Although a great deal is known biochemically about peroxiredoxins (Prxs), little is known about their real physiological function. We show here that two cytosolic yeast Prxs, cPrxI and II, which display diversity in structure and apparent molecular weights (MW), can act alternatively as peroxidases and molecular chaperones. The peroxidase function predominates in the lower MW forms, whereas the chaperone function predominates in the higher MW complexes. Oxidative stress and heat shock exposure of yeasts causes the protein structures of cPrxI and II to shift from low MW species to high MW complexes. This triggers a peroxidase-to-chaperone functional switch. These in vivo changes are primarily guided by the active peroxidase site residue, Cys(47), which serves as an efficient "H(2)O(2)-sensor" in the cells. The chaperone function of these proteins enhances yeast resistance to heat shock.

Although biochemical properties of 2-Cys peroxiredoxins (Prxs) have been extensively studied, their real physiological functions in higher eukaryotic cells remain obscure and certainly warrant further study. Here we demonstrated that human (h) PrxII, a cytosolic isotype of human 2-Cys Prx, has dual functions as a peroxidase and a molecular chaperone, and that these different functions are closely associated with its adoption of distinct protein structures. Upon exposure to oxidative stress, hPrxII assumes a high molecular weight complex structure that has a highly efficient chaperone function. However, the subsequent removal of stressors induces the dissociation of this protein structure into low molecular weight proteins and triggers a chaperone-toperoxidase functional switch. The formation of a high molecular weight hPrxII complex depends on the hyperoxidation of its N-terminal peroxidatic Cys residue as well as on its C-terminal domain, which contains a "YF motif" that is exclusively found in eukaryotic 2-Cys Prxs. A C-terminally truncated hPrxII exists as low and oligomeric protein species and does not respond to oxidative stress. Moreover, this C-terminal deletion of hPrxII converted it from an oxidation-sensitive to a hyperoxidation-resistant form of peroxidase. When functioning as a chaperone, hPrxII protects HeLa cells from H 2 O 2 -induced cell death, as measured by a terminal deoxynucleotidyltransferase-mediated dUTP nickend labeling assay and fluorescence-activated cell sorting analysis.

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A cDNA, PHCC-TPx, specifying a protein highly homologous to known phospholipid hydroperoxide glutathione peroxidases was isolated from a Chinese cabbage cDNA library.