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Originally published In Press as doi:10.1074/jbc.M607011200 on August 21, 2006

J. Biol. Chem., Vol. 281, Issue 42, 31705-31712, October 20, 2006
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Site-specific Phosphorylation Differentiates Active from Inactive Forms of the Human T-cell Leukemia Virus Type 1 Tax Oncoprotein*

Sarah S. Durkin, Michael D. Ward, Kimberly A. Fryrear, and O. John Semmes1

From the Department of Microbiology and Molecular Cell Biology, Center for Biomedical Proteomics, Eastern Virginia Medical School, Norfolk, Virginia 23507

Received for publication, July 24, 2006 , and in revised form, August 18, 2006.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
The human T-cell leukemia virus type 1 oncoprotein Tax is a phosphoprotein with a predominately nuclear subcellular localization that accomplishes multiple functions via protein-protein interactions. It has been proposed that regulation of this protein's pleiotropic functions may be accomplished through phosphorylation of specific amino acid residues. We have conducted a phosphoryl mapping of mammalian-expressed Tax protein using a combination of affinity purification, liquid chromatography tandem mass spectrometry, and site-directed substitution mutational analysis. We achieved physical coverage of 77% of the Tax sequence and identified four novel sites of phosphorylation at Thr-48, Thr-184, Thr-215, and Ser-336. Previously identified potential serine phosphorylation sites at Ser-10, Ser-77, and Ser-274 could not be confirmed by mass spectrometry. The functional significance of these novel phosphorylation events was evaluated by mutational analysis and subsequent evaluation for activity via both CREB and NF-{kappa}B-responsive promoters. Our results demonstrate that phosphorylation at Thr-215 is associated with loss of both Tax functions, phosphorylation at Thr-48 was specifically deficient for activation via NF-{kappa}B, and phosphorylation at Thr-184 and Ser-336 had no effect on these Tax functions. Semiquantitation of phosphopeptides revealed that the majority of Tax was phosphorylated at Thr-48, Thr-184, Thr-215, and Ser-336, whereas only a minor population of Tax was phosphorylated at either Ser-300 or Ser-301. These results suggest that both positive and negative phosphorylation signals result in the maintenance of a subfraction of Tax as "active" protein.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
Human T-cell leukemia virus type 1 (HTLV-1)2 is a human transforming retrovirus. Infection with HTLV-1 can give rise to adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis as well as other subneoplastic conditions (1-5). Although cellular transformation can be achieved by expression of a single viral transactivating protein, Tax, the exact mechanism of transformation is not known (6). Tax is thought to induce genomic instability and, thus, to contribute to cellular transformation through interaction with cellular proteins involved in cell cycle control and the DNA damage repair response (7-11). In addition, Tax can activate or repress a variety of cellular genes predominately through the CREB (cAMP-response element-binding protein) and NF-{kappa}B pathways (6, 12). Thus, uncovering the regulatory mechanism for controlling the various Tax activities is critical to understanding HTLV-1-mediated cellular transformation.

There have been several structure-function studies of the Tax protein predominately utilizing molecular biology techniques. 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-{kappa}B-responsive promoters (13). In addition to phosphoryl-specific post-translational modifications, recent studies have uncovered an important regulatory role for ubiquitin/sumoyl modifications (21, 22). Clearly, Tax structural domains contribute to its regulation.

Phosphorylation is a common reversible regulatory event with a central role in controlling trans-acting/transcription (23). Earlier studies have demonstrated that Tax is a phosphoprotein (24) and that phosphorylation can regulate Tax activation of the HTLV-1 LTR (25, 26). We conducted the first attempt toward identification of Tax phosphorylation sites using a scanning serine to alanine substitution mutational analysis. This study identified that substitution of serine residue at position 10 or 274 resulted in a Tax mutant that lost the ability to transactivate both CREB- and NF-{kappa}B-responsive promoters (13). A separate study showed that a serine to alanine substitution at Tax amino acid 77 significantly reduced transactivation of a CREB-dependent promoter (27). However, two-dimensional mapping of tryptic fragments suggested that this site is not phosphorylated in vivo. Using tryptic peptide analysis, Bex et al. (29) identified Tax phosphorylations on two serine residues at positions 300 and 301 and showed that they are critical for transcriptional activation by Tax. Although the role of phosphorylation in the regulation of Tax is important, a complete map of the sites of phosphorylation within Tax has not been reported.

