Proper ion channel function often requires specific combinations of pore-forming α and regulatory β subunits, but little is known about the mechanisms that regulate the surface expression of different channel combinations. Our studies of ATP-sensitive K⁺ channel (K_ATP) trafficking reveal an essential quality control function for a trafficking motif present in each of the α (Kir6.1/2) and β (SUR1) subunits of the K_ATP complex. We show that this novel motif for endoplasmic reticulum (ER) retention/retrieval is required at multiple stages of K_ATP assembly to restrict surface expression to fully assembled and correctly regulated octameric channels. We conclude that exposure of a three amino acid motif (RKR) can explain how assembly of an ion channel complex is coupled to intracellular trafficking.