Trans-activation between 7TM domains: implication in heterodimeric GABAB receptor activation

Seven-transmembrane domain (7TM) receptors have important functions in cell-cell communication and can assemble into dimers or oligomers. Such complexes may allow specific functional cross-talk through trans-activation of interacting 7TMs, but this hypothesis requires further validation. Herein, we used the GABAB receptor, which is composed of two distinct subunits, GABAB1, which binds the agonist, and GABAB2, which activates G proteins, as a model system. By using a novel orthogonal-labelling approach compatible with time-resolved FRET and based on ACP- and SNAP-tag technologies to verify the heterodimerization of wild-type and mutated GABAB subunits, we demonstrate the existence of a direct allosteric coupling between the 7TMs of GABAB heterodimers. Indeed, a GABAB receptor, in which the GABAB2 extracellular domain was deleted, was still capable of activating G proteins. Furthermore, synthetic ligands for the GABAB2 7TM could increase agonist affinity at the GABAB1 subunit in this mutated receptor. In addition to bringing new information on GABAB receptor activation, these data clearly demonstrate the existence of direct trans-activation between the 7TM of two interacting proteins.