A conserved motif for the transport of G protein-coupled receptors from the endoplasmic reticulum to the cell surface

The structural determinants for the export trafficking of G protein-coupled receptors are poorly defined. In this report, we determined the role of carboxyl termini (CTs) of alpha(2B)-adrenergic receptor (AR) and angiotensin II type 1A receptor (AT1R) in their transport from the endoplasmic reticulum (ER) to the cell surface. The alpha(2B)-AR and AT1R mutants lacking the CTs were completely unable to transport to the cell surface and were trapped in the ER. Alanine-scanning mutagenesis revealed that residues Phe(436) and Ile(443)-Leu(444) in the CT were required for alpha(2B)-AR export. Insertion or deletion between Phe(436) and Ile(443)-Leu(444) as well as Ile(443)-Leu(444) mutation to FF severely disrupted alpha(2B)-AR transport, indicating there is a defined spatial requirement, which is essential for their function as a single motif regulating receptor transport from the ER. Furthermore, the carboxyl-terminally truncated as well as Phe(436) and Ile(443)-Leu(444) mutants were unable to bind ligand and the alpha(2B)-AR CT conferred its transport properties to the AT1R mutant without the CT in a Phe(436)- Ile(443)-Leu(444)-dependent manner. These data suggest that the Phe436 and Ile(443)-Leu(444) may be involved in both proper folding and export from the ER of the receptor. Similarly, residues Phe(309) and Leu(316)-Leu(317) in the CT were identified as essential for AT1R export. The sequence F(X)(6)LL (where X can be any residue, and L is leucine or isoleucine) is highly conserved in the membrane-proximal CTs of many G protein-coupled receptors and may function as a common motif mediating receptor transport from the ER to the cell surface.