Role of Membrane‐proximal Basic Residues in Cell Surface Trafficking of GPR15

Membrane‐proximal basic residues (MPBRs) in the cytoplasmic region are one of the important factors that determine the topology of membrane proteins during biogenesis in the endoplasmic reticulum (ER), known as a positive‐inside rule. However, MPBRs have been also found to be critically involved in the ER export of certain membrane proteins such as Golgi‐resident glycosyltransferases. Here we investigated the role of cytoplasmic MPBRs (Arg310Arg311) in the cell surface trafficking of GPR15, a G protein‐coupled receptor that serves as an HIV co‐receptor. The mutation of Arg310Arg311 to Ala did not alter the topology but abolished the O‐glycosylation and cell surface expression of GPR15. Immunocytochemistry indicated that Ala mutant can exit ER and reach ER‐Golgi intermediate compartment (ERGIC) and cis‐Golgi, but not trans‐Golgi. Our study suggests the possibility that MPBRs play a pivotal role in the anterograde trafficking of a broad range of membrane proteins.