Do NSm virulence factors in the Bunyavirales viral order originate from Gn gene duplication?

Abstract

Several viral members of the large viral order of the Bunyavirales carry a NSm protein that acts as a virulence factor. Here we used AlphaFold to predict the structures of these NSm proteins and surprisingly found that the cytosolic domain of the Nairoviridae family NSm (NSmcyto) is predicted to have a very similar fold to the cytosolic domain of glycoprotein N (Gncyto). This observation is particularly striking for CCHFV (a member of the Nairoviridae family), for which the NMR structure of the Gncyto domain has already been described in the literature and shows a double zinc finger. We show that while the sequence identity between the NSmcyto and Gncyto domains is generally weak, the strict conservation of the two zinc-finger forming CCCH motifs in CCHFV NSmcyto explains the full structural conservation predicted by AlphaFold. Interestingly, a similar observation is made for the predictions of NSm in another group of the order Bunyavirales, the family Peribunyaviridae, where the Gncyto structures have not yet been described experimentally. Our findings suggest that NSm is the result of a gene duplication event in these viruses, and indicate that such events may indeed be common in the recent evolutionary history of RNA viruses. Importantly, our predictions provide a first insight into the long-unknown structure of NSm and its link to virulence.