The changes in gene expression underlying the yeast adaptive stress response to H 2 O 2 were analyzed by comparative two-dimensional gel electrophoresis of total cell proteins. The synthesis of at least 115 proteins is stimulated by H 2 O 2 , whereas 52 other proteins are repressed by this treatment. We have identified 71 of the stimulated and 44 of the repressed targets. The kinetics and dose-response parameters of the H 2 O 2 genomic response were also analyzed. Identification of these proteins and their mapping into specific cellular processes give a distinct picture of the way in which yeast cells adapt to oxidative stress. Aerobic organisms have to maintain a reduced cellular redox environment in the face of the prooxidative conditions characteristic of aerobic life. The incomplete reduction of oxygen to water during respiration leads to the formation of redox-active oxygen intermediates (ROI) 1 such as the superoxide anion radical (O 2 Ϫ ), hydrogen peroxide (H 2 O 2 ), and the hydroxyl radical (OH ⅐ ) (for review, see Refs. 1-3). ROI are also produced during the ␤-oxidation of fatty acids, and upon exposure to radiation, light, metals, and redox active drugs. Oxidative stress results from abnormally high levels of ROI which perturb the cell redox status and leads to damage to lipids, proteins, DNA, and eventually cell death. Living organisms constantly sense and adapt to such redox perturbations by the induction of batteries of genes or stimulons whose products act to maintain the cellular redox environment (4 (9,10). Yeast has the same defense mechanisms as higher eukaryotes (for review, see Refs. 11 and 12) and offers the power of genome-wide experimental approaches owing to the availability of the complete sequence of its genome. It therefore represents an ideal eukaryotic model in which to study the cellular redox control and ROI metabolism. We recently established a general method to identify yeast proteins based on two-dimensional gel electrophoresis (13). We used this genome-wide experimental approach to characterize proteins whose expression is altered upon exposure to low doses of H 2 O 2 . Such an oxidative stress challenge results in a dramatic genomic response involving at least 167 proteins. Identification of these proteins and their mapping into cellular processes give a global view of the ubiquitous cellular changes elicited by H 2 O 2 and provides the framework for understanding the mechanisms of cellular redox homeostasis and H 2 O 2 metabolism. ura3-52 lys2-801 amber ade2-101 ochre trp1-⌬1 leu2-⌬1) was used for the analysis of the H 2 O 2 response. The strain S288C (15) was used for protein spot identification. Strains were grown at 30°C in a medium containing 0.67% yeast nitrogen base without amino acids (Difco), 2% glucose, buffered to pH 5.8 with 1% succinate and 0.6% NaOH. For YPH98, uracil, adenine, lysine, tryptophan and leucine (30 mg/liter) were added to the culture medium. MATERIALS AND METHODS Strains and Growth Conditions-The yeast strain YPH98 (14) (MATaIdentification of P...

After invading host cells, Toxoplasma gondii multiplies within a parasitophorous vacuole (PV) that is maintained by parasite proteins secreted from organelles called dense granules. Most dense granule proteins remain within the PV, and few are known to access the host cell cytosol. We identify GRA16 as a dense granule protein that is exported through the PV membrane and reaches the host cell nucleus, where it positively modulates genes involved in cell-cycle progression and the p53 tumor suppressor pathway. GRA16 binds two host enzymes, the deubiquitinase HAUSP and PP2A phosphatase, which exert several functions, including regulation of p53 and the cell cycle. GRA16 alters p53 levels in a HAUSP-dependent manner and induces nuclear translocation of the PP2A holoenzyme. Additionally, certain GRA16-deficient strains exhibit attenuated virulence, indicating the importance of these host alterations in pathogenesis. Therefore, GRA16 represents a potentially emerging subfamily of exported dense granule proteins that modulate host function.