Membrane assembly of the cannabinoid receptor 1: impact of a long N-terminal tail

The human cannabinoid receptor 1 (CB1) belongs to the G protein-coupled receptor (GPCR) family. Among the members of GPCR family, it has an exceptionally long extracellular N-terminal domain (N-tail) of 116 amino acids but has no typical signal sequence. This poses questions of how the long N-tail affects the biosynthesis of the receptor and of how it is inserted into the endoplasmic reticulum (ER) membrane. Here we have examined the process of membrane assembly of CB1 in the ER membrane and the maturation of the receptor from the ER to the plasma membrane. We find that the long N-tail cannot be efficiently translocated across the ER membrane, causing the rapid degradation of CB1 by proteasomes; this leads to a low level of expression of the receptor at the plasma membrane. The addition of a signal peptide at the N terminus of CB1 or shortening of the long N-tail greatly enhances the stability and cell surface expression of the receptor without affecting receptor binding to a cannabinoid ligand, CP-55,940. We propose that the N-tail translocation is a crucial early step in biosynthesis of the receptor and may play a role in regulating the stability and surface expression of CB1.