JC Virus Small t Antigen Binds Phosphatase PP2A and Rb Family Proteins and Is Required for Efficient Viral DNA Replication Activity

BACKGROUND: The human polyomavirus, JC virus (JCV) produces five tumor proteins encoded by transcripts alternatively spliced from one precursor messenger RNA. Significant attention has been given to replication and transforming activities of JCV's large tumor antigen (TAg) and three T' proteins, but little is known about small tumor antigen (tAg) functions. Amino-terminal sequences of tAg overlap with those of the other tumor proteins, but the carboxy half of tAg is unique. These latter sequences are the least conserved among the early coding regions of primate polyomaviruses. METHODOLOGY AND FINDINGS: We investigated the ability of wild type and mutant forms of JCV tAg to interact with cellular proteins involved in regulating cell proliferation and survival. The JCV P99A tAg is mutated at a conserved proline, which in the SV40 tAg is required for efficient interaction with protein phosphatase 2A (PP2A), and the C157A mutant tAg is altered at one of two newly recognized LxCxE motifs. Relative to wild type and C157A tAgs, P99A tAg interacts inefficiently with PP2A in vivo. Unlike SV40 tAg, JCV tAg binds to the Rb family of tumor suppressor proteins. Viral DNAs expressing mutant t proteins replicated less efficiently than did the intact JCV genome. A JCV construct incapable of expressing tAg was replication-incompetent, a defect not complemented in trans using a tAg-expressing vector. CONCLUSIONS: JCV tAg possesses unique properties among the polyomavirus small t proteins. It contributes significantly to viral DNA replication in vivo; a tAg null mutant failed to display detectable DNA replication activity, and a tAg substitution mutant, reduced in PP2A binding, was replication-defective. Our observation that JCV tAg binds Rb proteins, indicates all five JCV tumor proteins have the potential to influence cell cycle progression in infected and transformed cells. It remains unclear how these proteins coordinate their unique and overlapping functions

Stability and Function of JC Virus Large T Antigen and T′ Proteins Are Altered by Mutation of Their Phosphorylated Threonine 125 Residues

JC virus (JCV), a human polyomavirus, exhibits oncogenic activity in rodents and primates. The large tumor antigens (TAgs) of the polyomaviruses play key roles in viral replication and oncogenic transformation. Analyses of JCV TAg phosphorylation mutants indicated that the amino-terminal phosphorylation site at threonine 125 (T125) is critical to TAg replication function. This site is also conserved in the TAg splice variants T′(135), T′(136), and T′(165). By constructing stable cell lines expressing JCV T125A and T125D mutants, we show that mutation of this phosphorylation site to alanine generates an unstable TAg; however, the stability of the three T′ proteins is unaffected. JCV T125A mutant proteins bind the retinoblastoma protein (RB) family members p107 and p130 with slightly reduced efficiencies and fail to induce the release of transcriptionally active E2F from RB-E2F complexes. On the other hand, cell lines expressing JCV T125D mutant proteins produce stable TAg and T′ proteins which bind p107 and p130 more efficiently than do the wild-type proteins. In addition, T125D mutant proteins efficiently induce the release of E2F from RB-E2F complexes. T125D mutant cell lines, unlike the T125A mutant lines, continue to grow under conditions of low serum concentration and anchorage independence. Finally, both T125A and T125D mutant viruses are replication defective. Phosphorylation of the T125 site is likely mediated by a cyclin-cyclin-dependent kinase, suggesting that JCV TAg and T′ protein functions that mediate viral replication and oncogenic transformation events are regulated in a cell cycle-dependent manner

JCV tAg fails to complement replication of a tAg-deficient JCV genome in <i>trans</i>.

