YBP1 and Its Homologue YBP2/YBH1 Influence Oxidative-Stress Tolerance by Nonidentical Mechanisms in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, the transcription factor Yap1p is a central determinant of resistance to oxidative stress. Previous work has demonstrated that Yap1p is recruited from the cytoplasm to the nucleus upon exposure to the oxidants diamide and H2O2 in a process that requires the transient covalent linkage of the glutathione peroxidase Gpx3p to Yap1p. Genetic and biochemical analyses indicate that while both oxidants trigger nuclear accumulation of Yap1p, the function and regulation of this transcription factor is different under these two different oxidative stresses. Ybp1p (Yap1p-binding protein) has recently been demonstrated to be required for Yap1p-mediated H2O2 resistance but not diamide resistance. A Ybp1p homologous protein (Ybh1p/Ybp2p) was also detected in the S. cerevisiae genome. Here we compare the actions of these two closely related proteins and provide evidence that while both factors influence H2O2 tolerance, they do so by nonidentical mechanisms. A double mutant strain lacking both YBP1 and YBH1 genes is more sensitive to H2O2 and more defective in activation of Yap1p-dependent gene expression than either single mutant. Ybp1p has a more pronounced effect on these phenotypes than does Ybh1p. Protein-protein interactions between Yap1p and Ybp1p could be detected by either the yeast two-hybrid or coimmunoprecipitation approach while neither technique could demonstrate Yap1p-Ybh1p interactions. Overexpression experiments indicated that high levels of Ybh1p but not Ybp1p could bypass the H2O2 hypersensitivity of a gpx3Delta strain. Together, these data argue that these two homologous proteins act as parallel positive regulators of H2O2 tolerance.