Monoclonal antibodies against simian virus 40 tumor antigens: analysis of antigenic binding sites, using adenovirus type 2-simian virus 40 hybrid viruses

Regulation of the U3-, U8-, and U13snoRNA Expression by the DEAD Box Proteins Ddx5/Ddx17 with Consequences for Cell Proliferation and Survival

Small nucleolar RNAs (snoRNAs) in cooperation with their associated proteins (snoRNPs) contribute to the maturation of ribosomal RNA, transfer RNA, and other transcripts. Most snoRNPs mediate chemical base modifications of their RNA substrates, and a few others, like those formed by the C/D snoRNAs U3, U8, and U13, are needed for the structural organization and maturation of primary transcripts. The U3-, U8-, and U13snoRNAs are encoded by autonomous genes, and our knowledge about their expression regulation is limited. In this study, a significant increase in the concentrations of U3-, U8-, and U13snoRNA after a knockdown of DEAD box proteins Ddx5/Ddx17 in HeLa cells is observed. These alterations are shown to be caused by transcriptional suppression mediated by Ddx5/Ddx17 via histone deacetylase 1 in a promoter-dependent way. The biological function of this expression control may be related to the role of Ddx5/Ddx17 in cell proliferation. The U3snoRNA is shown here to be essential for the proliferation and viability of human cells. Moreover, it was found that U3snoRNA interacts with Argonaute 2 in the RNA-induced silencing complexes (RISC), pointing to a microRNA-like function. For this reason, the 3′ untranslated region of the A-kinase anchor protein 9 (AKAP9)-mRNA could be identified as a potential target.

SV40 utilizes ATM kinase activity to prevent non-homologous end joining of broken viral DNA replication products

Simian virus 40 (SV40) and cellular DNA replication rely on host ATM and ATR DNA damage signaling kinases to facilitate DNA repair and elicit cell cycle arrest following DNA damage. During SV40 DNA replication, ATM kinase activity prevents concatemerization of the viral genome whereas ATR activity prevents accumulation of aberrant genomes resulting from breakage of a moving replication fork as it converges with a stalled fork. However, the repair pathways that ATM and ATR orchestrate to prevent these aberrant SV40 DNA replication products are unclear. Using two-dimensional gel electrophoresis and Southern blotting, we show that ATR kinase activity, but not DNA-PK_cs kinase activity, facilitates some aspects of double strand break (DSB) repair when ATM is inhibited during SV40 infection. To clarify which repair factors associate with viral DNA replication centers, we examined the localization of DSB repair proteins in response to SV40 infection. Under normal conditions, viral replication centers exclusively associate with homology-directed repair (HDR) and do not colocalize with non-homologous end joining (NHEJ) factors. Following ATM inhibition, but not ATR inhibition, activated DNA-PK_cs and KU70/80 accumulate at the viral replication centers while CtIP and BLM, proteins that initiate 5′ to 3′ end resection during HDR, become undetectable. Similar to what has been observed during cellular DSB repair in S phase, these data suggest that ATM kinase influences DSB repair pathway choice by preventing the recruitment of NHEJ factors to replicating viral DNA. These data may explain how ATM prevents concatemerization of the viral genome and promotes viral propagation. We suggest that inhibitors of DNA damage signaling and DNA repair could be used during infection to disrupt productive viral DNA replication.

Viruses from both Polyomaviridae and Papillomaviridae families share several characteristics. These include common modes of DNA replication and an accumulation of DNA damage signaling and repair proteins at replicating viral DNA. Several DNA repair proteins, with unknown functions during viral DNA replication, associate with the viral replication centers of the polyomavirus simian virus 40 (SV40). In this study we examined the mechanisms that regulate and recruit DNA repair machinery to replicating viral DNA during permissive SV40 infection. We found that the virus deploys DNA repair to broken viral DNA using cellular DNA damage signaling pathways. Our results shed light on why both Polyomaviridae and Papillomaviridae DNA replication elicits DNA damage signaling and repair. As no effective treatments currently exist for the Polyomaviridae family, our data identify pathways that might be therapeutically targeted to inhibit productive viral replication. Additionally, we categorize distinct functions for DNA repair and damage signaling pathways during viral replication. The results provide insights into how viruses exploit cellular processes to overwhelm the cell and propagate.

