Intramitochondrial transfer of phospholipids in the yeast, Saccharomyces cerevisiae

Translocation of phosphatidylinositol, which is synthesized on the outer aspect of the outer membrane of isolated yeast mitochondria, to the inner membrane is linked to phosphatidylinositol synthesis and is therefore a vectorial process. Phosphatidylinositol once integrated into the inner mitochondrial membrane is not transferred back to the mitochondrial surface. Phosphatidylserine is also translocated from the outer to the inner mitochondrial membrane, where it is decarboxylated to phosphatidylethanolamine. We made use of this metabolic modification to characterize the intramitochondrial transfer of phosphatidylserine and phosphatidylethanolamine. Intramitochondrial phosphatidylserine transfer is insensitive to the uncoupler carbonyl cyanide m-chlorophenylhydrazone and to valinomycin and is thus independent of an electrochemical gradient across the inner membrane. Transfer of phosphatidylserine from the outer to the inner mitochondrial membrane occurs not only in intact mitochondria but also in mitoplasts which are devoid of intermembrane space proteins but have the outer membrane still adherent to the inner membrane. This result suggests that specific contact sites are involved in the intramitochondrial translocation of phospholipids. 3H-Labeled phosphatidylethanolamine synthesized from [3H]serine in isolated mitochondria is readily exported from the inner to the outer mitochondrial membrane without prior mixing with the pool of phosphatidylethanolamine of the inner membrane.