Lipid flippases ATP9A and ATP9B form a complex and contribute to the secretory pathway from the Golgi apparatus

Type IV P-type ATPases (P4-ATPases) serve as lipid flippases, translocating membrane lipids from the exoplasmic (or luminal) leaflet to the cytoplasmic leaflet of lipid bilayers. In mammals, these P4-ATPases are localized to distinct subcellular compartments. ATP8A1 and ATP9A, both members of the P4-ATPase family, are involved in endosome-mediated membrane trafficking, although the roles of P4-ATPases in the secretory pathway remain to be clarified. ATP9A and ATP9B are located in the trans-Golgi network, with ATP9A also present in endosomal compartments. This study unveiled the overlapping roles of ATP9A and ATP9B in transporting VSVG from the Golgi to the plasma membrane within the secretory pathway. Furthermore, we demonstrated that the flippase activities of ATP9A and ATP9B were crucial for transport process. Notably, we discovered the formation of homomeric and/or heteromeric complexes between ATP9A and ATP9B. The existence of the heteromeric complex notably contributed to the retention of ATP9A in the Golgi. Therefore, ATP9A and ATP9B play a role in the secretory pathway from the Golgi to the plasma membrane, forming either homomeric or heteromeric complexes.