TY - JOUR
T1 - Cryo-EM structures of a lipid-sensitive pentameric ligand-gated ion channel embedded in a phosphatidylcholine-only bilayer
AU - Kumar, Pramod
AU - Wang, Yuhang
AU - Zhang, Zhening
AU - Zhao, Zhiyu
AU - Cymes, Gisela D.
AU - Tajkhorshid, Emad
AU - Grosman, Claudio
N1 - Funding Information:
ACKNOWLEDGMENTS. Part of this work was performed at the Simons Electron Microscopy Center and National Resource for Automated Molecular Microscopy located at the New York Structural Biology Center, supported by grants from the Simons Foundation (349247) and the US NIH/National Institute of General Medical Sciences (GM103310), with additional support from Agouron Institute (F00316) and NIH (OD019994). Molecular simulations were performed using supercomputers from Blue Waters (ACI-1713784) (to E.T.) and Extreme Science and Engineering Discovery Environment (XSEDE) (TG-MCA06N060) (to E.T.). Blue Waters is supported by the National Science Foundation (OCI-0725070 and ACI-1238993) and the State of Illinois. XSEDE is supported by the National Science Foundation (ACI-1548562). This work was also supported by NIH Grants P41-GM104601, U54-GM087519, and R01-GM123455 (to E.T.) and R01-NS042169 (to C.G.), and a Richard and Margaret Romano Professorial Scholarship (to C.G.).
Publisher Copyright:
© 2020 National Academy of Sciences. All rights reserved.
PY - 2020/1/21
Y1 - 2020/1/21
N2 - The lipid dependence of the nicotinic acetylcholine receptor from the Torpedo electric organ has long been recognized, and one of the most consistent experimental observations is that, when reconstituted in membranes formed by zwitterionic phospholipids alone, exposure to agonist fails to elicit ion-flux activity. More recently, it has been suggested that the bacterial homolog ELIC (Erwinia chrysanthemi ligand-gated ion channel) has a similar lipid sensitivity. As a first step toward the elucidation of the structural basis of this phenomenon, we solved the structures of ELIC embedded in palmitoyl-oleoyl-phosphatidylcholine- (POPC-) only nanodiscs in both the unliganded (4.1-Å resolution) and agonist-bound (3.3 Å) states using single-particle cryoelectron microscopy. Comparison of the two structural models revealed that the largest differences occur at the level of loop C-at the agonist-binding sites-and the loops at the interface between the extracellular and transmembrane domains (ECD and TMD, respectively). On the other hand, the transmembrane pore is occluded in a remarkably similar manner in both structures. A straightforward interpretation of these findings is that POPC-only membranes frustrate the ECD-TMD coupling in such a way that the "conformational wave" of liganded-receptor gating takes place in the ECD and the interfacial M2-M3 linker but fails to penetrate the membrane and propagate into the TMD. Furthermore, analysis of the structural models and molecular simulations suggested that the higher affinity for agonists characteristic of the open- and desensitized-channel conformations results, at least in part, from the tighter confinement of the ligand to its binding site; this limits the ligand's fluctuations, and thus delays its escape into bulk solvent.
AB - The lipid dependence of the nicotinic acetylcholine receptor from the Torpedo electric organ has long been recognized, and one of the most consistent experimental observations is that, when reconstituted in membranes formed by zwitterionic phospholipids alone, exposure to agonist fails to elicit ion-flux activity. More recently, it has been suggested that the bacterial homolog ELIC (Erwinia chrysanthemi ligand-gated ion channel) has a similar lipid sensitivity. As a first step toward the elucidation of the structural basis of this phenomenon, we solved the structures of ELIC embedded in palmitoyl-oleoyl-phosphatidylcholine- (POPC-) only nanodiscs in both the unliganded (4.1-Å resolution) and agonist-bound (3.3 Å) states using single-particle cryoelectron microscopy. Comparison of the two structural models revealed that the largest differences occur at the level of loop C-at the agonist-binding sites-and the loops at the interface between the extracellular and transmembrane domains (ECD and TMD, respectively). On the other hand, the transmembrane pore is occluded in a remarkably similar manner in both structures. A straightforward interpretation of these findings is that POPC-only membranes frustrate the ECD-TMD coupling in such a way that the "conformational wave" of liganded-receptor gating takes place in the ECD and the interfacial M2-M3 linker but fails to penetrate the membrane and propagate into the TMD. Furthermore, analysis of the structural models and molecular simulations suggested that the higher affinity for agonists characteristic of the open- and desensitized-channel conformations results, at least in part, from the tighter confinement of the ligand to its binding site; this limits the ligand's fluctuations, and thus delays its escape into bulk solvent.
KW - Allosteric proteins
KW - Cys-loop receptors
KW - Lipid membranes
KW - Molecular dynamics
KW - Structure-function relationships
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U2 - 10.1073/pnas.1906823117
DO - 10.1073/pnas.1906823117
M3 - Article
C2 - 31911476
AN - SCOPUS:85078137932
SN - 0027-8424
VL - 117
SP - 1788
EP - 1798
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 3
ER -