The Nuclear Import of the Human T Lymphotropic Virus Type I (HTLV-1) Tax Protein Is Carrier- and Energy-independent

Abstract

HTLV-1 is the etiologic agent of the adult T cell leukemialymphoma (ATLL). The viral regulatory protein Tax plays a central role in leukemogenesis as a transcriptional transactivator of both viral and cellular gene expression, and this requires Tax activity in both the cytoplasm and the nucleus. In the present study, we have investigated the mechanisms involved in the nuclear localization of Tax. Employing a GFP fusion expression system and a range of Tax mutants, we could confirm that the N-terminal 60 amino acids, and specifically residues within the zinc finger motif in this region, are important for nuclear localization. Using an in vitro nuclear import assay, it could be demonstrated that the transportation of Tax to the nucleus required neither energy nor carrier proteins. Specific and direct binding between Tax and p62, a nucleoporin with which the importin beta family of proteins have been known to interact was also observed. The nuclear import activity of wild type Tax and its mutants and their binding affinity for p62 were also clearly correlated, suggesting that the entry of Tax into the nucleus involves a direct interaction with nucleoporins within the nuclear pore complex (NPC). The nuclear export of Tax was also shown to be carrier independent. It could be also demonstrated that Tax it self may have a carrier function and that the NF-B subunit p65 could be imported into the nucleus by Tax. These studies suggest that Tax could alter the nucleocytoplasmic distribution of cellular proteins, and this could contribute to the deregulation of cellular processes observed in HTLV-1 infection.Human T cell lymphotropic virus type-I (HTLV-1) 2 is the etiologic agent of the malignant disorder adult T cell leukemialymphoma (ATLL) (1, 2). Whereas the pathogenesis of ATLL is unclear, the HTLV-1 regulatory protein Tax is thought to play a central role in leukemogenesis. Tax has been shown to immortalize human T cells (3) and transform fibroblast cells (4) in vitro, and transgenic animals expressing Tax have developed a range of malignancies (5-8). The mechanisms of the transformation are not fully understood, but have been shown to be related to the ability of Tax to dysregulate the transcription of genes involved in cellular proliferation, cell-cycle control, and apoptosis (9 -11). Tax is a potent transcriptional transactivator not only of viral but also of cellular gene expression. The protein physically interacts with a number of cellular transcription factors, which including components of the NF-B-Rel signaling complex, and persistent and constitutive activation of NF-B is central to the development and maintenance of the malignant phenotype in ATLL (10 -12).Activation of NF-B involves Tax activity in both the cytoplasm and nucleus. In the cytoplasm, Tax activates the kinase activity of IKK complex by directly interacting with IKK␥/ NEMO subunit. I B␣, which sequesters NF-B in the cytoplasm, is phosphorylated by the Tax-IKK complex and subsequently degraded allowing the nuclear translocation of NF-B (13). In t...

Interaction of HTLV-1 Tax with minichromosome maintenance proteins accelerates the replication timing program

“…Importantly, this 2 to 22 deletion does not affect the zinc finger motif of Tax, required for its nuclear localization. 20,21 Tax was able to associate with endogenous members of the MCM2-7 complex but failed to bind other replication regulators, such as MCM10 or HBO1 ( Figure 1B; and data not shown). The physiologic relevance of the interaction between Tax and endogenous MCM3 was further validated in HTLV-1-infected cell lines ( Figure 1C).…”

Polycomb-dependent epigenetic landscape in adult T-cell leukemia

“…51 The functional importance of NF-kB and cAMP response element-binding protein activities in this process was supported by results from Tax mutants. 52,53 EZH2 inhibition completely prevented cell growth, indicating that the enzymatic activity of EZH2 is necessary for Tax-dependent cell growth and subsequent immortalization, which is a relevant surrogate for studying the epigenome of preleukemic cells. In the immortalized cells (Taxcells), EZH2 expression was increased and miR-31 expression was reciprocally decreased ( Figure 6C).…”

Functional interaction of HTLV-1 Tax protein with the POZ domain of the transcriptional repressor BCL6

“…The expression plasmids, pCAGGS Tax 1, pEGFP-C1 Tax 1 and pEGFP-C1 GST, have been previously described Tsuji et al, 2007). A PCR product encoding the Figure 7 Subcellular localization of BCL6 WT and B-cell lymphoma 6 (BCL6) mutants.…”

“…Glutathione S-transferase pull-down assays GST and GST fusion proteins were expressed and purified as previously described (Tsuji et al, 2007). Briefly, transformed E. coli BL21 cells were grown to an OD 600 of 1.2 at 37 1C and induced with 0.3 mM IPTG (isopropyl-1-thio-b-d-galactopyranoside) for 12 h at 18 1C.…”