REGULATED ENDOCYTOSIS OF G-PROTEIN-COUPLED RECEPTORS BY A BIOCHEMICALLY AND FUNCTIONALLY DISTINCT SUBPOPULATION OF CLATHRIN-COATED PITS

beta-2 Adrenergic receptors (B2ARs) are endocytosed by clathrin coated pits. This process serves specialized functions in signal transductio n and receptor regulation, raising the question of whether B2ARs are a ssociated with biochemically specialized membrane vesicles during thei r endocytic trafficking. Here we show that B2ARs are endocytosed by a distinct subpopulation of clathrin-coated pits, which represent a limi ted subset of coated pits present in the plasma membrane, even in cell s overexpressing both B2ARs and p-arrestin. Coated pits mediating agon ist-induced endocytosis of B2ARs differ from other coated pits mediati ng constitutive endocytosis of transferrin receptors in their temperat ure dependence for fission from the plasma membrane and in the associa tion of their membrane coats with p-arrestin. Endocytosis of these coa ted pits generates endocytic vesicles selectively enriched in B2ARs, w hich fuse within similar to 10 min after their formation with a common population of endosomes containing both B2ARs and transferrin recepto rs. These observations demonstrate, for the first time, the existence of a functionally and biochemically distinct subpopulation of clathrin -coated pits that mediate the agonist-regulated endocytosis of G-prote in-coupled receptors, and they suggest a new model for the formation o f compositionally specialized membrane vesicles at the earliest stage of the endocytic pathway.