  The G-protein-coupled receptor (GPCR) activated by the neurotransmitter GABA
is made up of two subunits, GABA(B1) and GABA(B2). GABA(B1) binds agonists,
whereas GABA(B2) is required for trafficking GABA(B1) to the cell surface,
increasing agonist affinity to GABA(B1), and activating associated G proteins.
These subunits each comprise two domains, a Venus flytrap domain (VFT) and a
heptahelical transmembrane domain (7TM). How agonist binding to the GABA(B1)
VFT leads to GABA(B2) 7TM activation remains unknown. Here, we used a glycan
wedge scanning approach to investigate how the GABA(B) VFT dimer controls
receptor activity. We first identified the dimerization interface using a
bioinformatics approach and then showed that introducing an N-glycan at this
interface prevents the association of the two subunits and abolishes all
activities of GABA(B2), including agonist activation of the G protein. We also
identified a second region in the VFT where insertion of an N-glycan does not
prevent dimerization, but blocks agonist activation of the receptor. These data
provide new insight into the function of this prototypical GPCR and demonstrate
that a change in the dimerization interface is required for receptor
activation.