The RNA helicase Ddx5/p68 binds to hUpf3 and enhances NMD of Ddx17/p72 and Smg5 mRNA

Non-sense-mediated mRNA decay (NMD) is a mechanism of translation-dependent mRNA surveillance in eukaryotes: it degrades mRNAs with premature termination codons (PTCs) and contributes to cellular homeostasis by downregulating a number of physiologically important mRNAs. In the NMD pathway, Upf proteins, a set of conserved factors of which Upf1 is the central regulator, recruit decay enzymes to promote RNA cleavage. In mammals, the degradation of PTC-containing mRNAs is triggered by the exon–junction complex (EJC) through binding of its constituents Upf2 and Upf3 to Upf1. The complex formed eventually induces translational repression and recruitment of decay enzymes. Mechanisms by which physiological mRNAs are targeted by the NMD machinery in the absence of an EJC have been described but still are discussed controversially. Here, we report that the DEAD box proteins Ddx5/p68 and its paralog Ddx17/p72 also bind the Upf complex by physical interaction with Upf3, thereby interfering with the binding of EJC. By activating the NMD machinery, Ddx5 is shown to regulate the expression of its own, Ddx17 and Smg5 mRNAs. For NMD triggering, the adenosine triphosphate-binding activity of Ddx5 and the 3′-untranslated region of substrate mRNAs are essential.

SV40 DNA replication: from the A gene to a nanomachine

Duplication of the simian virus 40 (SV40) genome is the best understood eukaryotic DNA replication process to date. Like most prokaryotic genomes, the SV40 genome is a circular duplex DNA organized in a single replicon. This small viral genome, its association with host histones in nucleosomes, and its dependence on the host cell milieu for replication factors and precursors led to its adoption as a simple and powerful model. The steps in replication, the viral initiator, the host proteins, and their mechanisms of action were initially defined using a cell-free SV40 replication reaction. Although our understanding of the vastly more complex host replication fork is advancing, no eukaryotic replisome has yet been reconstituted and the SV40 paradigm remains a point of reference. This article reviews some of the milestones in the development of this paradigm and speculates on its potential utility to address unsolved questions in eukaryotic genome maintenance.

Alternative use of a mini exon of the L1 gene affects L1 binding to neural ligands

Neural cell adhesion molecule L1 is a cell surface glycoprotein required for the correct development of the nervous system. L1 exists as two isoforms encoded by mRNA species that either collectively incorporate or exclude exons 2 and 27. Neurons utilize only the full-length isoform, whereas Schwann cells, kidney cells, and blood lymphocytes only express the short form of L1. Still other cells, oligodendrocytes, regulate L1 isoform expression in a maturation-dependent manner. The RSLE motif encoded by exon 27 is known to have a role in clathrin-mediated endocytosis of L1, but the function of the exon 2-encoded motif (YEGHHV) is unknown. Here we show that this motif is required for the optimal binding of L1 to several neural ligands and is likely to be important for nervous system development. Thus, alternative use of exon 2 is a mechanism for regulating ligand interactions with L1.

Packaging cells based on inducible gene amplification for the production of adeno-associated virus vectors

Although vectors based on adeno-associated virus (AAV) offer several unique advantages, their usage has been hampered by the difficulties encountered in vector production. In this report, we describe a new AAV packaging system based on inducible amplification of integrated helper and vector constructs containing the simian virus 40 (SV40) replication origin. The packaging and producer cell lines developed express SV40 T antigen under the control of the reverse tetracycline transactivator system, which allows inducible amplification of chromosomal loci linked to the SV40 origin. Culturing these cells in the presence of doxycycline followed by adenovirus infection resulted in helper and vector gene amplification as well as higher vector titers. Clonal producer cell lines generated vector titers that were 10 times higher than those obtained by standard methods, with approximately 10(4) vector particles produced per cell. These stocks were free of detectable replication-competent virus. The lack of a transfection step combined with the reproducibility of stable producer lines makes this packaging method ideally suited for the large-scale production of vector stocks for human gene therapy.

