Coat protein that is involved in the localization of trans-Golgi network (TGN) membrane proteins that contain acidic cluster sorting motifs. Controls the endosome-to-Golgi trafficking of furin and mannose-6-phosphate receptor by connecting the acidic-cluster-containing cytoplasmic domain of these molecules with the adapter-protein complex-1 (AP-1) of endosomal clathrin-coated membrane pits. Involved in HIV-1 nef-mediated removal of MHC-I from the cell surface to the TGN. Required for normal ER Ca2+ handling in lymphocytes. Together with WDR37, it plays an essential role in lymphocyte development, quiescence and survival. Required for stabilizing peripheral lymphocyte populations (By similarity).