The Signal Peptide of the G Protein-coupled Human Endothelin B Receptor Is Necessary for Translocation of the N-terminal Tail across the Endoplasmic Reticulum Membrane

The initial step of the intracellular transport of G protein-coupled receptors, their insertion into the membrane of the endoplasmic reticulum, follows one of two different pathways. Whereas one group uses the first transmembrane domain of the mature receptor as an uncleaved signal anchor sequence for this process, a second group possesses additional cleavable signal peptides. The reason this second subset requires the additional signal peptide is not known. Here we have assessed the functional significance of the signal peptide of the endothelin B (ETB) receptor in transiently transfected COS.M6 cells. A green fluorescent protein-tagged ETB receptor mutant lacking the signal peptide was nonfunctional and retained in the endoplasmic reticulum, suggesting that it has a folding defect. To determine the defect in more detail, ETB receptor fragments containing the N-terminal tail, first transmembrane domain, and first cytoplasmic loop were constructed. We assessed N tail translocation across the endoplasmic reticulum membrane in the presence and absence of a signal peptide and show that the signal peptide is necessary for N tail translocation. We postulate that signal peptides are necessary for those G protein-coupled receptors for which post-translational translocation of the N terminus is impaired or blocked by the presence of stably folded domains.