beta(2)-adrenergic receptor internalization, endosomal sorting, and plasmamembrane recycling are regulated by Rab GTPases

Rab GTPases are recognized as critical regulatory factors involved in vesic ular membrane transport and endosomal fusion. For example, Rab5 directs the transport and fusion of endocytic vesicles to and with early endosomes, wh ereas Rab4 is thought to control protein trafficking from early endosomes b ack to the plasma membrane. In the present study, we investigated the role of Rab5 and Rab4 GTPases in regulating the endocytosis, intracellular sorti ng, and the plasma membrane recycling of the beta(2)AR. In cells expressing the dominant-negative Rab5-S34N mutant, beta(2)AR internalization was impa ired, and beta(2)AR-bearlng endocytic vesicles remained in either close jux taposition or physically attached to the plasma membrane. In contrast, a co nstitutively active Rab5-Q79L mutant redirected internalized beta(2)AR to e nlarged endosomes but did not prevent beta(2)AR dephosphorylation and recyc ling, The expression of either wild-type Rab4 or a Rab4-N121I mutant did no t prevent beta(2)AR dephosphorylation. However, the dominant-negative Rab4- N121I mutant blocked beta(2)AR resensitization by blocking receptor recycli ng from endosomes back to the cell surface. Our data indicate that, in addi tion to regulating the intracellular trafficking and fusion of beta(2)AR-be aring endocytic vesicles, Rab5 also contributes to the formation and/or bud ding of clathrin coated vesicles. Furthermore, beta(2)AR dephosphorylation occurs as the receptor transits between Rab5- and Rab4-positive compartment s.