Figure 1. A model for the activation mechanism of the heterodimeric GABA_B receptor based on the solved three-dimensional structure of the dimeric extracellular domain of mGlu1.

GABA_B1 is in blue, GABA_B2 in yellow. Binding of GABA in the cleft of GABA_B1 Venus Flytrap domain (VFT) leads to domain closure and a reorientation of both VFTs in the dimer. This is expected to induce a relative movement of the seven transmembrane (7TM) domains, allowing activation of GABA_B2 7TM. A similar activation mechanism is proposed for the mGlu receptors even though these have an additional cystein-rich domain that links their VFT to their 7TM.