? DESCRIPTION (provided by applicant): G protein-coupled receptors (GPCRs) constitute a superfamily of cell surface receptors that regulate a variety of cell functions and are the most important targets for drugs to treat diseases. As the cell surface is the functional destination fo most GPCRs, defective cell surface transport leads to GPCR dysfunction and is clearly associated with the pathogenesis of many human diseases. However, in contrast to the extensively studied internalization, recycling and degradation pathways, the anterograde transport of newly synthesized GPCRs from the endoplasmic reticulum (ER) through the Golgi apparatus to the cell surface remains poorly understood. The focus of this project is to test the central hypothesis that many GPCRs are transported to the cell surface via a specialized transport machinery made up of specific Rab GTPases, regulators and effectors. Our hypothesis stems from the surprising observation that ?2B-adrenergic receptor (?2B-AR) biogenesis uses a non-classic route of transport that is independent of Rab1. Our preliminary studies have identified instead a novel function for Rab43 in ?2B-AR transport from the ER to the Golgi. We have also found that Rab26 regulates post-Golgi transport of ?2B-AR, and that both Rab26 and Rab43 directly interact with ?2B-AR. Furthermore, we have found that RN-tre and TBC1D6 regulate the cell surface transport of ?2B-AR. RN-tre is a well-known GTPase-activating protein (GAP) for Rab43 and TBC1D6 is a GAP for Rab26. These data suggest that discrete pairs of Rabs and RabGAPs control the cell surface export of ?2B-AR at distinct steps, Importantly, this specialized transport pathway is critical for the cell surface delivery of severa different receptors, suggesting it represents a general mechanism of GPCR trafficking. Accordingly, we will extend these preliminary findings with the following Specific Aims, using ?2B-AR as a model. Specific Aim 1 will determine the roles of Rab43/RN-tre and Rab26/TBC1D6 in regulating the kinetics of ?2B-AR export to the cell surface. Specific Aim 2 will determine the role of Rab43/RN-tre and Rab26/TBC1D6 in the cell surface targeting and signaling of endogenous ?2B-AR. Specific Aim 3 will elucidate the mechanisms underlying the function of Rab43 and Rab26 in ?2B-AR transport. Overall, these studies will reveal a novel molecular mechanism that governs the export trafficking of GPCRs. The information generated from these studies may create new avenues for drug design and treatment of diseases involving aberrant trafficking and function of GPCRs.