The Jembrana disease virus Rev protein: Identification of nuclear and novel lentiviral nucleolar localization and nuclear export signals

Retroviral hijacking of host transport pathways for genome nuclear export

Recent advances in the study of virus-cell interactions have improved our understanding of how viruses that replicate their genomes in the nucleus (e.g., retroviruses, hepadnaviruses, herpesviruses, and a subset of RNA viruses) hijack cellular pathways to export these genomes to the cytoplasm where they access virion egress pathways. These findings shed light on novel aspects of viral life cycles relevant to the development of new antiviral strategies and can yield new tractable, virus-based tools for exposing additional secrets of the cell. The goal of this review is to summarize defined and emerging modes of virus-host interactions that drive the transit of viral genomes out of the nucleus across the nuclear envelope barrier, with an emphasis on retroviruses that are most extensively studied. In this context, we prioritize discussion of recent progress in understanding the trafficking and function of the human immunodeficiency virus type 1 Rev protein, exemplifying a relatively refined example of stepwise, cooperativity-driven viral subversion of multi-subunit host transport receptor complexes.

The Disruption of a Nuclear Export Signal in the C-Terminus of the Herpes Simplex Virus 1 Determinant of Pathogenicity UL24 Protein Leads to a Syncytial Plaque Phenotype

UL24 of herpes simplex virus 1 (HSV-1) has been shown to be a determinant of pathogenesis in mouse models of infection. The N-terminus of UL24 localizes to the nucleus and drives the redistribution of nucleolin and B23. In contrast, when expressed alone, the C-terminal domain of UL24 accumulates in the Golgi apparatus; its importance during infection is unknown. We generated a series of mammalian expression vectors encoding UL24 with nested deletions in the C-terminal domain. Interestingly, enhanced nuclear staining was observed for several UL24-deleted forms in transient transfection assays. The substitution of a threonine phosphorylation site had no effect on UL24 localization or viral titers in cell culture. In contrast, mutations targeting a predicted nuclear export signal (NES) significantly enhanced nuclear localization, indicating that UL24 is able to shuttle between the nucleus and the cytoplasm. Recombinant viruses that encode UL24-harboring substitutions in the NES led to the accumulation of UL24 in the nucleus. Treatment with the CRM-1-specific inhibitor leptomycin B blocked the nuclear export of UL24 in transfected cells but not in the context of infection. Viruses encoding UL24 with NES mutations resulted in a syncytial phenotype, but viral yield was unaffected. These results are consistent with a role for HSV-1 UL24 in late cytoplasmic events in HSV-1 replication.

Identification of the nuclear and nucleolar localization signals of the Feline immunodeficiency virus Rev protein

Lentivirus genomes code for a regulatory protein essential for virus replication termed Rev. The Rev protein binds to partially spliced and unspliced viral RNAs and mediates their nuclear export. Therefore, Rev possesses functional domains that enable its shuttling between the cytoplasm and the nucleus. The Feline immunodeficiency virus (FIV), a lentivirus, can lead to an immunodeficiency syndrome after a long incubation period, similar to that associated with the human immunodeficiency virus type 1 (HIV-1). The FIV Rev functional domains have been predicted only by homology with those of HIV-1 Rev. In the present study, the nuclear and nucleolar localization signals (NLS and NoLS, respectively) of the FIV Rev were examined. A series of FIV Rev deletion mutants fused to the enhanced green fluorescent protein (EGFP) were used to localize the NLS in a region spanning amino acids (aa) 81-100. By using alanine substitution mutants, basic residues present between the amino acids (aa) 84-99 of the FIV Rev protein sequence were identified to form the NLS, whereas those between aa 82-95 were associated with the NoLS function. These results further enhance our understanding of how Rev exerts its role in the replication cycle of lentiviruses.

Characterization of Signal Sequences Determining the Nuclear/Nucleolar Import and Nuclear Export of the Caprine Arthritis-Encephalitis Virus Rev Protein

Caprine arthritis-encephalitis virus (CAEV), a lentivirus, relies on the action of the Rev protein for its replication. The CAEV Rev fulfills its function by allowing the nuclear exportation of partially spliced or unspliced viral mRNAs. In this study, we characterized the nuclear and nucleolar localization signals (NLS and NoLS, respectively) and the nuclear export signal (NES) of the CAEV Rev protein. These signals are key actors in the nucleocytoplasmic shuttling of a lentiviral Rev protein. Several deletion and alanine substitution mutants were generated from a plasmid encoding the CAEV Rev wild-type protein that was fused to the enhanced green fluorescent protein (EGFP). Following cell transfection, images were captured by confocal microscopy and the fluorescence was quantified in the different cell compartments. The results showed that the NLS region is localized between amino acids (aa) 59 to 75, has a monopartite-like structure and is exclusively composed of arginine residues. The NoLS was found to be partially associated with the NLS. Finally, the CAEV Rev protein’s NES mapped between aa 89 to 101, with an aa spacing between the hydrophobic residues that was found to be unconventional as compared to that of other retroviral Rev/Rev-like proteins.

High-throughput, microscope-based sorting to dissect cellular heterogeneity

Microscopy is a powerful tool for characterizing complex cellular phenotypes, but linking these phenotypes to genotype or RNA expression at scale remains challenging. Here, we present Visual Cell Sorting, a method that physically separates hundreds of thousands of live cells based on their visual phenotype. Automated imaging and phenotypic analysis directs selective illumination of Dendra2, a photoconvertible fluorescent protein expressed in live cells; these photoactivated cells are then isolated using fluorescence-activated cell sorting. First, we use Visual Cell Sorting to assess hundreds of nuclear localization sequence variants in a pooled format, identifying variants that improve nuclear localization and enabling annotation of nuclear localization sequences in thousands of human proteins. Second, we recover cells that retain normal nuclear morphologies after paclitaxel treatment, and then derive their single-cell transcriptomes to identify pathways associated with paclitaxel resistance in cancers. Unlike alternative methods, Visual Cell Sorting depends on inexpensive reagents and commercially available hardware. As such, it can be readily deployed to uncover the relationships between visual cellular phenotypes and internal states, including genotypes and gene expression programs.

Identification of a novel TLR5 agonist derived from the P97 protein of Mycoplasma hyopneumoniae

By modulating specific immune responses against antigens, adjuvants are used in many vaccine preparations to enhance protective immunity. The C-terminal domain of the protein P97 (P97c) of Mycoplasma hyopneumoniae, which is the etiologic agent of porcine enzootic pneumonia, has been shown to increase the specific humoral response against an antigen when this antigen is merged with P97c and delivered by adenovectors. However, the immunostimulating mechanism of this protein remains unknown. In the present study, recombinantly expressed P97c triggered a concentration-dependent TLR5 activation and stimulates the production of interleukin-8 from HEK-Blue mTLR5 cells. Circular dichroism spectroscopy and prediction of 3-dimensional conformation exposed a relevant secondary and tertiary structural homology between P97c and flagellin, the known potent TLR5 agonist. P97c adjuvanticity was evaluated by fusing the conserved epitope of the ectodomain matrix 2 protein (M2e) of the influenza A virus to the protein. Mice immunized with P97c-3M2e revealed a high antibody titer against the M2e epitope associated with a mixed Th1/Th2 immune response. Overall, this study identifies a novel agonist of the pattern recognition receptor TLR5 and reveals that P97c is a potential adjuvant through the activation of the innate immune system.