Saccharomyces cerevisiae Hog1 protein phosphorylation upon exposure to bacterial endotoxin.

The yeast Hog1 protein is both functionally and structurally similar to the mammalian p38, belonging to the same family of mitogen-activated protein (MAP) kinases and responding to extracellular changes in osmolarity. Since p38 mediates lipopolysaccharide (LPS) effects in mammalian cells, we now tested the responsiveness of Hog1 upon exposure of the yeast Saccharomyces cerevisiae to bacterial LPS. In the presence of Escherichia coli LPS (100 ng/ml) and an endotoxically active, hexaacylated, synthetic lipid A (compound 506; 100 ng/ml), Hog1 becomes phosphorylated with a maximum of phosphorylation between 3 and 6 h, whereas a tetraacylated, inactive form of lipid A (compound 406) did not cause any modification in the phosphorylation state of Hog1. A triple labeling immunocytochemical study showed that phosphorylated Hog1 translocates into the nucleus after a 90-min incubation and becomes sparsely located in the cytoplasm. The translocation of the phospho-Hog1 is preceded by an increased expression of the HOG1 gene and concomitant with the expression of the Hog1 target gene, GPD1. We also observed that cells unable to synthesize Hog1 do not resist LPS as efficiently as wild-type cells. We conclude that the yeast S. cerevisiae is able to respond to the presence of Gram-negative bacteria endotoxin and that Hog1 is involved in this response.

Expression of HPV16 E6 oncoprotein increases resistance to several stress conditions in Saccharomyces cerevisiae.

The E6 protein of human papillomavirus type 16 is essential for the oncogenic transformation process induced by these viruses. Here we expressed the E6 protein in Saccharomyces cerevisiae (which lacks p53) in order to determine if E6 interacts with normal cell functioning, independently of the p53 tumour suppressor factor. We observed a higher resistance to caffeine, hydrogen peroxide and to pheromone, but not to high temperature, starvation and osmostress. Measurement of the relative expression levels of target genes of the signalling pathways, involved in the latter stressful stimuli, led us to conclude that such pathways are differently regulated in the presence of E6.