Electroporation-mediated delivery of nucleolar targeting sequences from Semliki Forest virus nucleocapsid protein

The interaction of animal cytoplasmic RNA viruses with the nucleus to facilitate replication

A number of positive and negative strand RNA viruses whose primary site of replication is the cytoplasm use the nucleus and/or nuclear components in order to facilitate their replicative processes and alter host cell function. The nucleus itself is divided into a number of different sub-domains including structures such as the nucleolus. Many of the nuclear proteins that localise to these domains are involved in RNA processing, and because of their limited coding capacity, it may be necessary for RNA viruses to sequester such cellular factors in order to facilitate the replication, transcription and translation of their genomes. Amongst the best-studied examples of this are the picornaviruses, whose infection results in the redistribution of nuclear proteins to the cytoplasm and their interaction with the internal ribosome entry site (IRES) to facilitate translation of the picornavirus polyprotein. Examples can be found of other positive and also negative strand RNA virus proteins that localise to the nucleus and sub-domains (especially the nucleolus) during virus infection, and several localisation motifs have been defined. Apart from sequestering nuclear proteins for a role in replication, such viruses may also target the nucleus to disrupt nuclear functions and to inhibit antiviral responses.

The localization of porcine reproductive and respiratory syndrome virus nucleocapsid protein to the nucleolus of infected cells and identification of a potential nucleolar localization signal sequence

The nucleocapsid (N) protein of porcine reproductive and respiratory syndrome virus (PRRSV) possesses two regions in the N-terminal half of the protein that are enriched in basic amino acids. Presumably, these basic regions are important for packaging the RNA genome within the nucleocapsid of the virus. The PSORT computer program identified the same regions as nuclear localization signal (NLS) sequence motifs. N protein localization to the nucleus of infected MARC-145 and porcine pulmonary macrophages was observed following staining with SDOW-17 and SR-30 anti-N monoclonal antibodies. Furthermore, the co-localization of SR-30 antibody with human ANA-N autoimmune serum identified the nucleolus as the primary site for N protein localization within the nucleus. The localization of the N protein in the absence of infection was studied by following fluorescence in MARC-145 cells transfected with a plasmid, which expressed the nucleocapsid protein fused to an enhanced green fluorescent protein (N-EGFP). Similar to infected cells, N-EGFP localized to the cytoplasm and the nucleolus. Results following the transfection of cells with pEGFP fused to truncated portions of the N gene identified a region containing the second basic stretch of amino acids as the nucleolar localization signal (NoLS) sequence. Another outcome following transfection was the rapid disappearance of cells that expressed high levels of N-EGFP. However, cell death did not correlate with localization of N-EGFP to the nucleolus.

Proteins C and NS4B of the flavivirus Kunjin translocate independently into the nucleus

The subcellular locations in infected Vero cells of Kunjin (KUN) virus core protein C and NS4B were analyzed by immunofluorescence (IF) and by immunoelectron microscopy using monospecific antibodies. Selection of appropriate fixation methods for IF showed that both proteins were associated at all times with perinuclear membranes spreading outward in a reticular pattern and they entered the nucleus late during the latent period. Subsequently NS4B was also dispersed through the nucleoplasm, while C appeared in the nucleolus and the nucleoplasm. These nuclear locations were confirmed by immunogold labeling of cryosections of infected cells at 24 hr postinfection. Labeling of NS4B in cryosections was especially enriched in the perinuclear membranes of the endoplasmic reticulum. When C and NS4B were each expressed separately in stably transformed cell lines, both cytoplasmic and nuclear localization was observed by IF and confirmed by immunoelectron microscopy. Thus the two proteins translocated to the nucleus independently of each other and of other viral proteins. Dual IF with antibodies to double-stranded RNA showed that cytoplasmic locations of C and NS4B were apparently associated in part with the sites of viral RNA synthesis which were resistant to solubilization by Triton X-100.

Identification of mayaro virus nucleocapsid protein in nucleus of Aedes albopictus cells

The modifications in the pattern of nuclear proteins of Aedes albopictus cells in response to Mayaro virus infection were analysed early and late after infection. The viral capsid (C) protein of 34 kDa (p34) could be detected in the nuclear compartment 4 h after infection, soon after its synthesis in the cytoplasm. In addition to p34, a group of high molecular weight proteins was also present in this compartment late after infection. The exposition of infected cells to supra optimal temperature of growth modifies significantly the pattern of nuclear proteins. However, the stress condition does not inhibit the transport of p34 to the nucleus. The transport of proteins into nuclei was also followed under "in vitro' conditions by incubating radiolabeled post-mitochondrial extract of infected cells with unlabeled nuclei. Under these conditions, as observed "in vivo', a specific transport of viral C protein and of a group of proteins of high molecular weight to the nuclei was also detected. These results indicate that Mayaro virus infection modifies the nuclear protein pattern in invertebrate cells.