Poly(A)-binding protein binds to the non-polyadenylated 3 untranslated region of dengue virus and modulates translation efficiency. nuclear ribonucleoprotein (hnRNP) A1 significantly reduced the 5-3 conversation. Both PCBP2 and hnRNP A1 recombinant proteins stabilized the 5-3 interactions and formed ribonucleoprotein complexes with the 5 and 3 ends of the MNV-1 genomic RNA. Mutations within the 3 complementary sequences (CS) that disrupt the 5-3-end interactions resulted in a significant reduction of the viral titer, suggesting that this integrity of the 3-end sequence and/or the lack of complementarity with the 5 end is usually important for efficient virus replication. Small interfering RNA-mediated knockdown of PCBP2 or hnRNP A1 resulted in a reduction in virus yield, confirming a role for the observed interactions in efficient viral replication. PCBP2 and hnRNP A1 induced the circularization of MNV-1 RNA, as revealed by electron microscopy. This study provides evidence that PCBP2 and hnRNP A1 bind to the 5 and 3 ends of the MNV-1 viral RNA and contribute to RNA circularization, playing a role in the virus life cycle. INTRODUCTION Noroviruses (NoVs) are the causative brokers of nonbacterial gastroenteritis in humans and are responsible for almost all viral gastroenteritis outbreaks worldwide (1C3). The genus within the family comprises nonenveloped icosahedral viruses with a single-stranded positive-polarity RNA genome. NoV genomic RNA typically contains three open reading frames (ORFs): ORF1 encodes a polyprotein precursor that is processed to give rise to Rimantadine Hydrochloride 6 or 7 nonstructural proteins. ORF2 and ORF3 encode the major and minor capsid proteins VP1 and VP2, respectively. Both VP1 and VP2 are synthesized from a subgenomic RNA (4). In the case of murine norovirus 1 (MNV-1), a fourth potential ORF (ORF4) has been identified that is highly conserved between strains and encodes a protein (VF1) involved in the regulation of the innate immune response to NoV contamination (5). Attempts to grow human noroviruses (HuNoVs) in cell culture DHCR24 have been largely unsuccessful (6); therefore, the finer details of the NoV replication cycle remain unclear. However, the identification of the first MNV-1 strain and its routine laboratory propagation in the murine macrophage cell line Raw264.7 provide a cell culture system to investigate the molecular mechanisms of NoV translation and replication (6, 7). The NoV genome is usually flanked by a very short 5 untranslated region (UTR) covalently linked to the viral VPg protein and by a polyadenylated 3 UTR (8). VPg functions as a proteinaceous cap substitute and is able to bind to eukaryotic initiation factors to promote viral translation (9C12). MNV-1 is an enteric pathogen that shares many molecular and biological properties with HuNoV (13). The 5-end sequence of both the genomic and subgenomic RNAs is usually highly conserved among several members of the family (14, 15). Moreover, their 5- and 3-end regions contain several conserved RNA secondary structures, with various sizes and positions, implicated in viral replication (16C18), as well as viral pathogenesis (14). Translation and replication of the positive-strand RNA viruses take place in the cytoplasm of the infected cells. Several lines of evidence support the hypothesis that an conversation between the 5 and 3 ends of viral RNA genomes regulates the translation and RNA replication of many Rimantadine Hydrochloride viruses (19C21). However, the mechanism by which the Rimantadine Hydrochloride 5 and 3 ends associate and the fine details of how different conformations of the RNA participate in viral translation and replication remain unclear. Several variations in the way in which this conversation occurs have been observed, with each virus developing its own strategies to allow these 5-3-end contacts. Some of these interactions can occur via direct RNA-RNA contacts, as in the case of dengue virus genomic RNA, where complementary sequences (CS) present in the viral genome ends are necessary for RNA replication and viral viability (22, 23). Sequence complementarity is necessary but not sufficient in some cases to direct these 5-3-end interactions (24); therefore, cellular proteins act as facilitators to maintain the interactions. In the case of bovine viral diarrhea virus (BVDV) and hepatitis C virus (HCV), with RNA genomes that lack a 5 cap structure and a 3 poly(A) tail, 5-3 genomic-RNA contacts are mediated by the NFAR proteins (25, 26). More recently, the role of the cellular poly(rC) binding protein 2 (PCBP2) in the circularization.