ER-to-cell surface transport and signal regulation of GPCRs

DESCRIPTION (provided by applicant): The spatiotemporal regulation of intracellular trafficking and signaling of G protein-coupled receptors (GPCRs) is a critical aspect of integrated responses of the cell to hormones. Indeed, defective transport and dysfunction of GPCRs are associated with the pathogenesis of many human diseases. Our overall objective is to define the molecular mechanisms underlying the maturation and signal propagation of GPCRs and their roles in modulating cellular responses to hormones and drugs. Under this broad objective, the focus of the current proposal is to elucidate the mechanisms of nascent GPCR export from the endoplasmic reticulum (ER) to the cell surface and GPCR-mediated activation of the mitogen-activated protein kinase pathway in neuroblastoma-glioma NG108 and human embryonic kidney HEK293 cell lines by using alpha2B-adrenergic receptor (alpha2B-AR) as a model GPCR. We have demonstrated that alpha2B-AR export from the ER is modulated by a highly conserved triple arginine (3R) motif. The 3R motif mediates receptor interaction with selective Sec24 isoforms, components of COPII-coated transport vesicles. Our studies have also revealed a novel function for GGAs [monomeric Golgi-localizing, 3-adaptin ear domain homology, ADP ribosylation factor (ARF)-binding proteins]. GGAs associate with alpha2B-AR and are required for alpha2B-AR transport from the trans-Golgi network (TGN) to the plasma membrane. Furthermore, we have identified a novel signaling pathway in which the di-tryptophan motif-mediated, agonist-dependent interaction of alpha2B-AR with the small GTPase ARF1 dictates the activation of the conventional Raf1-MEK-ERK1/2 cascade by the receptor. The Specific Aims are: 1) to elucidate the mechanism of COPII vesicle-mediated alpha2B-AR export from the ER, 2) to determine the function of GGAs in alpha2B-AR transport from the TGN to the cell surface, and 3) to define the function and mechanism of alpha2B-AR- and ARF1-mediated activation of the Raf1-MEK-ERK1/2 pathway. Overall, these studies will reveal novel molecular mechanisms underlying export trafficking and signal regulation of GPCRs. The information generated from these studies may open new directions for designing drugs to treat diseases involving abnormal trafficking and functioning of GPCRs.