Editors' ChoiceCell Biology

A Partner for Smoothened in Cilia

Sci. Signal.  30 Oct 2012:
Vol. 5, Issue 248, pp. ec281
DOI: 10.1126/scisignal.2003735

Primary cilia are critical for Hedgehog (Hh) signaling in vertebrates, and defects in Hh signaling or cilium structure cause pleiotropic diseases known as ciliopathies in humans. Binding of a Hh ligand to the receptor Patched (Ptc) relieves Ptc-mediated inhibition of the seven-transmembrane protein Smoothened (Smo). Through a mechanism that is not entirely clear, this leads to accumulation of Smo in the cilium and subsequent recruitment of Suppressor of Fused (SuFu) and the Gli transcription factors into the cilium. The Glis are then processed into active forms that translocate into the nucleus to mediate Hh signaling outputs. Mutations in the gene encoding Ellis–van Crevid syndrome protein 2 (Evc2) cause inherited ciliopathy syndromes in which cilia of various cell types appear normal yet show impaired Hh signaling. Dorn et al. report that the cilium-localized protein Evc2 was required for Sonic hedgehog (Shh)–induced transcription of GLI1 in cultured human fibroblasts but had no function in ciliary architecture. A dominant-negative form of Evc2 (Evc2ΔW) prevented signaling by a constitutively activated form of Smo but did not affect Hh signaling induced by a dominant-negative form of protein kinase A (PKA) or in sufu–/– mouse embryonic fibroblasts, indicating that Evc2 acted upstream of these negative regulators of Hh signaling. Depletion of Evc2 by RNA interference did not affect the localization of Smo to cilia but reduced the accumulation of SuFu and Gli proteins in cilia upon pathway activation. Evc2 localized to a ring near the base of the cilium (the EvC zone) distal to the transition zone, which has been proposed to form a diffusion boundary between the cellular and ciliary membranes, and this localization was required for Evc2 activity. Two amino acids within the 43–amino acid W-peptide that is missing from the C terminus of Evc2ΔW were required for proper localization of Evc2, and the W-peptide conferred EvC zone localization to Smo when fused to the Smo C terminus. Cross-linking and immunoprecipitation experiments indicated that signaling induced by Shh or a Smo antagonist caused Evc2 to interact directly with Smo but not with SuFu, PKA, or Gli2. Smo colocalization with Evc2 in the EvC zone after Hh pathway activation was required for Gli-dependent transcription, and Evc2 mutants that failed to localize to the EvC zone dominantly inhibited this signaling output. Evc2-Smo complex formation is, therefore, important for Hh signal transduction, and further work will be required to determine whether the effect of Evc2 on Hh signaling is limited to its ability to localize Smo to a distinct domain within the cilium or whether it has additional effects on Smo activity.

K. V. Dorn, C. E. Hughes, R. Rohatgi, A Smoothened-Evc2 complex transduces the Hedgehog signal at primary cilia. Dev. Cell 23, 823–835 (2012). [PubMed]