Mapping of the viral mRNA encoding a super-T antigen of 115,000 daltons expressed in simian virus 40-transformed rat cell lines

New properties of simian virus 40 large T antigen

An 8,000-molecular-weight (8K) T antigen was found in all cells transformed by simian virus 40. The 8K T antigen was weakly labeled in vivo with [35S]methionine or 32Pi. A deletion in the human papovavirus BK genome, in the region coding for the carboxy-terminal end of the large T antigen, reduced the size of the 8K T antigen. The last 80 amino acids of the large T antigen include the sequence Asp-Asp-Asp-Asp unique to the activation peptide of trypsinogen. Large T antigen bound diisopropyl fluorophosphate and was retained by D-phenylalanine coupled to Sepharose beads, an affinity adsorbent that can retain chymotrypsin. The large T antigen and the recA protein of Escherichia coli, a known protease, have several properties in common as well as several similar sequences. Antibodies against large T antigen interacted with native recA protein.

New properties of simian virus 40 large T antigen

An 8,000-molecular-weight (8K) T antigen was found in all cells transformed by simian virus 40. The 8K T antigen was weakly labeled in vivo with [35S]methionine or 32Pi. A deletion in the human papovavirus BK genome, in the region coding for the carboxy-terminal end of the large T antigen, reduced the size of the 8K T antigen. The last 80 amino acids of the large T antigen include the sequence Asp-Asp-Asp-Asp unique to the activation peptide of trypsinogen. Large T antigen bound diisopropyl fluorophosphate and was retained by D-phenylalanine coupled to Sepharose beads, an affinity adsorbent that can retain chymotrypsin. The large T antigen and the recA protein of Escherichia coli, a known protease, have several properties in common as well as several similar sequences. Antibodies against large T antigen interacted with native recA protein.

Simian virus 40-transformed metallothionein null cells showed increased sensitivity to cadmium but not to zinc, copper, mercury or nickel

Primary cultured embryonic cells derived from mice with disrupted metallothionein (MT) I and II genes and from control mice were transformed with a plasmid encoding the simian virus 40 (SV40) large T antigen. The resulting MT-/- and MT+/+ cell strains showed similar cell morphology, cell cycle and no significant differences in glutathione levels or in the activities of glutathione-related enzymes and antioxidant enzymes. The MT-/- cells were more sensitive to Cd than MT+/+ cells, though no increase in the sensitivity to Zn, Cu, Hg or Ni were observed in MT-/- cells. MT+/+ cells accumulated more Cd than MT-/- cells but showed less lesion, suggesting the role of MT induced by Cd in MT+/+ cells as a scavenger of toxic Cd ion. These results suggest a dominant protective role of MT against Cd compared with other metals. SV40-transformed MT-/- cells seem to be a useful tool for the investigation of cellular function of MT.

New insights into the mechanism of inhibition of p53 by simian virus 40 large T antigen

Simian virus 40 (SV40) large tumor antigen (T antigen) has been shown to inhibit p53-dependent transcription by preventing p53 from binding to its cognate cis element. Data presented in this report provide the first direct functional evidence that T antigen, under certain conditions, may also repress p53-dependent transcription by a mechanism in which the transactivation domain of p53 is abrogated while DNA binding is unaffected. Specifically, p53 purified as a complex with T antigen from mouse cells was found to bind DNA as a transcriptionally inactive intact complex, while that purified from human cells was found to bind DNA independently of T antigen and could activate p53-dependent transcription. This difference in activity may be dependent on a different interaction of T antigen with mouse and human p53 and, in addition, on the presence of super T, which is found only in transformed rodent cells. These results suggest that subtle yet important differences exist between the inhibition of p53 by T antigen in mouse and human cells. The implications of this finding with respect to SV40-associated malignancies are discussed.

