Abstract
G-protein-coupled receptors (GPCRs) have central roles in intercellular communication1,2. Structural studies have revealed how GPCRs can activate G proteins. However, whether this mechanism is conserved among all classes of GPCR remains unknown. Here we report the structure of the class-C heterodimeric GABAB receptor, which is activated by the inhibitory transmitter GABA, in its active form complexed with Gi1 protein. We found that a single G protein interacts with the GB2 subunit of the GABAB receptor at a site that mainly involves intracellular loop 2 on the side of the transmembrane domain. This is in contrast to the G protein binding in a central cavity, as has been observed with other classes of GPCR. This binding mode results from the active form of the transmembrane domain of this GABAB receptor being different from that of other GPCRs, as it shows no outside movement of transmembrane helix 6. Our work also provides details of the inter- and intra-subunit changes that link agonist binding to G-protein activation in this heterodimeric complex.
Highlights
From the inactive G protein, and the activation of these proteins upon GTP binding[2]
Our results reveal a mode of G-protein coupling that differs from those that have previously been reported for G-protein-coupled receptors (GPCRs) of other classes; our structures reveal that small movements of TM3 and TM5 lead to changes in the intracellular loops (ICLs) that offer a binding site for the G protein on the side of the GB2 subunit of GABAB
Our cryo-electron microscopy (cryo-EM) structure of the GABAB–Gi complex stabilized with an agonist and a positive allosteric modulator (PAM) reveals an asymmetric activation process in which a single G protein interacts with GB2
Summary
From the inactive G protein, and the activation of these proteins upon GTP binding[2]. Our results reveal a mode of G-protein coupling that differs from those that have previously been reported for GPCRs of other classes; our structures reveal that small movements of TM3 and TM5 lead to changes in the intracellular loops (ICLs) that offer a binding site for the G protein on the side of the GB2 subunit of GABAB These data help to refine models that describe how agonist binding in the VFT domain of GB1 leads to the activation of the TMD of GB2, and how small molecules can act as PAMs. Using a modified version of a previously established protocol[11] (Extended Data Fig. 1), we assembled the GABAB–Gil complex by incubating purified GABAB with Gi1 in the presence of the agonist baclofen and the PAM R,S-5,7-di-tert-butyl-3-hydroxy-3-trifluoromethyl-3H-benz ofuran-2-one (BHFF)[19] (Fig. 1a, b).
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