New methods for affinity purification and liquid chromatography tandem mass spectrometry (LC-MS/MS) allow for direct interrogation of the presence of phosphoryl residues (28). In this study we combined LC-MS/MS analysis of affinity-purified Tax protein with a substitution mutational analysis to identify and functionally characterize phosphorylation sites. The LC-MS/MS analysis achieved 77% coverage of the Tax sequence and identified four novel sites of phosphorylation on serine and threonine residues. Phosphorylation at specific amino acid residues provided both positive and negative regulatory signals. These results highlight the importance of phosphorylation in the regulation of Tax activity.


   EXPERIMENTAL PROCEDURES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
RESULTS
 DISCUSSION
 REFERENCES
 
Mammalian Expression Plasmid—The S-tagged expression vector STaxGFP was constructed by inserting the tax-EGFP fusion open reading frame into the SmaI site of pTriEx4-Neo (Novagen, Madison, WI) in-frame with the amino-terminal S tag and His tag.

Cell Culture and Transfection—293T 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). Jurkat cells were maintained in RPMI 1640 medium supplemented with 10% fetal bovine serum and 1% penicillin-streptomycin (Invitrogen).

Transfections of 293T cells were performed by standard calcium phosphate precipitation. Cells were plated in 150-mm plates at 4 x 106 cells/plate. The following day 20 µg of plasmid DNA in 2 M CaCl2 and 2x Hepes-buffered saline were added dropwise to cells in fresh medium. Cells were incubated at 37 °C for 5 h, and fresh medium was added. The cells were harvested 48 h later after a single wash with 1x phosphate-buffered saline, in 500 µl of M-Per mammalian protein extraction reagent (Pierce) with protease inhibitor mixture (Roche Applied Science) and immediately frozen at -80 °C.

Transfections of Jurkat cells were performed by electroporation with Bio-Rad Gene Pulser II with Capacitance Extender (Bio-Rad). 1 x 107 cells were electroporated with 30 µg of HTLV 1 LTR Luc reporter vector and 30 µg of Tax plasmid in 250 µl of RPMI with 10% fetal bovine serum in a 0.2-cm electrode gap cuvette at 280 V and 960 microfarads. After electroporation cells were resuspended in 5 ml of complete media. The cells were harvested 48 h later for luciferase assays in reporter lysis buffer as described below.

Purification of Tax Protein—Prepared cell lysate (1.5 ml) was incubated with a 75-µl bed volume of S-protein agarose (Novagen) for 30 min at room temperature, then washed 3 times with 1 ml of Bind/wash buffer (20 mM Tris-HCl, pH 7.5, 150 mM NaCl, 0.1% Triton X-100). Washed beads were eluted by resuspension in 150 µl Laemmli sample buffer (Bio-Rad) with beta-mercaptoethanol followed by boiling for 5 min. Eluates were electrophoresed in a 10% SDS one-dimensional polyacrylamide gel and visualized by Coomassie Blue staining. Bands of interest were manually excised from the gel for further analysis.

Immunoblot Analysis—Cell extracts were derived as described above. Total protein concentrations were determined by protein assay (Bio-Rad). A normalized volume of Laemmli sample buffer (Bio-Rad) was added to the lysate and boiled for 5 min, and a normalized amount of total protein was loaded in each lane and electrophoresed through a 10% SDS-polyacrylamide gel. The proteins were transferred onto an Immobilon-P (Millipore, Billerica, MA) membrane by semidry electroblotting and probed with anti-green fluorescent protein (GFP) monoclonal antibody (Santa Cruz Biotechnology, Santa Cruz, CA) and secondary horseradish-peroxidase conjugated anti-mouse antibody (Bio-Rad). Immunoreactivity was detected via Immunstar enhanced chemiluminescence protein detection (Bio-Rad).

