Smoothened and classic G-coupled protein receptors are activated by a similar mechanism.
Major finding: Smoothened and classic G-coupled protein receptors are activated by a similar mechanism.
Mechanism: Cholesterol induces reorientation of the SMO CRD to drive the allosteric activation of the 7TM domain.
Impact: Inhibition of the remodeling of the 7TM site in SMO is a potential therapy for Hedgehog-active cancers.
The Hedgehog signaling pathway, which is frequently dysregulated in cancer, is triggered when the Hedgehog ligand binds and inhibits the PTCH surface receptor. This event relieves the repression that PTCH exerts on the transmembrane oncoprotein Smoothened (SMO), a member of Class F of the G-protein coupled receptor (GPCR) superfamily, leading to SMO activation. Having previously shown that cholesterol is the endogenous agonist that activates SMO by binding to its extracellular cysteine-rich domain (CRD), Huang and colleagues sought to determine the precise mechanism of SMO activation. Crystallization of full-length Xenopus SMO in complex with either the sterol alkaloid cyclopamine or cholesterol revealed the structure of sterol-activated SMO. Sterol binding did not alter the conformation of the hydrophobic cleft along the CRD, but caused a dramatic reorientation of the CRD relative to the 7TM domain of SMO. Further, the 7TM domain underwent a conformational change highly reminiscent of those reported for activation of Class A/B GPCRs, indicating that SMO and classic GPCRs are activated by a common conformational mechanism. Consistent with these findings, sterol-induced SMO CRD reorientation was required for allosteric activation of the 7TM domain. Structure-guided mutagenesis studies identified a π-cation lock between TM6 and TM7 of SMO which is critical for maintaining the inactive conformation, analogous to the TM3-TM6 locks in classic GPCRs, and is broken upon SMO activation; moreover, it is disrupted in an oncogenic SMO mutant found in basal cell carcinoma, thereby explaining its constitutive activity. Additional crystal structure analysis showed that both inactive and active SMO conformations are stabilized by a hydrogen bond network that connects 5 key residues. Finally, SMO activation results in remodeling of the 7TM site, causing a tunnel to open from the inner leaflet of the membrane to the extracellular space, that may permit upward cholesterol transport to the CRD. These results provide insight into the mechanisms underlying SMO activation and inactivation.
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