Figure EV5. UBQLN4 recognizes ER assembly defective forms of IL‐2Rα.

1. Schematic representation of the IL‐2Rα proteins used in this experiment. WT: wild type, ΔSS: SS‐deleted mutant, and ΔSSΔTM: SS‐ and TM‐deleted mutant. Note that the position of the T7‐tag in WT IL‐2Rα is upstream (N‐terminal) of the SS: thus, SS cleavage by signal peptidase that occurs immediately after ER luminal incorporation would result in the loss of the T7 signal.
2. A series of T7‐tagged IL‐2Rα substrates were expressed in HeLa cells with Flag‐tagged UBQLN4 and treated with (+) or without (−) 10 μM MG‐132 for 4 h. Flag‐UBQLN4 was immunoprecipitated, and its co‐precipitation with IL‐2Rα was analyzed. In contrast to the case in Fig 5, glycosylated bands of WT IL‐2Rα were never detected, since T7‐tagged SS for ER targeting is cleaved off immediately after its successful co‐translational insertion into the lumen of the ER. Thus, only cytoplasmic defective forms of IL‐2Rα can be detected as T7‐positive polypeptides in this experiment. Asterisks indicate non‐specific signals.

Source data are available online for this figure.