LC-MS/MS Analysis—Protein bands were excised from one-dimensional polyacrylamide gels. Gel slices were cut into 1-2-mm cubes, washed 3x with 500 µl of Ultra-pure water, and incubated in 100% acetonitrile for 45 min. The material was dried in a SpeedVac, rehydrated in a 12.5 ng/µl modified sequencing grade trypsin solution (Promega, Madison, WI), and incubated in an ice bath for 40-45 min. The excess trypsin solution was then removed and replaced with 40-50 µl of 50 mM ammonium bicarbonate, 10% acetonitrile, pH 8.0, and the mixture was incubated overnight at 37 °C. Elastase digestion was performed as described for trypsin at an enzyme concentration of 15 ng/µl without acetonitrile in the reaction buffer. Peptides were extracted 2x with 25 µl of 50% acetonitrile, 5% formic acid and dried in a SpeedVac. Digests were resuspended in 20 µl of buffer A (5% acetonitrile, 0.1% formic acid, 0.005% heptafluorobutyric acid), and 3-6 µl were loaded onto a 12-cm x 0.075-mm fused silica capillary column packed with 5 µM diameter C-18 beads (The Nest Group, Southboro, MA) using a N2 pressure vessel at 1100 p.s.i. Peptides were eluted over 55 min by applying a 0-80% linear gradient of buffer B (95% acetonitrile, 0.1% formic acid, 0.005% heptafluorobutyric acid) at a flow rate of 130 µl/min with a precolumn flow splitter resulting in a final flow rate of ~200 nl/min directly into the source. In some cases the gradient was extended to 150 min to acquire more MS/MS spectra. A LCQTM Deca XP (ThermoFinnigan, San Jose, CA) was run in an automated collection mode with an instrument method composed of a single segment and four data-dependent scan events with a full MS scan followed by three MS/MS scans of the highest intensity ions. Normalized collision energy was set at 30, and activation Q was 0.250 with minimum full scan signal intensity at 5 x 105 and a minimum MS2 intensity at 1 x 104. Dynamic exclusion was turned on utilizing a 3-min repeat count of 2 with the mass width set at 1.50 Da. Sequence analysis was performed with TurboSEQUESTTM (ThermoFinnigan, San Jose, CA) or MASCOT (Matrix Sciences, London, GB) using an indexed human subset data base of the non-redundant protein data base from National Center for Biotechnology Information web site (www.ncbi.nlm.nih.gov). An additional data base was created containing only the open reading frame sequence for the expressed Tax protein.

MALDI-TOF Analysis—Tryptic digests of purified Tax protein were resuspended in 20 µl of 70% acetonitrile, 0.1% formic acid, and 2 µl were applied to a MP384 steel MALDI target (Brucker Daltonics, Bilaricka, MA). Spectra were acquired in reflectron mode using 96.3-µJ laser power at 20 Hz with a total shot count of ~500.

Site-directed Mutagenesis—Site-directed mutagenesis was performed using QuikChange II (Stratagene, La Jolla, CA) site-directed mutagenesis kit to introduce single amino acid changes by altering one or two nucleotides at the mutation site in the STaxGFP template. Both forward and reverse primers were designed to contain the desired mutation according to the manufacturer's protocol. Methylated original STaxGFP plasmid derived from bacteria was used as the template, and mutagenic primers were extended with PfuUltra high fidelity DNA polymerase during a 16-cycle PCR, incorporating the desired mutation into newly synthesized strands. The remaining methylated template was digested with the DpnI provided, and the PCR product was used to transform XL-1 Blue competent bacteria. Bacterial colonies growing under ampicillin selection were selected for further processing, and correct plasmids were purified using the Qiagen Maxiprep system (Qiagen, Valencia, CA). Introduction of each mutation was confirmed by DNA sequence analysis (Davis Sequencing, Davis, CA), and expression was confirmed by transfection and Western analysis using anti-GFP monoclonal antibody (Santa Cruz Biotechnology).


Figure 1
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  		FIGURE 1.
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{gamma}, LIM, P/CAF, and PDZ are indicated (19, 43-45). Tax contains multiple leucine zipper-like regions and an activation-specific domain (17-19). Other regions are necessary for CREB2 contact and DNA contact (46, 47). Also shown are the zinc-finger domain and domains required for dimerization and NF-{kappa}B activation (20, 48, 49). Potential sites of phosphorylation at serine residues 10, 77, and 274 are indicated (13, 27). Serine residues 300 and 301 have been previously identified as sites of phosphorylation (29). CBP, CREB-binding protein.

