The G protein-coupling specificity of D2-like dopamine receptors

The G-protein coupling specificity of D2-like dopamine receptors was investigated using both receptor-G protein fusions and membranes of cells in which pertussis toxin-resistant mutants of individual Gi-family G proteins could be expressed in an inducible fashion. A range of ligands displayed agonism at the long isoform of the human dopamine D2 receptor. However, varying degrees of efficacy were observed for individual ligands as monitored by their capacity to load [35S] GTPγS onto each of Gi1, Gi2, Gi3 and Go1. By contrast, S-(-)-3-(3-hydroxyphenyl)-N-propylpiperidine (S-(-)-3PPP) was a partial agonist when Go1 was the target G protein but an antagonist/inverse agonist at Gi1, Gi2, Gi3. In ligand binding assays dopamine identified both high and low affinity states at each of the dopamine D2 receptor-G protein fusion proteins and the high affinity state was eliminated by guanine nucleotide. S-(-)-3PPP bound to an apparent single state of the constructs where the D2 receptor was fused to Gi1, Gi2 or Gi3. However, it bound to distinct high and low affinity states of the D2 receptor-Go1 fusion with the high affinity state being eliminated by guanine nucleotide. Similarly, although dopamine identified guanine nucleotide-sensitive high affinity states of the D2 receptor when expression of pertussis toxin-resistant forms of either Gi2 or Go1 was induced, S-(-)-3PPP identified a high affinity site only in the presence of Go1. These results demonstrate S-(-)-3PPP to be a protean agonist at the D2l receptor and may explain in vivo actions of this ligand. Furthermore, in agreement with previous studies, the ability of the dopamine D2l receptor to couple promiscuously to Gαi1-3, and Gαo1 was demonstrated. However, despite high homology between dopamine D2l and D3 receptors, the G protein-coupling specificity of the D3 receptor has not been well characterised. Again using both receptor-G protein fusions and membranes of cells in which pertussis toxin-resistant mutants of individual Gi-family G proteins could be expressed in an inducible fashion, we confirmed the selective coupling of the D3 receptor to Gαo1. A range of ligands displayed agonism at the D2l receptor and the D3 receptor when coupled to Gαo1. As a general trend, agonists, including dopamine, displayed a higher potency at the D3 receptor. This perhaps reflects the role of D3 as an autoreceptor. Of particular interest was the demonstration that S-(-)-3PPP has both a higher efficacy and potency at the D3 receptor when coupled to Gαo1. The investigations into dopamine receptor-G protein coupling highlighted the utility of the [35S]GTPγS binding assay as a method of directly measuring receptor catalysed nucleotide exchange on the α subunit of G proteins. However, the expense associated with the use of radiolabels makes this assay less attractive, particularly for high-throughput screening programmes. In an attempt to develop a non-radioactive assay equivalent to the [35S] GTPγS assay an immunisation programme was initiated to generate antibodies selective against the active (GTP bound) conformation of G proteins. 4 way primary screening of 1632 hybridomas generated from mice immunized with GTPS-loaded Gi1 and isolated using an automated robotic colony picker, identified 3 antibodies that interacted with the constitutively active Q204L but neither the constitutively inactive G203A nor wild type form of Gi1. This profile extended to other closely related Gi-family G proteins but not to the less closely related Gs and Gq/G11 families. Each of these antibodies was, however, also able to identify wild type, GDP-bound Gi- family G proteins in the presence of AlF4- which mimics the presence of the terminal phosphate of GTP and hence generates an active conformation of the G protein. Stimulation of cells co-expressing a wild type Gisubunit and the dopamine D2 receptor with the agonist ligand nor-apomorphine also allowed these conformation selective antibodies to bind the G protein. Such reagents allow the development of label- free assays for G protein-coupled receptor-mediated activation of Gi- family G proteins.