Import of phosphatidylserine into isolated yeast mitochondria

A yeast phosphatidylserine transfer protein was used as a tool to transport radioactively labeled phosphatidylserine from unilamellar vesicles to isolated mitochondria of the yeast, Saccharomyces cerevisiae. Import of phosphatidylserine into mitochondria was monitored by the appearance of radioactively labeled phosphatidylethanolamine, which is produced from phosphatidylserine by the action of phosphatidylserine decarboxylase. This enzyme is located on the outer aspect of the inner mitochondrial membrane. Import of phosphatidylserine into mitochondria and formation of phosphatidylethanolamine does not require ATP or an electrochemical gradient, and is not affected by adriamycin. Evidence is presented that contact sites between the two mitochondrial membranes are zones of intramitochondrial translocation of phosphatidylserine and phosphatidylethanolamine. When phosphatidylserine decarboxylase is inhibited by hydroxylamine, transport of phosphatidylserine to the mitochondrial surface is unaffected. Under these conditions only a small amount of phosphatidylserine accumulates in the inner mitochondrial membrane indicating that the intramitochondrial transport of phosphatidylserine and its metabolic conversion to phosphatidylethanolamine are linked processes.