Cell Biology

Directing Receptors into Cilia

Sci. Signal.  20 Nov 2012:
Vol. 5, Issue 251, pp. ec299
DOI: 10.1126/scisignal.2003787

Guanosine triphosphatases (GTPases) function in protein trafficking. Mutations in the gene encoding the GTPase ARL13B are associated with the ciliopathy Joubert syndrome. To identify interacting partners of this GTPase, Li et al. turned to the nematode Caenorhabditis elegans, which has a single ortholog, ARL-13. A yeast two-hybrid screen identified the E2 SUMO-conjugating enzyme UBC-9 as an interacting partner, an interaction that was confirmed with in vitro assays. Both proteins were localized to the middle segments of cilia. In vitro SUMOylation assays indicated that ARL-13 was conjugated to SUMO, and mutational analysis identified two sites of modification (Lys239 and Lys328). A SUMOylation-deficient mutant (the K239,328R mutant) localized to the middle segments of cilia and rescued the cilia formation phenotype of the ARL-13–null worms, but it failed to rescue the behavioral defects related to inappropriate targeting of cilia-localized sensory proteins, PKD-2 (a mechanosensory receptor) and ODR-10 (an olfactory receptor). Indeed, the ciliary localization of these two receptors was not rescued by introduction of the SUMOylation-deficient ARL-13 mutant into the ARL-13–null animals. Introduction of a form of the K239,328R mutant with SUMO fused to the C terminus of the mutant protein restored the ciliary localization of PKD-2 and ODR-10 and rescued the behavioral defects. In vitro analysis showed that human UBC-9 and ARL13B interacted and SUMOylation of ARL13B could be detected when the proteins were overexpressed in cultured cells. A SUMOylation-deficient ARL13B mutant localized to cilia, but in cells in which wild-type ARL13B was knocked down, these mutants reduced the ciliary localization of polycystin-2, a protein associated with polycystic kidney disease. Thus, SUMO modification appears to have a role in protein trafficking in primary cilia, and aberrations in this process may contribute to various human disorders.

Y. Li, Q. Zhang, Q. Wei, Y. Zhang, K. Ling, J. Hu, SUMOylation of the small GTPase ARL-13 promotes ciliary targeting of sensory receptors. J. Cell Biol. 199, 589–598 (2012). [Abstract] [Full Text]