The stability of the Opi1p repressor for phospholipid biosynthetic gene expression in Saccharomyces cerevisiae is dependent on its interactions with Scs2p and Ino2p

The yeast Saccharomyces cerevisiae Opi1p negatively regulates phospholipid biosynthetic genes. Under derepressing conditions, Opi1p binds to the endoplasmic reticulum/nuclear membrane with the aid of the membrane protein Scs2p and phosphatidic acids under derepressing conditions. Under repressing conditions, it enters the nucleus to inhibit the positive transcription factors Ino2p and Ino4p. While the spatial regulation of Opi1p is understood, the regulation of its abundance remains unclear. We investigated the role of Scs2p and Ino2p in Opi1p stability by overexpressing these proteins in yeast cells. Opi1p was stable in the presence of Scs2p, but mutations in residues required for interaction with Scs2p caused Opi1p unstable. Even in the absence of Scs2p, Opi1p remained stable in the strain having a mutation to increase phosphatidic acid levels. Conversely, overproduction of Ino2p reduced Opi1p stability, whereas a mutant Ino2p that cannot interact with Opi1p did not. Additionally, Opi1p was stable in strains lacking Ino2p or with a mutated Ino2p-binding domain. These findings suggest that regulation, adding another layer to the regulation of phospholipid biosynthetic gene expression by Opi1p.