IF1 distribution in HepG2 cells in relation to ecto-F0F1ATPsynthase and calmodulin

F(0)F(1)ATPsynthase is now known to be expressed as a plasma membrane receptor for several extracellular ligands. On hepatocytes, ecto-F(0)F(1)ATPsynthase binds apoA-I and triggers HDL endocytosis concomitant with ATP hydrolysis. Considering that inhibitor protein IF1 was shown to regulate the hydrolytic activity of ecto-F(0)F(1)ATPsynthase and to interact with calmodulin (CaM) in vitro, we investigated the subcellular distributions of IF1, calmodulin (CaM), OSCP and beta subunits of F(0)F(1)ATPsynthase in HepG2 cells. Using immunofluorescence and Western blotting, we found that around 50% of total cellular IF1 is localized outside mitochondria, a relevant amount of which is associated to the plasma membrane where we also found Ca2+-CaM, OSCP and beta. Confocal microscopy showed that IF1 colocalized with Ca2+-CaM on plasma membrane but not in mitochondria, suggesting that Ca2+-CaM may modulate the cell surface availability of IF1 and thus its ability to inhibit ATP hydrolysis by ecto-F(0)F(1)ATPsynthase. These observations support a hypothesis that the IF1-Ca2+-CaM complex, forming on plasma membrane, functions in the cellular regulation of HDL endocytosis by hepatocytes.