Sterols in an intramolecular channel of Smoothened mediate Hedgehog signaling
- Nat Chem Biol. 2020 Dec;16(12):1368-1375. doi: 10.1038/s41589-020-0646-2.
- 1. Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX, USA.
- 2. Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, TX, USA. [email protected].
- 3. Department of Biophysics, University of Texas Southwestern Medical Center, Dallas, TX, USA. [email protected].
Smoothened (Smo), a class Frizzled G protein-coupled receptor (class F GPCR), transduces the Hedgehog signal across the cell membrane. Sterols can bind to its extracellular cysteine-rich domain (CRD) and to several sites in the seven transmembrane helices (7-TMs) of Smo. However, the mechanism by which sterols regulate Smo via multiple sites is unknown. Here we determined the structures of SMO-Gi complexes bound to the synthetic Smo agonist (SAG) and to 24(S),25-epoxycholesterol (24(S),25-EC). A novel sterol-binding site in the extracellular extension of TM6 was revealed to connect Other sites in 7-TMs and CRD, forming an intramolecular sterol channel from the middle side of 7-TMs to CRD. Additional structures of two gain-of-function variants, SmoD384R and SmoG111C/I496C, showed that blocking the channel at its midpoints allows sterols to occupy the binding sites in 7-TMs, thereby activating Smo. These data indicate that sterol transport through the core of Smo is a major regulator of SMO-mediated signaling.