Development of a syngeneic bovine fibroblast cell line: implications for the study of bovine cytotoxic T lymphocytes

The study of T-cell-mediated cytotoxicity in domestic animals, especially in cattle, has been hampered by the lack of proper restimulatory as well as target systems. While the currently available bovine cell lines have not been typed for the major histocompatibility complex (MHC) class I molecules they express, methods to derive lines of cells obtained from animals that are MHC-typed have not been thoroughly explored. In the present study, we describe a method for the development of cell lines from MHC-typed animals. Cells obtained from the skin of a calf typed as bovine lymphocyte antigen-A11/-A13 were transfected with a plasmid containing the whole genome of simian vacuolating virus 40 (SV40). A cell line was derived from the resultant transfectants. This cell line expressed bovine MHC class I molecules on the cell surface, and SV40 large T antigen in the nucleus. The cells were permissive to the replicative cycle of bovine herpesvirus-1 (BHV-1), and the major glycoproteins of BHV-1 were expressed at expected times after infection. The present study should contribute to the study of cytotoxic T lymphocyte response of cattle to BHV-1 and other intracellular pathogens.

Simian virus 40 DNA replication correlates with expression of a particular subclass of T antigen in a human glial cell line

Immunocytochemistry and in situ hybridization were used to identify simian virus 40 (SV40) large T-antigen expression and viral DNA replication in individual cells of infected semipermissive human cell lines. SV40 infection aborts before T-antigen expression in many cells of each of the human cell lines examined. In all but one of the human cell lines, most of the T-antigen-producing cells replicated viral DNA. However, in the A172 line of human glial cells only a small percentage of the T-antigen-expressing cells replicated viral DNA. Since different structural and functional classes of T antigen can be recognized with anti-T monoclonal antibodies, we examined infected A172 cells with a panel of 10 anti-T monoclonal antibodies to determine whether viral DNA replication might correlate with the expression of a particular epitope of T antigen. One anti-T monoclonal antibody, PAb 100, did specifically recognize that subset of A172 cells which replicated SV40 DNA. The percentage of PAb 100-reactive A172 cells was dramatically increased by the DNA synthesis inhibitors hydroxyurea and aphidicolin. Removal of the hydroxyurea was followed by an increase in the percentage of cells replicating viral DNA corresponding to the increased percentage reactive with PAb 100. The pattern of SV40 infection in A172 cells was not altered by infection with viable viral mutants containing lesions in the small t protein, the agnoprotein, or the enhancer region. Finally, in situ hybridization was used to show that the percentage of human cells expressing T antigen was similar to the percentage transcribing early SV40 mRNA. Thus, the block to T-antigen expression in human cells is at a stage prior to transcription of early SV40 mRNA.

Effects of the cellular p53 protein on Simian-virus-40-T-antigen-catalyzed DNA unwinding in vitro

It is known that large T antigen, the regulatory protein encoded by Simian virus 40 (SV40), forms tight complexes with the cellular p53 protein in SV40-transformed rodent cells. Using immunoaffinity procedures we have purified large T antigen and, in separate experiments, the cellular p53 protein. The two proteins formed complexes in vitro which bound well to double-stranded DNA fragments although in a sequence-unspecific manner. Free, uncomplexed T antigen readily converted double-stranded DNA into a single-stranded form whereas in-vitro-formed p53-T-antigen complexes were inactive in this reaction. We conclude that one function of p53 in SV40-transformed mouse cells could be the inhibition of the replication initiating activity of T antigen.

Analysis of a large-T-antigen variant expressed in simian virus 40-transformed mouse cell line mKS-A

Earlier reports had suggested that the large T antigen expressed in simian virus 40 (SV40)-transformed mKS-A cells may be replication defective. Our experiments support these earlier observations showing that the mKS-A T antigen has a reduced DNA-unwinding activity in vitro. To investigate the molecular basis for this defect, we have isolated from an mKS-A genomic library an EMBL-3 bacteriophage clone carrying in its insert a full-length SV40 DNA element that most likely encodes the expressed T-antigen variant. DNA sequencing revealed only one nonconservative amino acid exchange, Asp to Asn at residue 636. Surprisingly, when a plasmid clone carrying the mKS-A T-antigen-coding sequence was transfected into monkey cells, we found that it replicated quite efficiently, probably suggesting that a high nuclear concentration of the variant T-antigen form compensates for the partial biochemical defect. However, a high nuclear concentration of T antigen was also found in mKS-A T-antigen-transformed mouse cells, yet a fusion of these cells to permissive monkey cells failed to induce in situ replication and excision of integrated SV40 DNA. We discuss possible reasons for the different behavior of T antigen in monkey cells and in mouse cells and suggest that one possibility for the replication-negative phenotype in transformed cells may be related to the fact that T antigen forms a tight complex with the cellular p53 protein in mouse cells but not in monkey cells.