Spontaneous rearrangement of integrated simian virus 40 DNA in nine transformed rodent cell lines

Frequencies of spontaneous DNA rearrangement within or near integrated simian virus 40 (SV40) DNA were measured in four transformed mouse and rat cell lines of independent origin and in five clones of the SV40-transformed mouse line SVT2. Rearrangements were detected as polymorphisms of restriction enzyme fragment length in subclones of the lines. At least 17% of the subclones of each line had detectable rearrangements. The rate of rearrangement was calculated to be at least 5 x 10(-3) events per cell per division. No rearrangements were detected in sequences of an immunoglobulin gene, part of the coding region of the mouse protein p53, and five proto-oncogenes. The possible role of recombination between duplicated segments of integrated SV40 DNA in generating rearrangements was studied in the five SVT2 clones, which differed in the number of duplications within a single SV40 DNA segment. The SVT2 clone that had no duplications, M3, became rearranged further at least as frequently as did closely related lines with one, two, or three duplications. Another line in this group that had one small duplication, X1, had a much higher frequency of rearrangement than did the others; integrated SV40 DNA of X1 became mostly rearranged within 100 cell divisions. The examples of M3 and X1 suggested that the high rate of rearrangement characteristic of integrated SV40 DNA was influenced more by the presence of particular sequences within or near integrated SV40 DNA than by the number or extent of duplicated sequences.

Multiple insertions and tandem repeats of origin-minus simian virus 40 DNA in transformed rat and mouse cells

Stable simian virus 40 (SV40) transformation requires integration and expression of the early region of the SV40 genome. We have examined the amount and state of integrated viral DNA of SV40-transformed NIH 3T3 mouse and F2408 rat fibroblast lines generated by transfection with either wild-type or origin-defective SV40 DNA. A functional SV40 replication origin was not required for multiple inserts and partial-repeat structures to form in NIH 3T3 mouse transformants. In contrast, partial repeats in F2408 rat transformants were rare when the SV40 replication origin was intact and not detected at all when it was defective.

Fluctuation of simian virus 40 (SV40) super T-antigen expression in tumors induced by SV40-transformed mouse mammary epithelial cells

Higher-molecular-weight forms of the simian virus 40 (SV40) large tumor antigen (T-Ag), designated super T-Ag, are commonly found in SV40-transformed rodent cells. We examined the potential role of super T-Ag in neoplastic progression by using a series of clonal SV40-transformed mouse mammary epithelial cell lines. We confirmed an association between the presence of super T-Ag and cellular anchorage-independent growth in methylcellulose. However, tumorigenicity in nude mice did not correlate with the expression of super T-Ag. In the tumors that developed in nude mice, super T-Ag expression fluctuated almost randomly. Cell surface iodination showed that super T-Ag molecules were transported to the epithelial cell surface. The biological functions of super T-Ag remain obscure, but it is clear that it is not important for tumorigenicity by SV40-transformed mouse mammary epithelial cells. Super T-Ag may be most important as a marker of genomic rearrangements by the resident viral genes in transformed cells.

Origin binding by a 100,000-dalton super-T antigen from SVT2 cells

The SVT2 line of simian virus 40-transformed mouse cells expresses little or no wild-type-size A protein (T antigen). Instead, a variant form is produced in these cells that is larger than normal-size A protein. This variant form has an Mr of 100,000 (100K super-T antigen) and is found primarily in complexes with the host-cell-coded p53 protein. Binding of the 100K super-T antigen to simian virus 40 origin region DNA was assayed by immunoprecipitation of super-T antigen-DNA complexes and then digestion with DNase I. DNA sequences associated with super-T antigen were protected from digestion and retained in the immune complex, while unprotected sequences were digested and released. The 100K super-T antigen efficiently protects DNA sequences in the previously defined regions I and II (P. Tegtmeyer, B. A. Lewton, A. L. DeLucia, V. G. Wilson, and K. Ryder, J. Virol. 46:151-161, 1983). Within region II (the origin of replication), the pattern and size of protected fragments are identical for super-T antigen and purified wild-type A protein. Thus, even though super-T antigen is larger than wild-type A protein, both must bind with the same alignment on origin DNA. Furthermore, complexes between the host-cell-coded p53 protein and the 100K super-T antigen also retain the ability to bind in regions I and II.

