Some picornaviruses, for example, poliovirus, increase bidirectional permeability of the nuclear envelope and suppress active nucleocytoplasmic transport. These activities require the viral protease 2Apro . Here, we studied nucleocytoplasmic traffic in cells infected with encephalomyocarditis virus (EMCV; a cardiovirus), which lacks the poliovirus 2A pro -related protein. EMCV similarly enhanced bidirectional nucleocytoplasmic traffic. By using the fluorescent "Timer" protein, which contains a nuclear localization signal, we showed that the cytoplasmic accumulation of nuclear proteins in infected cells was largely due to the nuclear efflux of "old" proteins rather than impaired active nuclear import of newly synthesized molecules. The nuclear envelope of digitonin-treated EMCV-infected cells permitted rapid efflux of a nuclear marker protein. Inhibitors of poliovirus 2A pro did not prevent the EMCV-induced efflux. Extracts from EMCV-infected cells and products of in vitro translation of viral RNAs contained an activity increasing permeability of the nuclear envelope of uninfected cells. This activity depended on the expression of the viral leader protein. Mutations disrupting the zinc finger motif of this protein abolished its efflux-inducing ability. Inactivation of the L protein phosphorylation site (Thr473Ala) resulted in a delayed efflux, while a phosphorylation-mimicking (Thr473Asp) replacement did not significantly impair the efflux-inducing ability. Such activity of extracts from EMCV-infected cells was suppressed by the protein kinase inhibitor staurosporine. As evidenced by electron microscopy, cardiovirus infection resulted in alteration of the nuclear pores, but it did not trigger degradation of the nucleoporins known to be degraded in the poliovirus-infected cells. Thus, two groups of picornaviruses, enteroviruses and cardioviruses, similarly alter the nucleocytoplasmic traffic but achieve this by strikingly different mechanisms.Picornaviruses, small nonenveloped icosahedral animal viruses with a single-stranded RNA genome of positive (mRNA) polarity, encompass the Enterovirus, Rhinoviruses, Cardiovirus, Aphthovirus, Parechovirus, and some other genera (64). All essential steps of their reproduction, such as translation, RNA synthesis, and encapsidation, take place in the cytoplasm of infected cells. The nonessential role of the nucleus for their reproduction follows from their ability to fulfill the complete infectious cycle in nuclei-free cytoplasts (31, 60) or cytoplasmic extracts (7,52,71). This fact, however, does not mean that the nuclei are not involved in the infectious process. Indeed, virusspecific proteins have been detected in the nuclei of poliovirusinfected (11, 29) and encephalomyocarditis virus (EMCV)-infected (5, 6) cells. Poliovirus proteases 2A and 3C are known to target a variety of nuclear transcription factors and histones (66,78,79,80). The EMCV 2A protein enters the nucleoli and interacts there with a ribosome precursor, contributing thereby to alterations in the translation c...

Representatives of several picornavirus genera have been shown previously to significantly enhance noncontrollable bidirectional exchange of proteins between nuclei and cytoplasm. In enteroviruses and rhinoviruses, enhanced permeabilization of the nuclear pores appears to be primarily due to proteolytic degradation of some nucleoporins (protein components of the pore), whereas this effect in cardiovirus-infected cells is triggered by the leader (L) protein, devoid of any enzymatic activities. Here, we present evidence that expression of L alone was sufficient to cause permeabilization of the nuclear envelope in HeLa cells. In contrast to poliovirus, mengovirus infection of these cells did not elicit loss of nucleoporins Nup62 and Nup153 from the nuclear pore complex. Instead, nuclear envelope permeabilization was accompanied by hyperphosphorylation of Nup62 in cells infected with wild-type mengovirus, whereas both of these alterations were suppressed in L-deficient virus mutants. Since phosphorylation of Nup62 (although less prominent) did accompany permeabilization of the nuclear envelope prior to its mitotic disassembly in uninfected cells, we hypothesize that cardiovirus L alters the nucleocytoplasmic traffic by hijacking some components of the normal cell division machinery. The variability and biological significance of picornaviral interactions with the nucleocytoplasmic transport in the infected cells are discussed.Productive viral interaction with host cells requires generation of an intracellular environment ensuring efficient synthesis and assembly of virally encoded macromolecules. A fundamental aspect of this process is overcoming a variety of host defensive tools, collectively named innate immunity. Some of the viral antidefensive measures are direct results of enzymatic activities of viral proteins, as exemplified by proteolytic degradation of host translation or transcription factors. However, the viral antidefensive armament in the case of small-genome viruses is usually relatively limited, and therefore such viruses have evolved the capacity to divert ("hijack") cellular proteins, pathways, and structures to perform proviral functions. A group of viruses relatively well studied in this respect, is represented by picornaviruses-small, nonenveloped animal viruses with a 7.2-to 8.5-kb RNA genome of positive polarity (46).The Picornaviridae family is composed of at least nine genera (48), which share general genome organization but differ from each other in some important aspects such as, for example, the capsid structure (and consequently the usage of cellular receptors), the structure of replicative and translational cis elements, and the presence or absence of structurally and functionally unrelated accessory (usually nonessential) proteins (2).In recent years, we have studied, among other activities exhibited by picornaviruses, their ability to trigger bidirectional permeabilization of the nuclear envelope, facilitating noncontrollable exchange of macromolecules between the nucleus and cytoplasm...

The life cycle of HIV-1 requires integration of a DNA copy into the genome of the host cell. Transcription of the viral genes generates RNAs that are exported to the cytoplasm with the contribution of viral and cellular factors to get translated or incorporated in the newly synthesized virions. It has been observed that highly effective antiretroviral therapy, which is able to reduce circulating virus to undetectable levels, cannot fully eradicate the virus from cellular reservoirs that harbor a transcriptionally latent provirus. Thus, persistence of latently infected cells is the major barrier to a cure for HIV-1 infection. In order to purge these reservoirs of latently infected cells, it has been proposed to activate transcription to stimulate the virus to complete its life cycle. This strategy is believed to unmask these reservoirs, making them vulnerable to the immune system. However, limited successes of this approach may indicate additional posttranscriptional restrictions that need to be overcome for full virus reactivation. In this work we identify the cellular protein MATR3 as an essential cofactor of viral RNA processing. Reactivation of HIV-1 transcription per se is not sufficient to allow completion of a full life cycle of the virus if MATR3 is depleted. Furthermore, MATR3 is poorly expressed in quiescent CD4+ T lymphocytes that are the major reservoir of latent HIV-1. Cells derived from aviremic HIV-1 patients under antiretroviral therapy didn’t express MATR3, and most importantly, latency-reversing agents proposed for the rescue of latent provirus were ineffective for MATR3 upregulation. To conclude, our work identifies a cellular factor required for full HIV-1 reactivation and points to the revision of the current strategies for purging viral reservoirs that focus only on transcription.