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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
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 addiction¹. A unique GPCR that is known to require heterodimerization for function^2,3,4,5,6, 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^7,8, while GABA_B2 couples with G proteins^{9,10,11,12,13,14}. Each subunit is characterized by an extracellular Venus flytrap (VFT) module, a descending peptide linker, a seven-helix transmembrane domain and a cytoplasmic tail¹⁵. Although the VFT heterodimer structure has been resolved¹⁶, 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 at atomic resolution, 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.