DUAL LOCALIZATION OF SQUALENE EPOXIDASE, ERG1P, IN YEAST REFLECTS A RELATIONSHIP BETWEEN THE ENDOPLASMIC-RETICULUM AND LIPID PARTICLES

Squalene epoxidase, encoded by the ERG1 gene in yeast, is a key enzyme of sterol biosynthesis. Analysis of subcellular fractions revealed th at squalene epoxidase was present in the microsomal fraction (30,000 X g) and also cofractionated with lipid particles. A dual localization of Erg1p was confirmed by immunofluorescence microscopy. On the basis of the distribution of marker proteins, 62% of cellular Erg1p could be assigned to the endoplasmic reticulum and 38% to lipid particles in l ate logarithmic-phase cells. In contrast, sterol Delta(24)-methyltrans ferase (Erg6p), an enzyme catalyzing a late step in sterol biosynthesi s, was found mainly in lipid particles cofractionating with triacylgly cerols and steryl esters. The relative distribution of Erg1p between t he endoplasmic reticulum and lipid particles changes during growth. Sq ualene epoxidase (Erg1p) was absent in an erg1 disruptant strain and w as induced fivefold in lipid particles and in the endoplasmic reticulu m when the ERG1 gene was overexpressed from a multicopy plasmid. The a mount of squalene epoxidase in both compartments was also induced appr oximately fivefold by treatment of yeast cells with terbinafine, an in hibitor of the fungal squalene epoxidase. In contrast to the distribut ion of the protein, enzymatic activity of squalene epoxidase was only detectable in the endoplasmic reticulum but was absent from isolated l ipid particles. When lipid particles of the wild-type strain and micro somes of an erg1 disruptant were mixed, squalene epoxidase activity wa s partially restored. These findings suggest that factor(s) present in the endoplasmic reticulum are required for squalene epoxidase activit y. Close contact between lipid particles and endoplasmic reticulum may be necessary for a concerted action of these two compartments in ster ol biosynthesis.