The transcription factor NF-B is critical for the induction of cancer, including adult T-cell leukemia, which is linked to infection by human T-cell leukemia virus type 1 and the expression of its regulatory protein Tax. Although activation of the NF-B pathway by Tax involves its interaction with the regulatory subunit of the IB kinase (IKK) complex, NEMO/IKK␥, the mechanism by which Tax activates specific cellular genes in the nucleus remains unknown. Here, we demonstrate that the attachment of SUMO-1 to Tax regulates its localization in nuclear bodies and the recruitment of both the RelA subunit of NF-B and free IKK␥ in these nuclear structures. However, this sumoylation step is not sufficient for the activation of the NF-B pathway by Tax. This activity requires the prior ubiquitination and colocalization of ubiquitinated Tax with IKK complexes in the cytoplasm and the subsequent migration of the RelA subunit of NF-B to the nucleus. Thus, the ubiquitination and sumoylation of Tax function in concert to result in the migration of RelA to the nucleus and its accumulation with IKK␥ in nuclear bodies for activation of gene expression. These modifications may result in targets for the treatment of adult T-cell leukemia.The human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia and a virus-associated neurodegenerative disease, HTLV-1-associated myelopathy/tropical spastic paraparesis (31,36,47). These two diseases have been linked to the expression of the HTLV-1 regulatory protein Tax (10,13,29,41). Tax is a potent transcriptional activator of viral genes as well as specific cellular genes and has a clear oncogenic potential since Tax is able to transform T lymphocytes and fibroblasts and induces tumors in transgenic mice (19). A substantial part of the oncogenic properties of Tax is associated with its ability to activate the expression of cellular genes that control T-cell proliferation and differentiation by inducing constitutive activation of the NF-B pathway (2, 38).In nonstimulated cells, inactive NF-B complexes composed of p50 and RelA heterodimers are retained in the cytoplasm by NF-B inhibitors of the IB family (15,23,40). Tax activation of the NF-B pathway involves its interaction with the regulatory subunit of the IB kinase (IKK) complex, NEMO/IKK␥, leading to the activation of the two catalytic subunits, IKK␣ and IKK␤ (8,14). Activation of the IB kinase complex by Tax determines the phosphorylation of the IB proteins, leading to their ubiquitination and degradation by the proteasome and to the migration of the NF-B complexes to the nucleus (18,45,46). It also determines the phosphorylation of the RelA subunit of NF-B, a prerequisite for activation of RelA and p50 heterodimers in the nucleus (30). However, Tax also colocalizes in discrete nuclear bodies (NB) with the two subunits of NF-B, p50 and RelA, in addition to RNA polymerase II, and the assembly of these nuclear structures correlates with Tax transcriptional activity (4-6, 37). Thus, Tax-mediated activation o...

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) is phosphorylated by Ins(1,4,5)P(3) 3-kinase, generating inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P(4)). The physiological function of Ins(1,3,4,5)P(4) is still unclear, but it has been reported to be a potential modulator of calcium mobilization. Disruption of the gene encoding the ubiquitously expressed Ins(1,4,5)P(3) 3-kinase isoform B (Itpkb) in mice caused a severe T cell deficiency due to major alterations in thymocyte responsiveness and selection. However, we were unable to detect substantial defects in Ins(1,4,5)P(3) amounts or calcium mobilization in Itpkb(-/-) thymocytes. These data indicate that Itpkb and Ins(1,3,4,5)P(4) define an essential signaling pathway for T cell precursor responsiveness and development.

To examine the potential role of the GAG precursor polyprotein in morphogenesis and assembly of the simian immunodeficiency virus (SIV), we have expressed the gag gene of SIVMac using a baculovirus expression vector. Infection of insect cells with recombinant virus containing the entire gag gene results in high expression of the GAG precursor protein, Pr57gag. The recombinant protein is myristylated and is released in the culture supernatant in an insoluble particulate form. A point mutation in the N‐terminal glycine codon (Gly‐‐‐‐Ala) inhibits myristylation. This mutated product is highly expressed but is not found in the culture supernatant. Electron microscopy and immunogold labelling of infected cells show that the native Pr57gag protein assembles into 100‐120 nm virus‐like particles that bud from the cell plasma membrane and are released in the culture supernatant. The unmyristylated protein also assembles into particulate structures which only accumulate inside the cells. These results demonstrate that the unprocessed GAG precursor of SIV can spontaneously assemble into particles in the absence of other viral proteins. Myristylation of the Pr57gag precursor is necessary for its association with the cell plasma membrane, for budding and for extracellular release.

No abstract