In various organisms, mutations in the orthologs of CCRK and those of the kinase ICK/CILK1, which is phosphorylated by CCRK, are known to result in cilia elongation. Furthermore, we recently showed that ICK regulates retrograde ciliary protein trafficking and/or the turnaround event at the ciliary tips, and that its mutations result in the elimination of intraflagellar transport (IFT) proteins that have overaccumulated at the bulged ciliary tips as extracellular vesicles, in addition to cilia elongation. However, how these proteins cooperate to regulate ciliary protein trafficking has remained unclear. We here show that the phenotypes of CCRK-knockout (KO) cells closely resemble those of ICK-KO cells; namely, the overaccumulation of IFT proteins at the bulged ciliary tips, which appear to be eliminated as extracellular vesicles, and the enrichment of GPR161 and Smoothened on the ciliary membrane. The abnormal phenotypes of CCRK-KO cells were rescued by the exogenous expression of wild-type CCRK but not its kinase-dead mutant or a mutant defective in BROMI binding. These results together indicate that CCRK regulates the turnaround process at the ciliary tips in concert with BROMI and probably via activating ICK.

Cell cycle-related kinase (CCRK) phosphorylates intestinal cell kinase (ICK). BROMI interacts with CCRK, CFAP20, and FAM149B1. In addition, FAM149B1 interacts directly with CCRK. We show that CCRK, BROMI, FAM149B1, and CFAP20 regulate the intraflagellar transport turnaround process under the control of ICK.