JOURNAL OF VIROLOGY, June 2009, p. 5339–5352 0022-538X/09/$08.00⫹0 doi:10.1128/JVI.00232-09 Copyright © 2009, American Society for Microbiology. All Rights Reserved.
Vol. 83, No. 11
Dimerization and a Novel Tax Speckled Structure Localization Signal Are Required for Tax Nuclear Localization䌤 Kimberly A. Fryrear, Sarah S. Durkin, Saurabh K. Gupta, Jessica B. Tiedebohl, and O. John Semmes* Department of Microbiology and Molecular Cell Biology, Eastern Virginia Medical School, 700 W. Olney Road, Norfolk, Virginia 23507 Received 2 February 2009/Accepted 2 March 2009
The human T-cell leukemia virus type 1 oncoprotein Tax has pleiotropic activities, a subset of which likely leads to immortalization of T cells. Tax is expressed and known to function in both the cell nucleus and the cytoplasm. Tax has defined nuclear localization (NLS) and nuclear export signals that enable shuttling between the two compartments. In this study, we identified a novel region in Tax that targets the protein to discrete nuclear foci that we have previously termed Tax speckled structures (TSS). We demonstrated that the identified region is both necessary and sufficient for directing proteins to TSS. This novel TSS localization signal (TSLS), spanning amino acids 50 to 75, is separable from and adjacent to the NLS of Tax. Coexpression of a Tax NLS mutant and a Tax TSLS mutant rescued the nuclear entry and subnuclear TSS targeting of both proteins, demonstrating that these signals are independent domains. Our analysis also revealed that Tax proteins deficient for dimerization fail to localize to the nucleus. Consequently, when we restored dimerization via induction of a heterologous “dimerizer” domain, nuclear localization was rescued. Thus, we defined additional domains in Tax specific for nuclear localization and subnuclear targeting. Our results reveal a more complex network for regulation of Tax subcellular localization and subsequent function. exportins involved in Tax transport have yet to be identified, but recent studies indicate that Tax can both import and export from the nucleus without the aid of carrier proteins or energy and can interact directly with the FG nucleoporins within the nuclear pore complex (67). Both the NLS and the NES of Tax are atypical. The NLS is large, involving the first 48 amino acids of Tax, and lacks the basic residues that define classical NLS (23, 53, 55). The NES of Tax contains a classic leucinerich region between amino acids 188 and 202 that is capable of mediating export of Tax via the CRM-1 pathway but displays notably slower kinetics (1, 11). This NES is believed to be masked in the native Tax protein, and there is strong evidence that ubiquitylation serves to “unmask” the NES, in response to DNA damage, for instance (21). In fact, “nuclear” Tax is predominately sumoylated whereas “cytoplasmic” Tax appears to be primarily ubiquitylated (37). While in the cytoplasm, Tax activates transcription via the NF-B pathway (61) by directly binding to the IB kinase ␥/NF-B essential modulator, leading to the phosphorylation and degradation of IB and the release of NF-B (38, 43, 62). In the nucleus, Tax interacts with cellular transcription factors to activate or repress transcription of cellular and viral genes via the ATF/CREB (42), AP-1 (19), and SRF (29) pathways (reviewed in reference 33). The list of nuclear proteins that bind to Tax and are functionally modulated is growing and includes APC (39), CBP (7, 48), Cdk4 (25), Chk1 (45), Chk2 (26), DNA protein kinase (15), MAD1 (32), c-Myc (50), and retinoblastoma protein (34). While the NLS and NES provide one level of regulation for Tax localization, it is likely that there are other mechanisms at work regulating Tax subcellular and subnuclear localization and associated functions. In addition to interaction with other proteins, Tax is also capable of dimerization/oligomerization, and studies indicate that optimal transcriptional trans-activation by Tax requires that it be in a dimeric/oligomeric form (5, 23, 31, 65).
Human T-cell leukemia virus type 1 (HTLV-1) is the etiologic agent of adult T-cell leukemia as well as subneoplastic disease states, the most prevalent being HTLV-1-associated myelopathy/tropical spastic paraparesis (22, 44, 47, 63, 68). Tax is a potent transcriptional transactivator of both viral and cellular genes and can physically associate with a number of cellular proteins, thus altering their functions (10). Expression of Tax results in the dysregulation of critical cell functions, including cell cycle regulation, proliferation, DNA damage recognition and repair, and apoptosis. Furthermore, although the pathogenesis of adult T-cell leukemia is not completely understood, the transformation of T cells infected with HTLV-1 can be logically attributed to genomic instability resulting from the pleiotropic activities of the viral protein Tax (30, 41). Of particular note for this study, Tax has documented nuclear and cytoplasmic functions; thus, the regulation of Tax localization to subcellular compartments is a critical aspect of overall Tax function (11, 23, 52). Tax contains a nuclear localization signal (NLS) sequence in the N-terminal region of the protein that is necessary and sufficient for nuclear localization (23, 53, 55). We previously showed that Tax “shuttles” between nuclear and cytoplasmic subcellular compartments and identified a consensus nuclear export signal (NES) sequence defined by amino acids 190 to 203 (11). Subsequently, a fine-mapping mutational analysis of this region by Alefantis et al. (1) clearly demonstrated a functional NES at this site. These signals presumably mediate interactions between Tax and karyopherins in the nuclear pore complex to allow Tax to translocate through the nuclear membrane (58, 64). The specific importins and * Corresponding author. Mailing address: Department of Microbiology and Molecular Cell Biology, Lewis Hall, 700 West Olney Road, Norfolk, VA 23507. Phone: (757) 446-5904. Fax: (757) 446-5766. Email:
[email protected]. 䌤 Published ahead of print on 25 March 2009. 5339
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In previous studies, we and others have demonstrated that Tax enters the nucleus and is directed to discrete nuclear foci that we termed Tax speckled structures (TSS) (6, 52). TSS coincide with interchromatin granules and consist of multiprotein complexes that partially overlap with subnuclear regions involved in splicing and transcription. We have also shown that Tax recruits cellular proteins involved in the DNA damage recognition and repair response into the TSS as well (15, 24, 26). The formation of TSS and the colocalization of spliceosome component 35 (SC35), DNA protein kinase, Chk2, and 53BP1 within the TSS places Tax near cellular machinery for transcription, splicing, DNA damage response, and checkpoint activation (26). This may explain how Tax is able to affect multiple cellular functions simultaneously (41). Although the NLS for Tax has been previously described, the domain in Tax that dictates TSS localization has not yet been defined. In this study, we identified the region in Tax containing the sequence that targets the protein into TSS. We fine mapped the TSS localization signal (TSLS) to a region containing amino acids 50 to 75. This sequence lies adjacent to but is completely separable from the Tax NLS. We also demonstrated that dimerization of Tax is required for nuclear localization. We showed that deletion of the dimerization domain of Tax prevents nuclear accumulation and results in the