Mutations in the mouse p (pink-eyed dilution) and human P genes lead to melanosomal defects and ocular developmental abnormalities. Despite the critical role played by the p gene product in controlling tyrosinase processing and melanosome biogenesis, its precise biological function is still not defined. We have expressed p heterologously in the yeast Saccharomyces cerevisiae to study its function in greater detail. Immunofluorescence studies revealed that p reaches the yeast vacuolar membrane via the prevacuolar compartment. Yeast cells expressing p exhibited increased sensitivity to a number of toxic compounds, including arsenicals. Similarly, cultured murine melanocytes expressing a functional p gene were also found to be more sensitive to arsenical compounds compared with p-null cell lines. Intracellular glutathione, known to play a role in detoxification of arsenicals, was diminished by 50% in p-expressing yeast. By using the glutathione-conjugating dye monochlorobimane, in combination with acivicin, an inhibitor of vacuolar gamma-glutamyl cysteine transpeptidase, involved in the breakdown of glutathione, we found that p facilitates the vacuolar accumulation of glutathione. Our data demonstrate that the pink-eyed dilution protein increases cellular sensitivity to arsenicals and other metalloids and can modulate intracellular glutathione metabolism.

Mating in haploid Saccharomyces cerevisiae occurs after activation of the pheromone response pathway. Biochemical components of this pathway are involved in other yeast signal transduction networks. To understand more about the coordination between signaling pathways, we used a "chemical genetic" approach, searching for compounds that would activate the pheromone-responsive gene FUS1 and RLM1, a reporter for the cell integrity pathway. We found that catecholamines (L-3,4-hydroxyphenylalanine [L-dopa], dopamine, adrenaline, and noradrenaline) elevate FUS1 and RLM1 transcription. N-Acetyl-cysteine, a powerful antioxidant in yeast, completely reversed this effect, suggesting that FUS1 and RLM1 activation in response to catecholamines is a result of oxidative stress. The oxidant hydrogen peroxide also was found to activate transcription of an RLM1 reporter. Further genetic analysis combined with immunoblotting revealed that Kss1, one of the mating mitogen-activated protein kinases (MAPKs), and Mpk1, an MAPK of the cell integrity pathway, participated in L-dopa-induced stimulation of FUS1 and RLM1 transcription. We also report that Mpk1 and Hog1, the high osmolarity MAPK, were phosphorylated upon induction by hydrogen peroxide. Together, our results demonstrate that cells respond to oxidative stress via different signal transduction machinery dependent upon the nature of the oxidant. INTRODUCTIONIn the haploid cells of the yeast Saccharomyces cerevisiae, four essential MAPK cascades (Figure 1) respond to different external signals to mediate specific responses. The mating pathway is activated by peptide pheromones and induces cell-cycle arrest and the morphological changes required for mating (Elion, 1998;Gustin et al., 1998). The invasive growth pathway is activated by starvation and induces foraging into the agar substratum. The high osmolarity glycerol (HOG) pathway increases intracellular glycerol levels in response to hypertonic stress, whereas the cell integrity pathway is activated by hypotonic stress, heat shock, or impaired cell wall synthesis. Recently, an additional MAPK cascade has been described: the Kss1 vegetative growth pathway (Lee and Elion, 1999). The Kss1 pathway may be activated by cell wall stress or changes in osmolarity (Cullen et al., 2000).The mating pathway is the most studied cellular response to an external signal. As a relatively simple G protein-coupled cascade, it is a widely used model to study mammalian G protein-coupled receptors. The mating cascade includes pheromone receptors (Ste2 and Ste3), G proteins (Gpa1, Ste4, and Ste18), the p21 activating protein kinase Ste20, a mitogen-activated protein kinase kinase kinase (MAPKKK) Ste11, a mitogen-activated protein kinase kinase (MAPKK) Ste7, a scaffolding protein Ste5 and two MAPKs (Fus3 and Kss1) (Gustin et al., 1998;Madhani and Fink, 1998;Farley et al., 1999). Targets of the terminal MAPK include Ste12, a factor required for transcription of pheromone-responsive genes, and Far1, a bifunctional protein required for polarization and G1...

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The study of some 4-aroyl-1-(2-chloroethyl)-1-nitrosohydrazinecarboxamides with a Saccharomyces cerevisiae mutagenicity test of increased sensitivity defined two of them, 4-(4-bromobenzoyl)-1-(2-chloroethyl)-1-nitrosohydrazinecarboxam ide and 4-(4-fluorophenyl)-1-(2-chloroethyl)-1-nitrosohydrazine carboxamide as typical cytostatic agents. At concentrations of 2-5 microg/ml the substances kill up to 60%-70% of cells without having any detectable recombinogenic and mutagenic effects. At the same concentrations, lomustine, well known as a cytostatic reference, demonstrated recombinogenic and mutagenic activity on yeast cells. The advantage of the newly synthesized substances is that, in a certain concentration range, their biological activity is mainly cytotoxic without induction of recombinogenic and mutagenic events in surviving cells.