Rearrangements of host and viral DNA in mouse cells transformed by simian virus 40

We have determined the structure of host DNA and viral DNA at the site of integration of Simian virus 40 (SV40) in a line of transformed Balb/c-3T3 cells (SVB400) isolated by single cell cloning after virus infection. Recombinant phage containing integrated viral DNA and flanking host DNA were purified from a genomic library and, in conjunction with restriction endonuclease cleavage analysis of the transformed cell DNA, were used to determine the organization of the integrated viral sequences. There is heterogeneity in the arrangement of the viral sequences resulting from tandem duplications of all or part of the SV40 genome with preservation of the viral-host junctions. The predominant arrangement is the result of tandem duplication of 41% of the SV40 genome from 0.64 to 0.23. Analysis of the structure of integrated viral DNA in SVB400 at different passage numbers and in single cell clones derived from the 20th passage indicated that rearrangements of viral DNA occur after the integration event and continue with passage of the cells. The organization of host sequences before and after the integration of SV40 was determined by restriction endonuclease cleavage analysis of parental 3T3 DNA and SVB400 DNA, and by analysis of recombinant phage isolated from genomic libraries. A deletion of at least 15 X 10(3) bases of host DNA occurred at the site of integration, which indicates that viral integration was not a result of a simple insertion of SV40. Nucleotide sequence analysis of the virus-host junctions showed that retained SV40 sequences were colinear with the viral genome, and that the junctions with SV40 DNA occurred at nucleotide numbers 1377 and 3610. There was no evidence of duplications of viral or host sequences at the junctions, and a comparison of the flanking mouse sequences with the deleted SV40 sequences revealed no significant homology at the point of joining of the two genomes.

Reacquisition of a functional early region by a mouse transformant containing only defective simian virus 40 DNA

Viral DNA in simian virus 40-transformed mouse cells is capable of rearranging with passage. In this report, we show that such rearrangement can include an alteration in viral protein expression. SVT2, a simian virus 40-transformed mouse BALB/c 3T3 cell line, synthesizes only a super T antigen of molecular weight 100,000 without synthesizing the lytic-size large T or small t antigens with molecular weights of 94,000 and 17,000, respectively. Analyses of the integrated viral DNA revealed an early region of 4.4 kilobases instead of the lytic-size 2.7 kilobases. However, upon subcloning in either plastic or agarose or after being in culture for several passages, the appearance of lytic-size large T and small t antigens was detected. Concurrently, an early region of 2.7 kilobases, in addition to one of 4.4 kilobases, was observed.

Reacquisition of a functional early region by a mouse transformant containing only defective simian virus 40 DNA

Viral DNA in simian virus 40-transformed mouse cells is capable of rearranging with passage. In this report, we show that such rearrangement can include an alteration in viral protein expression. SVT2, a simian virus 40-transformed mouse BALB/c 3T3 cell line, synthesizes only a super T antigen of molecular weight 100,000 without synthesizing the lytic-size large T or small t antigens with molecular weights of 94,000 and 17,000, respectively. Analyses of the integrated viral DNA revealed an early region of 4.4 kilobases instead of the lytic-size 2.7 kilobases. However, upon subcloning in either plastic or agarose or after being in culture for several passages, the appearance of lytic-size large T and small t antigens was detected. Concurrently, an early region of 2.7 kilobases, in addition to one of 4.4 kilobases, was observed.

Evidence of transcription from the late region of the integrated simian virus 40 genome in transformed cells: location of the 5' ends of late transcripts in cells abortively infected and in cells transformed by simian virus 40

By means of S1 mapping, we observed that spliced 16S and 19S viral late mRNAs--in addition to early mRNAs--were present in cytoplasmic polyadenylated RNA preparations from simian virus 40-transformed cell lines of rat or mouse origin containing no detectable amount of free viral DNA. The amounts of early and late virus-specific mRNAs in these lines were quantified by hybridization of radioactive cytoplasmic polyadenylated RNA with cloned region-specific restriction fragments. The relative amount of late viral mRNA produced in these transformed cells was found to be of the same order as that produced in simian virus 40-infected, nonpermissive baby mouse kidney cells. Moreover, by using the S1 nuclease protection method, we compared the 5' ends of late mRNAs produced (i) in transformed cells, (ii) in abortively infected mouse cells, and (iii) in the late phase of the lytic cycle. The 5' ends of late mRNAs both in abortively infected and in transformed cells were less heterogeneous than the 5' ends of late mRNAs produced during the lytic cycle; however, they were a subset of the 5' ends of late transcripts produced in the lytic cycle.

