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J. Cell Biol.,
Volume 144, Number 1, January 11, 1999 31-43
Department of Psychiatry and Department of Cellular and Molecular Pharmacology, University of California, San Francisco,
California 94143-0984
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 D1 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.
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