DNA-binding properties of simian virus 40 T-antigen mutants defective in viral DNA replication

Quantitative analysis of viral load per haploid genome revealed the different biological features of Merkel cell polyomavirus infection in skin tumor

Merkel cell polyomavirus (MCPyV) has recently been identified in Merkel cell carcinoma (MCC), an aggressive cancer that occurs in sun-exposed skin. Conventional technologies, such as polymerase chain reaction (PCR) and immunohistochemistry, have produced conflicting results for MCPyV infections in non-MCC tumors. Therefore, we performed quantitative analyses of the MCPyV copy number in various skin tumor tissues, including MCC (n = 9) and other sun exposure-related skin tumors (basal cell carcinoma [BCC, n = 45], actinic keratosis [AK, n = 52], Bowen’s disease [n = 34], seborrheic keratosis [n = 5], primary cutaneous anaplastic large-cell lymphoma [n = 5], malignant melanoma [n = 5], and melanocytic nevus [n = 6]). In a conventional PCR analysis, MCPyV DNA was detected in MCC (9 cases; 100%), BCC (1 case; 2%), and AK (3 cases; 6%). We then used digital PCR technology to estimate the absolute viral copy number per haploid human genome in these tissues. The viral copy number per haploid genome was estimated to be around 1 in most MCC tissues, and there were marked differences between the MCC (0.119–42.8) and AK (0.02–0.07) groups. PCR-positive BCC tissue showed a similar viral load as MCC tissue (0.662). Immunohistochemistry with a monoclonal antibody against the MCPyV T antigen (CM2B4) demonstrated positive nuclear localization in most of the high-viral-load tumor groups (8 of 9 MCC and 1 BCC), but not in the low-viral-load or PCR-negative tumor groups. These results demonstrated that MCPyV infection is possibly involved in a minority of sun-exposed skin tumors, including BCC and AK, and that these tumors display different modes of infection.

Wild-type p53 enhances efficiency of simian virus 40 large-T-antigen-induced cellular transformation

Abortive infection of BALB/c mouse embryo fibroblasts differing in p53 gene status (p53(+/+) versus p53(-/)(-)) with simian virus 40 (SV40) revealed a quantitatively and qualitatively decreased transformation efficiency in p53(-/-) cells compared to p53(+/+) cells, suggesting a supportive effect of wild-type (wt) p53 in the SV40 transformation process. SV40 transformation efficiency also was low in immortalized p53(-/-) BALB/c 10-1 cells but could be restored to approximately the level in immortalized p53(+/+) BALB/c 3T3 cells by reconstituting wt p53, but not mutant p53 (mutp53), expression. Stable expression of large T antigen (LT) in p53(+/+) 3T3 cells resulted in full transformation, while LT expression in p53(-/-) 10-1 cells could not promote growth in suspension or in soft agar to a significant extent. The helper effect of wt p53 is mediated by its cooperation with LT and resides in the p53 N terminus, as an N-terminally truncated p53 (DeltaNp53) could not rescue the p53-null phenotype. The p53 N terminus serves as a scaffold for recruiting transcriptional regulators like p300/CBP and Mdm2 into the LT-p53 complex. Consequently, LT affected global and specific gene expression in p53(+/+) cells significantly more than in p53(-/-) cells. Our data suggest that recruitment of transcriptional regulators into the LT-p53 complex may help to modify cellular gene expression in response to the needs of cellular transformation.

T antigen mutations are a human tumor-specific signature for Merkel cell polyomavirus

Merkel cell polyomavirus (MCV) is a virus discovered in our laboratory at the University of Pittsburgh that is monoclonally integrated into the genome of approximately 80% of human Merkel cell carcinomas (MCCs). Transcript mapping was performed to show that MCV expresses transcripts in MCCs similar to large T (LT), small T (ST), and 17kT transcripts of SV40. Nine MCC tumor-derived LT genomic sequences have been examined, and all were found to harbor mutations prematurely truncating the MCV LT helicase. In contrast, four presumed episomal viruses from nontumor sources did not possess this T antigen signature mutation. Using coimmunoprecipitation and origin replication assays, we show that tumor-derived virus mutations do not affect retinoblastoma tumor suppressor protein (Rb) binding by LT but do eliminate viral DNA replication capacity. Identification of an MCC cell line (MKL-1) having monoclonal MCV integration and the signature LT mutation allowed us to functionally test both tumor-derived and wild type (WT) T antigens. Only WT LT expression activates replication of integrated MCV DNA in MKL-1 cells. Our findings suggest that MCV-positive MCC tumor cells undergo selection for LT mutations to prevent autoactivation of integrated virus replication that would be detrimental to cell survival. Because these mutations render the virus replication-incompetent, MCV is not a "passenger virus" that secondarily infects MCC tumors.

