  The gamma-aminobutyric acid type B (GABAB) receptor is an allosteric complex
made of two subunits, GABAB1 (GB1) and GABAB2 (GB2). Both subunits are composed
of an extracellular Venus flytrap domain (VFT) and a heptahelical domain (HD).
GB1 binds GABA, and GB2 plays a major role in G-protein activation as well as
in the high agonist affinity state of GB1. How agonist affinity in GB1 is
regulated in the receptor remains unknown. Here, we demonstrate that GB2 VFT is
a major molecular determinant involved in this control. We show that isolated
versions of GB1 and GB2 VFTs in the absence of the HD and C-terminal tail can
form hetero-oligomers as shown by time-resolved fluorescence resonance energy
transfer (based on HTRF technology). GB2 VFT and its association with GB1 VFT
controlled agonist affinity in GB1 in two ways. First, GB2 VFT exerted a direct
action on GB1 VFT, as it slightly increased agonist affinity in isolated GB1
VFT. Second and most importantly, GB2 VFT prevented inhibitory interaction
between the two main domains (VFT and HD) of GB1. According to this model, we
propose that GB1 HD prevents the possible natural closure of GB1 VFT. In
contrast, GB2 VFT facilitates this closure. Finally, such inhibitory contacts
between HD and VFT in GB1 could be similar to those important to maintain the
inactive state of the receptor.
