Structures of metabotropic GABA_Breceptor

Abstract

GABA (γ-aminobutyric acid) stimulation of the metabotropic GABAB receptor results in prolonged inhibition of neurotransmission that is central to brain physiology. GABAB 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. GABAB, however, is unique in its function as an obligate heterodimer in which agonist binding and G protein activation take place on distinct subunits. Here we show structures of heterodimeric and homodimeric full-length GABAB receptors. Complemented by cellular signaling assays and atomistic simulations, the structures reveal an essential role for the GABAB 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 GABAB 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 GABAB 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.…”