Specific DNA binding activity of T antigen subclasses varies among different SV40-transformed cell lines

Large tumor antigen (T antigen) occurs in at least three different oligomeric subclasses in cells infected or transformed by simian virus 40 (SV40): 5-7 S, 14-16 S, and 23-25 S. The 23-25 S form is complexed with a host phosphoprotein (p53). The DNA binding properties of these three subclasses of T antigen from nine different cell lines and free p53 protein were compared using an immunoprecipitation assay. All three subclasses of T antigen bound specifically to SV40 DNA sequences near the origin of replication. However, the DNA binding activity varied between different cell lines over a 40- to 50-fold range. The 23-25 S and 14-16 S forms from most of the cell lines tested bound much less SV40 origin DNA than 5-7 S T antigen. The free p53 phosphoprotein did not bind specifically to any SV40 DNA sequences.

Biochemical properties of the 145,000-dalton super-T antigen from simian virus 40-transformed BALB/c 3T3 clone 20 cells

SV3T3 C120 cells contain a 145,000-dalton form of simian virus 40 (SV40) super-T antigen but little if any normal-sized large-T. The subcellular location of super-T, its DNA binding properties, and its interaction with nonviral tumor antigen (NVT) were examined. Immunofluorescence microscopy and subcellular fractionation indicated that super-T is almost exclusively nuclear. Chromatography on double-stranded DNA-cellulose showed that super-T binds to double-stranded DNA and has an elution profile indistinguishable from normal-sized large-T. Super-T also binds specifically to a fragment of SV40 DNA which contains the origin of DNA replication. However, immunoprecipitation of super-T or large-T either with anti-tumor cell serum or with anti-NVT serum from fractions obtained by sucrose density centrifugation of 32P-labeled or [35S]methionine-labeled extracts revealed clear differences in the sedimentation characteristics of these proteins. The bulk of labeled 145,000-dalton super-T sedimented between 4S and 10S, whereas the bulk of 32P-labeled large-T from normal SV40-transformed cells sedimented as two peaks at 23S to 25S and 16S to 18S. By contrast, the sedimentation properties of NVT from the SV3T3 C120 cells were similar to those normally observed with other SV3T3 cell lines. The reason for this apparent difference in complex formation between super-T and NVT and that normally observed with large-T is unclear, but it probably has no deleterious effect on the ability of super-T to maintain transformation.

Study of the functional activities concomitantly retained by the 115,000 Mr super T antigen, an evolutionary variant of simian virus 40 large T antigen expressed in transformed rat cells

Simian virus 40 (SV40) transformed V 11 F 1 clone 1 subclone 7 rat cells (subclone 7) do not synthesize normal-size large T antigen (M(r), 90,000); instead, they produce a 115,000 M(r) super T antigen (115K super T antigen). This super T antigen is SV40 virus coded, and its synthesis results from rearrangement and amplification of integrated viral DNA sequences in subclone 7 (May et al., Nucleic Acids Res. 9:4111-4128, 1981). In this study the functional activities of 115K super T antigen were compared with the functional activities of SV40 large T antigen. Transfection experiments were performed with (i) cosmid SVE 5 Kb and plasmid pSVsT, both containing the super T antigen gene and (ii) plasmids pSV1 and pSV40, both containing the large T antigen gene. Transfection of pSVsT DNA or SVE 5 Kb DNA into secondary cultures of rat kidney cells induced the formation of transformed cell foci with an efficiency that was about 50% of the efficiency of pSV1 DNA or pSV40 DNA. Concomitant with the transforming activity, two other activities were also retained by super T antigen, namely, the ability to enhance the level of host cellular protein p53 and the capacity to bind to p53. In contrast, pSVsT and SVE 5 Kb DNAs were markedly deficient in the capacity to support tsA58 DNA replication in CV1-P cells at a nonpermissive temperature (41 degrees C), as shown by cotransfection experiments. The yield of virus produced in these experiments was 400-fold less than the yield obtained in parallel experiments with pSV40 or pSV1. However, SVE 5 Kb and pSVsT have a functional SV40 replication origin, as shown by their efficient replication in COS 1 cells which provided functional large T antigen. Super T antigen also possesses a specific affinity for sequences of SV40 viral origin. Our results suggest that under certain conditions, evolutionary changes in T antigen take place and that these changes could be restricted to the phenotypic requirement of maintaining a structure that is able to induce cell transformation, to form a complex with p53, and to enhance the cellular level of p53. Therefore, there appears to be a close relationship among the activities of T antigen involved in transforming cells, in binding to p53, and in enhancing the p53 cellular level. Moreover, this set of activities appears to be separable from the replicative ability of T antigen, based on the observation that 115K super T antigen is markedly defective for initiating viral DNA synthesis.

