Phosphorylation of eIF2α is responsible for the failure of the picornavirus internal ribosome entry site to direct translation from Sindbis virus replicons

Translation directed by the poliovirus (PV) or encephalomyocarditis virus (EMCV) internal ribosome entry site ORES) is very inefficient when expressed from Sindbis virus (SV) replicons. This inhibition can be rescued by co-expression of PV 2A protease (2A(pro)). Inhibition correlates with the extensive phosphorylation of eukaryotic initiation factor (eIF) 2 alpha induced by SV replication. Confirmation that PV or EMCV IRES-driven translation can function when eIF2 alpha is not phosphorylated was obtained in dsRNA-activated protein kinase knockout mouse embryonic fibroblasts (PKR-/- MEFs), where SV replication cannot induce eIF2 alpha phosphorylation, and in variant S51A MEFs that express an unphosphonjlatable eIF2 alpha. In these cells, PV or EMCV IRES-dependent translation operated more efficiently than in wild-type MEFs. However, this translation was potently blocked when eIF2 alpha was phosphorylated by the addition of thapsigargin to PKR-/- MEFs. In addition, when wild-type eIF2 alpha was expressed in S51A MEFs or PKR was expressed in PKR-/- MEFs, PV IRES-dependent translation decreased. In both cases, the decrease in PV IRES-dependent translation correlated with the phosphorylation of eIF2 alpha. Notably, PV 2A(pro) expression rescued PV IRES-driven translation in thapsigargin-treated PKR-/- MEFs. Taken together, these results demonstrated that PV IRES-driven translation can take place from SV replicons if eIF2 alpha remains unphosphorylated. Remarkably, PV IRES-dependent translation was fully functional in this system when PV 2A(pro) was present, even if eIF2 alpha was phosphorylated.