Subcellular Localization of Fad1p in Saccharomyces cerevisiae: A Choice at Post-Transcriptional Level?

Abstract

FAD synthase is the last enzyme in the pathway that converts riboflavin into FAD. In Saccharomyces cerevisiae, the gene encoding for FAD synthase is FAD1, from which a sole protein product (Fad1p) is expected to be generated. In this work, we showed that a natural Fad1p exists in yeast mitochondria and that, in its recombinant form, the protein is able, per se, to both enter mitochondria and to be destined to cytosol. Thus, we propose that FAD1 generates two echoforms—that is, two identical proteins addressed to different subcellular compartments. To shed light on the mechanism underlying the subcellular destination of Fad1p, the 3′ region of FAD1 mRNA was analyzed by 3′RACE experiments, which revealed the existence of (at least) two FAD1 transcripts with different 3′UTRs, the short one being 128 bp and the long one being 759 bp. Bioinformatic analysis on these 3′UTRs allowed us to predict the existence of a cis-acting mitochondrial localization motif, present in both the transcripts and, presumably, involved in protein targeting based on the 3′UTR context. Here, we propose that the long FAD1 transcript might be responsible for the generation of mitochondrial Fad1p echoform.

Mitochondria: From Physiology to Pathology

“…However, a protein responsible for the synthesis of FAD had never been identified in yeast mitochondria. In this paper [15], the presence of two Fad1p echoforms, dually localised to the cytosol and mitochondria, was reported. Intriguingly, the authors demonstrated the existence of two pools of FAD1 mRNAs with 3' untranslated regions (UTRs) of different length and containing a mitochondrial targeting signal.…”

Mitochondrial transport and metabolism of the vitamin B‐derived cofactors thiamine pyrophosphate, coenzyme A,FADandNAD+, and related diseases: A review

“…170 In yeast, there is only one isoform of FADS, which may be localized both to the cytoplasm and mitochondria due to a cis-acting mitochondrial localization motif present in the 3 0 -untranslated region of its mRNA. 171,172 FAD is distributed in different cellular compartments: cytoplasm, peroxisomes, ER, nucleus, lysosomes and mitochondria. Very limited examples of processes in which FAD is involved are: cytochrome P450 reductase and nitric oxide synthase 2 in the cytosol 173,174 ; β-oxidation of fatty acids, D-amino acid and sarcosine oxidases in peroxisomes 145 ; degradation of proteins and formation of disulfide bonds in the ER 175,176 ; histone lysine demethylases in the nucleus 177 ; amino acid and prenyl-cysteine oxidases in lysosomes 178,179 ; and many enzymes involved in carbohydrate, fatty acid and amino acid catabolism, such as succinate dehydrogenase (complex II), in mitochondria.…”

“… 169 Whereas RFK is thought to be cytoplasmic, FADS has been suggested to have two isoforms in human, one cytoplasmic and one mitochondrial 170 . In yeast, there is only one isoform of FADS, which may be localized both to the cytoplasm and mitochondria due to a cis‐acting mitochondrial localization motif present in the 3′‐untranslated region of its mRNA 171,172 . FAD is distributed in different cellular compartments: cytoplasm, peroxisomes, ER, nucleus, lysosomes and mitochondria.…”