Transfer of phospholipids from microsomes to mitochondria in germinating castor bean endosperm

14C-labeled microsomes were prepared by feeding [1-14 C]acetate to endosperm tissues from 4-day-old seedlings of castor bean seeds and incubated with unlabeled mitochondria from the same tissues. The loss of 14C-lipids from the microsomes was accompanied by an increase of 14C-lipids in the mitochondria. The addition of 105,000×g supernatant and also pH 5.1-treated supernatant, both of which had been prepared from castor bean endosperms at the same stage, markedly enhanced the lipid transfer from microsomes to mitochondria. The activity in this fraction was precipitated by ammonium sulfate and lost with trypsin or heat treatment. The transfer of lipids was limited to phospholipids. Thus, it is concluded that in castor bean endosperms, phospholipids are transferred from the endoplasmic reticulum to the mitochondria by a phospholipid-exchange protein contained in the cytosol.