The yeast gene ERG6 is required for normal membrane function but is not essential for biosynthesis of the cell-cycle-sparking sterol.

Abstract

In Saccharomyces cerevisiae, methylation of the principal membrane sterol at C-24 produces the C-28 methyl group specific to ergosterol and represents one of the few structural differences between ergosterol and cholesterol. C-28 in S. cerevisiae has been suggested to be essential for the sparking function (W. J. Pinto and W. R. Nes, J. Biol. Chem. 258:4472-4476, 1983), a cell cycle event that may be required to enter G1 (C. Dahl, H.-P. Biemann, and J. Dahl, Proc. Natl. Acad. Sci. USA 84:4012-4016, 1987). The sterol biosynthetic pathway in S. cerevisiae was genetically altered to assess the functional role of the C-28 methyl group of ergosterol. ERG6, the putative structural gene for S-adenosylmethionine: delta 24-methyltransferase, which catalyzes C-24 methylation, was cloned, and haploid strains containing erg6 null alleles (erg6 delta 1 and erg6 delta ::LEU2) were generated. Although erg6 delta cells are unable to methylate ergosterol precursors at C-24, they exhibit normal vegatative growth, suggesting that C-28 sterols are not essential in S. cerevisiae. However, erg6 delta cells exhibit pleiotropic phenotypes that include defective conjugation, hypersensitivity to cycloheximide, resistance to nystatin, a severely diminished capacity for genetic transformation, and defective tryptophan uptake. These phenotypes reflect the role of ergosterol as a regulator of membrane permeability and fluidity. Genetic mapping experiments revealed that ERG6 is located on chromosome XIII, tightly linked to sec59.

The Physiological Roles of Membrane Ergosterol as Revealed by the Phenotypes ofsyr1/erg3Null Mutant ofSaccharomyces cerevisiae

“…4 ) It is likely that the tryptophan transporter requires ergosterol for proper function at low temperatures, and the degree of the temperature dependency differs by genetic background. The strains used in this study required histidine, leucine, uracil, and adenine (only in W303-1A), in addition to tryptophan for growth.…”

“…It would be of interest to see the physiological effects of other mutations, such as erg2 (defective C-8 sterol isomerase)13) and erg6 (defective C-24 methylase). 4) It is likely that fungi acquired the ability to synthesize ergosterol during evolution in order to cope with various environmental stresses. Curiously, however, membrane ergosterol is necessary for specific binding of some antifungal antibiotics, such as nystatin and syringomycin, and possibly several other antifungal agents.…”

“…4) The three strains used in this study were all tryptophan auxotrophs. Therefore, the defect of the growth of the Jsyr 1 strain at the low temperatures could be due to the defect of tryptophan uptake from the medium.…”

“…2) The non essentiality of ergosterol in yeast was unexpected, because ergosterol has a "sparking sterol" function that allows the progression through the G 1 phase of the cell cycle. 4 ) S. cerevisiae is generally highly tolerant of various drugs. High drug dosages (occasionally IOO-fold and higher concentrations over those used for mammalian cells) are required for inhibition of yeast.…”

The role of lipids in the biogenesis of integral membrane proteins

“…Yeast cells synthesize ergosterol as their main membrane sterol. Mutant cells that are deficient in a late step along the ergosterol biosynthetic pathway due to a defect in the S-adenosylmethionine Δ24 methyltransferase, Erg6, were known to have a reduced capacity to take up tryptophan (Gaber et al 1989). In an erg6 mutant, however, Tat2 is missorted to the vacuole even if tryptophan levels are low, thus explaining their reduced capacity to take up tryptophan (Umebayashi and Nakano 2003).…”