Copper-independent lysosomal localization of the Wilson disease protein ATP7B

In hepatocytes, the Wilson disease protein ATP7B resides on trans-Golgi network and traffics to peripheral lysosomes to export excess intracellular copper through lysosomal exocytosis. We found that in basal copper or even upon copper chelation, a significant amount of ATP7B persists in endolysosomal compartment of hepatocytes but not in non-hepatic cells. These ATP7B-harboring lysosomes lie in close proximity of ∼10nm to the TGN. ATP7B constitutively distributes itself between the sub-domain of the TGN with a lower pH and the TGN-proximal lysosomal compartments. Presence of ATP7B on TGN-Lysosome colocalizing sites upon Golgi disruption suggested possible exchange of ATP7B directly between the TGN and its proximal lysosomes. Manipulating lysosomal positioning significantly alters the localization of ATP7B in the cell. Contrary to previous understanding, we found that upon copper chelation in a copper-replete hepatocyte, ATP7B is not retrieved back to TGN from peripheral lysosomes; rather, ATP7B recycles to these TGN-proximal lysosomes to initiate the next cycle of copper transport. We report a hitherto unknown copper-independent lysosomal localization of ATP7B and the importance of TGN-proximal lysosomes but not TGN as the terminal acceptor organelle of ATP7B in its retrograde pathway. Synopsis Excess copper is toxic to the cell. In hepatocytes, the Wilson disease protein ATP7B has been reported to recycle between the trans-Golgi network and lysosomes to export copper in elevated copper conditions. We, for the first time, show that a large fraction of ATP7B constitutively resides on the lysosome and rather, the position of the lysosome shifts from TGN-proximal to the peripheral region of the cell in high copper to facilitate ATP7B-mediated lysosomal exocytosis of copper.