c Sequences and structures within the terminal genomic regions of plus-strand RNA viruses are targets for the binding of host proteins that modulate functions such as translation, RNA replication, and encapsidation. Using murine norovirus 1 (MNV-1), we describe the presence of long-range RNA-RNA interactions that were stabilized by cellular proteins. The proteins potentially responsible for the stabilization were selected based on their ability to bind the MNV-1 genome and/or having been reported to be involved in the stabilization of RNA-RNA interactions. Cell extracts were preincubated with antibodies against the selected proteins and used for coprecipitation reactions. Extracts treated with antibodies to poly(C) binding protein 2 (PCBP2) and heterogeneous nuclear ribonucleoprotein (hnRNP) A1 significantly reduced the 5=-3= interaction. 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 the integrity of the 3=-end sequence and/or the lack of complementarity with the 5= end is 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.

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