The Epstein–Barr virus protein (EB2) allows the nuclear export of a particular subset of early and late viral RNAs derived from intronless genes. EB2 is conserved among most herpesvirus members and its presence is essential for the production of infectious particles. Here we show that, besides its role as a nuclear export factor, EB2 strongly stimulates translation of unspliced mRNAs without affecting overall cellular translation. Interestingly, this effect can be reversed by the addition of an intron within the gene. The spliced mRNA is then efficiently exported and translated even in the absence of EB2. Moreover, we show that EB2 associates with translating ribosomes and increases the proportion of its target RNA in the polyribosomal fraction. Finally, testing of EB2 homolog proteins derived from EBV-related herpesviruses, shows that, even if they play similar roles within the replication cycle of their respective virus, their mechanisms of action are different.

Available data suggest that the Varicella-Zoster virus (VZV) IE4 protein acts as an important regulator on VZV and cellular genes expression and could exert its functions at post-transcriptional level. However, the molecular mechanisms supported by this protein are not yet fully characterized. In the present study, we have attempted to clarify this IE4-mediated gene regulation and identify some cellular partners of IE4. By yeast two-hybrid and immunoprecipitation analysis, we showed that IE4 interacts with three shuttling SR proteins, namely ASF/SF2, 9G8 and SRp20. We positioned the binding domain in the IE4 RbRc region and we showed that these interactions are not bridged by RNA. We demonstrated also that IE4 strongly interacts with the main SR protein kinase, SRPK1, and is phosphorylated in in vitro kinase assay on residue Ser-136 contained in the Rb domain. By Northwestern analysis, we showed that IE4 is able to bind RNA through its arginine-rich region and in immunoprecipitation experiments the presence of RNA stabilizes complexes containing IE4 and the cellular export factors TAP/NXF1 and Aly/REF since the interactions are RNase-sensitive. Finally, we determined that IE4 influences the export of reporter mRNAs and clearly showed, by TAP/NXF1 knockdown, that VZV infection requires the TAP/NXF1 export pathway to express some viral transcripts. We thus highlighted a new example of viral mRNA export factor and proposed a model of IE4-mediated viral mRNAs export.

Protein phosphatase (PP) 2A is a heterotrimeric enzyme regulated by specific subunits. The B56 (or B/PR61/PPP2R5) class of B-subunits direct PP2A or its substrates to different cellular locations, and the B56␣, -␤, and -⑀ isoforms are known to localize primarily in the cytoplasm. Here we studied the pathways that regulate B56␣ subcellular localization. We detected B56␣ in the cytoplasm and nucleus, and at the nuclear envelope and centrosomes, and show that cytoplasmic localization is dependent on CRM1-mediated nuclear export. The inactivation of CRM1 by leptomycin B or by siRNA knockdown caused nuclear accumulation of ectopic and endogenous B56␣. Conversely, CRM1 overexpression shifted B56␣ to the cytoplasm. We identified a functional nuclear export signal at the C terminus (NES; amino acids 451-469), and site-directed mutagenesis of the NES (L461A) caused nuclear retention of full-length B56␣. Active NESs were identified at similar positions in the cytoplasmic B56-␤ and ⑀ isoforms, but not in the nuclear-localized B56-␦ or ␥ isoforms. The transient expression of B56␣ induced nuclear export of the PP2A catalytic (C) subunit, and this was blocked by the L461A NES mutation. In addition, B56␣ co-located with the PP2A active (A) subunit at centrosomes, and its centrosome targeting involved sequences that bind to the A-subunit. Fluorescence Recovery after Photobleaching (FRAP) assays revealed dynamic and immobile pools of B56␣-GFP, which was rapidly exported from the nucleus and subject to retention at centrosomes. We propose that B56␣ can act as a PP2A C-subunit chaperone and regulates PP2A activity at diverse subcellular locations.Reversible protein phosphorylation is a key mechanism regulating a myriad of cellular processes. A delicate balance between the opposing effects of protein kinases and phosphatases determines the functional state of many proteins. Protein phosphatase 2A (PP2A) 3 refers to a major family of heterotrimeric serine-threonine phosphatase enzymes in the cell (1-3). The core enzyme is a dimer consisting of a 36-kDa catalytic C-subunit and a 65-kDa structural regulatory A-subunit, which acts as a scaffold to bring into proximity the C-subunit and protein substrates bound by the diverse regulatory B-subunits. There are four B-subunit gene families each with multiple genes that encode a range of splice variant peptides. The diverse nature of PP2A is inherent in its composition, which is potentially comprised of over 200 distinct protein complexes each containing different combinations of the A-, B-, and C-subunits, and hence allowing for variability and subtle regulation in phosphatase action (3).The B-subunits are postulated to regulate PP2A activity in different ways: (a) by targeting the holoenzyme to specific subcellular locations (e.g. B55␣ directs PP2A to microtubules (4)), (b) determining substrate specificity (e.g. PP2A complexes containing B55 or B72 B-subunits cause the activation or inhibition of SV40 DNA replication, respectively, because of differences in substrate recognition sites on...