Role of a G_i2 protein splice variant in the formation of an intracellular dopamine D₂ receptor pool

Treatment of D₂-receptor-expressing cells with specific drugs upregulates the receptor number at the cell surface independently of protein synthesis, leading to the concept of an intracellular receptor pool. However, how this pool is operating is still an enigma. Here, we report that a splice variant of the G_i2 protein, protein sG_i2, plays a crucial role in the maintenance of this D₂-receptor pool. Co-expression of sG_i2 with D₂ receptor reduced receptor localization to cell surface by one-third. This effect is associated with specific intracellular protein-protein interaction and the formation of a sG_i2-D₂-receptor complex. It has been suggested that the formation of this complex serves to prevent D₂ receptors from reaching the cell membrane. Treatment of D₂-receptor-expressing cells with agonists increased the number of cell surface D₂ receptors and coincided with a reduction in these receptors from intracellular complexes, suggesting that agonist treatment released D₂ receptors from the complex allowing them to localize to the cell membrane. Thus, in addition to elucidating how the intracellular pool of D₂ receptor functions, our findings uncover a novel mechanism regulating the density of cell surface D₂ receptors.