Class IA phosphoinositide 3-kinases (PI3Ks) are a family of p85/p110 heterodimeric lipid kinases that generate second messenger signals downstream of tyrosine kinases, thereby controlling cell metabolism, growth, proliferation, differentiation, motility, and survival. Mammals express three class IA catalytic subunits: p110α, p110β, and p110δ. It is unclear to what extent these p110 isoforms have overlapping or distinct biological roles. Mice expressing a catalytically inactive form of p110δ (p110δ D910A ) were generated by gene targeting. Antigen receptor signaling in B and T cells was impaired and immune responses in vivo were attenuated in p110δ mutant mice. They also developed inflammatory bowel disease. These results reveal a selective role for p110δ in immunity.

A striking characteristic of mRNA export factors is that they shuttle continuously between the cytoplasm and the nucleus. This shuttling is mediated by specific factors interacting with peptide motifs called nuclear export signals (NES) and nuclear localization signals. We have identified a novel CRM-1-independent transferable NES and two nuclear localization signals in the Epstein-Barr virus mRNA export factor EB2 (also called BMLF1, Mta, or SM) localized at the N terminus of the protein between amino acids 61 and 146. We have also found that a previously described double NES (amino acids 213-236) does not mediate the nuclear shuttling of EB2, but is an interaction domain with the cellular export factor REF in vitro. This newly characterized REF interaction domain is essential for EB2-mediated mRNA export. Accordingly, in vivo, EB2 is found in complexes containing REF as well as the cellular factor TAP. However, these interactions are RNase-sensitive, suggesting that the RNA is an essential component of these complexes.In cells infected by human herpesviruses, viral mRNAs and proteins are trafficked through the nuclear pore complex. Several cellular factors that mediate the nucleocytoplasmic transport of mRNAs have now been identified (1-6). Interestingly, some human herpesviruses carry, in their genome, genes whose products are also mRNA export factors, such as HSV-1 1 ICP27 (7) and the EBV BMLF1 early gene product originally called EB2 (8), but later called Mta (9) or SM (10). Such genes are conserved among all human herpesviruses, suggesting a conserved function for their products. At least for HSV-1 and EBV, the inactivation of ICP27 (11) or EB2 (12), respectively, abolishes the production of infectious viral particles, demonstrating that ICP27 and EB2 are essential factors for viral mRNA export and that their function cannot be trans-complemented by cellular factors. Moreover, EB2 appears to have an effect on cellular mRNAs because it has transforming properties when expressed both in established cell lines such as Rat1 and NIH3T3 and in primary rat fibroblasts (13).Most of the HSV-1 and EBV early and late mRNAs are transcribed from intronless genes. However, it is now clearly established that the nuclear export of mRNAs is dramatically increased when a splicing event occurs (14). In effect, splicing leads to the deposition on the mRNA of a multiprotein export complex (called EJC for exon-exon junction complex), including REF/Aly (Yra1 in yeast), Y14, RNPS1, SRm160, and Magoh, 20 -24 nucleotides upstream of the exon-exon junction (2-5). Such a complex is thought to export mRNAs by recruiting TAP/Mex67p (15) to cellular messenger RNPs (16 -18). For cellular mRNAs generated from intronless genes, they are likely to be exported to the cytoplasm by cellular factors through nonspecific interactions with mRNA-bound adapters like REF (19) or through sequence-specific interactions with SRp20 or 9G8 (20) or U2AF (21). It is therefore tempting to speculate that EB2, like its HSV-1 functional homolog ICP27 (7), is an...

Human herpesviruses encode posttranscriptional activators that are believed to up-regulate viral replication by facilitating early and late gene expression. We have reported previously that the Epstein-Barr virus protein EB2 (also called M or SM) promotes nuclear export of RNAs that are poor substrates for spliceosome assembly, an effect that closely resembles the human immunodeficiency virus type 1 Rev-dependent nuclear export of unspliced viral RNA. Here we present experimental data showing that EB2 efficiently promotes the nuclear export of unspliced RNA expressed from a Rev reporter construct. Site-directed mutagenesis as well as domain swapping experiments indicate that a leucine-rich region found in the EB2 protein, which matches the consensus sequence for the leucine-rich nuclear export signal, is not a nuclear export signal per se. Accordingly, leptomycin B (LMB), a specific Crm-1 inhibitor, impairs Rev-but not EB2-dependent nuclear export of unspliced RNA. Moreover, EB2 nucleocytoplasmic shuttling visualized by a heterokaryon assay is, unlike Rev shuttling, not affected by LMB. We also show that overexpression of an N-terminal deletion mutant of Nup214/can, a major nucleoporin of the nuclear pore complex involved in several aspects of nuclear transport, blocks both Rev-and EB2-dependent nuclear export of RNA. These results strongly suggest that EB2 nuclear export of unspliced RNA is mediated by a Crm-1-independent pathway. Epstein-Barr virus (EBV) is a human gamma-herpesviruswidely spread in the adult human population. This virus is associated with several malignancies such as Burkitt's lymphoma, nasopharyngeal carcinoma, Hodgkin's disease, gastric carcinoma, breast carcinoma, and B-and T-cell lymphomas and induces the permanent proliferation (immortalization) of quiescent B lymphocytes in vitro. In EBV-associated tumors in vivo as well as in EBV-infected B cells proliferating ex vivo, entry into a productive cycle is a rare event and the transcription of the EBV genome is usually restricted to a few genes defining a latent state of the viral cycle (for reviews, see references 25 and 36). Although the molecular events occurring during the switch from latency to the productive cycle are now partially understood for in vitro-immortalized B cells (42), the functions of many EBV gene products expressed during the lytic cycle have only partially been characterized, and very little is known about certain others. Among these, the early nuclear protein EB2 (7), which is also called M or SM (8), was originally described as a promiscuous transcription factor, as it activates transient expression of the chloramphenicol acetyltransferase (CAT) gene placed under the control of many different promoters (26). We reported recently that EB2 could activate cytoplasmic accumulation of unspliced RNAs, particularly when they are poor substrates for spliceosome assembly, which suggested an effect of EB2 on either splicing or RNA export or both (4). A recent report, using heterokaryon assays, has revealed that EB2, like its h...