The Nucleocapsid protein of Crimean Congo hemorrhagic fever virus interacts with eIF4A to promote the translation of viral mRNA in cells.

Abstract

Crimean-Congo hemorrhagic fever virus (CCHFV) is a tickborne nairovirus in the Bunyavirales order. Unlike many viral infections, CCHFV does not induce a host translation shutdown, posing the question of how its mRNAs are efficiently translated amidst competing host transcripts. Here, we show that the CCHFV nucleocapsid protein (N protein) enhances the translation of luciferase reporter mRNA with the help of the viral S-segment mRNA-derived 5' UTR. Chemical inhibition of eIF4E did not affect the N protein-mediated preferential translation of the reporter mRNA. However, translation shutdowns caused by either proteolytic cleavage of eIF4G or chemical inhibition of eIF4A abolished the N protein-mediated preferential translation of the reporter mRNA. These findings demonstrate that the CCHFV N protein requires both eIF4A and eIF4G to facilitate mRNA translation with the assistance of the viral mRNA 5' UTR. Randomization of the viral 5' UTR significantly reduced the translation efficiency of viral S-segment mRNA in cells. Our results demonstrate that wild type S-segment mRNA was heavily engaged with ribosomes, and N protein likely remained associated with the wild type 5' UTR, continuously facilitating ribosome loading, promoting polysome formation, and enhancing protein production. In contrast, most S-segment mRNA with a randomized 5' UTR was largely free from ribosome engagement, explaining the lower protein production from this transcript. Our results demonstrate that the N protein binds to eIF4A and likely reserves a population of eIF4A-eIF4G complexes that remain dedicated to selectively boost the translation of viral S-segment mRNA, thus avoiding competition from host cell transcripts for the same translation machinery.