 
Luciferase Assays—293T cells, plated at 2 x 105 cells per well of a 6-well plate, were transfected as described with 2-8 µg of Tax expression plasmid and 1 µg of HTLV-LTR-Luc or pNF- {kappa}B-Luc (Clontech, Mountain View, CA) reporter plasmid. The parental vector used for Tax expression, pTriEx4-Neo (Novagen), was added to normalize for the total amount of transfected DNA. Luciferase activity was measured 48 h after transfection. Cells were washed twice with 1x phosphate-buffered saline and then lysed in 1x reporter lysis buffer (Promega) and subjected to 1 freeze-thaw at -80 °C. Luciferase assay substrate was used according to the manufacturer's protocol, and activity was measured in a TD-20/20 luminometer (Turner Designs, Sunnyvale, CA). Luciferase activity was normalized for total cell protein determined from the whole cell extracts by protein assay (Bio-Rad) according to manufacturer's directions.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
DISCUSSION
 REFERENCES
 
Functional Domains of HTLV-1 Tax—As a reference point for structure-function determination, we present a composite representation of known functional domains of HTLV-1 Tax protein in Fig. 1. The specific sites/motifs were assembled from a manual search of the peer-reviewed literature. The potential phosphorylation sites at Ser-10 and Ser-274 were mapped based on substitution mutation analysis from our earlier studies (13). The Ser-77 site was defined via two-dimensional mapping of tryptic fragments (27). Serine residues 300 and 301 have been previously identified as sites of phosphorylation by combined tryptic peptide and substitution mutational analysis (29). None of the previous studies utilized mass spectrometry, and the physical mapping of the Ser-300/301 tryptic peptide did not formally rule out phosphorylation at four possible alternative sites that are predicted to reside in the same tryptic fragment.

Expression of Biologically Active Affinity-tagged Tax Protein—To facilitate the analysis of phosphopeptides in the Tax protein occurring in vivo, we created a tandem affinity tagged Tax construct that could be expressed in and purified from mammalian cells. The STaxGFP vector expresses full-length Tax protein fused to amino-terminal His6 and S tags and carboxyl-terminal GFP (Fig. 2A). We included the GFP subunit to facilitate protein expression and purification monitoring. The STaxGFP vector was transiently transfected into 293T cells, and the transfection efficiency and appropriate subcellular localization to Tax Speckled Structures was confirmed by visualizing the GFP expression (Fig. 2C). To determine whether the tandem affinity-tagged Tax-GFP retains functional activity, we compared it to untagged Tax expressed from the IEX vector in trans-activation assays. 293T cells were cotransfected with the HTLV-I LTR Luc reporter plasmid and either Tax-expressing vector IEX or STaxGFP. STaxGFP showed comparable transcriptional activity to untagged Tax expressed from the IEX vector (Fig. 2D). The activity of STaxGFP as measured by the ability to activate via NF-{kappa}B-responsive promoter and to induce cell cycle accumulation in 4N was also comparable with wild type Tax (data not shown). Thus, efficient expression of functionally active tagged Tax was achieved in mammalian cells.

Affinity Purification of Tax from Mammalian Cells—We successfully purified S-tagged Tax protein from transfected 293T cells using S-protein-agarose beads as described under "Experimental Procedures." The purification is based on the strong interaction between the 15-amino acid S-tag and S-protein immobilized on agarose beads, both of which are derived from RNase S (30). The eluted protein from the affinity purification was further resolved by SDS-PAGE, and a distinct STaxGFP band was detected by Coomassie Blue staining (Fig. 2B). This process produced sufficient quantities of highly purified Tax protein from mammalian cells and allowed for the subsequent post-translational modification analysis by LC-MS/MS.

Phosphopeptide Mapping of Tax Using LC-MS/MS—We employed several strategies in determining the phosphorylation sites in Tax. First, the purified STaxGFP protein was subjected to enzymatic digestion by trypsin resulting in ~15 peptides within the reproducible dynamic mass range of the ion trap accounting for nearly 50% of the sequence. Tryptic peptides that were too large to detect were either further digested with elastase or independently digested with elastase, resulting in an additional 27% sequence coverage. The combined analysis of all of the peptide fragments generated by digestion with either trypsin or elastase enabled us to obtain a detailed physical map covering 77% of the Tax sequence (Fig. 3A). We found only suggestive evidence that the previously reported Ser-300/301 was phosphorylated (29). Specifically, of the 23 experimental runs conducted, only a single instance of a base peak neutral loss from a non-tryptic peptide was found, indicating a phosphate residing on either Ser-300 or Ser-301 but not both (Fig. 4). There was no evidence for phosphorylation at Ser-10, -77, or -274.