Functional analysis of a simian virus 40 super T-antigen

The SV3T3 C120 line of simian virus 40-transformed mouse cells synthesizes no large T-antigen of molecular weight 94,000 but instead a super T-antigen of molecular weight 145,000. In the accompanying paper (Lovett et al., J. Virol. 44:963-973, 1982), we showed that the integrated viral DNA segment SV3T3-20-K contains a perfect, in-phase, tandem duplication of 1.212 kilobases within the large T-antigen coding sequences. Our data suggested that this integrated template encodes mRNAs of 3.9 and 3.6 kilobases, the smaller of which directs the synthesis of the super T-antigen of molecular weight 145,000. We transfected the DNA segment SV3T3-20-K into nonpermissive rat cells and into TK- mouse L cells and analyzed the T-antigens and viral mRNAs in the transfectants; these data prove directly the coding assignments suggested previously. The super T-antigen retained the ability to induce morphological transformation, and may even transform better than the wild-type protein. It also retained the ability to bind to the cell-coded p53 protein. Transfection into permissive CV-1 cells showed that the super T-antigen encoded by SV3T3-20-K was incapable of initiating DNA replication at the viral origin. The duplication in SV3T3-20-K thus defines a mutation which separates the transformation and DNA replication functions of large T-antigen. We discuss why such mutations may be selected in transformed cells.

Structure and synthesis of a simian virus 40 super T-antigen

Mouse cells transformed by simian virus 40 often contain virus-coded tumor antigens distinct from those synthesized in productively infected permissive cells. The SV3T3 C120 cell line produces no large T-antigen of apparent molecular weight 94,000 but instead a super T-antigen of apparent molecular weight 145,000. We used recombinant DNA techniques to isolate the template for this super T-antigen and determined its structure by DNA sequencing. The integrated viral early transcription unit contains an in-phase, perfect tandem duplication of 1,212 base pairs. Transfer hybridization and endonuclease S1 mapping experiments were performed to elucidate the structures of the stable, cytoplasmic mRNAs of SV3T3 C120 cells, mRNAs of 3.9 and 3.6 kilobases, containing the small t- and large T-antigen splices, respectively, were transcribed from the internally duplicated early transcription unit. We showed by in vitro translation that these mRNAs encode small t-antigen and the super T-antigen of molecular weight 145,000. Peptide mapping studies of the SV3T3 C120 super T-antigen were consistent with its being derived from an internally duplicated template, since the protein has methionine and cysteine tryptic fingerprints virtually identical to those of normal large T-antigen, with certain methionine peptides present in greater than one molar yield.

Nonlytic simian virus 40-specific 100K phosphoprotein is associated with anchorage-independent growth in simian virus 40-transformed and revertant mouse cell lines

Normal fibroblasts display two distinct growth controls which can be assayed as requirements for serum or for anchorage. Interaction of mouse 3T3 fibroblasts with simian virus 40 (SV40) thus generates four classes of transformed cells. We have examined viral gene expression in these four classes of cell lines. Immunoprecipitation of [35S]methionine-labeled cell extracts with an antiserum obtained from tumor-bearing hamsters detected the SV40 large T and small t proteins (94,000 molecular weight [94K], 17K) and the nonviral host 54K protein in all cell lines tested. A tumor antigen with an apparent molecular weight of 100,000 was also found in some, but not all, lines. Similar "super T" molecules have been found by others in many rodent transformed lines. We carried out an analysis of the relation of phenotype to relative amounts of these proteins in cell lines of the four classes, using the Spearman rank correlation test. The amount of the 100K T antigen relative to the 94K T antigen or to total viral protein was well correlated with the ability to form colonies in semisolid medium. No significant correlation was found between quantities of labeled 94K T antigen, 54K host antigen, or 17K t antigen and either serum or anchorage independence. Mouse cells transformed with the small t SV40 deletion mutant 884 synthesized a 100K T antigen, suggesting that small t is not required for the production of this protein. The 100K T antigen migrated more slowly than lytic T. Since mixtures of extracts from cells expressing and lacking the 100K T antigen yielded the expected amount of this protein, it is unlikely that the 100K T derives from the 94K protein by a posttranslational modification.

