Huang Pengxiang, Nedelcu Daniel, Watanabe Miyako, Jao Cindy, Kim Youngchang, Liu Jing, Salic Adrian
Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA.
Structural Biology Center, Biosciences Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
Cell. 2016 Aug 25;166(5):1176-1187.e14. doi: 10.1016/j.cell.2016.08.003. Epub 2016 Aug 18.
In vertebrates, sterols are necessary for Hedgehog signaling, a pathway critical in embryogenesis and cancer. Sterols activate the membrane protein Smoothened by binding its extracellular, cysteine-rich domain (CRD). Major unanswered questions concern the nature of the endogenous, activating sterol and the mechanism by which it regulates Smoothened. We report crystal structures of CRD complexed with sterols and alone, revealing that sterols induce a dramatic conformational change of the binding site, which is sufficient for Smoothened activation and is unique among CRD-containing receptors. We demonstrate that Hedgehog signaling requires sterol binding to Smoothened and define key residues for sterol recognition and activity. We also show that cholesterol itself binds and activates Smoothened. Furthermore, the effect of oxysterols is abolished in Smoothened mutants that retain activation by cholesterol and Hedgehog. We propose that the endogenous Smoothened activator is cholesterol, not oxysterols, and that vertebrate Hedgehog signaling controls Smoothened by regulating its access to cholesterol.
在脊椎动物中，固醇对于刺猬信号通路是必需的，该通路在胚胎发生和癌症中至关重要。固醇通过结合其富含半胱氨酸的细胞外结构域（CRD）来激活膜蛋白平滑受体（Smoothened）。主要未解决的问题涉及内源性激活固醇的性质及其调节平滑受体的机制。我们报告了与固醇复合以及单独存在时CRD的晶体结构，揭示固醇会诱导结合位点发生显著的构象变化，这足以激活平滑受体，并且在含CRD的受体中是独特的。我们证明刺猬信号通路需要固醇与平滑受体结合，并确定了固醇识别和活性的关键残基。我们还表明胆固醇本身能结合并激活平滑受体。此外，在保留由胆固醇和刺猬蛋白激活能力的平滑受体突变体中，氧化固醇的作用被消除。我们提出内源性平滑受体激活剂是胆固醇，而非氧化固醇，并且脊椎动物的刺猬信号通路通过调节平滑受体获取胆固醇来控制它。
