Elucidating CCRK’s function in ciliary assembly and cilium‐dependent signaling

Our studies are focused on the questions of how ciliary length and ciliary import of cell signaling regulators are controlled. Despite many advances, these questions have yet to be fully answered. One clue as to the mechanistic basis of these processes comes from studies on Cell Cycle‐Related Kinase (CCRK). Work on the Clamydomonas homolog, LF2, indicates that this kinase ensures that flagella are neither too long nor too short, likely mediated by regulated transport of ciliary tubulin ( Craft et al., 2015). We previously showed that mammalian CCRK is similarly required for ciliary length control and that it is needed for cells to achieve appropriate responses to Hedgehog (Hh) signals ( Snouffer et al., 2017). Additional work indicated that CCRK promotes the efficient transport of Hh pathway regulatory factors, such as Smoothened and Gli2, into cilia. However, the manner by which the kinase directs these processes is unknown. One target of CCRK phosphorylation is the ciliary length regulator Intestinal Cell Kinase (ICK); CCRK‐mediated phosphorylation in ICK’s TDY motif promotes ICK activity. Our analysis of Ccrk/Ick double mutants indicates that CCRK must also function via ICK‐independent mechanisms and that ICK promotes functions in the absence of CCRK‐mediated phosphorylation. To gain insight into the ICK‐independent CCRK functions in ciliary length control and ciliary cargo import, we performed phosphoproteomic screens for potential mediators of CCRK function. This screen has pointed to candidate mediators that regulate microfilament and microtubule dynamics. Consistent with a role for CCRK’s action via microfilament dynamics, Ccrk mutant, but not wild‐type, cells showed greatly reduced import of ciliary cargo upon treatment with sub‐threshold doses of the actin polymerization inhibitor Cytochalasin D. Our ongoing work on this mechanism will be presented. Support or Funding Information This work was supported by National Institute of Child Health and Human Development grant number R01HD050761 and by the Office of the Vice President of Research at the University of Georgia.ReferencesCraft et al. ( 2015 ). Tubulin transport by IFT is upregulated during ciliary growth by a cilium-autonomous mechanism . J Cell Biol. 2015 Jan 19; 208 ( 2 ): 223 – 37 .Snouffer et al., ( 2017 ). Cell Cycle-Related Kinase (CCRK) regulates ciliogenesis and Hedgehog signaling in mice . PLoS Genet . 2017 Aug 17; 13 ( 8 )