Analyses of the interaction between the origin binding domain from simian virus 40 T antigen and single-stranded DNA provide insights into DNA unwinding and initiation of DNA replication

DNA helicases are essential for DNA metabolism; however, at the molecular level little is known about how they assemble or function. Therefore, as a model for a eukaryotic helicase, we are analyzing T antigen (T-ag) the helicase encoded by simian virus 40. In this study, nuclear magnetic resonance (NMR) methods were used to investigate the transit of single-stranded DNA (ssDNA) through the T-ag origin-binding domain (T-ag OBD). When the residues that interact with ssDNA are viewed in terms of the structure of a hexamer of the T-ag OBD, comprised of residues 131 to 260, they indicate that ssDNA passes over one face of the T-ag OBD and then transits through a gap in the open ring structure. The NMR-based conclusions are supported by an analysis of previously described mutations that disrupt critical steps during the initiation of DNA replication. These and related observations are discussed in terms of the threading of DNA through T-ag hexamers and the initiation of viral DNA replication.

SV40 large T antigen targets multiple cellular pathways to elicit cellular transformation

DNA tumor viruses such as simian virus 40 (SV40) express dominant acting oncoproteins that exert their effects by associating with key cellular targets and altering the signaling pathways they govern. Thus, tumor viruses have proved to be invaluable aids in identifying proteins that participate in tumorigenesis, and in understanding the molecular basis for the transformed phenotype. The roles played by the SV40-encoded 708 amino-acid large T antigen (T antigen), and 174 amino acid small T antigen (t antigen), in transformation have been examined extensively. These studies have firmly established that large T antigen's inhibition of the p53 and Rb-family of tumor suppressors and small T antigen's action on the pp2A phosphatase, are important for SV40-induced transformation. It is not yet clear if the Rb, p53 and pp2A proteins are the only targets through which SV40 transforms cells, or whether additional targets await discovery. Finally, expression of SV40 oncoproteins in transgenic mice results in effects ranging from hyperplasia to invasive carcinoma accompanied by metastasis, depending on the tissue in which they are expressed. Thus, the consequences of SV40 action on these targets depend on the cell type being studied. The identification of additional cellular targets important for transformation, and understanding the molecular basis for the cell type-specific action of the viral T antigens are two important areas through which SV40 will continue to contribute to our understanding of cancer.

Mapping the transcriptional transactivation function of simian virus 40 large T antigen

T antigen is able to transactivate gene expression from the simian virus 40 (SV40) late promoter and from several other viral and cellular promoters. Neither the mechanisms of transactivation by T antigen nor the regions of T antigen required for this activity have been determined. To address the latter point, we have measured the ability of a set of SV40 large T antigen mutants to stimulate gene expression in CV-1 monkey kidney cells from the SV40 late promoter and Rous sarcoma virus (RSV) long terminal repeat (LTR) promoter. Transactivation, although reduced, was retained by an N-terminal 138-amino-acid fragment of T antigen. Mutants with alterations at various locations within the N-terminal 85 amino acids transactivated the RSV LTR promoter less well than did wild-type T antigen. Most of these were also partially defective in their ability to transactivate the SV40 late promoter. Two mutants with lesions in the DNA-binding domain that were unable to bind to SV40 DNA were completely defective for transactivation of both promoter, while a third mutant with a lesion in the DNA-binding domain which retained origin-binding activity transactivated both promoters as well as did wild-type T antigen. Only a low level of transactivation was seen with mutant T antigens which had lesions in or near the zinc finger region (amino acids 300 to 350). Mutations which caused defects in ATPase activity, host range/helper function, binding to p53, binding to the retinoblastoma susceptibility protein, or nuclear localization had little or no effect on transactivation. These results suggest that N-terminal portion of T antigen possesses an activation activity. The data are consistent with the idea that the overall conformation of T antigen is important for transactivation and that mutations in other regions that reduce or eliminate transactivation do so by altering the conformation or orientation of the N-terminal region so that its ability to interact with various targets is diminished or abolished.

