retrieval signal was first discovered in the ATP-sensitive inwardly rectifying K+ channel (Kir) subunit , Kir 6.2[1].This finding has lead to a proposed mechanism by which the in completely assembled channel proteins can be screened for a distinct peptide signal by the quality control system and returned to the ER.The RXR motif is recognized by a coatomer protein (COP) I complex that forms the Golgi-to-ER retrograde transport vesicles [1–3], and now numerous surface membrane proteins including ion channels and receptors are known to carry this signal [4–7].Moreover,multiple different peptide signals necessary for promoting cell surface trafficking have been also found in K+
channels [8–11]. These findings have supported a concept that the surface trafficking of membrane proteins is not a default process but is highly regulated by different sequence motifs that are displayed on the cargo proteins.