Degradation of oxidized proteins by the 20S proteasome

Oxidatively modified proteins are continuously produced in cells by reactiv e oxygen and nitrogen species generated as a consequence of aerobic metabol ism. During periods of oxidative stress, protein oxidation is significantly increased and map become a threat to cell survival. In eucaryotic cells th e proteasome has been shown (by purification of enzymatic activity, by immu nopreciptation and by antisense oligonucleotide studies) to selectively rec ognize and degrade mildly oxidized proteins in the cytosol, nucleus, and en doplasmic reticulum, thus minimizing their cytotoxicity. From in vitro stud ies it is evident that the 20S proteasome complex actively recognizes and d egrades oxidized proteins, but the 26S proteasome, even in the presence of ATP and a reconstituted functional ubiquitinylating system, is not very eff ective. Furthermore, relatively mild oxidative stress rapidly (but reversib ly) inactivates both the ubiquitin activating/conjugating system and 26S pr oteasome activity in intact cells, but does not affect 20S proteasome activ ity. Since mild oxidative stress actually increases proteasome-dependent pr oteolysis (of oxidized protein substrates) the 20S 'core' proteasome comple x would appear to be responsible. Finally, new experiments indicate that co nditional mutational inactivation of the E1 ubiquitin-activatin, enzyme doe s not affect the degradation of oxidized proteins, further strengthening th e hypothesis that oxidatively modified proteins are degraded in an ATP-inde pendent, and ubiquitin-independent, manner by the 20S proteasome. More seve re oxidative stress causes extensive protein oxidation, directly generating protein fragments, and cross-linked and aggregated proteins, that become p rogressively resistant to proteolytic digestion. In fact these aggregated, cross-linked, oxidized proteins actually bind to the 20S proteasome and act as irreversible inhibitors. It is proposed that aging, and various degener ative diseases, involve increased oxidative stress (largely from damaged an d electron 'leaky' mitochondria), and elevated levels of protein oxidation, cross-linking, and aggregation. Since these products of severe oxidative s tress inhibit the 20S proteasome, they cause a vicious cycle of progressive ly worsening accumulation of cytotoxic protein oxidation products. (C) 2001 Societe francaise de biochimie et biologie moleculaire / Editions scientif iques et medicales Elsevier SAS. All rights reserved.