Properties of the DNA-binding domain of the simian virus 40 large T antigen

T antigen (Tag) from simian virus 40 binds specifically to two distinct sites in the viral origin of replication and to single-stranded DNA. Analysis of the protein domain responsible for these activities revealed the following. (i) The C-terminal boundary of the origin-specific and single-strand-specific DNA-binding domain is at or near amino acid 246; furthermore, the maximum of these DNA-binding activities coincides with a narrow C-terminal boundary, spanning 4 amino acids (246 to 249) and declines sharply in proteins with C termini which differ by a few (4 to 10) amino acids; (ii) a polypeptide spanning residues 132 to 246 of Tag is an independent domain responsible for origin-specific DNA binding and presumably for single-stranded DNA binding; and (iii) a comparison of identical N-terminal fragments of Tag purified from mammalian and bacterial cells revealed differential specificity and levels of activity between the two sources of protein. A role for posttranslational modification (phosphorylation) in controlling the DNA-binding activity of Tag is discussed.

Properties of the DNA-binding domain of the simian virus 40 large T antigen

T antigen (Tag) from simian virus 40 binds specifically to two distinct sites in the viral origin of replication and to single-stranded DNA. Analysis of the protein domain responsible for these activities revealed the following. (i) The C-terminal boundary of the origin-specific and single-strand-specific DNA-binding domain is at or near amino acid 246; furthermore, the maximum of these DNA-binding activities coincides with a narrow C-terminal boundary, spanning 4 amino acids (246 to 249) and declines sharply in proteins with C termini which differ by a few (4 to 10) amino acids; (ii) a polypeptide spanning residues 132 to 246 of Tag is an independent domain responsible for origin-specific DNA binding and presumably for single-stranded DNA binding; and (iii) a comparison of identical N-terminal fragments of Tag purified from mammalian and bacterial cells revealed differential specificity and levels of activity between the two sources of protein. A role for posttranslational modification (phosphorylation) in controlling the DNA-binding activity of Tag is discussed.

Asp-286----Asn-286 in polyomavirus large T antigen relaxes the specificity of binding to the polyomavirus origin

We isolated revertants of a polyomavirus whose origin of DNA replication contains a point mutation in the palindrome to which large T antigen binds. Four independent second-site revertants contain an Asp-286----Asn-286 substitution in large T antigen. This mutant large T antigen activates replication of DNAs containing the mutant polyomavirus origin as well as replication of DNAs containing the wild-type origin; however, replication of DNAs with enhancer mutations is not activated by this large T antigen. The Asn-286 mutation occurs in a positively charge region of large T antigen near the location of several mutations which inactivate DNA replication. We suggest that this region of large T antigen is responsible for recognition of specific DNA sequences at the origin and that ionic forces are important for this interaction.

Expression of simian virus 40 T antigen in Escherichia coli: localization of T-antigen origin DNA-binding domain to within 129 amino acids

The genomic coding sequence of the large T antigen of simian virus 40 (SV40) was cloned into an Escherichia coli expression vector by joining new restriction sites, BglII and BamHI, introduced at the intron boundaries of the gene. Full-length large T antigen, as well as deletion and amino acid substitution mutants, were inducibly expressed from the lac promoter of pUC9, albeit with different efficiencies and protein stabilities. Specific interaction with SV40 origin DNA was detected for full-length T antigen and certain mutants. Deletion mutants lacking T-antigen residues 1 to 130 and 260 to 708 retained specific origin-binding activity, demonstrating that the region between residues 131 and 259 must carry the essential binding domain for DNA-binding sites I and II. A sequence between residues 302 and 320 homologous to a metal-binding "finger" motif is therefore not required for origin-specific binding. However, substitution of serine for either of two cysteine residues in this motif caused a dramatic decrease in origin DNA-binding activity. This region, as well as other regions of the full-length protein, may thus be involved in stabilizing the DNA-binding domain and altering its preference for binding to site I or site II DNA.

T-antigen-DNA polymerase alpha complex implicated in simian virus 40 DNA replication

We have combined in vitro DNA replication reactions and immunological techniques to analyze biochemical interactions between simian virus (SV40) large T antigen and components of the cellular replication apparatus. First, in vitro SV40 DNA replication was characterized with specific origin mutants. Next, monoclonal antibodies were used to demonstrate that a specific domain of T antigen formed a complex with cellular DNA polymerase alpha. Several antibodies were identified that coprecipitated T antigen and DNA polymerase alpha, while others were found to selectively prevent this interaction and concomitantly inhibit DNA replication. DNA polymerase alpha also bound efficiently to a T-antigen affinity column, confirming the immunoprecipitation results and providing a useful method for purification of the complete protein complex. Taken together, these results suggest that the T-antigen-polymerase association may be a key step in the initiation of SV40 DNA replication.

