Transfer of phosphatidic acid between microsomal and mitochondrial outer and inner membranes.

A protein fraction from rat liver cytoplasm, precipitable at 50-95% saturation of ammonium sulphate, binds phosphatidic acid from mitochondrial and microsomal membranes. Protein-bound phosphatidic acid was eluted from Sephadex G-75 in fractions corresponding to a molecular weight of about 10 000. No such binding was observed with mitochondrial soluble proteins, either total or precipitated with ammonium sulphate between 50 and 95% saturation. The transfer of phosphatidic acid from microsomes to mitochondria was increased by liver cytoplasmic proteins precipitable at 50-95% saturation of ammonium sulphate but not with mitochondrial soluble proteins. This increase by cytoplasmic proteins was pronounced in 200 mM sucrose but was negligible in 100 mM KCI where the spontaneous transfer was quite high. Cytoplasmic proteins stimulated the synthesis of cardiolipin and phosphatidylglycerol in mitochondria deprived of the outer membrane but not in intact mitochondria when phosphatidic acid was supplied either by microsomes or liposomes. It is suggested that the transfer of phosphatidic acid from the outer to the inner mitochondrial membrane is not mediated by transfer proteins but occurs either by direct contact of the membranes or as free diffusion through the aqueous phase.