Regulation of eukaryotic protein synthesis: selective influenza viral mRNA translation is mediated by the cellular RNA-binding protein GRSF-1.

To better understand regulation of eukaryotic protein synthesis, we studied cellular and viral mRNA translation in influenza virus-infected cells. Influenza virus infection results in a dramatic shut-off of cellular protein synthesis that is concomitant with selective viral mRNA translation. Earlier work showed that these events are mediated by viral and/or cellular factors binding to the 5' untranslated region (5' UTR) of viral mRNAs. To identify trans-acting cellular proteins responsible for selective viral protein synthesis, we employed the yeast three-hybrid system. Using the 5' UTR of the influenza virus nucleocapsid protein (NP) mRNA as bait, we identified the cellular RNA-recognition motif containing RNA-binding protein G-rich sequence factor 1 (GRSF-1) as a positive-acting translational regulatory factor. The in vivo yeast assay revealed GRSF-1 specifically bound to the NP 5' UTR but not select NP 5' UTR mutants or cellular RNA 5' UTRs. These data were confirmed by gel shift assays using recombinant GRSF-1. Importantly, recombinant GRSF-1 specifically stimulated translation of a NP 5' UTR-driven template in cell-free translation systems. Furthermore, translation efficiency of NP 5' UTR-driven templates was reduced markedly in GRSF-1-depleted HeLa cell extracts, but restored in GRSF-1-reconstituted extracts. GRSF-1 also stimulated translation of an NP 5' UTR-driven template in HeLa cell extracts that were depleted of essential factors by addition of RNA oligonucleotides representing the viral 5' UTR RNA. Taken together, these data document the functional demonstration of a cellular protein binding to influenza virus RNAs and, importantly, suggest that influenza virus may recruit GRSF-1 to the 5' UTR to ensure preferential translation of viral mRNAs in infected cells.