Trafficking into cilia
The primary cilium has been found to be associated with a number of cellular signaling pathways, such as vertebrate hedgehog signaling, and implicated in the pathogenesis of diseases affecting multiple organs including the neural tube, kidney and brain. The primary cilium is the site where a subset of the cell's membrane proteins is enriched. Processes determining the level of proteins in the ciliary membrane include entry into the compartment, removal, and retention by diffusion barriers such as the transition zone. However, pathways that target and concentrate membrane proteins in cilia are not well understood.
The tubby family proteins, Tulp3 and tubby (Tub) have been implicated as adapters in trafficking of multiple GPCRs into the ciliary membrane (Loktev and Jackson, 2013; Mukhopadhyay et al., 2013; Sun et al., 2012). These tubby family proteins have a N-terminal intraflagellar complex A (IFT-A) core binding conserved helix, and a C-terminal tubby domain that binds to PI(4,5)P2 (Mukhopadhyay et al., 2010; Santagata et al., 2001). Disrupting either of these domains prevents trafficking of these GPCRs to cilia, suggesting that Tulp3 “bridges” the GPCRs with IFT-A core in targeting them into cilia (Mukhopadhyay et al., 2010). We have recently shown that TULP3 to function as a general adapter for ciliary trafficking of structurally diverse integral membrane cargo, including multiple reported and novel rhodopsin family GPCRs, and polycystic kidney disease causing polycystin-1/2 complex. TUB also localizes to cilia similar to TULP3, and determines trafficking of a subset of these GPCRs to neuronal cilia. Using minimal ciliary localization sequences from GPCRs, and fibrocystin, also implicated in polycystic kidney disease, we demonstrate these motifs to be sufficient and TULP3-dependent for ciliary trafficking. We propose a three-step model for TULP3/TUB-mediated ciliary trafficking, including capture of diverse membrane cargo by tubby domain in a phosphoinositide 4,5-bisphosphate (PI(4,5)P2)-dependent manner, ciliary delivery by intraflagellar transport complex-A binding to TULP3/TUB N-terminus, and subsequent release into PI(4,5)P2-deficient ciliary membrane (Badgandi et al., 2017; Mukhopadhyay et al., 2010; Mukhopadhyay et al., 2013). Understanding the trafficking mechanisms that underlie the role of ciliary compartmentalization in signaling might provide unique approaches for intervention in progressive ciliopathies.
Badgandi, H.B., Hwang, S.H., Shimada, I.S., Loriot, E., and Mukhopadhyay, S. (2017). Tubby family proteins are adapters for ciliary trafficking of integral membrane proteins. J Cell Biol 216, 743-760.
Loktev, A.V., and Jackson, P.K. (2013). Neuropeptide Y family receptors traffic via the Bardet-Biedl syndrome pathway to signal in neuronal primary cilia. Cell Rep 5, 1316-1329.
Mukhopadhyay, S., Wen, X., Chih, B., Nelson, C.D., Lane, W.S., Scales, S.J., and Jackson, P.K. (2010). TULP3 bridges the IFT-A complex and membrane phosphoinositides to promote trafficking of G protein-coupled receptors into primary cilia. Genes Dev 24, 2180-2193.
Mukhopadhyay, S., Wen, X., Ratti, N., Loktev, A., Rangell, L., Scales, S.J., and Jackson, P.K. (2013). The ciliary G-protein-coupled receptor Gpr161 negatively regulates the Sonic hedgehog pathway via cAMP signaling. Cell 152, 210-223.
Santagata, S., Boggon, T.J., Baird, C.L., Gomez, C.A., Zhao, J., Shan, W.S., Myszka, D.G., and Shapiro, L. (2001). G-protein signaling through tubby proteins. Science 292, 2041-2050.
Sun, X., Haley, J., Bulgakov, O.V., Cai, X., McGinnis, J., and Li, T. (2012). Tubby is required for trafficking G protein-coupled receptors to neuronal cilia. Cilia 1, 21.