GABA B receptors are the G-protein-coupled receptors for GABA, the main inhibitory neurotransmitter in the brain. Two receptor subtypes, GABA B(1a,2) and GABA B(1b,2) , are formed by the assembly of GABA B1a and GABA B1b subunits with GABA B2 subunits. The GABA B1b subunit is a shorter isoform of the GABA B1a subunit lacking two N-terminal protein interaction motifs, the sushi domains. Selectively GABA B1a protein traffics into the axons of glutamatergic neurons, whereas both the GABA B1a and GABA B1b proteins traffic into the dendrites. The mechanism(s) and targeting signal(s) responsible for the selective trafficking of GABA B1a protein into axons are unknown. Here, we provide evidence that the sushi domains are axonal targeting signals that redirect GABA B1a protein from its default dendritic localization to axons. Specifically, we show that mutations in the sushi domains preventing protein interactions preclude axonal localization of GABA B1a . When fused to CD8␣, the sushi domains polarize this uniformly distributed protein to axons. Likewise, when fused to mGluR1a the sushi domains redirect this somatodendritic protein to axons, showing that the sushi domains can override dendritic targeting information in a heterologous protein. Cell surface expression of the sushi domains is not required for axonal localization of GABA B1a . Altogether, our findings are consistent with the sushi domains functioning as axonal targeting signals by interacting with axonally bound proteins along intracellular sorting pathways. Our data provide a mechanistic explanation for the selective trafficking of GABA B(1a,2) receptors into axons while at the same time identifying a well defined axonal delivery module that can be used as an experimental tool.

Functional asymmetry of G-protein-coupled receptor (GPCR) dimers has been reported for an increasing number of cases, but the molecular architecture of signalling units associated to these dimers remains unclear. Here, we characterized the molecular complex of the melatonin MT 1 receptor, which directly and constitutively couples to G i proteins and the regulator of G-protein signalling (RGS) 20. The molecular organization of the ternary MT 1 /G i /RGS20 complex was monitored in its basal and activated state by bioluminescence resonance energy transfer between probes inserted at multiple sites of the complex. On the basis of the reported crystal structures of G i and the RGS domain, we propose a model wherein one G i and one RGS20 protein bind to separate protomers of MT 1 dimers in a pre-associated complex that rearranges upon agonist activation. This model was further validated with MT 1 /MT 2 heterodimers. Collectively, our data extend the concept of asymmetry within GPCR dimers, reinforce the notion of receptor specificity for RGS proteins and highlight the advantage of GPCRs organized as dimers in which each protomer fulfils its specific task by binding to different GPCR-interacting proteins.