Common Mechanism for RNA Encapsidation by Negative-Strand RNA Viruses

113

The RNA genome of respiratory syncytial virus (RSV) is constitutively encapsidated by the viral nucleoprotein N, thus forming a helical nucleocapsid. Polymerization of N along the genomic and antigenomic RNAs is concomitant to replication and requires the preservation of an unassembled monomeric nucleoprotein pool. To this end, and by analogy with Paramyxoviridae and Rhabdoviridae, it is expected that the viral phosphoprotein P acts as a chaperone protein, forming a soluble complex with the RNA-free form of N (N 0 -P complex). Here, we have engineered a mutant form of N that is monomeric, is unable to bind RNA, still interacts with P, and could thus mimic the N 0 monomer. We used this N mutant, designated N mono , as a substitute for N 0 in order to characterize the P regions involved in the N 0 -P complex formation. Using a series of P fragments, we determined by glutathione S-transferase (GST) pulldown assays that the N and C termini of P are able to interact with N mono . We analyzed the functional role of amino-terminal residues of P by site-directed mutagenesis, using an RSV polymerase activity assay based on a human RSV minireplicon, and found that several residues were critical for viral RNA synthesis. Using GST pulldown and surface plasmon resonance assays, we showed that these critical residues are involved in the interaction between P[1-40] peptide and N mono in vitro. Finally, we showed that overexpression of the peptide P[1-29] can inhibit the polymerase activity in the context of the RSV minireplicon, thus demonstrating that targeting the N 0 -P interaction could constitute a potential antiviral strategy. IMPORTANCERespiratory syncytial virus (RSV) is the leading cause of lower respiratory tract illness in infants. Since no vaccine or efficient antiviral treatment is available against RSV, it is essential to better understand how the viral machinery functions in order to develop new antiviral strategies. RSV phosphoprotein P, the main RNA polymerase cofactor, is believed to function as a chaperon protein, maintaining N as a nonassembled, RNA-free protein (N 0 ) competent for RNA encapsidation. In this paper, we provide the first evidence, to our knowledge, that the N terminus of P contains a domain that binds specifically to this RNA-free form of N. We further show that overexpression of a small peptide spanning this region of P can inhibit viral RNA synthesis. These findings extend our understanding of the function of RSV RNA polymerase and point to a new target for the development of drugs against this virus. T he human respiratory syncytial virus (HRSV) is the leading cause of severe respiratory tract infections in newborn children worldwide (1). HRSV infects close to 100% of infants within the first 2 years of life and is the main cause of bronchiolitis. It is also recognized as a significant cause of severe respiratory infections in the elderly. The virus belongs to the Mononegavirales order and constitutes the prototype virus of the Pneumovirus genus of the Paramyxoviridae family. As f...

Section: Discussionsupporting

confidence: 73%

Section: Discussionmentioning

confidence: 41%

Identification and Characterization of the Binding Site of the Respiratory Syncytial Virus Phosphoprotein to RNA-Free Nucleoprotein

Galloux

Gabiane

Sourimant

et al. 2015

113

“…The common features among various structures are that the N protein has an N-terminal domain and C-terminal domain in its core, composed mostly of ␣-helices. When the N subunits assemble into a polymeric capsid, they are aligned in parallel in a linear fashion (13). There are extensive side-by-side interactions between the neighboring domains and domain swaps of extended loops and long termini.…”

mentioning

confidence: 99%

Releasing the Genomic RNA Sequestered in the Mumps Virus Nucleocapsid

Severin

Terrell

Zengel

et al. 2016

Self Cite

In a negative-strand RNA virus, the genomic RNA is sequestered inside the nucleocapsid when the viral RNA-dependent RNA polymerase uses it as the template for viral RNA synthesis. It must require a conformational change in the nucleocapsid protein (N) to make the RNA accessible to the viral polymerase during this process. The structure of an empty mumps virus (MuV) nucleocapsid-like particle was determined to 10.4-Å resolution by cryo-electron microscopy (cryo-EM) image reconstruction. By modeling the crystal structure of parainfluenza virus 5 into the density, it was shown that the ␣-helix close to the RNA became flexible when RNA was removed. Point mutations in this helix resulted in loss of polymerase activities. Since the core of N is rigid in the nucleocapsid, we suggest that interactions between this region of the mumps virus N and its polymerase, instead of large N domain rotations, lead to exposure of the sequestered genomic RNA. Some pathogens appear to reemerge in spite of available vaccines, such as mumps virus and measles virus (3-5). Effective controls are needed to combat these pathogens. In order to develop more effective countermeasures, the mechanism of NSV replication should be better understood. One of the unique features in NSVs is that the genomic RNA is sequestered in the nucleocapsid (6). During transcription and replication, the viral RNA-dependent RNA polymerase (vRdRp) must be able to gain access to the sequestered genomic RNA in order to use it as the template. For Rhabdoviridae and Paramyxoviridae, the virus encodes a single nucleocapsid protein (N) that polymerizes as a linear capsid to encapsidate the genomic RNA (7). The viral polymerase complex consists of the large protein (L) and the phosphoprotein (P). IMPORTANCE Mumps virus (MuVThe structure of the nucleocapsid or a nucleocapsid-like particle has been solved for several members of Rhabdoviridae and Paramyxoviridae by X-ray crystallography or cryo-electron microscopy (cryo-EM) three-dimensional (3D) reconstruction (8-12). The common features among various structures are that the N protein has an N-terminal domain and C-terminal domain in its core, composed mostly of ␣-helices. When the N subunits assemble into a polymeric capsid, they are aligned in parallel in a linear fashion (13). There are extensive side-by-side interactions between the neighboring domains and domain swaps of extended loops and long termini. The genomic RNA is encapsidated in a cavity formed between the two core domains. Most of the RNA bases are stacked, some of which face the exterior and some the interior of the N protein core. The tight assembly of the nucleocapsid clearly suggests that vRdRp must open the N protein core in order to unveil the genomic RNA. How this action is carried out remains to be discovered. The interaction of the polymerase cofactor P with the nucleocapsid may provide some insights on this subject. The C-terminal domain of vesicular stomatitis virus (VSV) P protein binds between the extended loops in the C-terminal domains ...

