Interactions between the dopamine D2 receptor and GTP-binding proteins

The dopamine D2 receptor in bovine brain striatum exists in a high- and a low-affinity state for dopamine. The high-affinity state is believed to arise from coupling with a guanine nucleotide-binding protein, to be disrupted by the addition of GTP. We found that antibodies specific to the C-terminal region of the alpha-subunit of Go and Gi cause a shift from high- towards low-affinity, demonstrating that the C-terminus of G alpha is involved in the receptor G protein contact. Purification of the D2 receptor by affinity chromatography via immobilized agonists or antagonists results in the copurification of both Go and Gi proteins. However, this copurification is of a nonspecific nature and prevents the detection of putative specific receptor.G protein complexes.

An anti-peptide antibody that recognizes the dopamine D2 receptor from bovine striatum

The bovine dopamine D2 receptor was purified by wheat-germ-agglutinin-Sepharose chromatography and affinity chromatography, using the D2-receptor-specific agonist N-0434. Purification yields a preparation with a major protein band of 95 kDa. In order to ascertain the identity of this protein, polyclonal antibodies against the dopamine D2 receptor have been raised using synthetic peptides based on the predicted amino acid sequence of the cloned D2 receptor. For the initial screening of these antibodies, three fusion proteins consisting of beta-galactosidase and receptor fragments were constructed. One antiserum reacted strongly with the corresponding D2 receptor fusion protein, both on Western blots and in immunoprecipitation experiments. In each case, recognition was inhibited by competition with free peptide. On Western blots of partially purified receptor preparations from bovine striatum, the antiserum specifically recognized a 95-kDa glycoprotein. From similar preparations, the antiserum precipitated a substantial proportion of active D2 receptor, as determined by a decrease in [3H]spiperone binding in the supernatant. Active receptor could be released from the immunoprecipitate by addition of free peptide. Immunocytochemical analysis of cells transiently transfected with DNA coding for the D2 receptor showed specific staining of transfected cells. The antibody raised against a sequence in the third intracellular loop is able to shift the affinity of the receptor for dopamine from high to low, indicating that the antiserum may be interfering with receptor-GTP-binding-protein interactions.