Protein import into peroxisomes and oxidative stress: a study to elucidate the potential functional role of the conserved cysteine in Pex5p

Abstract: The oxidation status of a cell plays a crucial role in aging. As cells get aged, their redox state gets increased. Pex5p is a peroxisomal recycling receptor which binds to newly synthesized cargo proteins in the cytosol and imports them across the peroxisomal membrane. During this transport event, Pex5p gets monoubiquitinated at a conserved cysteine (C11) residue. This C11 is very essential for the recycling of Pex5p from the peroxisomal membrane to back into the cytosol. If the cysteine is replaced by serine, Pex5p does not get recycled back to the cytosol and accumulates on the peroxisomal membrane. In the present study, we have investigated whether the C11 in Pex5p could act as a redox switch. We measured the redox state of the cytosol and the peroxisomal matrix as well as the subcellular localization of catalase in aging cells. We found that an increase in the redox state of peroxisomes (in WT) leads to an increase in the redox state of the cytosol, which ultimately results in the impairment of PTS1 import. Interestingly, in the C11K condition, we did not see an impairment of PTS1 import. These observations support our hypothesis that C11 may act as a redox switch. We also performed some challenging experiments with H2O2. The results of these experiments show that a) import of catalase into peroxisomes sensitizes the cytosol and b) catalase overexpression does have a protective effect against oxidative stress caused by H2O2. In summary the results of our experiments support our hypothesis. However, further evaluation is needed to reveal the precise role of C11 in Pex5p function during cellular aging.