Figure 2
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  		FIGURE 2.
Efficient expression of biologically active affinity-tagged Tax protein. A, a depiction of the STaxGFP expression vector construct. B, affinity purification of S-tagged Tax from mammalian cells. 293T cells were transfected with STaxGFP, purified on S-protein-agarose beads, eluted and resolved by SDS-PAGE, and detected by Coomassie staining. C, 293T cells were transiently transfected with STaxGFP (green), fixed with 4% paraformaldehyde, permeabilized with methanol, and stained nuclei with TOPRO-3-iodide (Molecular Probes, Eugene, OR) (blue). Fluorescent images were acquired with a Zeiss LSM 510 confocal microscope at 40x magnification using argon (488 nm) and HeNe2 (633 nm) lasers and imaged with LSM Image Browser software (Carl Zeiss, Jena, Germany). D, 293T cells were cotransfected with the HTLV-I LTR Luc reporter plasmid and increasing amounts of either Tax expressing vector IEX or STaxGFP. Luciferase activity is shown as -fold activation over non-Tax-expressing cells.

 
We next employed a directed chemical derivatization of serine or threonine residues to rule out poor ionization efficiency or lability of the phosphates at Ser-300/301 (31). This approach utilizes a beta-elimination/ethanediol addition that results in phosphoserine and phosphothreonine conversion to the more stable S-ethylcysteine or beta-methyl-S-ethylcysteine, respectively. These derivatives are then detected by an increase of 44 atomic mass units from the unmodified serine or threonine residues. This method failed to increase the incidence rate of the Ser-300/301 phosphorylation event, suggesting that the low incidence is not an artifact of ionization but is, rather, indicative of native abundance. Results from the combined analysis detected three phosphothreonine residues at positions 48, 184, and 215 and two phosphoserine residues at positions 300/301 and 336 (Fig. 3B).

Mutational Analysis of the Identified Tax Phosphorylation Sites—Tax trans-activates viral and cellular gene expression through either the activating transcription factor (ATF)/CREB or NF-{kappa}B activation pathways (13, 14, 32). Thus, we evaluated each substitution mutation for trans-activation activity via both CREB-dependent and NF-{kappa}B-dependent promoters. Tax mutants were transiently co-transfected into 293T cells with either HTLV-1 LTR Luc or NF-{kappa}B Luc reporter plasmid to assay for the ability of Tax to activate either the ATF/CREB or NF-{kappa}B pathways, respectively. To determine the possible regulatory role of phosphorylation at the specific sites determined by mass spectrometry, single amino acid substitutions were introduced at Thr-48 -> Ala, Thr-48 -> Asp, Thr-184 -> Ala, Thr-184 -> Asp, Thr-215 -> Ala, Thr-215 -> Asp, Ser-300/301 -> Ala, Ser-300/301 -> Asp, Ser-336 -> Ala, and Ser-336 -> Asp. The stability of the expressed protein containing the specific substitution mutations was determined by Western blot analysis (Fig. 5A). This data were also used to normalize the extracts for the biological assays.

The results of the trans-activation assays are shown in Fig. 5B. Substitution mutations at Thr-184 and Ser-336 showed only a slight reduction in activity, although each of these sites is clearly phosphorylated. Substitutions at Thr-48 had a more pronounced effect upon NF-{kappa}B activity than was measurable via CREB activation. Interestingly, the Thr-48 -> Ala mutation retained both CREB and NF-{kappa}B activation when compared with the phosphomimetic substitution Thr-48 -> Asp. This result implies that phosphorylation at this site provides a negative regulatory signal. Likewise, Tax Thr-215 -> Ala retained full activity in both assays where as the phosphomimetic substitution Thr-215 -> Asp was inactive for each assay, suggesting that phosphorylation at this site results in significant decrease of both Tax activities. Examination of the Ser-300/301 substitution mutations confirmed a previous report that Ser-300/301 -> Ala retains only partial activity for either NF-{kappa}B or CREB activation, and the phosphomimetic substitution Ser-300/301 -> Asp specifically recovers the CREB activity but not the NF-{kappa}B activity (29). To determine whether the functional activity of these mutants is retained in a cell type targeted by HTLV-1, we tested mutants in luciferase assays measuring transactivation of HTLV-1 LTR in Jurkat cells (Fig. 5B). We found that those mutants with significant differences in functional activity from wild type showed the same pattern of activity in both 293T cells and Jurkat cells.

