A subfraction of the yeast endoplasmic reticulum associates with the plasma membrane and has a high capacity to synthesize lipids

Large parts of the endoplasmic reticulum of the yeast, Saccharomyces cerevi siae, are located close to intracellular organelles, i.e. mitochondria and the plasma membrane, as shown by fluorescence and electron microscopy. Here we report the isolation and characterization of the subfraction of the end oplasmic reticulum that is closely associated with the plasma membrane. Thi s plasma membrane associated membrane (PAM) is characterized by its high ca pacity to synthesize phosphatidylserine and phosphatidylinositol. As such, PAM is reminiscent of MAM, a mitochondria associated membrane fraction of t he yeast [Gaigg, B., Simbeni, R., Hrastnik, C., Paltauf, F. & Daum, G. (199 5) Biochim. Biophys. Acta 1234, 214-220], although the specific activity of phosphatidylserine synthase and phosphatidylinositol synthase in PAM excee ds several-fold the activity in MAM and also in the bulk endoplasmic reticu lum. In addition, several enzymes involved in ergosterol biosynthesis, name ly squalene synthase (Erg9p), squalene epoxidase (Erg1p) and sterol Delta ( 24)-methyltransferase (Erg6p), are highly enriched in PAM. A possible role of PAM in the supply of lipids to the plasma membrane is discussed.