cytoplasmic expression of the protein. The induction of dimerization in a Tax dimerization mutant restored nuclear localization. These results elucidate two novel mechanisms in the regulation of Tax subcellular localization and the subsequent function. MATERIALS AND METHODS Plasmids. Generation of the pSTaxGFP construct has been described previously (16). STaxGFP mutants were created with PCR-based deletion mutagenesis, using a QuickChange XL mutagenesis kit (Stratagene). The primer sequences used for cloning and mutagenesis are available upon request. pSNLSTaxGFP constructs were created by inserting the simian virus 40 (SV40) T antigen NLS in frame between the S tag and the tax-gfp fusion, using a PCR-based ExCite mutagenesis kit (Stratagene). pSTax1-75GFP and pSNLSTax50-75GFP were constructed by PCR-based mutagenesis with a Phusion site-directed mutagenesis kit (Finnzymes, Inc.), using pSTaxGFP and pSNLSTaxGFP as the templates. A Myc peptide-tagged Tax construct (pMycTax) was a kind gift from Ralph Grassmann. A pMycTax(⌬30-52) construct was created by site-directed mutagenesis of pMycTax, using a QuickChange XL mutagenesis kit (Stratagene). The inducible dimerization construct pSTax(⌬100-150)-Fv-GFP was created by amplifying the Fv domain of pC4-Fv1E (Ariad Pharmaceuticals) and inserting it into the ClaI site within pSTax(⌬99-150)GFP. Cell culture and transient transfection. HEK 293 cells were maintained at 37°C in a humidified atmosphere of 5% CO2 in air in Iscove’s modified Dulbecco’s medium supplemented with 10% fetal bovine serum and 1% penicillin/ streptomycin (Invitrogen). Transfections were performed by the standard calcium phosphate precipitation method (11). The cells were washed 16 h posttransfection and incubated at 37°C until harvest. Cells were harvested 48 h posttransfection following a single wash with 1⫻ phosphate-buffered saline (PBS). Transcriptional trans-activation assays. HEK 293 cells were transfected with Tax-expressing plasmids and either HTLV-luciferase or the NF-B-luciferase reporter, using the calcium phosphate method. Cells were lysed in 400 l of reporter/lysis buffer (Promega), and lysates were immediately frozen at ⫺80°C. Samples were allowed to thaw on ice, collected, and quantified, using a Bradford assay (Bio-Rad). A total of 1 g of protein from each sample was applied to 100 l of luciferase substrate (Promega), and a luciferase assay was immediately read in a Turner BD 20/20 luminometer. Transcriptional activation was analyzed and expressed as the level of activation over that of the reporter alone (level of activation ⫽ 1). Immunofluorescence confocal microscopy. HEK 293 cells were seeded at 1 ⫻ 105 cells/well on ethanol-washed coverslips in 6-well plates. The cells were subjected to transient transfection to express the desired proteins. After 48 h, the
J. VIROL. cells were fixed by the paraformaldehyde/methanol method as previously described (52). The primary antibodies used were anti-SC35 mouse monoclonal S4045 (Sigma), anti-Myc rabbit polyclonal ab9106 (Abcam), and anti-Tax rabbit polyclonal (52) at a 1:1,000 dilution. The secondary antibodies were speciesappropriate Alexa Fluor 594-conjugated antibodies (Molecular Probes) at dilutions of 1:1,000. Nuclei were counterstained with To-Pro-3 iodide (Molecular Probes) diluted 1:1,000 in 3% bovine serum albumin–PBS. The cells were then mounted on glass slides, using Vectashield with 4⬘,6-diamidino-2-phenylindole (Vector Laboratories). Confocal fluorescent images were acquired on a Zeiss LSM 510 confocal microscope (Carl Zeiss), using argon (488 nm), HeNe1 (543 nm), and HeNe2 (633 nm) lasers, and were imaged with LSM Image Browser software. STaxGFP purification. HEK 293 cells expressing the STaxGFP fusion protein were lysed in 400 l of M-Per mammalian protein extraction reagent (Pierce) with protease inhibitor cocktail (Roche Applied Science) and immediately frozen at ⫺80°C. The lysates were assayed for total protein concentration using a Bradford protein assay and were normalized using a bovine serum albumin standard curve. The protein solution was brought to a total volume of 500 l with M-Per for each sample, and 150 l of S-protein agarose beads (Novagen) were applied to the sample. S-tagged proteins were purified from lysates as described previously (16). Supernatants containing isolated proteins were loaded onto an 8%- to 12%-gradient sodium dodecyl sulfate (SDS)-polyacrylamide gel and separated by electrophoresis, transferred to Immobilon-P membranes (Millipore) by semidry transfer, and subjected to immunoblot analysis. Immunoblot analysis. Proteins separated by electrophoresis were transferred to Immobilon-P membranes using the semidry transfer method with 400 milliamps applied for 50 min. The membranes were then blocked for 1 h at room temperature in 1⫻ Odyssey blocking buffer (Li-Cor Biosciences). Primary antibodies diluted in 1⫻ Odyssey blocking buffer were applied to the membranes and allowed to interact at 4°C overnight on an orbital shaker. Membranes were washed four times for 5 min with PBS-1% Tween. Li-Cor Odyssey secondary antibodies were diluted to a concentration of 1:20,000 in 1⫻ Odyssey blocking buffer containing 0.5% SDS and 0.5% Tween and then incubated for 1 h at room temperature on an orbital shaker while protected from light. The membranes were washed four times for 5 min with PBS-1% Tween and then stored in PBS and protected from light until analyzed. Blots were scanned and analyzed with a Li-Cor Odyssey scanner and software. Dimerization assay. Full-length MycTax protein was coexpressed with STaxGFP or mutant Tax protein in HEK 293 cells. The Tax-expressing cells were subjected to lysis and subsequent S-bead purification as described above. The isolated proteins were separated by SDS-polyacrylamide gel electrophoresis and subjected to immunoblot analysis. STaxGFP and the corresponding mutants were detected with mouse monoclonal anti-green fluorescent protein (GFP) primary antibody (Santa Cruz) at a 1:1,000 dilution followed by Odyssey goat anti-mouse 488 (Li-Cor Biosciences) secondary antibody at a 1:20,000 dilution. Copurified MycTax was detected by polyclonal anti-Myc peptide primary antibody (Abcam) at a dilution of 1:1,000 followed by Odyssey goat anti-rabbit 680 (Li-Cor) secondary antibody at a dilution of 1:20,000. Complementation analysis. Full-length MycTax was coexpressed with “test” mutant proteins in the STaxGFP backbone. The resulting cellular localization of the coexpressed proteins was visualized with immunofluorescence confocal microscopy as previously described. MycTax and Tax mutant proteins were detected by staining with rabbit polyclonal anti-Myc peptide primary antibody (Abcam) at a dilution of 1:1,000 followed by goat anti-rabbit secondary antibody conjugated to Alexa Fluor 594 (Molecular Probes) at a 1:1,000 dilution. Induced dimerization assay. STax(⌬100–150)-Fv-GFP was transiently transfected in HEK 293 cells as described above. Forty-eight hours after transfection, the cells were treated with AP20187 from an Argent homodimerization kit (Ariad Pharmaceuticals) at concentrations of 0 nM, 0.01 nM, 0.1 nM, 1.0 nM, 10 nM, 50 nM, and 100 nM for 24 h. Cells were then washed, fixed with 4% paraformaldehyde, permeabilized with methanol, and processed as previously described for confocal microscopy analysis. Images were analyzed for nuclear accumulation using MetaMorph image analysis software. Briefly, 100 cells from each condition were analyzed for nuclear fluorescence expressed as a percentage of total cell fluorescence.