Semiquantitation of Phosphorylated Tax Protein—Although not directly quantitative, it is possible to evaluate the stoichiometric relationship between phosphorylated and unphosphorylated protein by determining the frequency that a phosphoryl peptide is detected. We re-examined all spectra of sufficient quality to identify Tax phosphopeptides and counted the relative incidence of phosphorylated ions versus all ions arising from the same sequence. The results of this analysis are shown in Table 1. The establishment of an absolute percentage of the peptide form is not reliable through this method; however, a general conclusion can be drawn that the majority of Tax is phosphorylated at Thr-48 and Ser-336. In addition a minor but significant portion of the protein is phosphorylated at Thr-184 and Thr-215.


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  		TABLE 1
High quality spectra from 23 experimental runs were examined for the presence of the indicated ions

 


Figure 3
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  		FIGURE 3.
Tax phosphopeptide map. A, a compilation of the results obtained with LC-MS/MS analysis of Tax. The identified phosphorylation sites are indicated (red). The Tax amino acid sequence assessed by enzymatic digestion with trypsin (green) and/or elastase (dotted line) is marked. The total analyzed sequence is highlighted in yellow. The table (inset) shows % total amino acid, serine amino acids, and threonine amino acids coverage of LC-MS/MS analysis. B, the novel phosphorylation sites and the 300/301 site are shown with the appropriate Tax sequence.

 


Figure 4
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  		FIGURE 4.
Neutral loss ion map of non-tryptic peptide containing Ser-300/301. MS/MS spectrum of a phosphorylated base peak ion indicating a loss of HPO3 from the +3 parent ion. The inset shows the resulting matched ion series for the selected peptide. The data base search was conducted without an enzyme restriction against the Tax-GFP sequence.

 
The much broader dynamic mass range of MALDI-TOF allowed analysis of tryptic peptides too large for the ion trap, such as the 40-amino acid tryptic peptide containing Ser-300/301. In addition, because ion acquisition is temporally static and devoid of data-dependent activation, MALDI-TOF analysis facilitates ion counting for semiquantitation of modified peptides. We were able to easily observe several of the large predicted tryptic masses corresponding to unmodified/modified peptides as well as the most abundant Tax-specific ionic masses seen with the ion trap instrument. MASCOT searches of these peak lists yielded identification of Tax as well as the GFP fusion protein, confirming that the predominant analyte was our fusion protein. We conducted 5 identical experiments and compared the relative average area under each of the peaks to obtain an estimate of <5% of all peptides were phosphorylated. An example of the spectral trace from one experiment is shown in Fig. 6. All of the resolvable features were consistent with a single phosphorylation event. Thus, our data suggest that phosphorylation of Tax at Ser-300/301 occurs at a single site and is a rare event.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
REFERENCES
 
The oncoprotein, Tax, encoded by the HTLV-1, is implicated in viral pathogenicity through its multiple pleiotropic functions. Although this trans-acting factor is essential for effective viral replication through its ability to activate viral transcription, Tax can also activate and repress transcription of a variety of cellular genes. This process occurs through targeting of both the CREB and NF-{kappa}B pathways. In addition, there have been many models proposed in which Tax interacts with cellular proteins to promote cellular transformation via dysregulation of the cell cycle, inhibition of tumor suppressor proteins, and modulation of the DNA damage repair response (7, 33). The specific regulation of all or a subset of these activities represents the functional impact of Tax on cell proliferation, increased mutation rate, and ultimately cellular transformation and leukemogenesis. Clearly then, a complete understanding of Tax-mediated oncogenesis is dependent upon a better understanding of how the activities of the protein are regulated.


