Translation driven by picornavirus IRES is hampered from Sindbis virus replicons: rescue by poliovirus 2A protease.

Alphavirus replicons are very useful for analyzing different aspects of viral molecular biology. They are also useful tools in the development of new vaccines and highly efficient expression of heterologous genes. We have investigated the translatability of Sindbis virus (SV) subgenomic mRNA bearing different 5'-untranslated regions, including several viral internal ribosome entry sites (IRESs) from picornaviruses, hepatitis C virus, and cricket paralysis virus. Our findings indicate that all these IRES-containing mRNAs are initially translated in culture cells transfected with the corresponding SV replicon but their translation is inhibited in the late phase of SV replication. Notably, co-expression of different poliovirus (PV) non-structural genes reveals that the protease 2A (2A(pro)) is able to increase translation of subgenomic mRNAs containing the PV or encephalomyocarditis virus IRESs but not of those of hepatitis C virus or cricket paralysis virus. A PV 2A(pro) variant deficient in eukaryotic initiation factor (eIF) 4GI cleavage or PV protease 3C, neither of which cleaves eIF4GI, does not increase picornavirus IRES-driven translation, whereas L protease from foot-and-mouth disease virus also rescues translation. These findings suggest that the replicative foci of SV-infected cells where translation takes place are deficient in components necessary to translate IRES-containing mRNAs. In the case of picornavirus IRESs, cleavage of eIF4GI accomplished by PV 2A(pro) or foot-and-mouth disease virus protease L rescues this inhibition. eIF4GI co-localizes with ribosomes both in cells electroporated with SV replicons bearing the picornavirus IRES and in cells co-electroporated with replicons that express PV 2A(pro). These findings support the idea that eIF4GI cleavage is necessary to rescue the translation driven by picornavirus IRESs in baby hamster kidney cells that express SV replicons.