Oxysterol-binding protein homologs mediate sterol transport from the endoplasmic reticulum to mitochondria in yeast

Sterols are present in eukaryotic membranes and significantly affect membrane fluidity, permeability, and microdomain formation. They are synthesized in the endoplasmic reticulum (ER) and transported to other organelles and the plasma membrane. Sterols play important roles in the biogenesis and maintenance of mitochondrial membranes. However, the mechanisms underlying ER-to-mitochondrion sterol transport remain to be identified. Here, using purified yeast membrane fractions enriched in ER and mitochondria, we show that the oxysterol-binding protein homologs encoded by the OSH genes in the yeast Saccharomyces cerevisiae mediate sterol transport from the ER to mitochondria. Combined depletion of all seven Osh proteins impaired sterol transport from the ER to mitochondria in vitro; however, sterol transport was recovered at different levels upon adding one of the Osh proteins. Of note, the sterol content in the mitochondrial fraction was significantly decreased in vivo after Osh4 inactivation in a genetic background in which all the other OSH genes were deleted. We also found that Osh5–Osh7 bind cholesterol in vitro. We propose a model in which Osh proteins share a common function to transport sterols between membranes, with varying contributions by these proteins, depending on the target membranes. In summary, we have developed an in vitro system to examine intracellular sterol transport and provide evidence for involvement of Osh proteins in sterol transport from the ER to mitochondria in yeast.