RESULTS Deletion-scanning mutational analysis of Tax. Previous structure/function studies of Tax involving mutational analysis involved amino acid substitution or refined scanning-linker approaches so as to preserve the transcriptional activity of the
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FIG. 1. Tax mutants and corresponding transcriptional activity. (A) Diagram of STaxGFP depicting the deletion-scanning series of Tax mutants. Shown are the relative positions of the S peptide (S Tag), NLS, leucine zipper region (LZL), NES, activation domain (ACT), and GFP fusion (GFP). Also indicated are the CREB-interacting, zinc finger, SH3 binding, PDZ binding, dimerization, and P/CAF binding domains. (B) Efficient expression of Tax deletion mutants is comparable to that of wild-type protein. Shown is an immunoblot analysis of whole-cell lysates from 293 T cells transfected with the appropriate plasmids for expression of the indicated proteins. Rabbit polyclonal anti-GFP antibody was used to evaluate protein expression. (C) Transcriptional activity of Tax mutants on the HTLV-1-LTR-Luc and NF-B-Luc promoter/reporter constructs. HEK 293 cells were transiently transfected with the indicated reporter construct and either STaxGFP or the indicated deletion mutant, and lysates were analyzed for activity. The reported level of activation is relative to that of the control in which the indicated reporter construct was cotransfected with empty vector expression plasmid.
protein (53, 56). In this study, we constructed a series of consecutive 25- to 50-amino-acid deletions that covered the length of the Tax protein as depicted in Fig. 1A. Our experimental objective in generating these larger deletions was to alter cel-
lular localization. The parental construct, which we have previously described (16), provided an N-terminal S tag for protein purification and a C-terminal GFP fusion to facilitate localization studies and displays activity comparable to that of
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wild-type Tax. When examined for protein expression levels via Western analysis, the deletion mutants generated comparable amounts of protein (Fig. 1B). We next analyzed each of the mutants for transcriptional trans-activation activity and subcellular localization. Nearly all of the deletion mutants displayed no transcriptional trans-activation for either an HTLV-1-responsive or an NF-B-responsive promoter/reporter (Fig. 1C), consistent with previous observations that mutations within Tax ablate transcriptional activation (56). The one exception was the ⌬323–353 mutant, which deletes the C-terminal portion of Tax downstream of the sequences mutated in the classic M47 mutant. Confocal microscopy studies revealed that the mutants displayed one of three phenotypes: localization to discrete TSS foci, diffuse nuclear localization, or cytoplasmic localization (Fig. 2). One mutant protein with the Tax NLS deleted in part, the ⌬1–29 mutant, showed cytoplasmic expression with weak nuclear expression in TSS foci, while the second NLS mutant, the ⌬30–52 mutant, was completely confined to the cytoplasm. Our results confirm previous studies that define the Tax NLS between amino acids 18 and 52 (55). Unexpectedly, mutants with the midregion of Tax deleted, including the ⌬100–150, ⌬151–202, ⌬203–254, and ⌬255–289 mutants, were also confined to the cytoplasm. One C-terminal deletion mutant, the ⌬290–322 mutant, was expressed in the cytoplasm but also displayed nuclear expression and foci formation. The extreme C-terminal deletion mutant, the ⌬323–353 mutant, localized to the nucleus and formed nuclear foci. The ⌬323–353 mutant protein displayed unusually high steady-state expression levels compared to the wild-type protein. One mutant, the ⌬53–99 mutant, displayed a diffusely nuclear localization with no apparent foci. It should be noted that between 10% and 20% of cells that express wild-type Tax protein show cytoplasmic expression in addition to the classic nuclear TSS localization. In this context, the ⌬53–99 mutant protein was observed to have cytoplasmic localization in 43% of expressing cells in addition to the diffuse nuclear localization. In all cases, both wild-type protein and the ⌬53–99 mutant displayed nuclear expression. We next determined whether the foci formed by the nucleusexpressed Tax mutants were TSS, as defined by colocalization with the cellular protein SC35. Three of the four Tax mutants, which had displayed at least partial nuclear expression, were able to form TSS (Fig. 3), including the ⌬1–29 mutant and the two C-terminal Tax deletion mutants, the ⌬290–322 and ⌬323– 353 mutants. Again the ⌬53–99 Tax mutant was able to enter the nucleus but clearly did not form TSS. In addition, we observed that deletion of amino acids 53 to 99 does not impact NLS function but disrupts TSS localization. Determining loss of function for cytoplasm-expressed Tax mutants. In the scanning series of deletion mutants of Tax, there were five Tax mutants that were confined to the cytoplasm. One mutant (the ⌬30–52 mutant) has a deletion of the Tax NLS, and the remaining four mutants have deletions in the midregion of Tax. We hypothesized that if these deletions affected the functioning of the Tax NLS, the addition of an exogenous NLS would restore nuclear expression of the mutants. To test this hypothesis, we added the SV40 Tag NLS in frame at the N-terminal of each of the five cytoplasmic Tax mutants. As expected, the presence of an exogenous NLS was able to restore nuclear expression for the mutant that had a
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deletion of the Tax NLS, the ⌬30–52 mutant (Fig. 4). In fact, the NLSTax⌬30–52 protein was competent for TSS formation. However, all of the mutations in the midregion of Tax failed to be rescued by fusion to an exogenous NLS. This finding suggests that these mutant proteins were deficient for a function other than that fulfilled by a classic NLS sequence. Implied in this conclusion is that Tax nuclear localization is dependent on more than the presence of an NLS. A novel domain within Tax amino acid residues 50 to 75 is sufficient for localization to TSS. We have shown that the deletion of amino acids 53 to 99 allows the retention of nuclear expression but disrupts TSS formation. We also showed that the addition of a heterologous NLS to the ⌬30–52 Tax NLS mutant rescues nuclear expression and also rescues TSS formation, implying that the region required for TSS formation resides in the NLS mutant. These observations indicate that amino acids 53 to 99 contain the Tax TSS localization signal. In order to more fully characterize this putative TSLS, we designed a construct containing the first 75 amino acids of Tax fused to an N-terminal S tag and a C-terminal GFP. This construct, which contains the Tax NLS, was able to target GFP into TSS in a fashion that was indistinguishable from that of wild-type Tax (Fig. 5A and C). In comparison, a construct containing only the S tag and GFP was expressed diffusely throughout the cell, with no targeting to nuclear foci (Fig. 5B). The NLS for Tax is contained in the first 50 amino acids (23, 53, 55), and deletion of this region ablates nuclear localization. In order to determine if the TSLS is distinct from the NLS, we inserted an exogenous NLS from the SV40 T in frame and N-terminally to Tax1-75GFP. This insertion did not interfere with the targeting of S-NLS-Tax1-75GFP to TSS (Fig. 5D). We then effectively deleted the first 49 amino acids of Tax by creating S-NLS-Tax50-75GFP and observed that this construct was also able to target the GFP fusion into the TSS (Fig. 5E). This result established that the TSLS is outside of the Tax NLS and resides within amino acids 50 to 75 of Tax. Complementation analyses of NLS and TSLS mutants. We then hypothesized that if the Tax NLS and TSLS are independent sequences, then coexpression of the two mutant proteins should result in dimerization and rescue of Tax localization to TSS. We therefore coexpressed the MycTax⌬30–52 mutant with the ⌬53–99 mutant to test whether the mutants could rescue nuclear localization and foci formation. The MycTax⌬30–52 mutant failed to localize to the nucleus, and the ⌬53–99 mutant failed to form TSS, as expected (Fig. 6A). However, coexpression of the MycTax⌬30–52 and ⌬53–99 mutants resulted in normal nuclear expression and TSS formation (Fig. 6A). To confirm that the foci formed by the coexpression of the mutants were TSS, we expressed the GFP fusion version of each mutant either alone or in combination (Fig. 6B) and observed colocalization of each with SC35. As was the case for the previous experiment, expression of either the ⌬30–52 or ⌬53–99 mutant resulted in Tax protein that failed to localize to the nucleus or to form TSS, respectively. Coexpression of the ⌬30–52 and ⌬53–99 mutants resulted in nuclear localization and the formation of TSS that overlap with the expression of SC35. Complementation by these two different domain mutants clearly demonstrated that the Tax NLS and TSLS are separate and sufficient for directing specific subcellular localization.
