Hedgehog regulates smoothened activity by inducing a conformational switch

Abstract

Hedgehog (HH) morphogen is essential for metazoan development. The seven-transmembrane protein smoothened (SMO) transduces the HH signal across the plasma membrane, but how SMO is activated remains poorly understood. In Drosophila melanogaster, HH induces phosphorylation at multiple Ser/Thr residues in the SMO carboxy-terminal cytoplasmic tail, leading to its cell surface accumulation and activation. Here we provide evidence that phosphorylation activates SMO by inducing a conformational switch. This occurs by antagonizing multiple Arg clusters in the SMO cytoplasmic tail. The Arg clusters inhibit SMO by blocking its cell surface expression and keeping it in an inactive conformation that is maintained by intramolecular electrostatic interactions. HH-induced phosphorylation disrupts the interaction, and induces a conformational switch and dimerization of SMO cytoplasmic tails, which is essential for pathway activation. Increasing the number of mutations in the Arg clusters progressively activates SMO. Hence, by employing multiple Arg clusters as inhibitory elements counteracted by differential phosphorylation, SMO acts as a rheostat to translate graded HH signals into distinct responses.The HH morphogen controls many key development processes, with different thresholds specifying distinct outcomes [1][2][3][4] . In Drosophila wing discs, HH proteins secreted by posterior (P) compartment cells move into the anterior (A) compartment to form a local concentration gradient 5,6 . Low levels of HH suffice to induce the expression of decapentaplegic (dpp), whereas high levels are required to induce patched (ptc) and engrailed (en) ( Supplementary Fig. 1) [7][8][9] .The reception system for HH consists of a twelve-transmembrane protein, PTC, as the HH receptor and a seven-transmembrane protein smoothened (SMO) as the signal transducer 10-13 . In Drosophila, HH binding to PTC abrogates its inhibition on SMO and induces extensive phosphorylation of the SMO cytoplasmic tail by protein kinase A (PKA) and casein kinase I (CKI), leading to SMO cell surface accumulation and activation [14][15][16][17] . How phosphorylation promotes SMO cell surface accumulation is not understood. In addition, phosphorylation may regulate SMO activity through mechanism(s) other than controlling its cell surface abundance. Regulation of SMO by multiple Arg clustersOur previous study indicates that phosphorylation may regulate SMO cell surface abundance by either preventing its endocytosis and/or promoting its recycling 15 . To investigate further how SMO cell surface expression is regulated, we generated a set of C-terminally truncated SMO variants and examined their subcellular localization using a cell-based assay (Fig. 1). Deletion up to amino acid 818 did not significantly change SMO subcellular distribution; however, further deletions resulted in progressively increased cell surface expression (Fig. 1a, c), implying that multiple negative regulatory elements exist between amino acids 661-818.SMODC710 exhibits consistently higher cell surface...

Activation of the hedgehog-signaling pathway in human cancer and the clinical implications

“…There are two important changes during mammalian SMO signaling. First, SMO protein undergoes conformational changes to favor intermolecular interaction of SMO (Zhao et al, 2007). It is still not clear how this conformational change is regulated.…”

Smoothened transduces Hedgehog signal by forming a complex with Evc/Evc2

“…Although Smo is activated through a conserved mechanism in Drosophila and mammals [7,9], the mechanisms by which activated Smo transduces the signal to the downstream signaling components appear to diverge. In Drosophila, Smo forms a complex with the kinesinlike protein Costal2 (Cos2) and the Ser/Thr kinase Fused (Fu) in a manner regulated by Hh and Smo phosphorylation [16,[17][18][19][20][21].…”

“…In the absence of Hh ligands, Ptc blocks the activity of Smo and full-length Ci/Gli is phosphorylated by multiple kinases including PKA, CK1 and GSK3β, which targets it for Slimb/β-TRCP-mediated proteolysis to generate its repressor form (Ci R /Gli R ) [5]. In the presence of Hh, the Ptc inhibition of Smo is released through Hh binding to Ptc; Smo is phosphorylated by multiple kinases including PKA (Drosophila only), CK1 and Gprk2 kinases, which promotes its active conformation and changes its subcellular localization with Drosophila Smo accumulating on the cell surface and mammalian Smo accumulating in the primary cilium [6][7][8][9][10][11]. Ci/Gli phosphorylation and prote-olysis is blocked, leading to diminished Ci R /Gli R activity [12][13][14].…”

Structure du récepteur Smoothened

“…La protéine Smo de vertébrés forme aussi des dimères en culture cellulaire. Il conviendra de déterminer si la formation de tels dimères, voire d'oligomères qui ont aussi été décrits [25,26], participe aux phénomènes de coopérativité observés dans des expériences pharmacologiques [5,6,8,9].…”