Immortalization of rat embryo fibroblasts by mutant polyomavirus large T antigens deficient in DNA binding

We have identified a putative DNA-binding domain in polyomavirus large T antigen. Mutations introduced into the gene between amino acids 290 and 310 resulted in proteins that no longer bound to the high-affinity binding sites on the polyomavirus genome, showed no detectable nonspecific DNA binding, and were not able to initiate DNA replication from the viral origin. These mutant T antigen genes were introduced into rat embryo fibroblasts together with the neomycin resistance gene to allow selection for growth in the presence of G418. All the mutations tested facilitated the establishment of these cells in long-term culture at an efficiency indistinguishable from that of the wild-type protein.

A second domain of simian virus 40 T antigen in which mutations can alter the cellular localization of the antigen

Previous studies have demonstrated that mutations at amino acid position 128 of the simian virus 40 large T antigen can alter the subcellular localization of the antigen. A second domain in which mutations can alter localization of the nuclear antigen has been identified by mutations at amino acid positions 185, 186, and 199. Mutations in this region cause the polypeptide to accumulate in both the nucleus and cytoplasm of monkey cells. These T-antigen variants accumulate to near normal levels, but they don't bind to the simian virus 40 origin of DNA replication and are unable to mediate DNA replication. Furthermore, the altered tumor antigens can no longer transform secondary rat cells at normal efficiency, but they retain the ability to transform established mouse and rat cell lines.

Immortalization of rat embryo fibroblasts by mutant polyomavirus large T antigens deficient in DNA binding

We have identified a putative DNA-binding domain in polyomavirus large T antigen. Mutations introduced into the gene between amino acids 290 and 310 resulted in proteins that no longer bound to the high-affinity binding sites on the polyomavirus genome, showed no detectable nonspecific DNA binding, and were not able to initiate DNA replication from the viral origin. These mutant T antigen genes were introduced into rat embryo fibroblasts together with the neomycin resistance gene to allow selection for growth in the presence of G418. All the mutations tested facilitated the establishment of these cells in long-term culture at an efficiency indistinguishable from that of the wild-type protein.

Construction and characterization of hybrid polyomavirus genomes

Several studies have suggested that certain unique features of the JC virus (JCV) regulatory region are responsible for the restricted lytic and transforming activities of this virus in vitro. To pursue this possibility, we have constructed hybrid polyomavirus genomes by exchanging the regulatory sequences of JCV, BK virus (BKV), and simian virus 40 (SV40). The host range of JCV was not expanded by the substitution of the BKV or SV40 regulatory signals; such hybrids were nonviable even in primary human fetal glial cells, the sole permissive cell for JCV. However, chimeric DNAs containing JCV regulatory sequences and BKV- or SV40-coding sequences were lytically active, indicating that the BKV and SV40 T proteins were capable of effectively interacting with the JCV replication and transcription signals to yield infectious hybrid viruses. Although JCV regulatory sequences and coding sequences both contributed to the restricted lytic activity of this virus, it appears that the latter sequences, most likely hose encoding the T protein, have a greater influence on this behavior.

T-antigen-DNA polymerase alpha complex implicated in simian virus 40 DNA replication

We have combined in vitro DNA replication reactions and immunological techniques to analyze biochemical interactions between simian virus (SV40) large T antigen and components of the cellular replication apparatus. First, in vitro SV40 DNA replication was characterized with specific origin mutants. Next, monoclonal antibodies were used to demonstrate that a specific domain of T antigen formed a complex with cellular DNA polymerase alpha. Several antibodies were identified that coprecipitated T antigen and DNA polymerase alpha, while others were found to selectively prevent this interaction and concomitantly inhibit DNA replication. DNA polymerase alpha also bound efficiently to a T-antigen affinity column, confirming the immunoprecipitation results and providing a useful method for purification of the complete protein complex. Taken together, these results suggest that the T-antigen-polymerase association may be a key step in the initiation of SV40 DNA replication.

A second domain of simian virus 40 T antigen in which mutations can alter the cellular localization of the antigen

Previous studies have demonstrated that mutations at amino acid position 128 of the simian virus 40 large T antigen can alter the subcellular localization of the antigen. A second domain in which mutations can alter localization of the nuclear antigen has been identified by mutations at amino acid positions 185, 186, and 199. Mutations in this region cause the polypeptide to accumulate in both the nucleus and cytoplasm of monkey cells. These T-antigen variants accumulate to near normal levels, but they don't bind to the simian virus 40 origin of DNA replication and are unable to mediate DNA replication. Furthermore, the altered tumor antigens can no longer transform secondary rat cells at normal efficiency, but they retain the ability to transform established mouse and rat cell lines.

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.

