Comparisons of the OXR gene family indicate several key events have occurred during evolution of OXR domain proteins. S. cerevisiae
carries only one copy of OXR in its genome. It is 273 amino acids in length and includes only sequences corresponding to the C-terminal OXR domains of NCOA7 and OXR1. In higher organisms, the OXR domain has become associated with additional upstream protein coding sequences. This occurred prior to duplication, since there is a high degree of identity and similarity between NCOA7 and OXR1 throughout their sequences. The exceptions to this are their N termini, which, in NCOA7 contains a nuclear localization sequence, which is absent in the mitochondrially targeted OXR1. Portions of their largest central exons are also dissimilar. In NCOA7 its exon 8 is 357 amino acids in length and contains its estrogen receptor binding site [25
], whereas the corresponding exon 7 of OXR1 is only 255 amino acids in length and lacks the estrogen receptor binding sequences. OXR1 also contains several unique exons. These include exon 10, which has a readily recognizable mitochondrial targeting sequence [22
], and exon 11, which is found in only one OXR1 isoform (Fig. ).
The demonstration that the full length NCOA7 protein can function to prevent oxidative mutagenesis when expressed in bacteria suggests it may function in this manner in its native eukaryotic host. In bacteria, this may be a general function that results in detoxification of various ROS molecules. The key role for the C-terminal OXR domains in oxidation resistance is indicated by (1) the oxidation sensitivity resulting from deletion of the OXR1 gene of yeast [21
]; (2) the ability of mitochondrially targeted human OXR domain of OXR1 to complement the H2
sensitivity of the yeast oxr1
deletion mutant [22
]; and (3) the ability of the OXR domains of either OXR1 or NCOA7 to suppress the oxidative mutator phenotype of oxidation sensitive E. coli
] (and Figure ). Thus we refer to the C-terminal region of NCOA7 and OXR1 as the oxidation resistance, or OXR domain. Comparison of the OXR domains of OXR1 and NCOA7 with the yeast gene product, indicates both human genes are approximately equally similar to the yeast protein when their OXR domains are compared with the full length yeast protein; OXR1 has 27%identity and 44% similarity to yeast OXR1 and NCOA7 has 31% identity and 43% similarity. Although both human genes are equally similar to the single S. cerevisiae
OXR gene, the yeast OXR gene is functionally most similar to human OXR1, since both yeast and human OXR1 proteins are induced by hydrogen peroxide and heat stress, and localize to mitochondria [22
The association of the NCOA7 gene product with the estrogen receptor is curious for a gene product involved in protection from oxidative DNA damage. It is noteworthy that several DNA repair proteins have recently been identified as estrogen receptor associated proteins. These include the O6
-methylguanine methyltransferase DNA repair protein, the 3-methyladenine DNA N-glycosylase repair protein, and the TG specific mismatch repair protein TDG [29
]. The result that NCOA7 is another ER associated protein that has DNA maintenance properties, suggests that ER association of these related classes of proteins may be a common feature. It has been proposed that NCOA7 may sense the oxidative state of the cell and regulate responses to oxidative DNA damage and the result that NCOA7 can function to protect cells from oxidative DNA damage strengthens this hypothesis [25
]. It may also play a direct role in oxidation resistance, a possibility that is particularly intriguing in light of results indicating that estrogen metabolism causes oxidative DNA damage (for review see: [32
]). When estrogens, such as β-estradiol, are metabolized to catechol estrogen quinones and semiquinones, they enter into a redox cycling reaction in which the quinones are reduced to semiquinones. The semiquinones, in turn, spontaneously oxidize to back to quinones producing ROS [33
]. Oxidative DNA damage has been demonstrated to result as a by-product of estradiol metabolism [34
], thus it is possible that NCOA7 functions to mitigate oxidative DNA damage resulting from estrogen metabolism by bringing it in close proximity to estrogens upon import into the nucleus. Moreover, such an oxidation resistance mechanism of NCOA7 should be enhanced by the presence of estrogen, since this stimulates NCOA7 entry into the nucleus (Figure ).
Both NCOA7 and OXR1 gene products show their highest levels of expression in brain tissue [22
], suggesting they may play a critical role in protecting brain cells from oxidative DNA damage. Thus, it will be of interest to see if either or both of these proteins function to protect against neurodegenerative diseases that are affected by oxidative damage and apoptosis.