|Oxidative stress can cause toxic outcomes to cells/organisms’ survival. To counter this toxicity, multiple cellular mechanisms were evolved. Among them is the methionine sulfoxide reductase (Msr) system. Methionine sulfoxide reductase A (MsrA) plays a key role in protecting cells from oxidative stress. In our previous studies we discovered a novel role of MsrA in the regulation of ubiquitin (Ub) and Ub-like modification of proteins. To further understand the role of MsrA in posttranslational modification of proteins, a yeast-2-hybrid (Y2H) screening was performed to screen for potential substrates for MsrA in brain. Accordingly, one of the major identified substrates was a protein denoted as COP9 signalosome subunit 5 (CSN5, also known as Jab1 or COPS5), containing a unique binding site to MsrA. The COP9 signalosome complex (CSN) is an essential regulator of the ubiquitin conjugation pathway by mediating the removal of Nedd8, an Ub-like protein modifier, from proteins (deneddylation process). Importantly, the Csn5/Jab1 contains a domain that provides the catalytic center of the CSN complex. The known substrates for neddylation and deneddylation (by CSN) are the cullin subunits and their homologues (e.g. Cul-1 of the SCF-type E3 ligase complexes). Furthermore, we showed that MsrA interacts with Csn5/Jab1 in mouse brain following immunoprecipitations and pull-down experiments. This interaction was compromised in brain extracts of MsrA knockout (MsrA KO) compared with the parent wild type (WT) brains. A decrease in the levels of neddylated Cul-1 was also observed in liver extract of MsrA KO mouse, while the levels of Csn5/Jab1 in brain were the same in both the WT and MsrA KO strains. These data suggested that MsrA positively regulates Csn5/Jab1 deneddylation activity, presumably via reduction of this protein MetO residue/s. To further study the relationship between neddylation level and MsrA in vivo, we used yeast strains with various expression levels of MsrA. These yeast strains were WT, MsrA KO and MsrA overexpressed (OE) yeast strains. Neddylation levels of yeast under the condition of hydrogen peroxide or human Csn5/Jab1 inhibitor were investigated. The data showed that both oxidative stress and Csn5/Jab1 inhibitor caused inhibition of deneddylation activity of Csn5/Jab1 in the absence of MsrA, while MsrA-containing strains (i.e. WT and OE) showed a strong ability to protect against H2O2/Csn5/Jab1 inhibitor induced neddylation. For in vitro assays, we used artificial Nedd8 conjugates monitoring deneddylation activity in extracts of brains of WT and MsrA KO mice strains as function of incubation time. The acquired data showed that the deneddylation activity was dramatically reduced in the MsrA KO compared with the WT brain extracts. In conclusion, the presented data provide direct and indirect evidence to support our hypothesis that MsrA plays an important role in maintaining the Csn5/Jab1 deneddylation activity. Lastly, we investigated the potential role of MsrA in regulating of neddylation levels in brain cancer. Glioblastoma is one of the most aggressive brain cancers. This cancer type demonstrates a relative low level of neddylation compared with normal brain cells. Accordingly, Csn5/Jab1 could be one of the posttranslational regulated proteins that may contribute to this phenomenon. We have indeed found that the neddylation level was reduced in glioblastoma cells, while the level of Csn5/Jab1 was unchanged compared with normal brain cells. This observation pointed to the possible role of MsrA in enhancing Csn5/Jab1 activity in these cancer cells. This possibility will be further investigated by comparing the MsrA activities in both the normal and glioblastoma cells in the nearest future.