DNA-binding region of the simian virus 40 tumor antigen

The simian virus 40 (SV40) tumor (T) antigen was purified by immunoaffinity chromatography and cleaved with small amounts of trypsin, and the resulting fragments were subjected to SV40 DNA cellulose chromatography. A 44,000-molecular-weight fragment (44K fragment) from the left end of the molecule and a 30K fragment mapping from approximately Lys 131 to Lys 371 bound to the column and were eluted with 1 M NaCl. In a second series of experiments, T antigen was immunoprecipitated with hamster anti-T serum or various monoclonal antibodies and partially digested with trypsin. Fragments that were solubilized by this treatment were tested for DNA-binding activity by using an SV40 DNA fragment-binding assay. A 17K fragment which originated from the amino-terminal region of the polypeptide had no apparent binding activity in this assay. On the other hand, larger fragments (76K, 46K, and 30K) whose amino termini were mapped around Lys 131 did display DNA-binding activity. Finally, complexes consisting of SV40 DNA and T-antigen fragments were precipitated in the DNA-binding assay with monoclonal antibodies that recognize the central region of the protein; however, antibodies with specificities to the amino- or carboxy-terminal regions were inactive. These results strongly suggest that the DNA-binding region of T antigen lies approximately between Lys 131 and Lys 371, corresponding to 0.51 and 0.37 map units on the DNA.

Expression of 100,000-Mr simian virus 40 (SV40) tumor antigen in mouse fibroblasts transfected with replication-defective SV40 genomes

Simian virus 40 early region mutants which are partially or completely replication defective were tested for their ability to transform postcrisis mouse fibroblasts. All mutants tested were capable of generating anchorage-independent transformants. We have previously reported the presence of a variant tumor antigen of 100,000 Mr (100K protein) generated upon transformation by wild-type simian virus 40 virions which correlates with anchorage-independent growth (Chen et al., Mol. Cell. Biol. 1:994-1006, 1981). In this study, none of the mutants tested produced the 100K variant protein at early (before the fifth) passage. Long-term passage (greater than 20 weeks) permitted the expression of this 100K variant in half of the transformants. Thus the phenotype of these mutants is different from both wild-type simian virus 40 (frequently production of 100K by the third passage, and always by the tenth passage) and the origin-minus class of mutants (no production of 100K at any passage).

T antigen and template requirements for SV40 DNA replication in vitro

A cell-free system for replication of SV40 DNA was used to assess the effect of mutations altering either the SV40 origin of DNA replication or the virus-encoded large tumor (T) antigen. Plasmid DNAs containing various portions of the SV40 genome that surround the origin of DNA replication support efficient DNA synthesis in vitro and in vivo. Deletion of DNA sequences adjacent to the binding sites for T antigen either reduce or prevent DNA synthesis. This analysis shows that sequences that had been previously defined by studies in vivo to constitute the minimal core origin sequences are also necessary for DNA synthesis in vitro. Five mutant T antigens containing amino acid substitutions that affect SV40 replication have been purified and their in vitro properties compared with the purified wild-type protein. One protein is completely defective in the ATPase activity of T antigen, but still binds to the origin sequences. Three altered proteins are defective in their ability to bind to origin DNA, but retain ATPase activity. Finally, one of the altered T antigens binds to origin sequences and contains ATPase activity and thus appears like wild-type for these functions. All five proteins fail to support SV40 DNA replication in vitro. Interestingly, in mixing experiments, all five proteins efficiently compete with the wild-type protein and reduce the amount of DNA replication. These data suggest that an additional function of T antigen other than origin binding or ATPase activity, is required for initiation of DNA replication.

trans Activation of the simian virus 40 late transcription unit by T-antigen

We have investigated the role of simian virus 40 (SV40) T-antigen in the induction of late gene expression independent of its function in amplifying templates through DNA replication. Northern blot and S1 nuclease analyses showed that stimulation occurred at the transcriptional level. At least two template elements, the T-antigen-binding sites and the 72-base-pair repeats, appeared to be important for this induction. Using template mutants, we demonstrated that deletions within T-antigen-binding site II decreased T-antigen-mediated late gene expression approximately 10- to 20-fold. In addition, multiple point mutations within a single retained copy of the SV40 72-base-pair repeat decreased T-antigen-mediated late gene expression. Using in vivo competition studies, we demonstrated that competitor DNA fragments containing the SV40 control region (nucleotides 5171 through 272) quantitatively decreased SV40 late gene expression in COS-1 cells. In contrast, competition with a plasmid containing SV40 nucleotides 1 through 294 (which removes all of T-antigen-binding site I and half of site II) was much less efficient. Finally, we demonstrated that in vivo competition experiments employing competitor fragments distal to the T-antigen-binding sites within the late template region (SV40 nucleotides 180 through 2533) resulted in superinduction of late gene expression in COS-1 cells. This finding suggests that negative factors such as repressors or attenuators may modulate late SV40 gene expression before induction. Our results are consistent with a model in which induction of late gene expression involves an interaction of the SV40 origin region with DNA-binding proteins, one of which may be T-antigen. Activation of the SV40 late transcription unit may involve induction of the SV40 enhancer or removal of a repressor-like protein or both.

