Characterization of a Nuclear Export Signal within the Human T Cell Leukemia Virus Type I Transactivator Protein Tax

Abstract

Human T cell leukemia virus type I (HTLV-I) is the etiologic agent of adult T cell leukemia and HTLV-Iassociated myelopathy/tropical spastic paraparesis. The HTLV-I transactivator protein Tax plays an integral role in the etiology of adult T cell leukemia, as expression of Tax in T lymphocytes has been shown to result in immortalization. In addition, Tax is known to interface with numerous transcription factor families, including activating transcription factor/cAMP response elementbinding protein and nuclear factor-B, requiring Tax to localize to both the nucleus and cytoplasm. In this report, the nucleocytoplasmic localization of Tax was examined in Jurkat, HeLa, and U-87 MG cells. The results reported herein indicate that Tax contains a leucinerich nuclear export signal (NES) that, when fused to green fluorescent protein (GFP), can direct nuclear export via the CRM-1 pathway, as determined by leptomycin B inhibition of nuclear export. However, cytoplasmic localization of full-length Tax was not altered by treatment with leptomycin B, suggesting that native Tax utilizes another nuclear export pathway. Additional support for the presence of a functional NES has also been shown because the NES mutant Tax(L200A)-GFP localized to the nuclear membrane in the majority of U-87 MG cells. Evidence has also been provided suggesting that the Tax NES likely exists as a conditionally masked signal because the truncation mutant Tax⌬214-GFP localized constitutively to the cytoplasm. These results suggest that Tax localization may be directed by specific changes in Tax conformation or by specific interactions with cellular proteins leading to changes in the availability of the Tax NES and nuclear localization signal.

Regulation of NF-κB2 p100 Processing by Its cis-Acting Domain

“…These data are correlated with high nuclear/cytoplasmic shuttling activity of Tax (Fig. 6A, third panel from top, lanes 5 and 6) (45). In support of the results of subcellular immunoblot assays, our immunofluorescence assays showed that, in the presence of Tax, p100 exhibited a whole-cell expression pattern (Fig.…”

The tax protein from the primate T-cell lymphotropic virus type 3 is expressed in vivo and is functionally related to HTLV-1 Tax rather than HTLV-2 Tax

“…Interestingly, the two amino acids corresponding to the M22 domain (aa 130-131) (Smith and Greene, 1990) are modified in the Tax3 vs Tax1 protein. The stretch of amino acid that is analogous to the previously reported Nuclear Export Signal of Tax1 (aa 188-202) (Alefantis et al, 2003) also contains several changes, but none of them corresponding to an essential leucine or isoleucine residue. Some changes are also present in the putative Tax3 S CR2-binding sequence (aa 312-319).…”

Site-specific Phosphorylation Differentiates Active from Inactive Forms of the Human T-cell Leukemia Virus Type 1 Tax Oncoprotein

“…Functional domains and potential sites of phosphorylation in Tax. Tax contains a nuclear localization signal (NLS) (13) and a nuclear export signal (NES) (15). Domains for binding CREB, SH3, p300/CBP, IKK␥, LIM, P/CAF, and PDZ are indicated (19,(43)(44)(45).…”

“…Specifically, with respect to the regulation of multiple Tax activities, it has been noted that mutation or deletion of individual domains results in a selective loss of function (13). Important domains for Tax function include a nuclear localization signal (13,14), nuclear export signal (15,16), activation specific region (17), two leucine zip-like domains (18,19), and a zinc-finger domain (20). Mutation of a number of individual serine residues in Tax have been shown to alter its ability to trans-activate either CREB or NF-B-responsive promoters (13).…”