A phospholipid exchange protein (PLEP) functioning between the endoplasmic reticulum and the mitochondrion was purified from the cytosolic fraction of germinated castor bean endosperms. In the protein fraction eluted from Sephadex G-100 column, the exchange rate reached 7.3μgphospholipids exchanged/mg protein/15 min, which was 60-fold that of pota to tuber PLEP. The lipid transfer by this protein was specific for phosphatidyl choline and the transfer rate from microsomes to mitochondria was as high as that from mitochondria to microsomes. Castor bean PLEP transferred phospholipid from castor bean microsomes to mitochondria from other sources such as potato tubers, cauliflower inflorescences, pumpkin hypocotyls and rat livers, and to liposomes, but not toAvenaetioplasts. In addition, it transferred phospholipid from potato microsomes to potato mitochondria
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