Site-directed mutagenesis of the simian virus 40 large T-antigen gene: replication-defective amino acid substitution mutants that retain the ability to induce morphological transformation

We used a heteroduplex deletion loop mutagenesis procedure for directing sodium bisulfite-induced mutations to specific sites on viral or plasmid DNA to generate a series of SV40 large T-antigen point mutants. The mutations were directed to a region of the T-antigen gene, 0.5 map units, that is thought to be important for interaction of the protein with the viral origin of DNA replication. Of the 16 mutants reported here, 10 had lost the ability to replicate their DNA, and 3 others showed a reduced level of replication compared to wild type. All of the mutants tested were capable of transforming rat cells in culture by the dense focus assay. We conclude that the sequences of the early region around 0.5 map units are critical for the replication of viral DNA but not for the transformation function of T antigen.

trans Activation of the simian virus 40 late transcription unit by T-antigen

We have investigated the role of simian virus 40 (SV40) T-antigen in the induction of late gene expression independent of its function in amplifying templates through DNA replication. Northern blot and S1 nuclease analyses showed that stimulation occurred at the transcriptional level. At least two template elements, the T-antigen-binding sites and the 72-base-pair repeats, appeared to be important for this induction. Using template mutants, we demonstrated that deletions within T-antigen-binding site II decreased T-antigen-mediated late gene expression approximately 10- to 20-fold. In addition, multiple point mutations within a single retained copy of the SV40 72-base-pair repeat decreased T-antigen-mediated late gene expression. Using in vivo competition studies, we demonstrated that competitor DNA fragments containing the SV40 control region (nucleotides 5171 through 272) quantitatively decreased SV40 late gene expression in COS-1 cells. In contrast, competition with a plasmid containing SV40 nucleotides 1 through 294 (which removes all of T-antigen-binding site I and half of site II) was much less efficient. Finally, we demonstrated that in vivo competition experiments employing competitor fragments distal to the T-antigen-binding sites within the late template region (SV40 nucleotides 180 through 2533) resulted in superinduction of late gene expression in COS-1 cells. This finding suggests that negative factors such as repressors or attenuators may modulate late SV40 gene expression before induction. Our results are consistent with a model in which induction of late gene expression involves an interaction of the SV40 origin region with DNA-binding proteins, one of which may be T-antigen. Activation of the SV40 late transcription unit may involve induction of the SV40 enhancer or removal of a repressor-like protein or both.

DNA sequence of the leftward junction in the adenovirus-simian virus 40 hybrid Ad2+D2 and determination of the structure of the D2-T antigen

The nucleotide sequence of the junction between the simian virus 40 early region and the adenovirus type 2 late region L4 in the hybrid virus Ad2+D2 was determined. The deduced amino acid sequence suggests that the D2-T antigen is a chimeric protein sharing 594 amino acids with the C-terminal end of the simian virus 40 T antigen and 104 amino acids with the N terminus of the adenovirus type 2 33,000-molecular-weight protein. The predicted structure of the D2-T antigen was confirmed by an immunoprecipitation analysis.

Association of simian virus 40 T antigen with the nuclear matrix of infected and transformed monkey cells

The subnuclear distribution of simian virus 40 large T antigen within nuclei of transformed Cos and C6 monkey cells was examined. Cos cells express wild-type T antigen but lack viral sequences required for DNA replication, whereas C6 cells contain a functional viral origin but express a replication-defective mutant T antigen which is unable to bind specifically to viral DNA. Discrete subpopulations of T antigen were isolated from the soluble nucleoplasm, chromatin, and nuclear matrix of both cell lines. Although only a small quantity (2 to 12%) of the total nuclear T antigen from Cos cells was associated with the nuclear matrix, a high proportion (25 to 50%) of C6 T antigen was bound to this structure. Results obtained from lytically infected monkey cells showed that early in infection, before viral replication was initiated, a higher proportion (22%) of T antigen was found associated with the nuclear matrix compared with amounts found associated with this structure later in infection (5 to 8%). These results suggest that an increased association of T antigen with this structure is not correlated with viral replication. T antigen isolated from the C6 nuclear matrix was more highly phosphorylated than was soluble C6 T antigen and was capable of binding to the host p53 protein. C6 DNA contains three mutations: two corresponding to N-terminal changes at amino acid positions 30 and 51 and a third located internally at amino acid position 153. By analysis of the subnuclear distribution of T antigen from rat cells transformed by C6 submutant T antigens, it was determined that one or both of the mutations at the NH2 terminus are responsible for the increased quantity of C6 T antigen associated with the nuclear matrix. These results suggest that neither a functional viral DNA replication origin nor the origin binding property of T antigen is required for association of this protein with the nuclear matrix.