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FIG. 2. Localization of Tax deletion mutants. Confocal microscopy images of Tax mutants transiently expressed in HEK 293 cells, fixed, and counterstained with To-Pro-3⬘ iodide (1:1,000). Tax expression is detected via the fluorescence of the GFP fusion (green). In the top left panel, an “untagged” version of Tax (HpX) was expressed and fixed as above but visualized via indirect immunofluorescence using a rabbit polyclonal anti-Tax primary antibody and a goat anti-rabbit Alexa Fluor 488 secondary antibody. For each protein examined, 50 expressing cells were observed. Shown is a representative of the expression pattern observed in ⬎85% of cells.
Full-length Tax rescues nuclear localization of Tax midregion mutants. Previous studies have established that Tax forms dimers and that optimal transcriptional trans-activation by Tax requires Tax dimerization (5, 31). The described dimerization domain spans approximately 150 amino acids in the midregion of Tax. Our observation that Tax mutants with deletions in this
region are expressed in the cytoplasm suggests a possible role for dimerization in nuclear localization. Three of our four midregion mutants of Tax that were nuclear excluded contained deletions within the Tax dimerization domain. This large domain reportedly contains three subdomains, I, II, and III, that are involved in Tax dimerization, but the interdepen-
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FIG. 3. Colocalization of Tax deletion mutants with TSS. Shown are confocal microscopy images of fixed HEK 293 cells transiently expressing “nuclear” Tax mutants. Tax proteins (green) were observed via fluorescence of the GFP fusion. SC35 (red) was detected with mouse anti-SC35 primary antibody (1:5,000) and goat anti-mouse Alexa Fluor 594-conjugated secondary antibody (1:1,000). The nuclei were counterstained with To-Pro-3⬘ iodide (1:1,000). The overlap (white) was displayed by merging the two images.
dence of these three subdomains for efficient dimerization has never been evaluated (5). In our original mutational analysis, we created constructs that were each missing one of these subdomains of the Tax dimerization region. The ⌬100–150 mutant is missing the first subdomain, the ⌬151–202 mutant is missing the second subdomain, and the ⌬203–254 mutant is missing the third subdo-
main. In addition, the double mutant, the ⌬100–150 ⌬203–254 mutant, is missing the first and third dimerization subdomains. As shown in Fig. 7, immunofluorescence confocal microscopy studies revealed that coexpression of full-length Tax was able to partially restore nuclear localization of those mutants missing only one dimerization subdomain (Fig. 7A to D) but not of the mutant missing two dimerization subdomains (Fig. 7E).
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FIG. 4. Rescue of Tax NLS mutant by exogenous NLS tagging. An NLS sequence from SV40 Tag was added to all nucleus-excluded Tax mutants, and the constructs were transiently expressed in HEK 293 cells. Tax proteins (green) were observed via fluorescence of the GFP fusion. SC35 (red) was detected with mouse anti-SC35 primary antibody (1:5,000) and goat anti-mouse Alexa Fluor 594-conjugated secondary antibody (1:1,000). Nuclei were counterstained with To-Pro-3⬘ iodide (1:1,000). The overlap (white) was displayed by merging the two images.
Mutual complementation by coexpression of different singlesubdomain mutants revealed that none of the mutants could be rescued for nuclear localization (data not shown). These results suggest that the midregion Tax mutants are impaired for dimerization and that presentation of a dimer interface is a prerequisite to nuclear localization. Dimerization subdomain Tax mutants are deficient for Tax binding. To directly test whether the subdomain mutants were deficient for dimerization, we coexpressed MycTax with STaxGFP or a Tax mutant missing one or two dimerization subdomains, the ⌬100–150 or ⌬100–150 ⌬203–254 mutants, respectively. We then assessed dimerization function by puri-
fication of STaxGFP, the ⌬100–150 mutant, or the ⌬100–150 ⌬203–254 mutant, followed by immunoblot analysis for the presence of coprecipitated MycTax (Fig. 8A). Full-length STaxGFP was able to bind to MycTax, demonstrating dimerization between the two full-length proteins (Fig. 8A, lane 7). When the ⌬100–150 mutant was tested, a measurable but reduced level of MycTax was detected (Fig. 8A, lane 6), consistent with weak dimerization with full-length Tax. However, when the ⌬100–150 ⌬203–254 mutant was examined, no evidence of dimerization with full-length Tax could be detected (Fig. 8A, lane 5). MycTax did not bind to the S beads alone (Fig. 8A, lane 8). We quantitated the dimerization by deter-
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FIG. 5. Tax peptide comprising amino acid residues 50 to 75 is sufficient for targeting to TSS. HEK 293 cells transiently expressing STaxGFP, SGFP, or N-terminal regions of Tax fused to GFP were assayed for TSS formation as confirmed by colocalization with SC35. Tax proteins (green) were observed via fluorescence of the GFP fusion. SC35 (red) was detected with mouse anti-SC35 primary antibody (1:5,000) and goat anti-mouse Alexa Fluor 594-conjugated secondary antibody (1:1,000). The nuclei were counterstained with To-Pro-3⬘ iodide (1:1,000). The peptide protein (green) and SC35 (red) and merged images (white) are also shown.
