Altered sterol metabolism in budding yeast affects mitochondrial iron–sulfur (Fe-S) cluster synthesis

Abstract

Ergosterol synthesis is essential for cellular growth and viability of the budding yeast , and intracellular sterol distribution and homeostasis are therefore highly regulated in this species. Erg25 is an iron-containing C4-methyl sterol oxidase that contributes to the conversion of 4,4-dimethylzymosterol to zymosterol, a precursor of ergosterol. The gene encodes an endoplasmic reticulum (ER)-associated protein, and here we identified a role for Erg29 in the methyl sterol oxidase step of ergosterol synthesis. deletion resulted in lethality in respiring cells, but respiration-incompetent (Rho or Rho) cells survived, suggesting that Erg29 loss leads to accumulation of oxidized sterol metabolites that affect cell viability. Down-regulation of expression in Δ cells indeed led to accumulation of methyl sterol metabolites, resulting in increased mitochondrial oxidants and a decreased ability of mitochondria to synthesize iron-sulfur (Fe-S) clusters due to reduced levels of Yfh1, the mammalian frataxin homolog, which is involved in mitochondrial iron metabolism. Using a high-copy genomic library, we identified suppressor genes that permitted growth of Δ cells on respiratory substrates, and these included genes encoding the mitochondrial proteins Yfh1, Mmt1, Mmt2, and Pet20, which reversed all phenotypes associated with loss of Of note, loss of Erg25 also resulted in accumulation of methyl sterol metabolites and also increased mitochondrial oxidants and degradation of Yfh1. We propose that accumulation of toxic intermediates of the methyl sterol oxidase reaction increases mitochondrial oxidants, which affect Yfh1 protein stability. These results indicate an interaction between sterols generated by ER proteins and mitochondrial iron metabolism.

Regulation of Ergosterol Biosynthesis in Saccharomyces cerevisiae

“…Defects in this step lead to the accumulation of toxic intermediates of the methyl sterol oxidase reaction that increase mitochondrial oxidation and affect the stability of the yeast frataxin homolog Ffh1, which is implicated in mitochondrial iron metabolism [13]. As a consequence, erg29 mutants exhibit defects in iron-sulfur cluster assembly and mitochondrial iron accumulation [13,115]. These results emphasize the multiple connections between iron metabolism and ergosterol biosynthesis.…”

“…In the next steps, lanosterol is transformed to zymosterol through a complex process involving various demethylation, reduction and desaturation reactions catalyzed by the lanosterol 14-α-demethylase Erg11 (also known as Cyp51), the C-14 reductase Erg24 and the C-4 demethylation complex Erg25-Erg26-Erg27. Erg28 and Erg29 are likely to function in the C-4 demethylation complex reaction [8,13]. Zymosterol is the first intermediate of the biosynthesis pathway that can be incorporated into cellular membranes [14].…”

“…Yeast cells can overproduce ergosterol or sterol intermediates, which can be incorporated into cellular membranes to some extent, to modulate their physicochemical properties. However, the accumulation of excess free sterol intermediates may become toxic for cells since it alters mitochondrial respiration, resulting in the generation of toxic methyl sterol intermediates, which raise mitochondrial oxidants and decrease the ability to synthesize iron-sulfur (Fe-S) clusters [13,42,43]. To prevent these harmful effects, yeast cells utilize multiple detoxification mechanisms.…”

“…In addition to Dap1, mutants in other components of ergosterol biosynthesis, such as in the essential genes ERG25 and ERG29, also lead to growth defects in low iron and respiratory conditions, respectively, due to impaired ergosterol production [114,115]. A recent study has revealed that, similarly to Erg25, Erg29 participates in the methyl sterol oxidase step of ergosterol biosynthesis [13]. Defects in this step lead to the accumulation of toxic intermediates of the methyl sterol oxidase reaction that increase mitochondrial oxidation and affect the stability of the yeast frataxin homolog Ffh1, which is implicated in mitochondrial iron metabolism [13].…”

“…A recent study has revealed that, similarly to Erg25, Erg29 participates in the methyl sterol oxidase step of ergosterol biosynthesis [13]. Defects in this step lead to the accumulation of toxic intermediates of the methyl sterol oxidase reaction that increase mitochondrial oxidation and affect the stability of the yeast frataxin homolog Ffh1, which is implicated in mitochondrial iron metabolism [13]. As a consequence, erg29 mutants exhibit defects in iron-sulfur cluster assembly and mitochondrial iron accumulation [13,115].…”