Hancock Leandria C, Behta Ryan P, Lopes John M
Department of Biological Sciences, Wayne State University, Detroit, Michigan 48202, USA.
Genetics. 2006 Jun;173(2):621-34. doi: 10.1534/genetics.106.057489. Epub 2006 Apr 2.
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(-) phenotype). To better understand the mechanism of Opi1p repression, the viable yeast deletion set was screened to identify Opi(-) mutants. In total, 89 Opi(-) mutants were identified, of which 7 were previously known to have the Opi(-) phenotype. The Opi(-) 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(-) phenotype because of a buildup of PA. However, in this case the Opi(-) 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(-) 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(-) mutants were found to affect the UPR (yhi9, ede1, and vps74).
酿酒酵母的大多数磷脂生物合成基因会响应肌醇和胆碱而受到协调调控。肌醇会影响磷脂酸（PA）的细胞内水平。Opi1p是磷脂生物合成基因的阻遏物，它在内质网中特异性结合PA。在有肌醇存在时，PA水平降低，使Opi1p释放到细胞核中，在那里它会抑制转录。opi1突变体在缺乏肌醇和胆碱的情况下会过量产生并将肌醇分泌到生长培养基中（Opi(-)表型）。为了更好地理解Opi1p的阻遏机制，对酵母可行缺失文库进行了筛选以鉴定Opi(-)突变体。总共鉴定出89个Opi(-)突变体，其中7个先前已知具有Opi(-)表型。Opi(-)突变体集合包括在磷脂生物合成、转录、蛋白质加工/合成和蛋白质转运中起作用的基因。该集合中包括NuA4 HAT复合物的所有非必需成分以及Rpd3p-Sin3p HDAC复合物中的六种蛋白质。先前已经表明，磷脂酰胆碱合成缺陷（cho2和opi3）会由于PA的积累而产生Opi(-)表型。然而，在这种情况下，Opi(-)表型是有条件的，因为通过向生长培养基中添加胆碱，PA可以通过补救途径（肯尼迪途径）穿梭。Opi(-)集合中存在的七个新突变体（fun26、kex1、nup84、tps1、mrpl38、mrpl49和opi10/yol032w）也被胆碱抑制，这表明这些突变体影响磷脂酰胆碱的合成。对肌醇的响应调节也与未折叠蛋白反应（UPR）相协调。与此一致的是，发现几个Opi(-)突变体影响UPR（yhi9、ede1和vps74）。