mining the ratio of MycTax that coprecipitated with STaxGFP or the STaxGFP-derived deletion mutant. When this analysis was performed, the single deletions between amino acids 100 to 289 resulted in similarly reduced dimerization compared to that of the wild-type protein (Fig. 8B). The double deletion that removes two dimerization subdomains resulted in a further reduction in binding. These results confirm that this region is required for dimerization and that deletion of the dimerization subdomains has an additive effect. Nuclear localization requires Tax dimerization activity. We have shown that deletion of Tax dimerization subdomains im-
pairs protein-protein interactions in a dimerization assay and results in cytoplasmic expression of Tax protein. Clearly, these results strongly support that dimerization is a prerequisite to nuclear localization of Tax. However, we wanted to determine if restoration of dimerization activity is sufficient to rescue nuclear localization of Tax. To accomplish this, we designed a Tax dimerization mutant protein that contains an inducible dimerization domain, STax(⌬100–150)-Fv-GFP. The inducible dimerization cassette is based on the binding domain of the human protein FK506 binding protein (FKBP) and its ability to bind to the immunosuppressive drugs FK506 and rapamy-
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FIG. 6. Complementation of Tax NLS and TSLS mutant proteins. (A) Confocal microscopy images of the expression of MycTax⌬30–52 (left panels) and ⌬53–99 (middle panels) expressed alone or in combination (right panels). MycTax⌬30–52 (red) was detected with rabbit anti-Myc primary antibody at a dilution of 1:1,000 and Alex Fluor 594 goat anti-rabbit secondary antibody (1:1,000). ⌬53–99 (green) was detected via fluorescence of the GFP fusion. The nuclei were counterstained with To-Pro-3⬘ iodide (1:1,000). (B) Colocalization with SC35 of ⌬30–52, ⌬53–99, or ⌬30–52 coexpressed with ⌬53–99 (right panels). Tax proteins (green) were observed via fluorescence of the GFP fusion. SC35 (red) was detected with mouse anti-SC35 primary antibody (1:5,000) and goat anti-mouse Alexa Fluor 594-conjugated secondary antibody (1:1,000). The nuclei were counterstained with To-Pro-3⬘ iodide (1:1,000).
cin. Fv is a modified version of the FKBP binding domain containing a phenylalanine-to-valine substitution that increases the affinity of a rapamycin derivative, AP20187, for the Fv fusion protein by 1,000-fold over that for the wild-type protein. Thus, AP20187 is able to associate any two proteins containing the FKBP binding domain by inducing dimerization. The expression of STax(⌬100–150)-Fv-GFP in the absence of the chemical dimerizer AP20187 resulted in cytoplasmic localization of the protein. Upon the addition of increasing concentrations of the dimerizer, the Tax protein accumulated in the nucleus (Fig. 9A). The nuclear form of the protein dimer does not target TSS because the Fv region was inserted into the TSLS region using the unique ClaI site within the Tax open reading frame. Semiquantitative analyses of the percentages of Tax expression in the nucleus revealed dose-dependent responses of from less than 1% to more than 40% nuclear accumulation (Fig. 9B). This result clearly shows that dimerization function is required for Tax nuclear localization. DISCUSSION Compartmentalization is a well-established mechanism for regulating protein function (14, 60). Clearly, this critical relationship between cellular targeting and function goes beyond nuclear-versus-cytoplasmic accumulation. For instance, a formidable number of nuclear proteins target specific subnuclear domains, many of which denote specific cellular functions (13,
35, 36). It is also understood that since localization is dynamic (for instance, proteins may exhibit compartmental “shuttling”), the stoichiometry is critical to overall protein function and an ideal target for protein regulation. In the case of the HTLV-1 Tax protein, localization is influenced by the presence of both NLS and NES sequences, as well as by the action of posttranslational modifications such as sumoylation and ubiquitylation (1, 11, 21, 23, 37, 53, 55). Our results from the current study provide an additional regulatory sequence, the TSLS, which allows for accumulation of Tax within the subnuclear structure TSS. We have further shown that dimerization of Tax is required for overall nuclear accumulation of this important viral protein. Although nuclear localization is delineated from cytoplasmic localization by a clear nuclear membrane, subnuclear localization is largely dictated by interactions with protein complexes present in the nucleus, with membership in these complexes defining the specific structure. All of the well-known subnuclear structures, represented by nuclear speckles (36), paraspeckles (36), Cajal bodies (36), gems (20), and ND10/ PML bodies (4, 59), are dynamic structures that contain characteristic sets of nuclear proteins and protein complexes that reside in distinct subnuclear regions (46). Nearly all of these nuclear structures contain subpopulations of nuclear factors, but each is distinguished by the presence of a nuclear protein(s) unique to each structure (36). One specific class of
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FIG. 7. Complementation of deletion mutants with full-length Tax. Full-length MycTax was coexpressed with the indicated Tax midregion mutants deficient for nuclear localization. Shown are single-subdomain mutants (A through D) and one double-subdomain mutant (E). Tax proteins (green) were observed via fluorescence of the GFP fusion. MycTax was observed by using mouse anti-Myc primary antibody (1:500) and goat anti-mouse Alexa Fluor 594-conjugated secondary antibody (1:1,000). Nuclei were counterstained with To-Pro-3⬘ iodide (1:1,000).
structures, called nuclear speckles, are interchromatin granule clusters that contain the pre-mRNA splicing machinery, including small nuclear ribonucleic proteins, non-small nuclear ribonucleic protein splicing factors, and spliceosome subunits such as SC35 (59). We previously observed that Tax localized to specific TSS, so named because of their overlap with nuclear speckles, colocalized with SC35, did not contain promyelocytic leukemia protein, and did not colocalize with nucleoli (3). However, TSS are distinct from nuclear speckles in that they contain various other nonsplicing cellular proteins. These proteins include DNA protein kinase, Chk2, 53BP1, and ␥H2AX, proteins involved in DNA damage recognition and repair (15, 24, 26). The colocalization of Tax with cellular machinery for
transcription/splicing, DNA damage response, and checkpoint activation suggests that targeting to TSS is an important integrating event for the varied functions of Tax. Subnuclear protein targeting to nuclear speckles appears to be mediated by at least two separable trafficking signals: one for nuclear import (the NLS) and one for mediating inclusion into speckles (12, 28, 49, 54). As might be expected, the identified domains are structurally variable and include examples such as ankyrin repeats in the IBL, FF domains in CA150, or interaction with RNA and are believed to represent modalities for protein-protein interactions. In our study, we found that the TSLS that is required for localization to TSS was physically distinct from the NLS and could function independently to
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FIG. 8. Midregion Tax mutants are deficient for dimerization. (A) MycTax was coexpressed with STaxGFP or Tax mutants missing one (⌬100–150) or two (⌬100–150, ⌬203–254) dimerization subdomains. Dimerization of STaxGFP and Tax mutants with MycTax was assayed by S-bead purification of protein followed by immunoblot analysis for coprecipitated MycTax. Starting levels of each protein are shown in the left panel (input). Analyses of the isolated Tax complexes are shown in the right panel (S-bead purification). Protein complexes were normalized to levels of GFP expressed by Tax fusions (lanes 5, 6, and 7). Corresponding levels of coprecipitated MycTax are indicated. (B) Quantitation of the dimerization assay. Following the isolation of the Tax complexes shown in the right panel of panel A above, the ratios of MycTax and the test proteins were determined for relative quantitation of dimerization ability. The bar graph shows the percentages of MycTax relative to those of the isolated STaxGFP or mutant proteins as indicated (ratio).
direct Tax to its specific intranuclear site. There are no remarkable features of the 25-amino-acid TSLS except that it is enriched for proline residues, a characteristic shared by many nuclear speckle proteins, but it does not contain an arginine-
serine (RS) motif common to targeting signals of splicing speckle components (8, 9, 17, 18, 27). This may reflect the unique nuclear address of TSS and the ability of Tax to form novel protein complexes.
