Yeast mutant defective in phosphatidylcholine synthesis

Area of Science:

• Molecular Biology
• Yeast Genetics
• Lipid Metabolism

Background:

• Phosphatidylcholine (PC) is a vital phospholipid in cell membranes.
• PC can be synthesized via the CDP-choline pathway or by methylation of phosphatidylethanolamine (PE).
• The methylation pathway involves sequential addition of methyl groups to PE, forming phosphatidylmonomethylethanolamine (PME) and phosphatidyldimethylethanolamine (PDME) before PC.

Purpose of the Study:

• To characterize the Saccharomyces cerevisiae opi3-3 mutant.
• To investigate the role of the OPI3 gene in phosphatidylcholine biosynthesis via PE methylation.
• To determine the specific methylation step affected in the opi3-3 mutant.

Main Methods:

• Genetic analysis of the opi3-3 mutation.
• Phospholipid compositional analysis of wild-type and mutant yeast strains.
• Radioactive labeling studies using [methyl-14C]methionine to trace methyl group incorporation.
• Growth studies with and without inositol or choline supplementation.

Main Results:

• The opi3-3 mutant is defective in PC synthesis from PE methylation, with PC levels dropping from 40-50% to 2-3% in the absence of choline.
• The mutant accumulates PME and PDME intermediates.
• Methionine incorporation into PC is significantly reduced (80-90%) in the mutant, with label appearing in PME and PDME.
• The opi3-3 mutation is recessive, linked to ino4, and stimulated by inositol, not choline.
• PC synthesis from exogenous choline is unaffected in the opi3-3 mutant.

Conclusions:

• The opi3-3 mutation impairs the third (and possibly second) methylation step in the PE to PC pathway.
• The first methylation step appears functional, potentially even elevated.
• The OPI3 gene product is essential for efficient PE methylation in Saccharomyces cerevisiae.
• The mutant's growth phenotype is linked to its defect in PC biosynthesis via methylation.