The DExD/H box RNA helicase retinoic acid-inducible gene I (RIG-I) and the melanoma differentiation-associated gene 5 (MDA5) are key intracellular receptors that recognize virus infection to produce type I IFN. A third helicase gene, Lgp2, is homologous to Rig-I and Mda5 but lacks a caspase activation and recruitment domain. We generated Lgp2-deficient mice and report that the loss of this gene greatly sensitizes cells to cytosolic polyinosinic/polycytidylic acid-mediated induction of type I IFN. However, negative feedback inhibition of IFN-β transcription was found to be normal in the absence of LGP2, indicating that LGP2 is not the primary negative regulator of type I IFN production. Our data further indicate that Lgp2−/− mice exhibited resistance to lethal vesicular stomatitis virus infection, a virus whose replicative RNA intermediates are recognized specifically by RIG-I rather than by MDA5 to trigger the production of type I IFN. However, mice lacking LGP2 were observed to exhibit a defect in type I IFN production in response to infection by the encephalomyocarditis virus, the replication of which activates MDA5-dependent innate immune responses. Collectively, our data indicate a disparate regulatory role for LGP2 in the triggering of innate immune signaling pathways following RNA virus infection.

We report here that the alternatively spliced nuclear factors associated with double-stranded RNA, NFAR-1 (90 kDa) and -2 (110 kDa), are involved in retaining cellular transcripts in intranuclear foci and can regulate the export of mRNA to the cytoplasm. Furthermore, the NFAR proteins were found to remain associated with exported ribonucleoprotein complexes. Loss of NFAR function, which was embryonic-lethal, caused an increase in protein synthesis rates, an effect augmented by the presence of the mRNA export factors TAP, p15, or Rae1. Significantly, NFAR depletion in normal murine fibroblasts rendered these cells dramatically susceptible to vesicular stomatitis virus replication. Collectively, our data demonstrate that the NFARs exert influence on mRNA trafficking and the modulation of translation rates and may constitute an innate immune translational surveillance mechanism important in host defense countermeasures against virus infection.innate immunity ͉ mRNA export ͉ vesicular stomatitis virus

Eukaryotic translation initiation factor 2B (eIF2B) is a heteropentameric guanine nucleotide exchange factor that converts protein synthesis initiation factor 2 (eIF2) from a GDP-bound form to the active eIF2-GTP complex. Cellular stress can repress translation initiation by activating kinases capable of phosphorylating the alpha subunit of eIF2 (eIF2␣), which sequesters eIF2B to prevent exchange activity. Previously, we demonstrated that tumor cells are sensitive to viral replication, possibly due to the occurrence of defects in eIF2B that overcome the inhibitory effects of eIF2␣ phosphorylation. To extend this analysis, we have investigated the importance of eIF2B␣ function and report that this subunit can functionally substitute for its counterpart, GCN3, in yeast. In addition, a variant of mammalian eIF2B␣ harboring a point mutation (T41A) was able overcome translational inhibition invoked by amino acid depravation, which activates Saccharomyces cerevisiae GCN2 to phosphorylate the yeast eIF2␣ homolog SUI2. Significantly, we also demonstrate that the loss of eIF2B␣, or the expression of the T41A variant in mammalian cells, is sufficient to neutralize the consequences of eIF2␣ phosphorylation and render normal cells susceptible to virus infection. Our data emphasize the importance of eIF2B␣ in mediating the eIF2 kinase translation-inhibitory activity and may provide insight into the complex nature of viral oncolysis.The initiation of translation requires the coordinate actions of several protein factors to complete a multiphase process that culminates in the formation of an 80S ribosome on a nascent mRNA molecule (30). The first of these steps is the formation of a stable ternary complex composed of one charged initiator Met-tRNA i molecule, GTP, and eukaryotic initiation factor 2 (eIF2), comprised of its ␣, ␤, and ␥ subunits. Configuration of the ternary complex is required to bring the Met-tRNA i molecule into contact with the 40S ribosomal subunit, which subsequently recognizes mRNA molecules through the eIF4F cap-binding complex (35). These events lead to the recognition of the AUG start codon of the mRNA and the binding of the 60S ribosomal subunit to the 40S subunit (26). At the end of initiation and upon the creation of the 80S ribosome, the accompanying protein factors are released from the ribosome-mRNA molecule to be recycled for future use. In addition to initiation factor release, the GTP bound to eIF2 is hydrolyzed by the action of eIF5, such that the resultant eIF2 is liberated as an inactive binary complex bound to GDP (31, 36). In order for subsequent rounds of initiation to occur, it is necessary to generate new active eIF2-GTP from the existing eIF2-GDP, a reaction catalyzed by the heteropentameric guanine nucleotide exchange factor (GEF) eIF2B (41).eIF2B provides a key regulatory mechanism for cells to decrease protein synthesis rates during periods of stress. Selected cellular stress, such as the accumulation of viral RNA species following infection or misfolded proteins in the endoplasmic...