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FIG. 9. Induced dimerization of a dimerization-deficient Tax mutant restores nuclear localization. (A) Confocal microscopy images of cells expressing STax(⌬100–150)-Fv-GFP treated with 0 nM, 1 nM, 10 nM, or 100 nM of the chemical inducer of dimerization AP20187. Cells were fixed and counterstained with To-Pro-3⬘ iodide (1:1,000). STax(⌬100–150)-Fv-GFP (green) was visualized via fluorescence from the GFP fusion. Overlapping expression of stained nuclei and GFP fusion protein is shown (merged). (B) Quantitation of the nuclear accumulation of STax(⌬100– 150)-Fv-GFP with increasing concentrations of dimerizer. The amounts of nuclear fluorescence are expressed as percentages of total fluorescence.
It is also important to point out that the 25-amino-acid sequence comprising the TSLS is sufficient for targeting to TSS when fused to either the native Tax NLS or a heterologous NLS. This implies that sumoylation of Tax is not a requirement for either nuclear localization or the formation of TSS. Likely, the role for sumoylation is in the context of the whole protein and may involve the removal of a block to entry or facilitate protein-protein interactions that enable nuclear accumulation. Such a model is reminiscent of that established for the tumor suppressor Wilms’ tumor gene (WT1) in which modification by
SUMO proteins is separable from nuclear speckle localization (57). As with Tax, although sumoylation is required for nuclear entry of WT1, an independent sequence is sufficient for targeting to nuclear speckles in the absence of a SUMO protein target. It may also be that sumoylation serves to mask the function of the NES, as has been suggested previously (37). Our results also demonstrate that the dimerization of Tax is a necessary prerequisite for nuclear localization/accumulation. There are numerous examples of proteins that must dimerize or oligomerize prior to nuclear translocation, including the
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human cytomegalovirus protein ppUL44 and the cellular protein p53 (2, 66). Other proteins, such as those of the AP-1 family, including c-Jun, JunD, JunB, and c-Fos, enter the nucleus as monomers but require heterodimerization in order to remain in the nucleus (40). In our studies, we were able to link the inability of Tax mutants to dimerize with a failure to accumulate in the nucleus in spite of a competent NLS. In fact, the addition of a heterologous NLS failed to rescue this defect. The relationship between dimerization efficiency and nuclear accumulation is linear in that weakly dimerizing proteins were able to weakly accumulate in the nucleus. Specifically, we generated a deletion of two subdomains within the Tax dimerization domain as well as single-subdomain mutations. The singlesubdomain Tax mutant retained a slight dimerization ability, whereas the Tax mutant in which the larger dimerization domain is removed showed no dimerization. The rescue of singlesubdomain dimerization mutants by wild-type Tax protein supports a complementation phenotype, and our observation that none of the subdomain mutants could complement distal subdomain mutants implies a requirement for a dimer interface. Finally, we showed that induction of dimerization in a previously cytoplasm-restricted Tax dimerization subdomain mutant resulted in restored nuclear accumulation. Although our studies definitively link dimerization as a prerequisite to nuclear accumulation, additional studies are required to determine if this dimerization occurs in the cytoplasm as a prerequisite to nuclear entry or within the nucleus as a means of retaining the protein within its subnuclear address. Previous studies on the dimerization of Tax have suggested that the N-terminal zinc finger domain (51) is important for Tax self association (5, 31). These findings were based on yeast two-hybrid assays where an N-terminal deletion of Tax failed to interact with wild-type Tax. In mammalian cells, as used in our system, deletion of the zinc finger region (⌬30-52) was rescued for nuclear localization by the coexpression of wildtype protein, demonstrating the ability to interact. Our functional complementation assay between the NLS mutant, the ⌬30-52 mutant, which is missing the zinc finger domain, and the TSLS mutant, the ⌬50-75 mutant, demonstrated that the zinc finger domain is not required for dimerization, since a complementation phenotype requires interaction. Likewise, the fusion of a heterologous NLS to the ⌬30-52 mutant was able to restore nuclear localization and subsequent TSS accumulation, providing further evidence that the zinc finger domain is dispensable for Tax self association in our system. Our results also allow for some inferences regarding the stoichiometry of Tax self association. The complementation assays between the NLS mutant and TSLS mutant suggest that at the minimum, a single copy of each, NLS or TSLS, is required for nuclear accumulation of Tax in TSS. Thus, Tax exists at least in a one-to-one dimer state. In addition, the inability of the dimerization subdomain mutants to complement distal subdomain mutants implies that at least two intact molecules of Tax are needed to form the required dimer interface. This finding is supported by findings in the induced dimerization assay. The design of the heterologous “dimerizer” domain is such that only dimers, and not nonpaired higher-order oligomers (for instance, trimers), are formed upon the addition of the chemical dimerizer. Although induced dimerization was able to increase the nuclear accumu-
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lation of the mutant protein, it did not result in a total restoration of nuclear accumulation to wild-type levels. This suggests the necessity of the presentation of specific dimer interfaces for normal Tax nuclear accumulation. Perhaps these specific interfaces mediate interaction with specific proteins. Interestingly, the ability of wild-type Tax to rescue dimerization subdomain mutants suggests the existence of a higherorder (⬎2) oligomeric structure. This conclusion derives from the reasoning that if dimer interfaces are required for nuclear accumulation and the mutants are incapable of forming interfaces, then wild-type Tax must provide them. In order for this to occur, the rescued Tax complex must contain at least 2⫻ molar equivalents of the wild-type protein. Thus, it is likely that Tax is capable of forming a homotetrameric complex. Higherordered Tax oligomerization could provide for more-complex regulation of Tax functions. Oligomerization could generate new intermolecular interfaces to improve stability, control the accessibility and specificity of active sites, and increase the number of cellular binding partners for Tax. The complexity of such a model helps explain how a single protein displays such a wide range of activities. ACKNOWLEDGMENT This work was supported by Public Health Service grant CA76959 (to O.J.S.) from the National Cancer Institute. REFERENCES 1. Alefantis, T., K. Barmak, E. W. Harhaj, C. Grant, and B. Wigdahl. 2003. Characterization of a nuclear export signal within the human T cell leukemia virus type I transactivator protein Tax. J. Biol. Chem. 278:21814–21822. 2. Alvisi, G., D. A. Jans, and A. Ripalti. 2006. Human cytomegalovirus (HCMV) DNA polymerase processivity factor ppUL44 dimerizes in the cytosol before translocation to the nucleus. Biochemistry 45:6866–6872. 