Phosphatidylinositol movement between isolated microsomal and mitochondrial membranes.

Transfer of membrane-bound phosphatidyl-[2'-3H]inositol from microsomal to unlabelled mitochondrial and from mitochondrial to unlabelled microsomal membranes was studied using partially purified cytosol proteins isolated from guinea pig liver cytosol. In the absence and presence of these proteins the amounts of phosphatidylinositol transfer from microsomal to mitochondrial membranes were approximately 21 and 33%, respectively, and the amounts from mitochondrial to microsomal membranes were approximately 31 and 39%, respectively. The release of phosphatidyl-[2'-3H]inositol from microsomal membranes in the absence of mitochondria was dependent on concentration of cytosol proteins. Two mechanisms for movement between membranes are proposed. In cytosol-protein-independent movement of phosphatidyl-[2'-3H]inositol from microsomal to mitochondrial membranes, a direct contact between membranes is required, since phosphatidyl-[2'-3H]inositol was not detected in the reaction medium. In the cytosol-protein-catalyzed transfer, formation of phosphatidyl-[2'-3H]inositol - cytosol protein complex is postulated, since phosphatidyl-[2'-3H]inositol was released into the reaction medium and its movement proceeded from mitochondrial to microsomal membranes in the presence of partially purified cytosol proteins. Thus, contact between the two membranes is probably not necessary for this transfer. Implications for the movement of phospholipids between biological membranes are discussed.