Helicase, DNA-binding, and immunological properties of replication-defective simian virus 40 mutant T antigens

Simian virus 40 T antigen (TAg) exhibits nonspecific and origin-specific DNA binding (ori binding) and ATPase and helicase activities, all of which are related to its roles in viral DNA replication. We have characterized some of the properties of four replication-defective but transformation-competent mutant TAgs, C6-2, T22, C11, and C8A. C6-2 and T22 TAgs were each previously determined to lack ori-binding properties, while C11 TAg was reported to lack ATPase activity. The C8A TAg did not exhibit defects in either ori-binding or ATPase functions. We have analyzed additional aspects of these mutant TAgs pertaining to their helicase, DNA-binding, and immunological properties. With the exception of the C11 TAg, all the other TAgs exhibited helicase activity. The lack of helicase activity by C11 TAg was consistent with its previously shown inability to hydrolyze ATP or to replicate viral DNA. These results therefore show that ori-binding and helicase activities are separate functions of TAg. Wild-type and mutant TAgs bound with similar efficiency to either native or denatured calf thymus DNA-cellulose, indicating no marked differences in their nonspecific DNA-binding properties. We also tested the binding of wild-type and mutant TAgs to a monoclonal antibody, PAb 100, that was previously shown to recognize an extremely small class of TAg that may represent a unique conformational form of the protein. Interestingly, while less than 10% of the wild-type, C6-2, C11, and T22 mutant TAgs were recognized by PAb 100, more than 60% of the C8A mutant TAg was bound by this antibody. Therefore, although no defect in biochemical function was observed with the C8A TAg, its deficiency in viral DNA replication may be related to an unusual conformation, as detected by its dramatically increased recognition by PAb 100. These results show that the helicase activity of TAg is not required for its transformation function.

Characterization of simian virus 40 large T antigen by using different monoclonal antibodies: T-p53 complexes are preferentially ATPase active and adenylylated

We used 21 monoclonal antibodies (PAbs 100 to 117, 405, 419, and KT3) specific for different determinants in simian virus 40 (SV40) large T antigen (T) and one antibody specific for p53 that coprecipitates T complexed with p53 (T-p53) to analyze T in SV40-infected CV1 cells. We measured the ATPase specific activity, extent of adenylylation, and p53 content of T precipitated by antibodies directed against the N-terminal region I (0.65 to 0.62 map units), the midregion III (0.43 to 0.28 map units) containing both the ATPase- and nucleotide-binding sites, and the C-terminal region IV (0.28 to 0.17 map units) of T. Lytic T appeared to exist in three different forms with respect to p53 binding and ATPase activity. The most ATPase-active form of T was that precipitated by PAb 122. This T-p53 complex contained only 6% of the total T but contributed 35% of the ATPase activity, on average. Free p53 isolated from 3T6, Ann-1, or L929 cells had no apparent ATPase activity. A second form of T precipitated by several antibodies had little associated p53 but appreciable ATPase activity, accounting for 15 to 20% of total T and 60 to 70% of the ATPase activity. The rest of T constituted the third form and was also depleted in p53 but had a decreased ATPase specific activity. Thus, the remaining 75 to 80% of T had 15 to 20% of the ATPase specific activity. Antibodies specific for region III precipitated T with both altered ATPase activity and altered amounts of bound p53. PAbs 104 and 114 reacted with ATPase-active T but inhibited ADP hydrolysis, suggesting that they were inactivating antibodies. T that was preferentially adenylylated in vitro corresponded to T that was also preferentially ATPase active. T bound to p53 was adenylylated to a higher specific activity than total T. In addition, p53 itself was significantly adenylylated under these conditions. The results suggest that ATPase activity and p53 binding are structurally and functionally related and that p53 alters biochemical activities of T and plays a role in productive infection.

