Guangyu Wu

Export from the endoplasmic reticulum (ER) represents an initial step in intracellular trafficking of G protein-coupled receptors (GPCRs). However, the underlying molecular mechanisms remain poorly understood. We have previously demonstrated that a highly conserved Leu residue on the first intracellular loop (ICL1) is required for exit of several GPCRs from the ER. Here we found that, in addition to Leu64 residue in the ICL1, the neighboring positively charged residue Lys65also modulates the cell-surface transport of α_2A-adrenergic receptor (α_2A-AR). Mutation of Lys65 to Ala, Glu and Gln significantly attenuated, whereas mutation of Lys65 to Arg strongly augmented α_2A-AR expression at the cell surface. Consistent with the effects on the cell-surface expression of α_2A-AR, mutation of Lys65 to Ala and Arg produced opposing effects on α_2A-AR-mediated ERK1/2 activation. Furthermore, confocal microscopy revealed that the α_2A-AR mutant K65A displayed a strong intracellular expression pattern and was extensively co-localized with the ER marker DsRed2-ER, suggestive of ER accumulation. These data provide the first evidence indicating an important function for a single Lys residue on the ICL1 in the ER export and cell-surface expression of α_2A-AR. These data also suggest that the ICL1 may possess multiple signals that control the cell-surface targeting of GPCRs via distinct mechanisms.

Anterograde trafficking of newly synthesized G protein-coupled ­receptors (GPCRs) from the endoplasmic reticulum to the cell surface represents a crucial checkpoint in controlling the amount of the functional receptors at the cell surface and the strength of signaling initiated by the receptors. In contrast to the extensively studied, well-understood endocytic and recycling pathways, the molecular mechanisms underlying the cell-surface targeting of the receptors remain poorly defined. In this chapter, I will discuss current advances in understanding post-Golgi transport of GPCRs by focusing on specific motifs or sequences that may function as sorting signals regulating export from the Golgi and subsequent transport to the plasma membrane of GPCRs.

Recent studies have demonstrated that cargo exit from the endoplasmic reticulum (ER) may be directed by ER export motifs recognized by components of the coat protein II (COPII) vesicles. However, little is known about ER export motifs and vesicle targeting of the G protein‐coupled receptor (GPCR) superfamily. Here, we have demonstrated that a triple Arg (3R) motif in the third intracellular loop functions as a novel ER export signal for α_2B‐adrenergic receptor (α_2B‐AR). The 3R motif mediates α_2B‐AR interaction with Sec24C/D and modulates ER exit, cell surface transport and function of α_2B‐AR. Furthermore, export function of the 3R motif is independent of its position within α_2B‐AR and can be conferred to CD8 glycoprotein. These data provide the first evidence implicating that export of GPCRs is controlled by code‐directed interactions with selective components of the COPII transport machinery.