Replication of cloned DNA containing the Alu family sequence during cell extract-promoting simian virus 40 DNA synthesis

The replicating activity of several cloned DNAs containing putative origin sequences was examined in a cell-free extract that absolutely depends on simian virus 40 (SV40) T antigen promoting initiation of SV40 DNA replication in vitro. Of the three DNAs containing the human Alu family sequence (BLUR8), the origin of (Saccharomyces cerevisiae plasmid 2 micron DNA (pJD29), and the yeast autonomous replicating sequence (YRp7), only BLUR8 was active as a template. Replication in a reaction mixture with BLUR8 as a template was semiconservative and not primed by a putative RNA polymerase III transcript synthesized on the Alu family sequence in vitro. Pulse-chase experiments showed that the small-sized DNA produced in a short-term incubation was converted to full-length closed circular and open circular DNAs in alkaline sucrose gradients. DNA synthesis in extracts began in a region of the Alu family sequence and was inhibited 80% by the addition of anti-T serum. Furthermore, partially purified T antigen bound the Alu family sequence in BLUR8 by the DNA-binding immunoassay. These results suggest that SV40 T antigen recognizes the Alu family sequence, similar to the origin sequence of SV40 DNA, and initiates semiconservative DNA replication in vitro.

Simian virus 40 large T-antigen point mutants that are defective in viral DNA replication but competent in oncogenic transformation

The large T antigen of simian virus 40 (SV40) is a multifunctional protein that is essential in both the virus lytic cycle and the oncogenic transformation of cells by SV40. To investigate the role of the numerous biochemical and physiological activities of T antigen in the lytic and transformation processes, we have studied DNA replication-deficient, transformation-competent large T-antigen mutants. Here we describe the genetic and biochemical analyses of two such mutants, C2/SV40 and C11/SV40. The mutants were isolated by rescuing the integrated SV40 DNA from C2 and C11 cells (CV-1 cell lines transformed with UV-irradiated SV40). The mutant viral early regions were cloned into the plasmid vector pK1 to generate pC2 and pC11. The mutations that are responsible for the deficiency in viral DNA replication were localized by marker rescue. Subsequent DNA sequencing revealed point mutations that predict amino acid substitutions in the carboxyl third of the protein in both mutants. The pC2 mutation predicts the change of Lys----Arg at amino acid 516. pC11 has two mutations, one predicting a change of Pro----Ser at residue 522, and another predicting a Pro----Arg change at amino acid 549. The two C11 mutations were separated from each other to form two distinct viral genomes in pC11A and pC11B. pC2, pC11, pC11A, and pC11B are able to transform both primary and established rodent cell cultures. The C11 and C11A T antigens are defective in ATPase activity, suggesting that wild-type levels of ATPase activity are not necessary for the oncogenic transformation of cells by T antigen.

Replication of cloned DNA containing the Alu family sequence during cell extract-promoting simian virus 40 DNA synthesis

The replicating activity of several cloned DNAs containing putative origin sequences was examined in a cell-free extract that absolutely depends on simian virus 40 (SV40) T antigen promoting initiation of SV40 DNA replication in vitro. Of the three DNAs containing the human Alu family sequence (BLUR8), the origin of (Saccharomyces cerevisiae plasmid 2 micron DNA (pJD29), and the yeast autonomous replicating sequence (YRp7), only BLUR8 was active as a template. Replication in a reaction mixture with BLUR8 as a template was semiconservative and not primed by a putative RNA polymerase III transcript synthesized on the Alu family sequence in vitro. Pulse-chase experiments showed that the small-sized DNA produced in a short-term incubation was converted to full-length closed circular and open circular DNAs in alkaline sucrose gradients. DNA synthesis in extracts began in a region of the Alu family sequence and was inhibited 80% by the addition of anti-T serum. Furthermore, partially purified T antigen bound the Alu family sequence in BLUR8 by the DNA-binding immunoassay. These results suggest that SV40 T antigen recognizes the Alu family sequence, similar to the origin sequence of SV40 DNA, and initiates semiconservative DNA replication in vitro.