Application of an immunoprecipitation procedure to the study of SV40 tumor antigen interaction with mouse genomic DNA sequences

Simian Virus 40 (SV40) large T antigen is a DNA binding protein with high affinity for segments of the viral genome. To find out whether T antigen also binds to sequences of genomic cellular DNA we mixed T antigen and SAU 3 A restricted mouse DNA under stringent DNA binding conditions. Resulting protein-DNA complexes were immunoprecipitated using T antigen specific monoclonal or polyclonal antibodies. The DNA fragments in the immunoprecipitates were cloned in plasmid vectors. Four plasmid clones were selected for a detailed investigation of the inserted mouse DNA fragments. Nucleotide sequencing and DNase I footprint experiments showed that T antigen binds to sites in these fragments consisting of two tandemly oriented G(A)AGGC pentamers separated by AT rich spacers of different lengths. The cellular binding sites are very similar in their architecture to the SV40-DNA binding site I. The isolated cellular DNA fragments with T antigen binding sites occur only once or a few times in the mouse genome. Our data help to further define the structure of T antigen's DNA binding sites. The genetic functions of the isolated cellular DNA elements are not known.

DNA helicase activity of SV40 large tumor antigen

Large tumor antigen (T antigen) was extracted from SV40-infected African Green Monkey cells and purified to homogeneity by immunoaffinity chromatography. The purified T antigen preparations unwind DNA duplices of greater than 120 bp in a reaction which is dependent on magnesium ions and ATP hydrolysis. Based on these and other properties of the reaction we classify this newly discovered enzymatic activity as a eukaryotic DNA helicase. The helicase and the known ATPase function of T antigen cosediment with the mono- or dimeric 4-6 S form of T antigen, but not with higher T antigen aggregates. The helicase activity seems to be an intrinsic function of SV40 T antigen. First, several different T antigen-specific monoclonal antibodies interfere with the DNA unwinding activity; monoclonals which are known to reduce the T antigen-specific ATPase most strongly inhibited the helicase reaction. Second, mutant T antigens with impaired ATPase function also showed a reduced DNA unwinding activity.

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.

Similar regulation of the synthesis of adenovirus fiber and of simian virus 40-specific proteins encoded by the helper-defective Ad2+SV40 hybrid viruses Ad2+ND5 and Ad2+ND4del

Human adenoviruses fail to multiply effectively in monkey cells. The block to the replication of these viruses can be overcome by coinfection with simian virus 40 (SV40) or when part of the SV40 genome is integrated into and expressed as part of the adenovirus type 2 (Ad2) genome, as occurs in several Ad2+SV40 hybrid viruses, such as Ad2+ND1, Ad2+ND2, and Ad2+ND4. The SV40 helper-defective Ad2+SV40 hybrid viruses Ad2+ND5 and Ad2+ND4del were analyzed to determine why they are unable to grow efficiently in monkey cells even though they contain the appropriate SV40 genetic information. Characterization of the Ad2+ND5-SV40-specific 42,000-molecular-weight (42K) protein revealed that this protein is closely related, but not identical, to the SV40-specific 42K protein of the SV40 helper-competent Ad2+ND2 hybrid virus. Although the minor differences between these proteins may be sufficient to account for the poor growth of Ad2+ND5 in monkey cells, the most striking difference between helper-competent Ad2+ND2 and helper-defective Ad2+ND5 is in the production of the SV40-specific protein after infection of monkey cells. Whereas synthesis of the SV40-specific proteins of Ad2+ND2 is very similar in human and in monkey cells, production of the 42K protein of Ad2+ND5 is dramatically reduced in monkey cells compared with human cells. Similarly, the synthesis of the SV40-specific proteins of Ad2+ND4del is markedly reduced in monkey cells. Thus, it is likely that both Ad2+ND5 and Ad2+ND4del are helper defective because of a block in the production of their SV40-specific proteins rather than because their SV40-specific proteins are nonfunctional. This block, like the block to adenovirus fiber synthesis, is overcome by coinfection with SV40, with helper-competent hybrid viruses, or with host range mutants of adenoviruses. This suggests that the synthesis of fiber and the synthesis of SV40-specific proteins are similarly regulated in Ad2+SV40 hybrid viruses.

