Heterozygous mutations in the TERT gene impair telomerase activity by haploinsufficiency and may be risk factors for marrow failure.

Summary Lassa fever virus (LASV) causes thousands of deaths yearly and is a biological threat agent, for which there is no vaccine and limited therapy1. The nucleoprotein (NP) of LASV plays essential roles in viral RNA synthesis and immune suppression2-6, the molecular mechanisms of which are poorly understood. Here, we report the crystal structure of LASV NP at 1.80 Angstrom resolution, which reveals N- and C-domains with structures unlike any of the reported viral NPs7-10. The N domain folds into a novel structure with a deep cavity for binding the m7GpppN cap structure that is required for viral RNA transcription, whereas the C domain contains 3′-5′ exoribonuclease activity involved in suppressing interferon induction. This is the first X-ray crystal structure solved for an arenaviral NP, which reveals its unexpected functions and suggests unique mechanisms in cap binding and immune evasion. These findings provide great potential for vaccine and drug development.

RIG-I (Retinoic acid-inducible gene I) and MDA5 (Melanoma Differentiation-Associated protein 5), collectively known as the RIG-I-like receptors (RLRs), are key protein sensors of the pathogen-associated molecular patterns (PAMPs) in the form of viral double-stranded RNA (dsRNA) motifs to induce expression of type 1 interferons (IFN1) (IFNα and IFNβ) and other pro-inflammatory cytokines during the early stage of viral infection. While RIG-I and MDA5 share many genetic, structural and functional similarities, there is increasing evidence that they can have significantly different strategies to recognize different pathogens, PAMPs, and in different host species. This review article discusses the similarities and differences between RIG-I and MDA5 from multiple perspectives, including their structures, evolution and functional relationships with other cellular proteins, their differential mechanisms of distinguishing between host and viral dsRNAs and interactions with host and viral protein factors, and their immunogenic signaling. A comprehensive comparative analysis can help inform future studies of RIG-I and MDA5 in order to fully understand their functions in order to optimize potential therapeutic approaches targeting them.

The NF-B signaling pathway has previously been shown to be required for efficient influenza A virus replication, although the molecular mechanism is not well understood. In this study, we identified a specific step of the influenza virus life cycle that is influenced by NF-B signaling by using two known NF-B inhibitors and a variety of influenza virus-specific assays. The results of time course experiments suggest that the NF-B inhibitors Bay11-7082 and ammonium pyrrolidinedithiocarbamate inhibited an early postentry step of viral infection, but they did not appear to affect the nucleocytoplasmic trafficking of the viral ribonucleoprotein complex. Instead, we found that the levels of influenza virus genomic RNA (vRNA), but not the corresponding cRNA or mRNA, were specifically reduced by the inhibitors in virus-infected cells, indicating that NF-B signaling is intimately involved in the vRNA synthesis. Furthermore, we showed that the NF-B inhibitors specifically diminished influenza virus RNA transcription from the cRNA promoter but not from the vRNA promoter in a reporter assay, a result which is consistent with data obtained from virus-infected cells. The overexpression of the p65 NF-B molecule could not only eliminate the inhibition but also activate influenza virus RNA transcription from the cRNA promoter. Finally, using p65-specific small interfering RNA, we have shown that p65 knockdown reduced the levels of influenza virus replication and vRNA synthesis. In summary, we have provided evidence showing, for the first time, that the NF-B host signaling pathway can differentially regulate influenza virus RNA synthesis, which may also offer some new perspectives into understanding the host regulation of RNA synthesis by other RNA viruses.

APOBEC3G (APO3G) is a host cytidine deaminase that is incorporated into human immunodeficiency virus type 1 (HIV-1) particles. We report here that viral RNA promotes stable association of APO3G with HIV-1 nucleoprotein complexes (NPC). A target sequence located within the 5-untranslated region of the HIV-1 RNA was identified to be necessary and sufficient for efficient APO3G packaging. Fine mapping revealed a sequence normally involved in viral genomic RNA dimerization and Gag binding to be important for APO3G packaging and association with viral NPC. Our data suggest that packaging of APO3G into HIV-1 NPC is enhanced by viral RNA.Replication of human immunodeficiency virus type 1 (HIV-1) in primary cells is dependent on the expression of Vif protein, which counteracts the activity of the host cytidine deaminases APOBEC3G (APO3G) and APOBEC3F (4,25,29,32). In the absence of Vif, APO3G is incorporated into virus particles (11,16,19,20,26,27,30), resulting in hypermutation of the viral genome (19) or degradation of mutated cDNA (14, 18, 31) via a DNA repair mechanism (reviewed in references 3 and 12). Interestingly, human APO3G is not only packaged into human immunodeficiency viruses but also incorporated into simian immunodeficiency viruses and murine leukemia virus (9,18,19). Packaging of APO3G into such diverse viruses suggests that it either is a relatively nonspecific process or involves signals shared by these viruses. APO3G can bind RNA in vitro (10). Indeed, several reports have noted that the presence of viral RNA enhanced APO3G encapsidation (28); however, others (17, 23) suggested that viral RNA was not essential for APO3G packaging (2,5,8,17,23,28).To further study the role of viral RNA in the packaging of APO3G into HIV-1 virions, we first compared the packaging of APO3G into either the wild-type infectious NL4-3 virus or a helper virus (C-Help) whose RNA genome is not packaged due to a 33-base deletion in the putative RNA packaging signal (21). Virus stocks were prepared by transient cotransfection of HeLa cells with either the pNL4-3 plasmid (1) or the C-Help vector DNA and the APO3G-expressing plasmid pcDNA-APO3G (11). Viruses were collected 48 h after transfection and purified by two rounds of sucrose gradient centrifugation. Cell lysates and concentrated virus preparations were analyzed by immunoblotting (Fig. 1A). We found that packaging of APO3G into helper virus was reduced by Ͼ3.5-fold compared to packaging into NL4-3 virus (Fig. 1B). Thus, viral RNA contributes to the specific packaging of APO3G into HIV-1 virions, consistent with data reported by Svarovskaia et al. (28).If encapsidation of APO3G and viral RNA are linked, the APO3G packaging defect observed with the helper virus ( Fig. 1A and B) should be overcome by the coexpression of packaging-competent vector-derived RNA. To test this hypothesis, several packaging vectors were constructed based on the lentiviral packaging vector pHRЈCMVGFP (15). This vector contains both HIV-1 long terminal repeats (LTRs), the 5Ј-untranslated region, 35...