Association between the endoplasmic reticulum and mitochondria of yeast facilitates interorganelle transport of phospholipids through membrane contact

Membrane association between mitochondria and the endoplasmic reticulum of the yeast Saccharomyces cerevisiae is probably a prerequisite for phospholi pid translocation between these two organelles. This association was visual ized by fluorescence microscopy and computer-aided three-dimensional recons truction of electron micrographs from serial ultrathin sections of yeast ce lls. A mitochondria-associated membrane (MAM), which is a subfraction of th e endoplasmic reticulum, was isolated and re-associated with mitochondria i n vitro. In the reconstituted system, phosphatidylserine synthesized in MAM was imported into mitochondria independently of cytosolic factors, bivalen t cations, ATP, and ongoing synthesis of phosphatidylserine. Proteolysis of mitochondrial surface proteins by treatment with proteinase K reduced the capacity to import phosphatidylserine. Phosphatidylethanolamine formed in m itochondria by decarboxylation of phosphatidylserine is exported to the end oplasmic reticulum where part of it is converted into phosphatidylcholine. In contrast with previous observations with permeabilized yeast cells [Achl eitner, G., Zweytick, D., Trotter, P., Voelker, D. & Daum, G. (1995) J. Bio l. Chem. 270, 29836-29842], export of phosphatidylethanolamine from mitocho ndria to the endoplasmic reticulum was shown to be energy-independent in th e reconstituted yeast system.