Figure 5
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  		FIGURE 5.
Mutational analysis of Tax phospho-specific mutants. A, Western blot analysis of the expression of Tax mutants in cells compared with wild type Tax-expressing cells. 10 µg of wild type STaxGFP DNA (STaxGFP) or 20 µg of mutant STaxGFP DNA (indicated) was transfected into 293T cells. Equal amounts of whole cell lysate were loaded on the gel and blotted with anti-GFP antibody. B, trans-activation assay. 293T cells were co-transfected with either the HTLV-I LTR Luc or the NF-{kappa}B Luc reporter plasmid (indicated) and vectors expressing either wild type STaxGFP or a Tax mutant (indicated). The specific amino acid substitution for each Tax mutant is shown. Luciferase activity is shown as a percentage of the activity relative to wild type Tax. In some experiments (indicated by an asterisk) Jurkat cells were used to analyze activation of HTLV-I LTR Luc plasmid. The values presented are the median of replicate measurements with an average CV of 0.083 (CV range from 0.012 to 0.2).

 
As a central molecular regulatory mechanism, phosphorylation has been shown to modulate transcription factor function by multiple mechanisms including regulation of DNA binding, cellular localization, protein-protein interactions, and protein stability (34). Examples can be found in the scientific literature of both positive and negative regulation of transcription factors by phosphorylation. Regulation of transcription factor DNA binding activity may be negatively regulated due to the introduction of a negative charge within or nearby a DNA binding domain, generating electrostatic repulsion with the phosphates on the DNA and, thus, inhibiting DNA binding (34, 35). In other cases the phosphorylation sites are located distal to the DNA binding domain, and regulation of DNA binding is mediated via phosphorylation-induced alteration in protein conformation. An example of negative regulation of DNA binding by distal site phosphorylation has been described for the transcription factor c-myb, in which phosphorylation at sites ~50 residues from the DNA binding domain inhibit transcription factor docking (35). The DNA binding activity of the transcriptional repressor, Wilms' tumor gene product WT1, is similarly inhibited by phosphorylation within a distal zinc-finger region (34, 36). Sometimes phosphorylation of transcription factors regulates transactivation function via post-binding mechanisms. For example, phosphorylation of yeast transcription factor ADR1 does not affect DNA binding activity but, instead, negatively regulates the interaction with general transcriptional machinery and, thus, controls transactivation function (37).

Given the preponderance of regulatory mechanisms involving phosphorylation, there have been some efforts directed at determining the role of phosphorylation in the regulation of HTLV-1 Tax. The first investigation, resulting in the determination that the native protein was phosphorylated predominately on serine, utilized two-dimensional thin layer chromatography and tryptic peptide analysis (24). Subsequently, we conducted a mutational analysis of Tax protein that targeted all 47 serine residues (13). This study was followed by a combined mutational analysis and two-dimensional mapping of Tax tryptic fragments (27). Neither of these studies could conclusively identify functionally relevant phosphorylation sites within Tax. However, Bex et al. (29) identified a specific phosphorylation of serine residues 300/301 by reverse-phase high performance liquid chromatography of Tax tryptic fragments. These investigators also employed substitution analysis directed at amino acids 300 and 301 to demonstrate that active Tax protein required phosphorylation at this site. However, there were several theoretical scenarios, such as phosphorylation at other sites within the same tryptic fragment and dual phosphorylation at 300/301, which were not formally ruled out. In our current study we report the phosphorylation of Tax at four novel sites, threonine residues 48, 184, and 215 and serine residue 336, and confirm the phosphorylation at either serine residue 300 or 301. One explanation for why the other phosphorylation sites were not identified in this earlier study may be that the high performance liquid chromatography fractionation procedure may have resulted in a loss of other phosphopeptides in the protein. The selective loss of phosphopeptides can result from the addition of a phosphate group, thus reducing hydrophobicity, which may cause it to fail to be retained on the reversed-phase material used in purification (38). An additional consideration is that this study analyzed Tax protein derived from BHK21 (hamster kidney) cells, which may produce alternative post-translational modification patterns when compared with human cells. Although not without its own limitations, our approach provides a more robust method for the comprehensive mapping of phosphorylation sites (28, 38-40). We were able to identify phosphorylated Tax peptides as well as assign the sites of phosphorylation to specific residues by peptide sequencing using tandem mass spectrometry. Contrary to previous reports, however, our studies show that Tax is predominantly phosphorylated on threonine residues.

