The genome of influenza virus (vRNA) is associated with nucleoprotein (NP) and viral RNA-dependent RNA polymerases, and forms helical viral ribonucleoprotein (vRNP) complexes. NP-vRNA complex is the biologically active template for RNA synthesis by the viral polymerase. Previously, we identified human pre-mRNA processing factor 18 (Prp18) as a stimulatory factor for viral RNA synthesis using an yeast replicon system and single-gene deletion library of yeast (Naito T, Kiyasu Y, Sugiyama K, Kimura A, Nakano R, Matsukage A, Nagata K. Proc. Natl. Acad. Sci., USA, 104:18235-18240, 2007). In infected Prp18 knock-down (KD) cells, synthesis of vRNA, cRNA, and viral mRNAs was reduced. Prp18 was found to stimulate the in vitro viral RNA synthesis through its interaction with NP. Analyses using in vitro RNA synthesis reactions revealed that Prp18 dissociates newly synthesized RNA from template after early elongation step to stimulate the elongation reaction. We found that Prp18 functions as a chaperone for NP to facilitate the formation of NP-RNA complexes. Based on these results, it is suggested that Prp18 accelerates the influenza viral RNA synthesis as an NP chaperone for the processive elongation reaction.

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Templates for viral RNA synthesis of negative-stranded RNA viruses are not naked RNA but rather RNA encapsidated by viral nucleocapsid proteins forming vRNP complexes. However, viral basic proteins tend to aggregate under the physiological ionic strength without chaperones. We identified pre-mRNA processing factor Prp18 as a stimulatory factor for influenza virus RNA synthesis. We found that one of targets of Prp18 is NP. Prp18 facilitates the elongation reaction of viral polymerases by preventing the deleterious annealing of newly synthesized RNA to template. Prp18 functions as a chaperone for NP to stimulate the formation of NP-RNA complexes. Based on these results, we would propose that Prp18 may be required to maintain the structural integrity of vRNP for processive template reading.