Structures of metabotropic GABAB receptor

Abstract

GABA (γ-aminobutyric acid) stimulation of the metabotropic GABA B receptor results in prolonged inhibition of neurotransmission that is central to brain physiology 1 . GABA B belongs to the Family C of G protein-coupled receptors (GPCRs), which operate as dimers to relay synaptic neurotransmitter signals into a cellular response through the binding and activation of heterotrimeric G proteins 2 , 3 . GABA B , however, is unique in its function as an obligate heterodimer in which agonist binding and G protein activation take place on distinct subunits 4 , 5 . Here we show structures of heterodimeric and homodimeric full-length GABA B receptors. Complemented by cellular signaling assays and atomistic simulations, the structures reveal an essential role for the GABA B extracellular loop 2 (ECL2) in relaying structural transitions by ordering the linker connecting the extracellular ligand-binding domain to the transmembrane region. Furthermore, the ECL2 of both GABA B subunits caps and interacts with the hydrophilic head of a phospholipid occupying the extracellular half of the transmembrane domain, thereby providing a potentially crucial link between ligand binding and the receptor core that engages G protein. These results provide a starting framework to decipher mechanistic modes of signal transduction mediated by GABA B dimers and have important implications for rational drug design targeting these receptors.

The GABAB Receptor—Structure, Ligand Binding and Drug Development

“…Mao et al further suggest that G-proteins may bind to one of the subunits at a time due to sterically hindrance, favoring the GABA B2 as this was the most frequent populated distribution found in processing the cryo-EM data [6]. The recent cryo-EM structures describe that the extracellular half of the 7TM region of both monomers are occupied by a phospholipid, in both the active and inactive conformations, at a site corresponding to the orthosteric binding sites in most family A GPCRs [6,8,9]. Moreover, a "lid" is formed over the lipids in each subunit by residues located in the ECL2 of GABA B1b , and all ECLs in GABA B2 including the linker [8,9].…”

“…The recent cryo-EM structures describe that the extracellular half of the 7TM region of both monomers are occupied by a phospholipid, in both the active and inactive conformations, at a site corresponding to the orthosteric binding sites in most family A GPCRs [6,8,9]. Moreover, a "lid" is formed over the lipids in each subunit by residues located in the ECL2 of GABA B1b , and all ECLs in GABA B2 including the linker [8,9]. The lipids are suggested to play a role in receptor activation and structural integrity as seen by mutational studies of interacting residues in GABA B1b , trying to displace the lipid tail and interfere with coordination of the head group [8,9].…”

“…Moreover, a "lid" is formed over the lipids in each subunit by residues located in the ECL2 of GABA B1b , and all ECLs in GABA B2 including the linker [8,9]. The lipids are suggested to play a role in receptor activation and structural integrity as seen by mutational studies of interacting residues in GABA B1b , trying to displace the lipid tail and interfere with coordination of the head group [8,9]. The presence of the lipid also blocks the access of allosteric modulators to the 7TM domains, which would have to displace the lipid or bind to an alternative binding site [8].…”

“…The lipids are suggested to play a role in receptor activation and structural integrity as seen by mutational studies of interacting residues in GABA B1b , trying to displace the lipid tail and interfere with coordination of the head group [8,9]. The presence of the lipid also blocks the access of allosteric modulators to the 7TM domains, which would have to displace the lipid or bind to an alternative binding site [8]. Park et al suggests that the lipid in GABA B1b acts as a negative allosteric modulator (NAM) by stabilizing the inactive conformation [9].…”

“…The paper by Shaye and coworkers [7] describes the full-length inactive apo receptor, an active receptor conformation bound to the agonist SKF97541 and the PAM GS39783, and two intermediate agonist-bound receptor conformations. The latest papers by Papasergi-Scott et al and Park et al describes the cryo-EM structure of the full-length inactive receptor [8,9]. These structures may provide new opportunities for understanding receptor mechanisms and for drug discovery.…”