<p>PHFG cells in 60 mm plates were co-transfected in duplicate with 400 ng of a tAg-deficient (T⁺/t⁻/T'⁺) JCV genome and 400 ng of a JCV DNA construct expressing either wild type tAg (T⁻/t⁺/T′⁻) or a J domain mutant tAg (T⁻/H42Qt⁺/T′⁻) under the control of the JCV promoter-enhancer. Cells were also transfected with 400 ng of either a T′-deficient (T⁺/t⁺/T'⁻) JCV genome or a positive replication control (T⁺/t⁺/T'⁺). Duplicate, independent samples representing each DNA construct were extracted by the method of Hirt <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0010606#pone.0010606-Hirt1" target="_blank">[39]</a> on days 0, 7, and 10 p.t. and analyzed using the Dpn1 assay as described in the legend to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0010606#pone-0010606-g005" target="_blank">Figure 5</a>. The marker shown in the first lane of each blot is 1 ng of linear JCV DNA (5130 bp), and the position of <i>Dpn</i>1-resistent replicating genomes is denoted by an arrow at days 7 and 10 p.t. <i>Dpn</i>1- and <i>Eco</i>RI-sensitive input DNAs are noted at the day 0 time point.</p

Interaction of mutant JCV tAgs with PP2A.

<p>Interactions between PP2A and wild-type tAg and (A) tAg-mutant P99A or (B) C157A were compared. 3T3 cells were stably transfected with DNA constructs expressing JCV early proteins under the control of SV40 promoter-enhancer signals. Lysates of these cells were subjected to IP with the anti-T monoclonal antibody PAb 962 (α-T) or anti-PP2A antibody (α-PP2A). The amount of total cell protein subjected to IP with anti-PP2A antibody (lanes 5–8, Panel A; lanes 6–10, Panel B) was five times that used with the anti-T antibody (lanes 1–4, Panel A; lanes 1–5, Panel B). Samples were electrophoresed on 18% SDS-polyacrylamide gels and transferred to nitrocellulose membranes. WB was performed using a cocktail of anti-T monoclonal antibodies. Results for two independently-derived cells lines (#1, #2) containing DNA constructs that either encode all 5 JCV wild type early proteins (T⁺/t⁺/T′⁺; Panel A, lanes 1,2,5,6, Panel B, lanes 1,2,6,7) or wild type TAg, T′₁₆₅, T′₁₃₆ and T′₁₃₅ plus tAg mutant P99A (T⁺/P99At⁺/T′⁺; Panel A, lanes 3,4,7,8) or C157A (T⁺/C157At⁺/T′⁺; Panel B, lanes 3,4,8,9) are shown. A single 3T3 cell line expressing tAg only (T⁻/t⁺/T′⁻) was isolated and tested for PP2A binding (Panel B, lanes 5, 10). Panels A and B of this figure each represent proteins electrophoresed on a single gel and transferred to a membrane, which was then cut in half and each half developed for different lengths of time.</p

Wild type tAg interacts with cellular phosphatase PP2A in cells expressing JCV early proteins.

<p>JCV early proteins, expressed in Rat 2 (R2) or MEF cells transformed with pSR:T⁺/t⁺/T'⁺ or in G418-selected 3T3 cells transfected with pSR:T⁺/t⁺/T'⁺ (encodes all 5 JCV early proteins) or pSR:T⁻/t⁺/T'⁻ (encodes JCV tAg only) were incubated with anti-T monoclonal antibody PAb 962 (α-T; lanes 4–7) or anti-PP2A antibody (α-PP2A; lanes 9–12). The amount of total cell protein subjected to IP in lanes 9–12 was four times that employed in the corresponding samples in lanes 4–7. Immunoprecipitated proteins were separated on a 20% SDS-polyacrylamide gel, and WB analysis was performed using a cocktail of anti-T monoclonal antibodies to detect JCV early proteins either expressed in the different cell lines (lanes 4–7) or expressed and bound to PP2A (lanes 9–12). Untransfected 3T3 and Rat 2 cells were included as negative controls (no JCV T proteins are present; lanes 1, 2), and α-mouse IgG was used in the IP step with the R2:T⁺/t⁺/T'⁺ cell extract to test for non-specific binding (lanes 3, 8). The asterisks denote antibody light and heavy chains. This figure represents proteins electrophoresed on a single gel and transferred to a membrane, which was then cut in half and each half developed for different lengths of time.</p