Protein–protein interactions among C-4 demethylation enzymes involved in yeast sterol biosynthesis

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Abstract
A Saccharomyces cerevisae microarray expression study indicated that an ORF, YER044C, now designated ERG28, was strongly coregulated with ergosterol biosynthesis. Disruption of the ERG28 gene results in slow growth and accumulation of sterol intermediates similar to those observed in erg26 and erg27 null strains, suggesting that the Erg28p may interact with Erg26p and/or Erg27p. In this study, a peptide from human hemagglutinin protein (HA) epitope tag was added to ERG26 and ERG27 genes, and a Myc tag was added to the ERG28 gene to detect interactions between Erg28p and Erg26p/Erg27p. Differential centrifugation showed that Erg26p, Erg27p, and Erg28p are all membrane-associated proteins. Green fluorescent protein–fusion protein localization studies showed that Erg26p, Erg27p, and Erg28p are all located in the endoplasmic reticulum. Solubilized membrane protein coimmunoprecipitation studies using rabbit anti-Erg25p indicated that Erg25p coimmunoprecipitates with both Erg27p and Erg28p. Erg28p was also shown to reciprocally coimmunoprecipitate with Erg27p. However, no coimmunoprecipitation was observed with Erg26p, most likely because of the poor solubilization of this protein. Sucrose gradient ultracentrifugation studies suggested that Erg25p/Erg26p/Erg27p/Erg28p, along with other proteins in sterol biosynthesis, might form a complex between 66 and 200 kDa. Using an anti-HA column with Erg27p-HA and Erg26p-HA as target proteins, a complex containing Erg25p/Erg26p/Erg27p/Erg28p was identified. Thus, we suggest that Erg28p works as a transmembrane scaffold to tether Erg27p and possibly other C-4 demethylation proteins (Erg25p, Erg26p), forming a demethylation complex in the endoplasmic reticulum.