Association of simian virus 40 T antigen with the nuclear matrix of infected and transformed monkey cells

The subnuclear distribution of simian virus 40 large T antigen within nuclei of transformed Cos and C6 monkey cells was examined. Cos cells express wild-type T antigen but lack viral sequences required for DNA replication, whereas C6 cells contain a functional viral origin but express a replication-defective mutant T antigen which is unable to bind specifically to viral DNA. Discrete subpopulations of T antigen were isolated from the soluble nucleoplasm, chromatin, and nuclear matrix of both cell lines. Although only a small quantity (2 to 12%) of the total nuclear T antigen from Cos cells was associated with the nuclear matrix, a high proportion (25 to 50%) of C6 T antigen was bound to this structure. Results obtained from lytically infected monkey cells showed that early in infection, before viral replication was initiated, a higher proportion (22%) of T antigen was found associated with the nuclear matrix compared with amounts found associated with this structure later in infection (5 to 8%). These results suggest that an increased association of T antigen with this structure is not correlated with viral replication. T antigen isolated from the C6 nuclear matrix was more highly phosphorylated than was soluble C6 T antigen and was capable of binding to the host p53 protein. C6 DNA contains three mutations: two corresponding to N-terminal changes at amino acid positions 30 and 51 and a third located internally at amino acid position 153. By analysis of the subnuclear distribution of T antigen from rat cells transformed by C6 submutant T antigens, it was determined that one or both of the mutations at the NH2 terminus are responsible for the increased quantity of C6 T antigen associated with the nuclear matrix. These results suggest that neither a functional viral DNA replication origin nor the origin binding property of T antigen is required for association of this protein with the nuclear matrix.

Simian virus 40 large T-antigen point mutants that are defective in viral DNA replication but competent in oncogenic transformation

The large T antigen of simian virus 40 (SV40) is a multifunctional protein that is essential in both the virus lytic cycle and the oncogenic transformation of cells by SV40. To investigate the role of the numerous biochemical and physiological activities of T antigen in the lytic and transformation processes, we have studied DNA replication-deficient, transformation-competent large T-antigen mutants. Here we describe the genetic and biochemical analyses of two such mutants, C2/SV40 and C11/SV40. The mutants were isolated by rescuing the integrated SV40 DNA from C2 and C11 cells (CV-1 cell lines transformed with UV-irradiated SV40). The mutant viral early regions were cloned into the plasmid vector pK1 to generate pC2 and pC11. The mutations that are responsible for the deficiency in viral DNA replication were localized by marker rescue. Subsequent DNA sequencing revealed point mutations that predict amino acid substitutions in the carboxyl third of the protein in both mutants. The pC2 mutation predicts the change of Lys----Arg at amino acid 516. pC11 has two mutations, one predicting a change of Pro----Ser at residue 522, and another predicting a Pro----Arg change at amino acid 549. The two C11 mutations were separated from each other to form two distinct viral genomes in pC11A and pC11B. pC2, pC11, pC11A, and pC11B are able to transform both primary and established rodent cell cultures. The C11 and C11A T antigens are defective in ATPase activity, suggesting that wild-type levels of ATPase activity are not necessary for the oncogenic transformation of cells by T antigen.

Stimulation of simian virus 40 late gene expression by simian virus 40 tumor antigen

The early simian virus 40 (SV40) gene product, large tumor (T) antigen, is responsible for the initiation of viral DNA replication and the autoregulation of early gene expression through direct protein-DNA interactions. We investigated the role of T antigen in late viral gene expression, independent of its function in amplifying templates through DNA replication. SV40 DNA was transfected into BSC-1 and COS-1 cells and cultured in the presence of inhibitors of DNA replication. Electrophoretic immunoblot analysis indicated that both the onset and the extent of SV40 late gene expression is increased in COS-1 cells, which constitutively express SV40 T antigen. Blot hybridization analysis of poly(A)-selected RNA demonstrated that the level of synthesis of the major late structural protein VP-1 in COS-1 cells was due to increased transcription. Similar results were obtained when plasmids that contain the SV40 late gene but lack both the origin for viral DNA replication and the early gene coding region were transfected onto COS-1 cells. Using lines of SV40-transformed monkey kidney cells that express altered T antigens, we found that enhanced expression of the late gene product is correlated with the ability of T antigen to bind SV40 DNA. These results indicate that large T antigen plays a role in the stimulation of late viral gene expression.