RNase L functions as a tumor suppressor protein due to its role in apoptosis during viral infections and various agents treatment. RNase L-mediated apoptosis is accompanied by cytochrome c release from mitochondria and requires caspase-3 activity. It was reported that RNase L is involved in JNK activation during viral infections, and that JNK is essential for apoptosis in response to RNase L activation. However, the proximal signals of RNase L that trigger JNK activation have not as yet been identified, and it is possible that the interactions between RNase L and other proteins play an important role in the RNase L-JNK apoptotic signal pathway. To investigate this hypothesis, we attempted to identify the proteins associated with RNase L using coimmunoprecipitation. In this study, we found that RNase L was associated with other proteins. Here we identified the protein X by LC-MS/MS analysis. We demonstrated that this association was increased in the presence of activated RNase L. Moreover, Protein X was phosphorylated during the activation of RNase L by treatment with cytotoxic agent, ECyd, 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl) cytosine and 2-5A. To reveal the role of protein X in RNase L-mediated apoptosis, we decreased the level of protein X by a small interfering RNA (siRNA). As a result, protein X deficient cells became resistant to the apoptosis mediated by RNase L, suggesting that protein X is related to RNase L-mediated apoptosis following JNK activation. Therefore, in this study, we report the identification of a novel protein, protein X that transduces between RNase L and JNK signals.
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