The lactate dehydrogenase-elevating virus capsid protein is a nuclear-cytoplasmic protein

Characterization of Localization and Export Signals of Bovine Torovirus Nucleocapsid Protein Responsible for Extensive Nuclear and Nucleolar Accumulation and Their Importance for Virus Growth

Torovirus (ToV) has recently been classified into the new family Tobaniviridae, although historically, it belonged to the Coronavirus (CoV) family. The nucleocapsid (N) proteins of CoVs are predominantly localized in the cytoplasm, where the viruses replicate, but in some cases the proteins are partially located in the nucleolus. Many studies have investigated the subcellular localization and nucleocytoplasmic trafficking signals of the CoV N proteins, but little is known about ToV N proteins. Here, we studied the subcellular localization of the bovine ToV (BToV) N protein (BToN) and characterized its nucleocytoplasmic trafficking signals. Unlike other CoVs, BToN in infected cells was transported mainly to the nucleolus during early infection but was distributed predominantly in the nucleoplasm rather than in the nucleolus during late infection. Interestingly, a small quantity of BToN was detected in the cytoplasm during infection. Examination of a comprehensive set of substitution or deletion mutants of BToN fused with enhanced green fluorescent protein (EGFP) revealed that clusters of arginine (R) residues comprise nuclear/nucleolar localization signals (NLS/NoLS), and the C-terminal region served as a chromosomal maintenance 1 (CRM1)-independent nuclear export signal (NES). Moreover, recombinant viruses with mutations in the NLS/NoLS, but retaining nuclear accumulation, were successfully rescued and showed slightly reduced growth ability, while the virus that lost the NLS/NoLS-mediated nuclear accumulation of BToN was not rescued. These results indicate that BToN uniquely accumulates mainly in nuclear compartments during infection, regulated by an R-rich NLS/NoLS and a CRM1-independent NES, and that the BToN accumulation in the nuclear compartment driven by NLS/NoLS is important for virus growth.IMPORTANCE ToVs are diarrhea-causing pathogens detected in many species, including humans. BToV has spread worldwide, leading to economic loss, and there is currently no treatment or vaccine available. Positive-stranded RNA viruses, including ToVs, replicate in the cytoplasm, and their structural proteins generally accumulate in the cytoplasm. Interestingly, BToN accumulated predominantly in the nucleus/nucleolus during all infectious processes, with only a small fraction accumulating in the cytoplasm despite being a major structural protein. Furthermore, we identified unique nucleocytoplasmic trafficking signals and demonstrated the importance of NLS/NoLS for virus growth. This study is the first to undertake an in-depth investigation of the subcellular localization and intracellular trafficking signals of BToN. Our findings additionally suggest that the NLS/NoLS-mediated nuclear accumulation of BToN is important for virus replication. An understanding of the unique features of BToV may provide novel insights into the assembly mechanisms of not only ToVs but also other positive-stranded RNA viruses.

Nucleocytoplasmic transport of nucleocapsid proteins of enveloped RNA viruses

Most viruses with non-segmented single stranded RNA genomes complete their life cycle in the cytoplasm of infected cells. However, despite undergoing replication in the cytoplasm, the structural proteins of some of these RNA viruses localize to the nucleus at specific times in the virus life cycle, primarily early in infection. Limited evidence suggests that this enhances successful viral replication by interfering with or inhibiting the host antiviral response. Nucleocapsid proteins of RNA viruses have a well-established, essential cytoplasmic role in virus replication and assembly. Intriguingly, nucleocapsid proteins of some RNA viruses also localize to the nucleus/nucleolus of infected cells. Their nuclear function is less well understood although significant advances have been made in recent years. This review will focus on the nucleocapsid protein of cytoplasmic enveloped RNA viruses, including their localization to the nucleus/nucleolus and function therein. A greater understanding of the nuclear localization of nucleocapsid proteins has the potential to enhance therapeutic strategies as it can be a target for the development of live-attenuated vaccines or antiviral drugs.

The bovine immunodeficiency virus Rev protein: identification of a novel nuclear import pathway and nuclear export signal among retroviral Rev/Rev-like proteins

The Rev protein is essential for the replication of lentiviruses. Rev is a shuttling protein that transports unspliced and partially spliced lentiviral RNAs from the nucleus to the cytoplasm via the nucleopore. To transport these RNAs, the human immunodeficiency virus type 1 (HIV-1) Rev uses the karyopherin β family importin β and CRM1 proteins that interact with the Rev nuclear localization signal (NLS) and nuclear exportation signal (NES), respectively. Recently, we reported the presence of new types of bipartite NLS and nucleolar localization signal (NoLS) in the bovine immunodeficiency virus (BIV) Rev protein. Here we report the characterization of the nuclear import and export pathways of BIV Rev. By using an in vitro nuclear import assay, we showed that BIV Rev is transported into the nucleus by a cytosolic and energy-dependent importin α/β classical pathway. Results from glutathione S-transferase (GST) pulldown assays that showed the binding of BIV Rev with importins α3 and α5 were in agreement with those from the nuclear import assay. We also identified a leptomycin B-sensitive NES in BIV Rev, which indicates that the protein is exported via CRM1 like HIV-1 Rev. Mutagenesis experiments showed that the BIV Rev NES maps between amino acids 109 to 121 of the protein. Remarkably, the BIV Rev NES was found to be of the cyclic AMP (cAMP)-dependent protein kinase inhibitor (PKI) type instead of the HIV-1 Rev type. In summary, our data showed that the nuclear import mechanism of BIV Rev is novel among Rev proteins characterized so far in lentiviruses.

Modulation of host cell responses and evasion strategies for porcine reproductive and respiratory syndrome virus

The immune surveillance system protects host cells from viral infection, and viruses have evolved to escape this system for efficient proliferation in the host. Host cells produce cytokines and chemokines in response to viral infection, and among such effector molecules, type I interferons are the principal antiviral cytokines and therefore effective targets for viruses to disarm host surveillance. Porcine reproductive and respiratory syndrome virus (PRRSV) expresses proteins that circumvent the IFN response and other cellular processes, and to compensate the small coding capacity of PRRSV, these proteins are multifunctional. To date, at least four viral proteins have been identified and studied as viral antagonists of host defenses: N as a structural protein and three non-structural proteins, Nsp1 (Nsp1α and Nsp1β), Nsp2, and Nsp11. Among these, N and Nsp1 are nuclear-cytoplasmic proteins distributed in both the nucleus and cytoplasm of cells. Nsp1 and Nsp2 are viral proteases while Nsp11 is an endoribonuclease. This review describes the current understanding of the role of these proteins in modulating the host innate immune responses. Blocking against virus-mediated inhibition of the innate response may lead to the future development of effective vaccines. The understanding of viral mechanisms modulating the normal cellular processes will be a key to the design of an effective control strategy for PRRS.