β2-Adrenergic Receptor Internalization, Endosomal Sorting, and Plasma Membrane Recycling Are Regulated by Rab GTPases

Rab GTPases are recognized as critical regulatory factors involved in vesicular membrane transport and endosomal fusion. For example, Rab5 directs the transport and fusion of endocytic vesicles to and with early endosomes, whereas Rab4 is thought to control protein trafficking from early endosomes back to the plasma membrane. In the present study, we investigated the role of Rab5 and Rab4 GTPases in regulating the endocytosis, intracellular sorting, and the plasma membrane recycling of the β 2 AR. In cells expressing the dominant-negative Rab5-S34N mutant, β 2 AR internalization was impaired, and β 2 AR-bearing endocytic vesicles remained in either close juxtaposition or physically attached to the plasma membrane. In contrast, a constitutively active Rab5-Q79L mutant redirected internalized β 2 AR to enlarged endosomes but did not prevent β 2 AR dephosphorylation and recycling. The expression of either wild-type Rab4 or a Rab4-N121I mutant did not prevent β 2 AR dephosphorylation. However, the dominant-negative Rab4-N121I mutant blocked β 2 AR resensitization by blocking receptor recycling from endosomes back to the cell surface. Our data indicate that, in addition to regulating the intracellular trafficking and fusion of β 2 AR-bearing endocytic vesicles, Rab5 also contributes to the formation and/or budding of clathrin-coated vesicles. Furthermore, β 2 AR dephosphorylation occurs as the receptor transits between Rab5- and Rab4-positive compartments.