In the yeast, Saccharomyces cerevisiae, similar to higher eukaryotes most phospholipids are synthesized in microsomes. Mitochondria contribute to the cellular biosynthesis of phospholipids insofar as they harbor phosphatidylethanolamine decarboxylase, and enzymes of phosphatidylglycerol and cardiolipin synthesis. In this paper we present evidence that certain enzymes of phospholipid biosynthesis, namely phosphatidylserine and phosphatidylinositol synthase, are enriched in a special microsomal fraction associated with mitochondria, which we named MAM. This fraction was isolated and characterized with respect to marker enzymes, protein and phospholipid composition, and enzymes of phospholipid synthesis. According to these analyses MAMs are a specialized subfraction of the endoplasmic reticulum, which is distinct from other microsomal subfractions. Phosphatidylserine synthesized in MAMs can be readily imported into mitochondria and converted to phosphatidylethanolamine. Reassociation of MAMs with purified mitochondria led to reconstitution of the import of phosphatidylserine into mitochondria. Organelle contact is suggested as a possible mechanism of this process.

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