3. Ariumi, Y., T. Ego, A. Kaida, M. Matsumoto, P. P. Pandolfi, and K. Shimotohno. 2003. Distinct nuclear body components, PML and SMRT, regulate the trans-acting function of HTLV-1 Tax oncoprotein. Oncogene 22:1611–1619. 4. Ascoli, C. A., and G. G. Maul. 1991. Identification of a novel nuclear domain. J. Cell Biol. 112:785–795. 5. Basbous, J., A. Bazarbachi, C. Granier, C. Devaux, and J. M. Mesnard. 2003. The central region of human T-cell leukemia virus type 1 Tax protein contains distinct domains involved in subunit dimerization. J. Virol. 77:13028–13035. 6. Bex, F., A. McDowall, A. Burny, and R. Gaynor. 1997. The human T-cell leukemia virus type 1 transactivator protein Tax colocalizes in unique nuclear structures with NF-B proteins. J. Virol. 71:3484–3497. 7. Bex, F., M. J. Yin, A. Burny, and R. B. Gaynor. 1998. Differential transcriptional activation by human T-cell leukemia virus type 1 Tax mutants is mediated by distinct interactions with CREB binding protein and p300. Mol. Cell. Biol. 18:2392–2405. 8. Bickmore, W. A., and H. G. Sutherland. 2002. Addressing protein localization within the nucleus. EMBO J. 21:1248–1254. 9. Birney, E., S. Kumar, and A. R. Krainer. 1993. Analysis of the RNArecognition motif and RS and RGG domains: conservation in metazoan pre-mRNA splicing factors. Nucleic Acids Res. 21:5803–5816. 10. Boxus, M., J. C. Twizere, S. Legros, J. F. Dewulf, R. Kettmann, and L. Willems. 2008. The HTLV-1 Tax interactome. Retrovirology 5:76. 11. Burton, M., C. D. Upadhyaya, B. Maier, T. J. Hope, and O. J. Semmes. 2000. Human T-cell leukemia virus type 1 Tax shuttles between functionally discrete subcellular targets. J. Virol. 74:2351–2364. 12. Calado, A., and M. Carmo-Fonseca. 2000. Localization of poly(A)-binding protein 2 (PABP2) in nuclear speckles is independent of import into the nucleus and requires binding to poly(A) RNA. J. Cell Sci. 113:2309–2318. 13. Cardoso, M. C., and H. Leonhardt. 1998. Protein targeting to subnuclear higher order structures: a new level of regulation and coordination of nuclear processes. J. Cell. Biochem. 70:222–230. 14. Chubb, J. R., and W. A. Bickmore. 2003. Considering nuclear compartmentalization in the light of nuclear dynamics. Cell 112:403–406. 15. Durkin, S. S., X. Guo, K. A. Fryrear, V. T. Mihaylova, S. K. Gupta, S. M. Belgnaoui, A. Haoudi, G. Kupfer, and O. J. Semmes. 2008. HTLV-1 Tax oncoprotein subverts the cellular DNA damage response via binding to DNA protein kinase. J. Biol. Chem. 283:36311–36320. 16. Durkin, S. S., M. D. Ward, K. A. Fryrear, and O. J. Semmes. 2006. Site-specific phosphorylation differentiates active from inactive forms of the human T-cell leukemia virus type 1 Tax oncoprotein. J. Biol. Chem. 281:31705–31712.
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FRYREAR ET AL.
17. Eilbracht, J., and M. S. Schmidt-Zachmann. 2001. Identification of a sequence element directing a protein to nuclear speckles. Proc. Natl. Acad. Sci. USA 98:3849–3854. 18. Fu, X.-D. 1995. The superfamily of arginine/serine-rich splicing factors. RNA 1:663–680. 19. Fujii, M., H. Tsuchiya, T. Chuhjo, T. Akizawa, and M. Seiki. 1992. Interaction of HTLV-1 Tax1 with p67SRF causes the aberrant induction of cellular immediate early genes through CArG boxes. Genes Dev. 6:2066–2076. 20. Gall, J. G. 2000. Cajal bodies: the first 100 years. Annu. Rev. Cell Dev. Biol. 16:273–300. 21. Gatza, M. L., T. Dayaram, and S. J. Marriott. 2007. Ubiquitination of HTLV-I Tax in response to DNA damage regulates nuclear complex formation and nuclear export. Retrovirology 4:95. 22. Gessain, A., F. Barin, J. C. Vernant, O. Gout, L. Maurs, A. Calender, and G. de The´. 1985. Antibodies to human T-lymphotropic virus type-I in patients with tropical spastic paraparesis. Lancet ii:407–410. 23. Gitlin, S. D., P. F. Lindholm, S. J. Marriott, and J. N. Brady. 1991. Transdominant human T-cell lymphotropic virus type I TAX1 mutant that fails to localize to the nucleus. J. Virol. 65:2612–2621. 24. Gupta, S. K., X. Guo, S. S. Durkin, K. F. Fryrear, M. D. Ward, and O. J. Semmes. 2007. Human T-cell leukemia virus type 1 Tax oncoprotein prevents DNA damage-induced chromatin egress of hyperphosphorylated Chk2. J. Biol. Chem. 282:29431–29440. 25. Haller, K., Y. Wu, E. Derow, I. Schmitt, K. T. Jeang, and R. Grassmann. 2002. Physical interaction of human T-cell leukemia virus type 1 Tax with cyclin-dependent kinase 4 stimulates the phosphorylation of retinoblastoma protein. Mol. Cell. Biol. 22:3327–3338. 26. Haoudi, A., R. C. Daniels, E. Wong, G. Kupfer, and O. J. Semmes. 2003. Human T-cell leukemia virus-I tax oncoprotein functionally targets a subnuclear complex involved in cellular DNA damage-response. J. Biol. Chem. 278:37736–37744. 27. Hedley, M. L., H. Amrein, and T. Maniatis. 1995. An amino acid sequence motif sufficient for subnuclear localization of an arginine/serine-rich splicing factor. Proc. Natl. Acad. Sci. USA 92:11524–11528. 28. Herrmann, A., K. Fleischer, H. Czajkowska, G. Mu ¨ller-Newen, and W. Becker. 2007. Characterization of cyclin L1 as an immobile component of the splicing factor compartment. FASEB J. 21:3142–3152. 29. Jeang, K. T., R. Chiu, E. Santos, and S. J. Kim. 1991. Induction of the HTLV-I LTR by Jun occurs through the Tax-responsive 21-bp elements. Virology 181:218–227. 30. Jeang, K. T., C. Z. Giam, F. Majone, and M. Aboud. 2004. Life, death, and tax: role of HTLV-I oncoprotein in genetic instability and cellular transformation. J. Biol. Chem. 279:31991–31994. 31. Jin, D.-Y., and K.-T. Jeang. 1997. HTLV-I Tax self-association in optimal trans-activation function. Nucleic Acids Res. 25:379–387. 32. Jin, D.-Y., F. Spencer, and K.-T. Jeang. 1998. Human T cell leukemia virus type 1 oncoprotein Tax targets the human mitotic checkpoint protein MAD1. Cell 93:81–91. 33. Kashanchi, F., and J. N. Brady. 2005. Transcriptional and post-transcriptional gene regulation of HTLV-1. Oncogene 24:5938–5951. 34. Kehn, K., L. Fuente Cde, K. Strouss, R. Berro, H. Jiang, J. Brady, R. Mahieux, A. Pumfery, M. E. Bottazzi, and F. Kashanchi. 2005. The HTLV-I Tax oncoprotein targets the retinoblastoma protein for proteasomal degradation. Oncogene 24:525–540. 35. Lamond, A. I., and J. E. Sleeman. 2003. Nuclear substructure and dynamics. Curr. Biol. 13:R825–R828. 36. Lamond, A. I., and D. L. Spector. 2003. Nuclear speckles: a model for nuclear organelles. Nat. Rev. Mol. Cell Biol. 4:605–612. 37. Lamsoul, I., J. Lodewick, S. Lebrun, R. Brasseur, A. Burny, R. B. Gaynor, and F. Bex. 2005. Exclusive ubiquitination and sumoylation on overlapping lysine residues mediate NF-B activation by the human T-cell leukemia virus Tax oncoprotein. Mol. Cell. Biol. 25:10391–10406. 38. Li, X. H., K. M. Murphy, K. T. Palka, R. M. Surabhi, and R. B. Gaynor. 1999. The human T-cell leukemia virus type-1 Tax protein regulates the activity of the IB kinase complex. J. Biol. Chem. 274:34417–34424. 39. Liu, B., S. Hong, Z. Tang, H. Yu, and C. Z. Giam. 2005. HTLV-I Tax directly binds the Cdc20-associated anaphase-promoting complex and activates it ahead of schedule. Proc. Natl. Acad. Sci. USA 102:63–68. 40. Malnou, C. E., T. Salem, F. Brockly, H. Wodrich, M. Piechaczyk, and I. Jariel-Encontre. 2007. Heterodimerization with Jun family members regulates c-Fos nucleocytoplasmic traffic. J. Biol. Chem. 282:31046–31059. 41. Marriott, S. J., and O. J. Semmes. 2005. Impact of HTLV-I Tax on cell cycle progression and the cellular DNA damage repair response. Oncogene 24:5986– 5995.
