Stoichiometry of large T antigen and pp53 in complexes isolated from simian virus 40-transformed rat cells

Characterization of the in vitro interaction between SV40 T antigen and p53: mapping the p53 binding site

An efficient in vitro system for generating soluble complexes between simian virus 40 T antigen and the cellular protein p53 was developed. A p53 cDNA was inserted 3' to the SP6 promoter in pGEM-1 (Promega-Biotec) and transcribed by SP6 polymerase. In vitro translation of the cRNA generated p53 which was immunoprecipitable with all five monoclonal antibodies tested (PAb122, PAb421, PAb242, PAb246, and PAb248). The p53 sedimented at about 8-10 S in sucrose gradients, possibly corresponding to a tetramer. T-antigen-p53 complexes were produced by the addition of immunoaffinity-purified T antigen to p53-containing translation lysates. Equivalent amounts of p53 were immunoprecipitated with the anti-T-antigen antibodies PAb416, PAb419, and PAb101, suggesting that in vitro made p53 complexed mostly to a population of T-antigen molecules that had matured at least 15 min in the cell. The complexes sedimented at 18-20 S in sucrose gradients. In order to map the p53 binding site on T antigen, p53 was complexed in vitro to labeled proteolytic fragments of T antigen. A 46K fragment, spanning residues 131-517, was immunoprecipitated with the anti-p53 monoclonal PAb122 and therefore is likely to contain the p53 binding site. This region contains T-antigen sequences necessary for the efficient transformation of nonpermissive cells and for the induction of cellular rRNA synthesis. It also contains the binding sites for DNA polymerase alpha and ATP. We suggest a possible role for T-p53 complexes in T-antigen-associated functions.

Dimers and complexes with p53 are the prevalent oligomeric forms of a transforming nonkaryophilic T antigen of simian virus 40

The oligomers formed by a mutant nonkaryophilic large T antigen of simian virus 40, which lacks residues 110 through 152 of normal large T antigen and transforms only established cells (L. Fischer-Fantuzzi and C. Vesco, Proc. Natl. Acad. Sci. USA 82:1891-1895, 1985), were found to consist predominantly of dimers. Anti-p53 antibodies precipitated 14 to 16S complexes containing the mutant nonkaryophilic large T antigen and p53 from extracts of transformed cells, and anti-p53 indirect immunofluorescence stained these cells in the cytoplasm.

Complex formation of simian virus 40 large T antigen with cellular protein p53

We investigated the formation of native complexes between simian virus 40 large T antigen and the cellular protein p53 (T-p53) by using simian virus 40 tsA58-transformed mouse fibroblasts (tsA58 F2b). We observed that newly synthesized p53 bound to all structural subclasses of large T antigen detectable on sucrose density gradients. This led to various intermediates of T-p53 complexes which converted within 2 h into typical mature aggregates. The final levels of stable T-p53 complexes seemed to be determined by p53 rather than by large T antigen.

The transformation-related protein p53 is not bound to the SV40 T antigen in BALB 3T12 cells expressing T antigen

In most murine cells transformed by the SV40 virus, virtually all of the cellular phosphoprotein p53 is in a complex with the SV40 T antigen. Here, we report that, in SV40-infected T-antigen-positive Balb 3T12 mouse cells, most (approximately 80%) of the p53 is not in complex. Complex formation was determined by measuring the amounts of [35S]methionine-labeled p53 which coprecipitated with T antigen when using monoclonal antibody to T antigen. The amount of complex formation was expressed as a percentage of total p53 present, measured by the amount of p53 precipitated with the monoclonal antibody to the p53. The values were confirmed by Western blotting procedure, in which the steady-state levels of the proteins were measured. In these measurements after complete precipitation with antibody to T antigen, the residual p53 in the supernatant was precipitated by antibody to p53, and this amount was denoted as free p53. There was no significant difference seen between the [35S]methionine-labeled tryptic peptides of complexed and the free p53 (or between complexed and free T antigens) as determined by two-dimensional gel electrophoresis and chromatography. Virus rescue experiments and retransformation by the rescued virus showed that there was no mutation in the SV40 DNA coding for the T antigen which could account for the lack of complex formation. Both p53 and T antigen were underphosphorylated in cells which exhibited reduced complex formation. Tumorigenicity in syngeneic mice and anchorage-independent cell growth in culture of various cloned mouse cells with or without T antigen expression was compared. The changes in the biologic properties were explainable solely on the basis of known or expected effects of expression of the T antigen and were independent of complex formation or of absence of complex formation between p53 and T antigen.

Free and viral chromosome-bound simian virus 40 T antigen: changes in reactivity of specific antigenic determinants during lytic infection

