Protein and RNA Trafficking by PV 2A pro . PV infection strongly impacts on hostcell protein localization, giving rise to an unusual cytoplasmic distribution of nuclear proteins [188] . This particular effect has been characterized by different laboratories for a number of nuclear factors involved in several cellular processes and containing different types of NLSs [189] . Nevertheless, not all nuclear proteins are re-localized after PV infection, evidencing the presence of a viral-specific mechanism affecting protein subcellular distribution [188] . Gustin and colleagues proposed the inhibition of nuclear protein import machinery as the cause of the cytoplasmic accumulation of nuclear proteins in PV-infected cells (Figure 4 ). They demonstrated that PV impairs protein trafficking across the NPC by expressing GFP proteins encoding classical or transporting NLSs in mock and infected cultured cells [15, 16, 159] . These recombinant proteins accumulate in the cytoplasm after PV infection, being almost completely depleted from the nucleus. Nevertheless, PV does not affect GFP distribution when the NLS is mutated or deleted, since the small size of GFP allows its inefficient efflux throughout the NPC [15] . Interestingly, cell-free nuclear import assays demonstrated that NLS-containing GFP is unable to traverse the NPC when cells are previously infected with PV [15] . In agreement with these findings, shuttling endogenous proteins, such as heterogeneous nuclear ribonucleoprotein (hnRNP) A1 and hnRNP K, are detected in the cytoplasm of infected cells from 3 hpi but are undetectable in the nucleus at 4.5 hpi. However, cytoplasmic accumulation of nuclear resident proteins such as hnRNP C requires longer times of infection [15] . These findings support the idea that the distribution of a protein that shuttles between nucleus and cytoplasm may be strongly altered by the disruption of protein trafficking pathways, as compared to nuclear resident proteins. However, not all the nuclear factors are redistributed to the cytoplasm of PV-infected cells. This is the case for SC35 (a serine/arginine-rich splicing factor), fibrillarin or TATA-binding protein (TBP), which remain in the nucleus for the duration of infection. The different behaviour of these groups of proteins could arise as a consequence of different turnover; thus, the distribution of highly stable nuclear proteins might be less affected by the inhibition of protein nuclear import than those proteins with low stability. Alternatively, PV might not impair all protein import pathways, and some might remain operative, thereby allowing the nuclear import of several families of nuclear proteins. Interestingly, some nuclear factors such as La antigen [190] , PTB [191] , Sam68 [192] , or nucleolin [188] have been shown to interact with PV RNA or viral proteins. These proteins accumulate in the cytoplasm of PV-infected cells and, consequently, their availability for viral replication is increased [188, [193] [194] [195] . Cytoplasmic accumulation of nuclear factors might be produced mostly by the blockade of nuclear-cytoplasmic trafficking. However, loss of the NLS after the cleavage of PTB and La proteins by PV 3C pro may also contribute to the subcellular relocalization of those proteins in PV-infected cells [193, 195] . Redistribution of nuclear proteins as well as impairment of cellular import machinery was also observed in cells infected with other picornaviruses such as HRV [159] or EMCV [160] . These findings indicate that most picornaviruses might share similar strategies to impair nuclear-cytoplasmic trafficking machinery.