The mechanism by which newly synthesized sterols are transported from their site of synthesis, the endoplasmic reticulum (ER), to the sterol-enriched plasma membrane (PM) is not fully understood. Studies in mammalian cells suggest that newly synthesized cholesterol is transported to the PM in Golgi-bypassing vesicles and/or via a nonvesicular process. Using the yeast Saccharomyces cerevisiae as a model system, we now rule out an essential role for known vesicular transport pathways in transporting the major yeast sterol, ergosterol, from its site of synthesis to the PM. We use a cyclodextrin-based sterol capture assay to show that transport of newly synthesized ergosterol to the PM is unaltered in cells defective in Sec18p, a protein required for almost all intracellular vesicular trafficking events; we also show that transport is not blocked in cells that are defective in formation of transport vesicles at the ER or in vesicle fusion with the PM. Our data suggest instead that transport occurs by equilibration (t(1/2) approximately 10-15 min) of ER and PM ergosterol pools via a bidirectional, nonvesicular process that is saturated in wild-type exponentially growing yeast. To reconcile an equilibration process with the high ergosterol concentration of the PM relative to ER, we note that a large fraction of PM ergosterol is found condensed with sphingolipids in membrane rafts that coexist with free sterol. We propose that the concentration of free sterol is similar in the PM and ER and that only free (nonraft) sterol molecules have access to a nonvesicular transport pathway that connects the two organelles. This is the first description of biosynthetic sterol transport in yeast.
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Evidence ID | Analyze ID | Gene/Complex | Systematic Name/Complex Accession | Qualifier | Gene Ontology Term ID | Gene Ontology Term | Aspect | Annotation Extension | Evidence | Method | Source | Assigned On | Reference |
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Evidence ID | Analyze ID | Gene | Gene Systematic Name | Phenotype | Experiment Type | Experiment Type Category | Mutant Information | Strain Background | Chemical | Details | Reference |
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Evidence ID | Analyze ID | Gene | Gene Systematic Name | Disease Ontology Term | Disease Ontology Term ID | Qualifier | Evidence | Method | Source | Assigned On | Reference |
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Evidence ID | Analyze ID | Regulator | Regulator Systematic Name | Target | Target Systematic Name | Direction | Regulation of | Happens During | Regulator Type | Direction | Regulation Of | Happens During | Method | Evidence | Strain Background | Reference |
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Site | Modification | Modifier | Source | Reference |
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Evidence ID | Analyze ID | Interactor | Interactor Systematic Name | Interactor | Interactor Systematic Name | Allele | Assay | Annotation | Action | Phenotype | SGA score | P-value | Source | Reference | Note |
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Evidence ID | Analyze ID | Interactor | Interactor Systematic Name | Interactor | Interactor Systematic Name | Assay | Annotation | Action | Modification | Source | Reference | Note |
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Complement ID | Locus ID | Gene | Species | Gene ID | Strain background | Direction | Details | Source | Reference |
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Evidence ID | Analyze ID | Dataset | Description | Keywords | Number of Conditions | Reference |
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