J. VIROL. 42. Mesnard, J. M., and C. Devaux. 1999. Multiple control levels of cell proliferation by human T-cell leukemia virus type 1 Tax protein. Virology 257:277–284. 43. Nicot, C., F. Tie, and C. Z. Giam. 1998. Cytoplasmic forms of human T-cell leukemia virus type 1 Tax induce NF-B activation. J. Virol. 72:6777–6784. 44. Osame, M., K. Usuku, S. Izumo, N. Ijichi, H. Amitani, A. Igata, M. Matsumoto, and M. Tara. 1986. HTLV-I associated myelopathy, a new clinical entity. Lancet i:1031–1032. 45. Park, H. U., J. H. Jeong, J. H. Chung, and J. N. Brady. 2004. Human T-cell leukemia virus type 1 Tax interacts with Chk1 and attenuates DNA-damage induced G2 arrest mediated by Chk1. Oncogene 23:4966–4974. 46. Phair, R. D., and T. Misteli. 2000. High mobility of proteins in the mammalian cell nucleus. Nature 404:604–609. 47. Poiesz, B. J., F. W. Ruscetti, A. F. Gazdar, P. A. Bunn, J. D. Minna, and R. C. Gallo. 1980. Detection and isolation of type C retrovirus particles from fresh and cultured lymphocytes of a patient with cutaneous T-cell lymphoma. Proc. Natl. Acad. Sci. USA 77:7415–7419. 48. Ramírez, J. A., and J. K. Nyborg. 2007. Molecular characterization of HTLV-1 Tax interaction with the KIX domain of CBP/p300. J. Mol. Biol. 372:958–969. ´ lvarez, M., A. C. Goldstrohm, M. A. Garcia-Blanco, and C. Sun 49. Sa ´nchez-A ˜´e. 2006. Human transcription elongation factor CA150 localizes to splicing factor-rich nuclear speckles and assembles transcription and splicing components into complexes through its amino and carboxyl regions. Mol. Cell. Biol. 26:4998–5014. 50. Semmes, O. J., J. F. Barret, C. V. Dang, and K.-T. Jeang. 1996. Human T-cell leukemia virus type I Tax masks c-Myc function through a cAMP-dependent pathway. J. Biol. Chem. 271:9730–9738. 51. Semmes, O. J., and K. T. Jeang. 1992. HTLV-I Tax is a zinc-binding protein: role of zinc in Tax structure and function. Virology 188:754–764. 52. Semmes, O. J., and K. T. Jeang. 1996. Localization of human T-cell leukemia virus type 1 tax to subnuclear compartments that overlap with interchromatin speckles. J. Virol. 70:6347–6357. 53. Semmes, O. J., and K.-T. Jeang. 1992. Mutational analysis of human T-cell leukemia virus type I Tax: regions necessary for function determined with 47 mutant proteins. J. Virol. 66:7183–7192. 54. Semple, J. I., S. E. Brown, C. M. Sanderson, and R. D. Campbell. 2002. A distinct bipartite motif is required for the localization of inhibitory B-like (IBL) protein to nuclear speckles. Biochem. J. 361:489–496. 55. Smith, M. R., and W. C. Greene. 1992. Characterization of a novel nuclear localization signal in the HTLV-I tax transactivator protein. Virology 187:316–320. 56. Smith, M. R., and W. C. Greene. 1990. Identification of HTLV-I tax transactivator mutants exhibiting novel transcriptional phenotypes. Genes Dev. 4:1875–1885. 57. Smolen, G. A., M. T. Vassileva, J. Wells, M. J. Matunis, and D. A. Haber. 2004. SUMO-1 modification of the Wilms’ tumor suppressor WT1. Cancer Res. 64:7846–7851. 58. Sorokin, A. V., E. R. Kim, and L. P. Ovchinnikov. 2007. Nucleocytoplasmic transport of proteins. Biochemistry (Moscow) 72:1439–1457. 59. Spector, D. L. 2001. Nuclear domains. J. Cell Sci. 114:2891–2893. 60. Strouboulis, J., and A. P. Wolffe. 1996. Functional compartmentalization of the nucleus. J. Cell Sci. 109:1991–2000. 61. Sun, S. C., and D. W. Ballard. 1999. Persistent activation of NF-B by the Tax transforming protein of HTLV-1: hijacking cellular IB kinases. Oncogene 18:6948–6958. 62. Sun, S. C., and S. Yamaoka. 2005. Activation of NF-B by HTLV-I and implications for cell transformation. Oncogene 24:5952–5964. 63. Takatsuki, K. 2005. Discovery of adult T-cell leukemia. Retrovirology 2:16. 64. Terry, L. J., E. B. Shows, and S. R. Wente. 2007. Crossing the nuclear envelope: hierarchical regulation of nucleocytoplasmic transport. Science 318:1412–1416. 65. Tie, F., N. Adya, W. C. Greene, and C.-Z. Giam. 1996. Interaction of the human T-lymphotropic virus type 1 Tax dimer with CREB and the viral 21-base-pair repeat. J. Virol. 70:8368–8374. 66. Trostel, S. Y., D. L. Sackett, and T. Fojo. 2006. Oligomerization of p53 precedes its association with dynein and nuclear accumulation. Cell Cycle 5:2253–2259. 67. Tsuji, T., N. Sheehy, V. W. Gautier, H. Hayakawa, H. Sawa, and W. W. Hall. 2007. The nuclear import of the human T lymphotropic virus type I (HTLV-1) tax protein is carrier- and energy-independent. J. Biol. Chem. 282:13875–13883. 68. Yoshida, M., I. Miyoshi, and Y. Hinuma. 1982. Isolation and characterization of retrovirus from cell lines of human adult T-cell leukemia and its implication in the disease. Proc. Natl. Acad. Sci. USA 79:2031–2035.
