Biochemical characterization and subcellular localization of the sterol C-24(28) reductase, Erg4p, from the yeast Saccharomyces cerevisiae

The yeast ERG4 gene encodes sterol C-24(28) reductase which catalyzes the f inal step in the biosynthesis of ergosterol. Deletion of ERG4 resulted in a complete lack of ergosterol and accumulation of the precursor ergosta-5,7, 22,24(28)-tetraen-3 beta-ol. An erg4 mutant strain exhibited pleiotropic de fects such as hypersensitivity to divalent cations and a number of drugs su ch as cycloheximide, miconazole, 4-nitroquinoline, fluconazole, and sodium dodecyl sulfate. Similar to erg6 mutants, erg4 mutants are sensitive to the Golgi-destabilizing drug brefeldin A. Enzyme activity measurements with is olated subcellular fractions revealed that Erg4p is localized to the endopl asmic reticulum, This view was confirmed in vivo by fluorescence microscopy of a strain expressing a functional fusion of Erg4p to enhanced green fluo rescent protein. We conclude that ergosterol biosynthesis is completed in t he endoplasmic reticulum, and the final product is supplied from there to i ts membranous destinations. (C) 2000 Federation of European Biochemical Soc ieties.