Simian virus 40 (SV40) large T antigen (TAg), both free and bound to mature 70S and replicating 90S SV40 chromosomes, was prepared from lytically infected cells. The relative reactivity of the different TAg-containing fractions toward 10 monoclonal antibodies directed against three different regions in SV40 TAg and toward an antibody against the p53 protein was measured. The results for free TAg indicated that all of the determinants in both the amino-terminal (0.65 to 0.62 map units) and carboxy-terminal (0.28 to 0.17 map units) regions were highly reactive, whereas all five determinants located between 0.43 and 0.28 map units in the midregion of TAg were poorly reactive. For TAg bound to replicating chromosomes, all but one of the antibodies specific for TAg were highly reactive. Thus, antigenic sites in the middle of TAg, the region important for nucleotide binding and ATP hydrolysis (an activity required for viral DNA replication), were more accessible in TAg-replicating DNA complexes. As replicating molecules matured into 70S chromosomes, three or more determinants at different locations in TAg bound to chromatin became two- to fivefold less reactive, indicating other changes in TAg structure. Overall, at least nine different antigenic determinants in the TAg molecule were identified. Anti-p53 was reactive with about 10% of the free TAg and the same amount of SV40 chromosomes of all ages, suggesting that p53-TAg complexes are not preferentially associated with either replicating or mature viral chromosomes. When the reactivity of both mature and replicating labeled SV40 chromosomes with polyclonal tumor anti-T was measured as a function of time after purification, TAg bound to mature chromosomes appeared to dissociate about fourfold faster than that bound to replicating chromosomes. The relative amount of TAg in various subcellular fractions was measured by an enzyme-linked immunosorbent assay. Approximately 1.3% of the total TAg was estimated to be associated with SV40 chromosomes in infected cells. Based on the relative amounts of TAg and viral DNA in the 70S and 90S fractions, replicating chromosome-TAg complexes were estimated to bind 4.8 times more TAg per DNA molecule, on the average, than mature chromosome-TAg complexes. Together, these results are consistent with major differences in TAg structure when free and associated with replicating and nonreplicating SV40 chromosomes.

Simian virus 40 large T antigen oligomers: analysis of electrophoresis in the absence of detergent

Large T antigen of simian virus 40 is found as monomeric and oligomeric species in transformed cells. These can be demonstrated in cell extracts by velocity centrifugation in sucrose gradients. We analyzed them further in a transformed human line cell (SV80) and a transformed mouse line cell (SVT2). Individual fractions from sucrose gradients were subjected to polyacrylamide gel electrophoresis in the absence of detergent. T-antigen species were then detected by protein blotting and antibody overlay with polyclonal anti-D2 T antibody or monoclonal Pab419, Pab101, or Pb1700 antibody. The rapidly sedimenting species (14S and larger) of large T antigen from both cell lines reproducibly showed two major bands with estimated molecular weights of 670,000 and 850,000. A third band of 1,200,000 was more prominent in SVT2 cells than in SV80 cells. In SV80 cells the slowly sedimenting species of large T antigen (5S to 11S) contained two reproducible bands. A band with a molecular weight of 95,000 was the predominant one in all fractions between 5S and 11S. A relatively minor band with a molecular weight of 230,000 was found in fractions between 9S and 11S. The low-molecular-weight forms were seen in SVT2 cells only when a prominent peak at 5S to 7S was present, that is, when extracts were stored before analysis. In fresh extracts, the low-molecular-weight bands and slowly sedimenting forms were absent.

Intracellular location and kinetics of complex formation between simian virus 40 T antigen and cellular protein p53

The intracellular location and kinetics at which the simian virus 40 T antigen and the cellular protein p53 associate with one another were determined for simian virus 40-transformed mouse (215) and rat (14B) cells. Cells were labeled under pulse-chase conditions and fractionated into nuclear and cytoplasmic components, and the proteins were immunoprecipitated with monoclonal antibodies (pAb 416, 101, and 122). We found that newly made T antigen and p53 migrated to the nucleus of these cells independently; that is, in uncomplexed form. Newly made p53 was transported to the nucleus more rapidly than T antigen in both cell lines and formed a complex with a mature form of T antigen recognizable by pAb 101. This association was very rapid in both cell lines (t 1/2, 5 to 15 min). In contrast, the time course of complex formation between newly made T antigen and the p53 in the nucleus varied with the ratio of T antigen to p53 of the cell line studied. In 215 cells, where the ratio was 3.6, the kinetics were quite slow (t 1/2, 30 min), whereas in 14B cells, where the ratio was 1.7, they were quite rapid (t 1/2, 5 min). We suggest that a competition between newly made and uncomplexed T antigen for the p53 in the nucleus is the major determinant of the rate of complex formation for newly made T antigen. Our studies indicate that this macromolecular interaction is extremely dynamic.

Immunoprecipitation of some forms of simian virus 40 large-T antigen by antibodies to synthetic peptides

Antibodies were raised against six synthetic peptides corresponding to overlapping amino acid sequences (106 through 145) from a putative DNA binding domain in simian virus 40 (SV40) large-T antigens. All six antipeptide sera immunoprecipitated large-T from crude extracts of SV40-transformed cells, but the efficiency varied widely; in general, antibodies to the longer peptides produced the strongest anti-large-T activity. Antisera were purified by immunoaffinity chromatography on immobilized peptide. The purified antisera recognized only some forms of large-T; full-sized large-T from transformed cells, super-T from SV3T3 C120 cells, and 70,000-dalton T-antigen from Taq-BamHI cells were immunoprecipitated, whereas large-T from productively infected cells reacted irreproducibly, and the full-sized protein, synthesized in vitro or eluted from sodium dodecyl sulfate-containing gels, and the 33,000- and 22,000-dalton truncated large-Ts from Swiss SV3T3 and MES2006 cells, respectively, were not immunoprecipitated. This pattern of reactivity was explained when extracts were fractionated by sucrose density centrifugation, and it was found that only rapidly sedimenting forms of large-T were immunoprecipitated by the antipeptide sera; that is, large-T complexed with nonviral T antigen was detected, whereas lighter forms were not detected. Cascade immunoprecipitations did not support the view that this result was caused by the low affinity of the peptide antisera for large-T, and Western blotting experiments confirmed that the peptide antisera react directly with immobilized, monomeric large-T but not with nonviral T antigen. Immunoprecipitation assays to detect large-T:nonviral T antigen complexes bound specifically to fragments of SV40 DNA showed that under conditions of apparent antibody excess, DNA still bound to the complex.