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Structure of human GABA_B receptor in an inactive state

Jinseo Park, Ziao Fu, Aurel Frangaj, Jonathan Liu, Lidia Mosyak, Tong Shen, Vesna N. Slavkovich, Kimberly M. Ray, Jaume Taura, Baohua Cao, Yong Geng, Hao Zuo, Yongjun Kou, Robert Grassucci, Shaoxia Chen, Zheng Liu, Xin Lin, Justin P. Williams, William J. Rice, Edward T. Eng, Rick K. Huang, Rajesh K. Soni, Brian Kloss, Zhiheng Yu, Jonathan A. Javitch, Wayne A. Hendrickson, Paul A. Slesinger, Matthias Quick, Joseph Graziano, Hongtao Yu, Oliver Fiehn, Oliver B. Clarke, Joachim Frank &
Qing R. Fan

• ABSTRACT
  (without references)
  The human GABA_B receptor—a member of the class C family of G-protein-coupled receptors (GPCRs)—mediates inhibitory neurotransmission and has been implicated in epilepsy, pain and addiction. A unique GPCR that is known to require heterodimerization for function, the GABA_B receptor has two subunits, GABA_B1 and GABA_B2, that are structurally homologous but perform distinct and complementary functions. GABA_B1 recognizes orthosteric ligands, while GABA_B2 couples with G proteins. Each subunit is characterized by an extracellular Venus flytrap (VFT) module, a descending peptide linker, a seven-helix transmembrane domain and a cytoplasmic tail. Although the VFT heterodimer structure has been resolved, the structure of the full-length receptor and its transmembrane signalling mechanism remain unknown. Here we present a near full-length structure of the GABA_B receptor, captured in an inactive state by cryo-electron microscopy. Our structure reveals several ligands that preassociate with the receptor, including two large endogenous phospholipids that are embedded within the transmembrane domains to maintain receptor integrity and modulate receptor function. We also identify a previously unknown heterodimer interface between transmembrane helices 3 and 5 of both subunits, which serves as a signature of the inactive conformation. A unique ‘intersubunit latch’ within this transmembrane interface maintains the inactive state, and its disruption leads to constitutive receptor activity.

I’m proud of being part of this study as it demonstrates highly successful transfer of technology from my lab to Qing’s at Columbia University:
Qing Fan, a former postdoc of Wayne Hendrickson, is an associate professor with experience in X-ray crystallography. Ziao (Jack) Fu, who was a graduate student in my group until a year ago (now postdoc at Rockefeller) trained the third author Aurel Frangaj in every aspect of cryo-EM and the two managed to get the first reconstructions at about 5 Å resolution after a 2-year effort. Assistant professor Oliver Clarke, also one of Wayne’s former postdocs, who got his cryo-EM training earlier from my former postdoc Amedee des Georges (now at CUNY) in the Ryanodine receptor project, then applied the most recent tools of refinement and drive the resolution up to the range of 3.3 Å that is sufficient for modeling the molecule.