A large-tumor-antigen-specific monoclonal antibody inhibits DNA replication of simian virus 40 minichromosomes in an in vitro elongation system

In productively infected cells, a fraction of large-tumor antigen (T antigen) is tightly bound to replicating simian virus 40 (SV40) minichromosomes and does not dissociate at salt concentrations of greater than 1 M NaCl. We present electronmicrograms demonstrating the presence of T antigen on the replicated sections of replicating SV40 minichromosomes. We also show that the fraction of tightly bound T antigen is recognized by antibodies from mouse tumor serum and, more specifically, by a particular T-antigen-specific monoclonal antibody, PAb 1630. A second T-antigen-specific monoclonal antibody, PAb 101, does not react with the T-antigen fraction remaining on replicating SV40 chromatin at high salt concentrations. We used an in vitro replication system which allows, via semiconservative DNA replication, the completion of in vivo-initiated replicative intermediate DNA molecules. We show that monoclonal antibody PAb 1630, but not monoclonal antibody PAb 101, inhibits viral DNA replication. We discuss the possibility that SV40 T antigen may play a role in chain elongation during SV40 chromatin replication.

Plasma membrane orientation of simian virus 40 T antigen in three transformed cell lines mapped with monoclonal antibodies

Simian virus 40 large T antigen transforms cells from several species. Recent studies show that it is present on the cell surface. As in other tumor virus systems, this may be important for transformation. We have used a radioimmunoassay to map antigenic determinants on living and formaldehyde-fixed transformed cells with six different monoclonal antibodies to T antigen. Nonrelevant monoclonal antibodies of the same subclasses served as controls. With the transformed mouse line SVT2, antibody PAb 101, which reacts with the C-terminal region of T antigen, and PAb 1700, which is directed against an internal region of T, reacted with both formaldehyde-fixed and living cells. Antibodies PAb 402 (C terminus) and 419 (N terminus) reacted only with living cells, their determinants being destroyed upon formaldehyde fixation. Antibodies PAb 405 (C terminus) and 100 (internal) fail to react on either fixed or living cells. Similar results were obtained on the simian virus 40-transformed human line SV80 and the fixed hamster line CHLwt23, although all antibodies failed to react with living CHLwt23 cells. The data suggest that T antigen is inserted into the plasma membrane of transformed cells in a specific, nonrandom manner, with the C and N termini exposed on the cell surface and the midportion either buried in the lipid bilayer, hidden by the tertiary structure of T antigen, or masked by a post-translational modification such as fatty acid acylation.

Monoclonal antibodies specific for the carboxy terminus of simian virus 40 large T antigen

Three mouse hybridomas secreting antibodies against the undecapeptide Lys-Pro-Pro-Thr-Pro-Pro-Pro-Glu-Pro-Glu-Thr, corresponding to the carboxy terminus of simian virus 40 large T antigen, were isolated and cloned. A sensitive enzyme-linked immunosorbent assay was used to characterize the properties of the monoclonal antibodies. All three hybridomas, designated KT1, KT3, and KT4, produced antibodies that immunoprecipitated large T. The antibodies differed in their affinities for the peptide and for the native protein. Antibodies from KT3 precipitated large T better than those from KT1 or KT4. KT3 antibodies also had the highest affinity for the free peptide (5.2 X 10(6) M-1) as determined by radioimmunoassay; KT1 and KT4 antibodies had ca. 5- and 1,000-fold lower affinities, respectively. Inhibition studies with shorter peptides, overlapping the undecapeptide, revealed the approximate regions recognized by the different monoclonal antibodies. KT3 antibodies bound to a region within the carboxy-terminal six amino acids of large T. Antibodies from KT1 and KT4 reacted with sequences located further towards the amino terminus of the undecapeptide. Surprising results were obtained with KT4 antibodies. Their binding to the undecapeptide was completely inhibited by the undecapeptide itself or the carboxy-terminal hexapeptide. The carboxy-terminal pentamer, on the other hand, slightly enhanced binding, and the carboxy-terminal tetramer, Glu-Pro-Glu-Thr, was strongly stimulatory. A model for this effect is proposed. Using the enzyme-linked immunosorbent assay, we confirmed previous studies (W. Deppert and G. Walter, Virology 122:56-70, 1982) which found that antiserum against sodium dodecyl sulfate-denatured large T reacts strongly with the carboxy terminus of large T. By inhibition studies, we identified the approximate region within the undecapeptide recognized by anti-sodium dodecyl sulfate-denatured large T and compared this region with the region identified by antipeptide serum.

