ИСТИНА

eIF2D (formerly called ligatin) is a mysterious translation factor that is able to facilitate tRNA binding to the P-site of the ribosome in a GTP-independent manner. Its activity requires an AUG codon of mRNA properly positioned in the P-site before the tRNA binding. In a reconstituted system of eukaryotic translation initiation, eIF2D can replace both eIF2 and eIF5B in reactions of 48S and 80S complex formation, respectively. However, in contrast to eIF2-mediated initiation which is limited to Met-tRNAi, this alternative pathway may operate with elongator tRNAs. The physiological function of eIF2D is yet unknown. In yeast, eIF2D ortholog TMA64 has genetic interactions and expression profile that predict its involvement in control of the cell cycle and adaptation to stress. In addition to translation-related domains (N-terminal PUA and C-terminal SUI1), eIF2D possesses a much less conserved central part with similarity to Kin17_mid and SWIB/MDM2 domains. This feature suggests that eIF2D may be a component of protein complexes that coordinate cell cycle checkpoints in response to stress. To investigate a putative role of eIF2D in such events, we analyzed its localization within cultured mammalian cells under variety of conditions. We found that under normal conditions eIF2D was diffusely distributed in the cytoplasm. However, upon induction of severe oxidative stress by sodium arsenite, both endogenous eIF2D and transiently expressed GFP-eIF2D formed two clearly defined foci in the cytoplasm in close proximity to the nucleus. These foci were clearly distinct from stress granules that were extensively formed under these conditions and contained several components of translation machinery. They were positively stained with antibodies against gamma-tubulin, the marker of centrosome. Similar localization pattern was observed under a number of stress conditions. The centrosomal localization of eIF2D was determined by the middle portion of the protein. We concluded that upon induction of stress response, eIF2D is relocated to the centrosome to participate in some specific translation events. This spatially restricted activity of eIF2D may be related to control of the cell cycle progression. D.S.M., P.G.S. and P.A.I. contributed equally to this work.