Genomic Analysis of the Opi− Phenotype

Abstract

Most of the phospholipid biosynthetic genes of Saccharomyces cerevisiae are coordinately regulated in response to inositol and choline. Inositol affects the intracellular levels of phosphatidic acid (PA). Opi1p is a repressor of the phospholipid biosynthetic genes and specifically binds PA in the endoplasmic reticulum. In the presence of inositol, PA levels decrease, releasing Opi1p into the nucleus where it represses transcription. The opi1 mutant overproduces and excretes inositol into the growth medium in the absence of inositol and choline (Opi À phenotype). To better understand the mechanism of Opi1p repression, the viable yeast deletion set was screened to identify Opi À mutants. In total, 89 Opi À mutants were identified, of which 7 were previously known to have the Opi À phenotype. The Opi À mutant collection included genes with roles in phospholipid biosynthesis, transcription, protein processing/synthesis, and protein trafficking. Included in this set were all nonessential components of the NuA4 HAT complex and six proteins in the Rpd3p-Sin3p HDAC complex. It has previously been shown that defects in phosphatidylcholine synthesis (cho2 and opi3) yield the Opi À phenotype because of a buildup of PA. However, in this case the Opi À phenotype is conditional because PA can be shuttled through a salvage pathway (Kennedy pathway) by adding choline to the growth medium. Seven new mutants present in the Opi À collection ( fun26, kex1, nup84, tps1, mrpl38, mrpl49, and opi10/yol032w) were also suppressed by choline, suggesting that these affect PC synthesis. Regulation in response to inositol is also coordinated with the unfolded protein response (UPR). Consistent with this, several Opi À mutants were found to affect the UPR (yhi9, ede1, and vps74).

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“…In this regard, MRPL49, the homolog of NFD1 in yeast, appears to be required for phosphatidylcholine (PC) biosynthesis (Hancock et al, 2006). At this time, it is unclear whether mrpl49 mutants are affected in PC biosynthesis directly or indirectly through an effect on mitochondria.…”

Lipid synthesis and membrane contact sites: a crossroads for cellular physiology

“…As discussed above, enrichment of PA at the ER upon PAH1 inactivation prevents Opi1 repression of UAS INO -regulated genes. Derepressed levels of INO1 and OPI3, among other UAS INO -regulated genes, were reported for both fld1 and ldb16 cells, suggesting that the absence of the seipin complex perturbs PA homeostasis and interferes with Opi1 function (51,59,63). However, unrestricted PL synthesis might not be the only cause for abnormal nuclear ER because opi1 cells exhibit wild-type nuclear ER (as well as normal LD morphology).…”

Pathways to doubled haploidy: chromosome doubling during androgenesis

“…NFD1 has a homolog in yeasts ( MRPL49 ) involved in phosphatidylcholine (PC) biosynthesis (Hancock et al, 2006). PC is the main lipid constituent of the plant nuclear envelope (Philipp et al, 1976) and is known to stabilize lipid membranes so that in PC-rich membranes fusion is inhibited (Duzgunes et al, 1981).…”