“…For several negative-strand RNA viruses, assembly of the RNP has been shown to involve nucleocapsid protein polymerization to enwrap the entire viral RNA genome (43)(44)(45)(46)(47). Arenavirus nucleoprotein homo-oligomerization, which has been documented for members of both the Old World (LASV and LCMV) and New World (TCRV) groups, has been proposed to be an essential element for arenavirus RNP assembly (11,(23)(24)(25).…”

Section: Discussionmentioning

confidence: 99%

“…In this regard, it is interesting to note that common structural features have been recognized in nucleocapsid-like particles from representative members of three negative-strand RNA virus families (Rhabdoviridae), Paramyxoviridae (genus Pneumovirus), and Bunyaviridae (genera Phlebovirus and Orthobunyavirus). These features include parallel orientation of nucleoprotein monomers in the linear nucleocapsid and extensive interactions among protein subunits, involving either one or both of the two (N-ter and C-ter) viral nucleoprotein domains (43).…”

Section: Discussionmentioning

confidence: 99%

Differential Contributions of Tacaribe Arenavirus Nucleoprotein N-Terminal and C-Terminal Residues to Nucleocapsid Functional Activity

D'antuono

Loureiro

Foscaldi

et al. 2014

The arenavirus nucleoprotein (NP) is the main protein component of viral nucleocapsids and is strictly required for viral genome replication mediated by the L polymerase. Homo-oligomerization of NP is presumed to play an important role in nucleocapsid assembly, albeit the underlying mechanism and the relevance of NP-NP interaction in nucleocapsid activity are still poorly understood. Here, we evaluate the contribution of the New World Tacaribe virus (TCRV) NP self-interaction to nucleocapsid functional activity. We show that alanine substitution of N-terminal residues predicted to be available for NP-NP interaction strongly affected NP self-association, as determined by coimmunoprecipitation assays, produced a drastic inhibition of transcription and replication of a TCRV minigenome RNA, and impaired NP binding to RNA. Mutagenesis and functional analysis also revealed that, while dispensable for NP self-interaction, key amino acids at the C-terminal domain were essential for RNA synthesis. Furthermore, mutations at these C-terminal residues rendered NP unable to bind RNA both in vivo and in vitro but had no effect on the interaction with the L polymerase. In addition, while all oligomerization-defective variants tested exhibited unaltered capacities to sustain NP-L interaction, NP deletion mutants were fully incompetent to bind L, suggesting that, whereas NP self-association is dispensable, the integrity of both the N-terminal and C-terminal domains is required for binding the L polymerase. Overall, our results suggest that NP self-interaction mediated by the N-terminal domain may play a critical role in TCRV nucleocapsid assembly and activity and that the C-terminal domain of NP is implicated in RNA binding. IMPORTANCEThe mechanism of arenavirus functional nucleocapsid assembly is still poorly understood. No detailed information is available on the nucleocapsid structure, and the regions of full-length NP involved in binding to viral RNA remain to be determined. In this report, novel findings are provided on critical interactions between the viral ribonucleoprotein components. We identify several amino acid residues in both the N-terminal and C-terminal domains of TCRV NP that differentially contribute to NP-NP and NP-RNA interactions and analyze their relevance for binding of NP to the L polymerase and for nucleocapsid activity. Our results provide insight into the contribution of NP self-interaction to RNP assembly and activity and reveal the involvement of the NP C-terminal domain in RNA binding. V iruses belonging to the Arenaviridae family are classified into two main groups, Old World (OW) and New World (NW), according to phylogeny, serological properties, and geographical distribution (1). Several members of this family, such as the NW Junin virus (JUNV) and the OW Lassa virus (LASV), produce serious hemorrhagic fever in humans. Tacaribe virus (TCRV), the prototype of NW arenaviruses, is nonpathogenic, closely related to JUNV, and has been used as a model in numerous studies (1-4). Arenaviruses ar...