Genome encapsidation by orthobunyavirus nucleoproteins.

All negative-sense, single-stranded RNA (−ssRNA) viruses encode a nucleoprotein (NP), the major function of which is to bind viral RNAs and encapsidate them as ribonucleoprotein complexes (RNPs) (1, 2). Consequently, the genomes of −ssRNA viruses do not exist as naked RNA but rather as protein–RNA complexes with high-order structures. In PNAS, Li et al. (3), Niu et al. (4), and Reguera et al. (5) report the crystal structures of NP-RNA complexes from three orthobunyaviruses, namely the Bunyawera virus (BUNV), Leanyer virus (LEAV), and La Crosse virus (LACV). Meanwhile, the apo-NP structure from another orthobunyavirus, the Schmallenberg virus (SBV), has been reported by Dong et al. (6). The structures of the BUNV NP-RNA complex and SBV apo-NP have also been reported by Ariza et al. (7). Altogether, these findings make a significant impact on our understanding of Orthobunyavirus genome encapsidation and condensation. Bunyaviridae is a large family of viruses encompassing more than 350 species that are organized into five genera, including Orthobunyavirus, Hantavirus, Nairovirus, Phlebovirus, and Tospovirus (8, 9). Orthobunyavirus, which is the focus of study by these five research teams listed above, includes more than 170 species and constitutes the largest genus within the family (9). Except for Tospovirus, members from the other four bunyavirus genera infect animals, and many of them are considered as serious human pathogens (e.g., the Crimean-Congo hemorrhagic fever virus, La Crosse virus, Rift valley fever virus, and Sin Nombre virus) (8, 9). For instance, the Yosemite hantavirus outbreak last summer caused ten confirmed infections with three deaths (10). Bunyaviruses are enveloped viruses that are generally spherical in shape, with a diameter of 80–120 nm (8) (Fig. 1). Their genome consists of three −ssRNA segments that encode four structural proteins (i.e., L, Gn, Gc, and N) and in some viruses also two nonstructural proteins (i.e., NSm and NSs) (8). Like in other −ssRNA viruses, the genomic RNAs of bunyaviruses are extensively coated by NP and associated with the viral RNA polymerase or L protein to form RNPs, which are responsible for viral RNA synthesis as well as viral genome packaging (1, 8, 9). Electron microscopy (EM) images show that bunyavirus RNPs form closed circular structures that are often loosely coiled without apparent symmetry (3, 4, 8). These five recent research papers (3–7) reveal that Orthobunyavirus NPs adopt a unique structural fold not previously observed in NPs from other −ssRNA viruses or even those from other genera within the same Bunyaviridae family (1). Each Orthobunyavirus NP folds into a compact structure that can be divided into four parts: the N-arm, N-terminal domain, C-terminal domain, and C-arm. The N-/C-terminal domains, formed by the N-/C-terminal halves of the polypeptide chain, comprise the NP structure core (3–7). The N-/C-arms are short sequences from the N/C termini, and they play important roles in mediating NP–NP interaction. In between the Nand C-terminal domains lies a positively charged RNA-binding groove. Although the core structure of NP is rigid, the N-/C-arms can adopt different conformations due to flexible hinge connections. The Orthobunyavirus NP–RNA complex structures by Li et al. (3), Niu et al. (4), Reguera et al. (5), and Ariza et al. (7) provide valuable insights into the mechanism of NP-mediated viral genome encapsidation. The bound RNAs in these complexes were either derived from the Escherichia coli expression host or reconstituted using synthetic oligos. All three Orthobunyavirus NP–RNA complexes were crystallized as tetramers, in which each NP interacts with two other NP molecules in a head-to-tail fashion through the N-/C-arms. The positive-charged grooves from neighboring protomers line