Structure and biochemical functions of four simian virus 40 truncated large-T antigens

The structure of four abnormal T antigens which are present in different simian virus 40 (SV40)-transformed mouse cell lines was studied by tryptic peptide mapping, partial proteolysis fingerprinting, immunoprecipitation with monoclonal antibodies, and in vitro translation. The results obtained allowed us to deduce that these proteins, which have apparent molecular weights of 15,000, 22,000, 33,000 and 45,000, are truncated forms of large-T antigen extending to different amounts into the amino acid sequences unique to large-T. The proteins are all phosphorylated, probably at a site between amino acids 106 and 123. The mRNAs coding for the proteins probably contain the normal large-T splice but are shorter than the normal transcripts of the SV40 early region. The truncated large-Ts were tested for the ability to bind to double-stranded DNA-cellulose. This showed that the 33,000- and 45,000-molecular-weight polypeptides contained sequences sufficient for binding under the conditions used, whereas the 15,000- and 22,000-molecular-weight forms did not. Together with published data, this allows the tentative mapping of a region of SV40 large-T between amino acids 109 and 272 that is necessary and may be sufficient for the binding to double-stranded DNA-cellulose in vitro. None of the truncated large-T species formed a stable complex with the host cell protein referred to as nonviral T-antigen or p53, suggesting that the carboxy-terminal sequences of large-T are necessary for complex formation.

Phosphorylation pattern of large T antigens in mouse cells infected by simian virus 40 wild type or deletion mutants

The phosphorylation sites of simian virus 40 (SV40) large tumor (T) antigens have been extensively studied in productive infection of monkey cells. In this study, we analyzed the phosphorylation sites of large T antigen from SV40-infected nonpermissive mouse cells by partial proteolysis fingerprints and analysis of the phosphoamino acids present in the resulting fragments. The wild-type virus and deletion mutants (dl1263, dl1265, dl2194, and dl2198) were used for infection. On the basis of our results and published data (M. Schwyzer, R. Weil, and H. Zuber, J. Biol. Chem. 225:5627-5634, 1980), a cleavage map of large T antigen was established. It was reported that at least four sites of phosphorylation were present. The amino-terminal part of the molecule contained both phosphoserine and phosphothreonine. One phosphothreonine residue was located in the prolinerich C-terminal end of the molecule at position 701 or 708. On the basis of the concensus as to the amino acid sequence surrounding the recognition sites for protein kinases, it was possible to more precisely locate this phosphothreonine at residue 701. Moreover, the C-terminal part of the molecule contained phosphoserine at a more internal position. In addition, this study firmly established the presence of a phosphothreonine in the N-terminal part of large T antigen. In conclusion, it was shown that the location of phosphorylation sites of large T antigen produced by nonpermissive mouse cells infected by SV40 is strikingly similar to that reported by other groups for large T antigen produced by SV40-infected permissive cells.

Transforming genes and gene products of polyoma and SV40

The small DNA-containing viruses, SV40 and polyoma, transform cells in vitro and induce tumors in vivo. For both viruses two genes required for transformation have been found. The genes required for transformation are also involved in productive infection. Although the two viruses are similar in their effects on cells, the organization of the transforming genes and gene products is different. The purpose of this review is to compare what is known about the biology and the biochemistry of the early regions of the two viruses. The genetic and biochemical studies defining the sequences important for transformation will be reviewed. Then, the products of the transforming genes, called T antigens, will be discussed in detail. There is a substantial body of descriptive information on those products, and studies on the function of the T antigens have also begun.

