A 55-kD organic anion binding protein (OABP) was identified previously in liver cell plasma membrane sinusoidal subfractions. Although this protein was localized to the surface of hepatocytes by immunofluorescence, immunoblot analysis revealed reactivity toward both plasma membrane and mito-chondrial fractions. To clarify these findings, an immunoreactive clone from a rat liver cDNA expression library was isolated, the 1,500-base pair cDNA insert was sequenced, and the corresponding β-galactosidase fusion protein was expressed and purified. The resulting sequence corresponded to that of the rat mitochondrial F1-adenosine triphosphatase (F1-ATP-ase) β-subunit. This protein and OABP are of similar size and are mutually immunologically cross-reactive. That the antigen was present on the cell surface as well as in mitochondria was suggested from studies of immunoprecipitation after cell-surface iodination, and light- and electron-microscopic immunocytochemistry. Photoaffinity labeling of bovine F1-ATPase with high-specific-activity [35S]sulfobromophthalein revealed binding only to the β-subunit. Hepatocyte uptake of bilirubin and sulfobromophthalein requires cellular ATP and mitochondria also transport these organic anions, which at high doses inhibit respiration. The presence of an organic anion binding site on the F1-ATPase β-subunit suggests that it may play a role in these processes.
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