Sterol assimilation by Saccharomyces cerevisiae Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4j03d375d

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  • Sterols of the yeast Saccharomyces cerevisiae are found in membranes with a free C-3,β hydroxyl group or stored in lipid droplets as esters of long-chain fatty acids. We have examined the distribution of cholesterol into free sterol and steryl ester pools of a sterol auxotrophic strain of Saccharomyces cerevisiae. Varying the amount of cholesterol available to cells has shown that steryl esters are accumulated as a small proportion of total sterol until a free sterol content of about 1 μg per milligram dry weight cells is attained. At cellular concentrations of free sterol above this threshold amount, the steryl ester pool receives a greater proportion of the total sterol incorporation, and the free sterol receives less. Since free sterol and steryl esters are known to be interconvertible within the cell, the occurence of a threshold of esterifiable sterol indicates that the cell attempts to maintain an optimal membrane sterol concentration through the regulation of components involved in this function. We have also conducted a study into a possible mechanism of sterol uptake control. Wild type yeast do not accumulate exogenous sterols under aerobic conditions, and a mutant allele conferring sterol auxotrophy (2a7), could only be isolated in strains with a heme deficiency. 6-Aminolevulinic acid (ALA), fed to a hem1 (ALA synthetase⁻) erg7 (2,3-oxidosqualene cyclase⁻), sterol auxotrophic yeast strain inhibits sterol uptake, and growth is negatively effected when intracellular sterol is depleted. The inhibition of sterol uptake (and growth of sterol auxotrophs) by ALA is dependent on the ability to synthesize heme from ALA. A procedure was developed which allows selection of strains which will take up exogenous sterols, but have no apparent defect in heme or ergosterol biosynthesis. One of these sterol uptake control mutants possesses an allele which allows phenotypic expression of sterol auxotrophy in a heme competent background.
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  • 1986
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