Functions and trafficking mechanisms of RIC-8 in C. elegans and mammalian cilia

Primary cilia exhibit conserved organization and contain structural and functional domains of unique molecular composition. The inversin compartment (InvC), which is found in the proximal ciliary segment of a subset of vertebrate and invertebrate cell types, concentrates different classes of signaling molecules. Mutations in genes encoding resident proteins of the InvC manifest in ciliopathies, highlighting the importance of the InvC in cilia biology. We previously showed that a chaperone of G proteins RIC-8 localizes to the InvC of C. elegans channel cilia; however, the mechanisms that regulate RIC-8 targeting to this ciliary sub-domain or RIC-8 function in the InvC remain unknown. Here, we build on our prior work to demonstrate that RIC-8 becomes restricted to the InvC during larval development and show that, while the RVxP motif and intact transition zone are required for its proper intraciliary distribution, RIC-8 localization to the cilium depends on intraflagellar transport. Using the ASH neuron as a model, we establish that RIC-8 modulates chemosensory responses mediated by channel cilia. Finally, we show that human RIC8A and RIC8B proteins are required for ciliogenesis in RPE-1 cells. Collectively, our results define ciliary trafficking mechanisms and novel functions for a highly conserved signaling protein.