Distinct dynamin-dependent and -independent mechanisms target structurallyhomologous 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 n onneural 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 rapi d kinetics, they differ in their regulation by agonist and deliver D1 and D 2 receptors specifically to different primary endocytic vesicles. After thi s segregation into different endocytic membranes, both DI and D2 receptors recycle to the plasma membrane. Similar results are observed in Neuro2A neu roblastoma cells coexpressing both receptors at high levels. These findings establish that "classical" dynamin-dependent and "alternative" dynamin-ind ependent endocytic mechanisms differ in their physiological regulation, sor t structurally homologous signaling receptors in the plasma membrane, and m ediate distinct early endocytic pathways leading to recycling endosomes. Ou r results also refute the previous hypothesis that dynamin-independent endo cytosis targets G protein-coupled receptors selectively to lysosomes, and t hey suggest a new role of endocytic sorting mechanisms in physically segreg ating structurally homologous signaling receptors at the cell surface.