Stereochemical specificity for sterols in Saccharomyces cerevisiae

Area of Science:

• Biochemistry
• Molecular Biology
• Yeast Genetics

Background:

• Sterols are essential membrane components in eukaryotes.
• Saccharomyces cerevisiae (yeast) sterol biosynthesis is crucial for growth.
• The role of specific sterol side-chain modifications in yeast function is not fully understood.

Purpose of the Study:

• To investigate the absolute requirement for specific sterol structures in yeast growth under inhibited sterol biosynthesis.
• To determine the impact of C-24 alkyl group configuration and size on sterol function.
• To elucidate the molecular basis for sterol specificity in yeast.

Main Methods:

• Utilized 2,3-iminosqualene, a 2,3-oxidosqualene cyclase inhibitor, to block sterol biosynthesis in Saccharomyces cerevisiae.
• Supplemented yeast cultures with various sterols, including those with different C-24 alkyl groups (e.g., 24 beta-methylcholesterol, cholesterol, campesterol, sitosterol).
• Analyzed yeast growth and identified endogenously synthesized sterols using analytical techniques.

Main Results:

• An absolute requirement for a sterol with a 24 beta-methyl group was demonstrated for yeast growth when sterol biosynthesis was inhibited.
• Sterols lacking the 24 beta-methyl group, such as cholesterol and its derivatives, did not support growth, except for desmosterol which was converted to a 24 beta-methylsterol.
• Endogenous synthesis of 24 beta-methylsterols (ergosterol, 22-dihydroergosterol) was observed when cholesterol supported growth, suggesting metabolic conversion.

Conclusions:

• Yeast exhibits a previously unrecognized absolute specificity for both the chirality and bulk of the C-24 alkyl group in sterols.
• This high specificity suggests that protein binding is involved in mediating sterol's essential roles in yeast.
• The findings provide critical insights into the structure-function relationships of sterols in a model eukaryotic organism.