Studies on the oxidative protein folding pathway in yeast

The oxidative protein folding (OPF) pathway is conserved across prokaryotic and eukaryotic organisms. In yeast, the OPF is catalysed by the essential proteins protein disulphide isomerase (PDI) and endoplasmic reticulum oxidoreductin (Ero1p) within the endoplasmic reticulum (ER). It is unclear why PDI1 is an essential gene, as upregulation of some yPDI homologues can restore viability to a ?pdi1 strain. The essentiality of PDI was therefore investigated to identify a potential suppressor of ?pdi1 lethality (supX), with the hypothesis that supX would be upregulation of yPDI homologues. As the purpose of the non-essential yPDI homologues is unknown this would help characterise the yeast OPF pathway as well as providing a potential mechanism to increase OPF in vivo. A genetic screen was carried out to search for potential extragenic suppressors of the lethality seen in a ?pdi1 mutant using a plasmid shuffling strategy. A number of viable mutants were obtained but these arose either via genomic recombination with the plasmid-borne PDI1 or through direct resistance to 5-FOA used in the screen. Two other PDI-less mutants obtained showed a loss of viability upon storage. Glutathione peroxidase 7 (GPx7) and glutathione peroxidase 8 (GPx8) are ER localised mammalian proteins that couple the detoxification of hydrogen peroxide (H2O2) to reoxidation of PDI, simultaneously alleviating H2O2 stress in the ER and increasing the rate of OPF. There are no known ER-localised peroxiredoxin proteins in S. cerevisiae and so GPx7 and GPx8 were expressed in yeast to remove accumulated H¬2O2 as this may be a limiting factor in OPF. Neither GPx7 nor GPx8 had any effect on levels of intracellular H2O2 in vivo or cellular sensitivity to H2O2. Furthermore there was no effect on the rate of OPF in vivo and in vitro. It is possible that there is no functional interaction between yeast PDI and the mammalian proteins GPx7 and GPx8.