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Cells possess mechanisms that permit survival and recovery from stress, several of which regulate the phosphorylation of eukaryotic translation initiation factor 2alpha (eIF2alpha). We identified the human OGFOD1 protein as a novel stress granule component that regulates the phosphorylation of eIF2alpha and the resumption of translation in cells recovering from arsenite-induced stress. Coimmunoprecipitation studies revealed that OGFOD1 associates with a small subset of stress granule proteins (G3BP1, USP10, Caprin1, and YB-1) and the ribosome in both unstressed and stressed cells. Overexpression of OGFOD1 led to increased abundance of phosphorylated eIF2alpha, both in unstressed cells and in cells exposed to arsenite-induced stress, and to accelerated apoptosis during stress. Conversely, knockdown of OGFOD1 resulted in smaller amounts of phosphorylated eIF2alpha and a faster accumulation of polyribosomes in cells recovering from stress. Finally, OGFOD1 interacted with both eIF2alpha and the eIF2alpha kinase heme-regulated inhibitor (HRI), which was identified as a novel stress granule resident. These findings argue that OGFOD1 plays important proapoptotic roles in the regulation of translation and HRI-mediated phosphorylation of eIF2alpha in cells subjected to arsenite-induced stress.

Summary MicroRNAs usually interact with 3′ noncoding regions (3′NCRs) of target mRNAs leading to downregulation of mRNA expression. In contrast, liver-specific microRNA miR-122 interacts with the 5′ end of the hepatitis C virus RNA genome, resulting in increased viral RNA abundance. We find that inserting the viral miR-122 binding site into the 3′ noncoding region of a reporter mRNA leads to downregulation of mRNA expression, indicating that the location of the miR-122 binding site dictates its effect on gene regulation. Furthermore, we discovered an adjacent, second miR-122 binding site, separated from the first by a highly conserved 14-nucleotide sequence. Mutational analysis demonstrates that both miR-122 binding sites in a single viral genome are occupied by the microRNA and function cooperatively to regulate target gene expression. These findings set a paradigm for dual, position-dependent functions of tandem microRNA-binding sites. Targeting an oligomeric microRNA complex offers potential as an antiviral-intervention strategy.

It has been known for some time that plants and insects use RNA interference (RNAi) as nucleic acid-based immunity against viral infections. However, it was unknown whether mammalian cells employ the RNA interference pathway as an antiviral mechanism as well. Over the past years, it has become clear that a variety of viruses, first in plants but recently in insect and mammalian viruses as well, encode suppressors of the RNAi pathway arguing for an antiviral role of this machinery. More recent findings have revealed that certain viruses encode their own microRNAs or microRNA-like RNA molecules, which are processed by the mammalian RNAi machinery. Furthermore, host-encoded microRNAs have been shown to both silence and enhance intracellular levels of viral RNAs. These findings argue that interactions between the RNAi pathway and viral genomes can profoundly affect the outcomes of the viral life cycles and contribute to the pathogenic signatures of the infectious agents.

Components of cytoplasmic processing bodies (P-bodies) and stress granules can be subverted during viral infections to modulate viral gene expression. Because hepatitis C virus (HCV) RNA abundance is regulated by P-body components such as microRNA miR-122, Argonaute 2 and RNA helicase RCK/p54, we examined whether HCV infection modulates P-bodies and stress granules during viral infection. It was discovered that HCV infection decreased the number of P-bodies, but induced the formation of stress granules. Immunofluorescence studies revealed that a number of P-body and stress granule proteins co-localized with viral core protein at lipid droplets, the sites for viral RNA packaging. Depletion of selected P-body proteins decreased overall HCV RNA and virion abundance. Depletion of stress granule proteins also decreased overall HCV RNA abundance, but surprisingly enhanced the accumulation of infectious, extracellular virus. These data argue that HCV subverts P-body and stress granule components to aid in viral gene expression at particular sites in the cytoplasm.

SummaryCellular responses to counter virus infection lead to the induction of cytoplasmic stress granules, which are composed of translationally stalled mRNAs. In this issue of Cell Host & Microbe, White and colleagues elucidate a mechanism where a poliovirus protease specifically cleaves a host cell factor involved in assembly of stress granules. This strategy ensures viral access to the limiting amounts of translation factors and interferes with host cell mRNA sorting.