In addition to the dominance of threonine phosphorylation, we identified phosphorylation sites that were found to be negative regulators of Tax activity. Specifically, whereas alanine substitution at threonine residues 48 or 215 resulted in Tax mutants that retained the ability to activate gene expression, substitution with the phosphomimetic aspartic acid residue resulted in a dramatic loss of activity. For threonine residue 48, this effect was more pronounced for activation of the NF-{kappa}B pathway than the CREB pathway. Although the Tax protein was abundantly phosphorylated at threonine residue 184 and serine residue 336, the mutational analysis revealed no significant effect on the ability of Tax to activate gene expression. This result is consistent with the known existence of functionally silent phosphorylation events.


Figure 6
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  		FIGURE 6.
Partial MALDI-TOF spectrum of a Tax tryptic digest. Peptide masses equivalent to the phosphorylated and non-phosphorylated tryptic peptide containing Ser-300/301 are shown. The area under the curve was calculated from the total isotopic distribution of each peptide. This spectrum represents the highest percentage of phosphorylated Ser-300/301 peptide observed in five separate experiments.

 
One logical model for our combined results is that phosphorylation at either Ser-300 or Ser-301 along with dephosphorylation at Thr-48 and Thr-215 confers an activation state for Tax. A second aspect of our analysis allowed for inference toward the relative stoichiometry of individual phosphorylation events by determining the relative abundance of specific ions across 23 separate experiments. This summarization clearly suggested that phosphorylation at Ser-300 or Ser-301 was a rare event and that phosphorylation at Thr-48 and Thr-215 was a common event. Thus, active Tax protein, which is phosphorylated at Ser-300/301 and dephosphorylated at both Thr-48 and Thr-215, is less abundant than "inactive" Tax protein. The fact that the active/inactive ratio of a biologically potent oncoprotein is skewed toward the inactive state has significant impact upon modeling Tax function. Hypothetically then, function of Tax with respect to promoter activation requires phosphorylation at Ser-300 or Ser-301. Additionally, selective activation of CREB-dependent promoters is achieved by phosphorylation at Thr-48 and dephosphorylation at Thr-215. Thus, discrete regulation of the pleiotropic functionality displayed by the Tax oncoprotein may be reflected in variations of its phosphorylation pattern. This variation is likely dependent upon the host cell type as well as the specific cellular physiological environment. The combined role of Tax post-translational modifications, including phosphorylation as we have discussed here, and ubiquitination/sumoylation as has been discussed elsewhere (21, 22, 41, 42), provide a wealth of signals controlling protein structure, stability, and localization.

In silico analysis of the sequence surrounding the identified phosphorylation sites in Tax using ELM and MotifScan revealed several consensus sites for kinase recognition. Serine 336 is located within a proline-dependent serine/threonine kinase consensus motif, and threonine 48 is located within a casein kinase 2 consensus motif. Our data suggest that phosphorylation at Thr-48 results in loss of the ability to transactivate via the NF-{kappa}B pathway. Thus, casein kinase 2 activity may provide an important regulatory signal for Tax-mediated activation of the NF-{kappa}B pathway. This phosphorylation site lies within the zinc-finger domain of Tax and may represent a negative regulatory mechanism similar to that described for WT1 (36). Future studies will be aimed at uncovering the kinetic relationship of Tax post-translational modifications and Tax function.


   FOOTNOTES
 
* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Back

1 To whom correspondence should be addressed: Dept. of Microbiology and Molecular Cell Biology, Eastern Virginia Medical School, 700 West Olney Rd., Norfolk, VA 23507-1696. Tel.: 757-446-5904; Fax: 757-446-5766; E-mail: semmesoj{at}evms.edu.

2 The abbreviations used are: HTLV-1, human T-cell leukemia virus type 1; CREB, cAMP-response element-binding protein; LC-MS/MS, liquid chromatography tandem mass spectrometry; GFP, green fluorescent protein; MALDI-TOF, matrix-assisted laser desorption ionization time-of-flight; LTR, long terminal repeat. Back



   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
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S. K. Gupta, X. Guo, S. S. Durkin, K. F. Fryrear, M. D. Ward, and O. J. Semmes
Human T-cell Leukemia Virus Type 1 Tax Oncoprotein Prevents DNA Damage-induced Chromatin Egress of Hyperphosphorylated Chk2
J. Biol. Chem., October 5, 2007; 282(40): 29431 - 29440.
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