A single conserved leucine residue on the first intracellular loop regulates ER export of G protein-coupled receptors

The intrinsic structural determinants for export trafficking of G protein-coupled receptors (GPCRs) have been mainly identified in the termini of the receptors. In this report, we determined the role of the first intracellular loop (ICL1) in the transport from the endoplasmic reticulum (ER) to the cell surface of GPCRs. The alpha(2B)-adrenergic receptor (AR) mutant lacking the ICL1 is unable to traffic to the cell surface and to initiate signaling measured as ERK1/2 activation. Mutagenesis studies identify a single Leu48 residue in the ICL1 modulates alpha(2B)-AR export from the ER. The ER export function of the Leu48 residue can be substituted by Phe, but not Ile, Val, Tyr and Trp, and is unlikely involved in correct folding or dimerization of alpha(2B)-AR in the ER. Importantly, the isolated Leu residue is remarkably conserved in the center of the ICL1s among the family A GPCRs and is also required for the export to the cell surface of beta(2)-AR, alpha(1B)-AR and angiotensin II type 1 receptor. These data indicate a crucial role for a single Leu residue within the ICL1 in ER export of GPCRs.