Nonlytic simian virus 40-specific 100K phosphoprotein is associated with anchorage-independent growth in simian virus 40-transformed and revertant mouse cell lines

Normal fibroblasts display two distinct growth controls which can be assayed as requirements for serum or for anchorage. Interaction of mouse 3T3 fibroblasts with simian virus 40 (SV40) thus generates four classes of transformed cells. We have examined viral gene expression in these four classes of cell lines. Immunoprecipitation of [35S]methionine-labeled cell extracts with an antiserum obtained from tumor-bearing hamsters detected the SV40 large T and small t proteins (94,000 molecular weight [94K], 17K) and the nonviral host 54K protein in all cell lines tested. A tumor antigen with an apparent molecular weight of 100,000 was also found in some, but not all, lines. Similar "super T" molecules have been found by others in many rodent transformed lines. We carried out an analysis of the relation of phenotype to relative amounts of these proteins in cell lines of the four classes, using the Spearman rank correlation test. The amount of the 100K T antigen relative to the 94K T antigen or to total viral protein was well correlated with the ability to form colonies in semisolid medium. No significant correlation was found between quantities of labeled 94K T antigen, 54K host antigen, or 17K t antigen and either serum or anchorage independence. Mouse cells transformed with the small t SV40 deletion mutant 884 synthesized a 100K T antigen, suggesting that small t is not required for the production of this protein. The 100K T antigen migrated more slowly than lytic T. Since mixtures of extracts from cells expressing and lacking the 100K T antigen yielded the expected amount of this protein, it is unlikely that the 100K T derives from the 94K protein by a posttranslational modification.

Phosphorylation patterns of tumour antigens in cells lytically infected or transformed by simian virus 40

The phosphorylation sites of simian virus 40 (SV40) large tumor (T) antigens have been analyzed by partial proteolysis peptide mapping and phosphoamino acid analysis of the resulting products. At least four sites were found to be phosphorylated. An amino-terminal part of the molecule contained both phosphoserine and phosphothreonine. One phosphothreonine residue was located in the proline-rich carboxy-terminal end of the molecule, either at position 701 or at position 708. The mutant dl 1265, which is defective in adenovirus helper function, lacked this phosphorylation site. In addition, the carboxy-terminal part of the molecule contained phosphoserine at a more central position. T-antigen-associated proteins of SV40-transformed cell (nonviral T; 51,000 to 55,000 daltons) also contained multiple phosphorylation sites involving at least two serine residues in mouse antigens and an additional threonine residue in rat, human, and monkey antigens. The latter residue and at least one phosphoserine residue were located near one terminus of the human NVT molecule. We did not find any evidence for phosphorylation of tyrosine residues in any of the multiple species of either large T or nonviral T molecules. Several forms of large T antigens were extracted from both SV40-transformed and SV40-infected permissive and nonpermissive cells, and their phosphorylation patterns were compared. No evidence was found for a different phosphorylation pattern of T antigen in transformed cells.

Characterization of a gene encoding a 115 K super T antigen expressed by a SV40-transformed rat cell line

It has been reported that SV40-transformed V 11 F 1 clone 1 subclone 7 rat cells (subclone 7) produce a super T antigen of 115,000 M. This super T antigen is entirely SV40 coded and is synthesized by translation of an elongated form of SV40 early mRNA (May, E., Kress, M. Daya-Grosjean, L., Monier, R. and May, P. (1981) J. Virol., 37, 24-35). The results reported here show that there is only one independent insertion of viral DNA in the cellular genome of subclone 7 cells. When DNA from subclone 7 cells was cleaved with Bam HI endonuclease two distinct SV40 sequence containing fragments were generated with sizes of 5 Kb and 10 Kb, respectively. Two recombinant cosmids were constructed by insertion of the 5 Kb and 10 Kb fragments, respectively, into cosmid pHC 79. Using restriction map analysis and nucleotide sequencing, we showed that the 5 Kb fragment actually contained the complete sequence of a gene encoding super T antigen. As compared to the normal SV40 early gene, the sequence of super T gene showed the following rearrangements: (i) The segment between nucleotides 4116 - 3544 was duplicated in a direct order and (ii) these two copies of 573 nucleotide sequence were separated by a 93 nucleotide tract which was a nearly perfect inverted repeat of the segment located between nucleotides 4868 and 4776 (nucleotide numbering used here = Weissmann number +17).