Distinct dynamin-dependent and -independent mechanisms target structurally homologous dopamine receptors to different endocytic membranes

D1 and D2 dopamine receptors are structurally homologous G protein-coupled receptors that serve distinct physiological functions both in neurons and nonneural cell types. We have observed that these receptors are selectively endocytosed in HEK293 cells by distinct dynamin-dependent and -independent mechanisms. Although these endocytic mechanisms operate with similarly rapid kinetics, they differ in their regulation by agonist and deliver D1 and D2 receptors specifically to different primary endocytic vesicles. After this segregation into different endocytic membranes, both DI and D2 receptors recycle to the plasma membrane. Similar results are observed in Neuro2A neuroblastoma cells coexpressing both receptors at high levels. These findings establish that "classical" dynamin-dependent and "alternative" dynamin-independent endocytic mechanisms differ in their physiological regulation, sort structurally homologous signaling receptors in the plasma membrane, and mediate distinct early endocytic pathways leading to recycling endosomes. Our results also refute the previous hypothesis that dynamin-independent endocytosis targets G protein-coupled receptors selectively to lysosomes, and they suggest a new role of endocytic sorting mechanisms in physically segregating structurally homologous signaling receptors at the cell surface.