In a study published in Structure, the teams of Drs. LI Dianfan, ZHAO Yun, and CONG Yao from the Center for Excellence in Molecular Cell Science, Shanghai Institute of Biochemistry and Cell Biology of the Chinese Academy of Sciences, and Dr. QU Qianhui's team from Fudan University, reported the cryo-electron microscopy structure of Patched (Ptc1) in the lipid nanodisc, a native-like environment that contains phospholipids and cholesterol.
The evolutionarily conserved Hedgehog (Hh) signaling pathway dictates morphogenesis for both embryonic and adult cells in metazoan. Ptc1, a negative modulator for Hh signaling, is central to this important pathway. It acts as a transporter to deplete sterols in the inner leaflet of the plasma membrane that are required for the activation of downstream regulators in this pathway. Binding to its proteinaceous ligand Hh inhibits this function and activates the Hh signaling pathway.
Ptc1 is clustered in caveolae, a vesicular organelle that is referred to as the ‘signaling hub’ in the cell. Caveolae is rich in sterol and sphingolipids and is highly curved. Previous studies have focused on Ptc1 in detergents and amphipols, amphipathic chemicals that mimic lipids.
The researchers in this study found that despite the overall similar shape with the previous structures in flat membrane mimetic, the nanodisc-reconstituted Ptc1 showed several conformational differences that may be related to its function.
Specifically, the transmembrane region near its sterol-sensing domain (part of the sterol-transporting conduit) is more compact and the ‘last stop’ in the sterol-transporting conduit displays flexibility, which are speculated to be related to the transport and release of sterols.
Although the three-dimensional structure was determined as a monomer, there exist significant amounts of Ptc1 particles in several distinct dimeric states. Remarkably, these dimeric states are coupled with different membrane curvatures. For the most bent membrane, the curvature is similar to that reported for native caveolae.
The interactions observed in this study, together with the versatile binding mode of Ptc1 reported in the scientific literature, provide intriguing clues about how Ptc1 clusters in caveolae for efficient signal transduction.