CHARACTERIZATION OF A MICROSOMAL SUBFRACTION ASSOCIATED WITH MITOCHONDRIA OF THE YEAST, SACCHAROMYCES-CEREVISIAE - INVOLVEMENT IN SYNTHESISAND IMPORT OF PHOSPHOLIPIDS INTO MITOCHONDRIA

In the yeast, Saccharomyces cerevisiae, similar to higher eukaryotes m ost phospholipids are synthesized in microsomes. Mitochondria contribu te to the cellular biosynthesis of phospholipids insofar as they harbo r phosphatidylethanolamine decarboxylase, and enzymes of phosphatidylg lycerol and cardiolipin synthesis. In this paper we present evidence t hat certain enzymes of phospholipid biosynthesis, namely phosphatidyls erine and phosphatidylinositol synthase, are enriched in a special mic rosomal fraction associated with mitochondria, which we named MAM. Thi s fraction was isolated and characterized with respect to marker enzym es, protein and phospholipid composition, and enzymes of phospholipid synthesis. According to these analyses MAMs are a specialized subfract ion of the endoplasmic reticulum, which is distinct from other microso mal subfractions. Phosphatidylserine synthesized in MAMs can be readil y imported into mitochondria and converted to phosphatidylethanolamine . Reassociation of MAMs with purified mitochondria led to reconstituti on of the import of phosphatidylserine into mitochondria. Organelle co ntact is suggested as a possible mechanism of this process.