Anterograde trafficking of G protein-coupled receptors: Function of the C-terminal F(X) ₆LL motif in export from the endoplasmic reticulum

We have reported previously that the F(X) ₆LL motif in the C termini is essential for export of aα _2B-adrenergic (α _2B-AR) and angiotensin II type 1 receptors (AT1Rs) from the endoplasmic reticulum (ER). Here, we further demonstrate that mutation of the F(X) ₆lL motif similarly abolished the cell-surface expression of α _2B-AR, AT1R, α _1B-AR, and β ₂- AR, suggesting that the F(X) ₆LL motif plays a general role in ER export of G protein- coupled receptors (GPCRs). Mutation of Phe to Val, Leu, Trp, and Tyr, and mutation of LL to FF and VV, markedly inhibited α _2B-AR transport, indicating that the F(X) ₆LL function cannot be fully substituted by other hydrophobic residues. The structural analysis revealed that the Phe residue in the F(X) ₆LL motif is buried in the transmembrane domains and possibly interacts with Ile58 in β ₂-AR and Val42 in α _2B-AR, whereas the LL motif is exposed to the cytosolic space. Indeed, mutation of Ile58 in β ₂-AR and Val42 in α _2B-AR markedly disrupted cell surface transport of the receptors. It is noteworthy that the Val and Ile residues are highly conserved among the GPCRs carrying the F(X) ₆LL motif. Furthermore, the Phe mutant exhibited a stronger interaction with ER chaperones and was more potently rescued by physical and chemical treatments than the LL mutant. These data suggest that the Phe residue is probably involved in folding of α _2B-AR and β ₂-AR, possibly through interaction with other hydrophobic residues in neighboring domains. These data also provide the first evidence implying crucial roles of the C termini possibly through modulating multiple events in an- terograde trafficking of GPCRs.