Antigenic structure of simian virus 40 large tumor antigen and association with cellular protein p53 on the surfaces of simian virus 40-infected and -transformed cells

The antigenic structure of simian virus 40 (SV40) large tumor antigen (T-ag) in the plasma membranes of SV40-transformed mouse cells and SV40-infected monkey cells was characterized as a step toward defining possible biological function(s). Wild-type SV40, as well as a deletion mutant of SV40 (dl1263) which codes for a truncated T-ag with an altered carboxy terminus, was used to infect permissive cells. Members of a series of monoclonal antibodies directed against antigenic determinants on either the amino or the carboxy terminus of the T-ag polypeptide were able to precipitate surface T-ag (as well as nuclear T-ag) from both SV40-transformed and SV40-infected cells. Cellular protein p53 was coprecipitated with T-ag by all T-ag-reactive reagents from the surface and nucleus of SV40-transformed cells. In contrast, T-ag, but not T-ag-p53 complex, was recovered from the surface of SV40-infected cells. These results confirm that nuclear T-ag and surface T-ag are highly related molecules and that a complex of SV40 T-ag and p53 is present at the surface of SV40-transformed cells. Detectable levels of such a complex do not appear to be present on SV40-infected cells. Both the carboxy and amino termini of T-ag are exposed on the surfaces of SV40-transformed and -infected cells. The possible relevance of the presence of a T-ag-p53 complex on the surface of SV40-transformed cells and its absence from SV40-infected cells is considered.

JC virus-induced owl monkey glioblastoma cells in culture: biological properties associated with the viral early gene product

JCV induces glioblastomas in owl monkeys 18-24 months or longer following intracranial inoculation (W. London, S. Houff, D. Madden, D. Fuccillo, M. Gravell, W. Wallen, A. Palmer, J. Sever, B. Padgett, D. Walker, G. Zu Rhein, and T. Ohashi, 1978, Science 201, 1246-1248). Cells from one brain tumor, owl monkey 26, were successfully established in culture and analyzed for phenotypic characteristics generally associated with persistence and expression of the papovavirus early region of its genome. Owl monkey 26 cells demonstrated nuclear JCV T protein detected by either SV40 hamster tumor sera or PAb 108, a monoclonal antibody to SV40 T protein. However, the JCV T protein was not detected in a complex with the host cell p53 protein as judged by immunoprecipitation using PAb 122, a monoclonal antibody directed to the mammalian p53 cellular protein. These cells also demonstrated increased plasminogen activator secretion and actin cable disorganization properties not before reported for JCV-induced tumor or transformed cells. Of the primate papovaviruses, JCV is unique in its ability to induce brain tumors in these primates. JCV early gene expression can be shown to persist in brain tumor cells once established in culture and correlates with cell phenotypes typical of papovavirus malignant transformation even though the time between virus inoculation and tumor development is usually several years.

Monoclonal antibodies against simian virus 40 nuclear large T tumour antigen: epitope mapping, papova virus cross-reaction and cell surface staining

Thirty six cloned hybridomas have been isolated which produce monoclonal antibodies directed against simian virus 40 (SV40) large T tumour antigen. They have been shown to recognize at least six different epitopes along the T antigen polypeptide according to their reaction with the various truncated forms of T antigen expressed by adenovirus-SV 40 hybrid viruses. Sixteen antibodies cross-react with cells infected by the closely related human BK virus. Only two antibodies, PAb1604 and PAb1614, directed against different epitopes of the SV40 T antigen, cross-react with polyoma large T tumour antigen which has a more limited amino acid sequence homology. This cross-reaction is rarely seen with polyclonal antibodies. Monoclonal antibody PAb1620 gave nuclear immunofluorescence only with murine cells transformed by SV40 and was found to react with a complex of T-antigen and 53 000-dalton host-coded protein. All the monoclonal antibodies react with nuclear T antigen and all but four antibodies stained the surface of SV40-transformed cells. These were four of the five antibodies directed against the central third of the T antigen. Thus the monoclonal antibodies show that cell surface T antigen differs from nuclear T antigen, either in accessibility or structure.

Protein kinase activities in immune complexes of simian virus 40 large T-antigen and transformation-associated cellular p53 protein

Immune complex kinase assays in the simian virus 40 system were performed by incubation of immunoprecipitates containing tumor antigens with [gamma-32P]ATP, followed by analysis of any phosphoacceptor proteins. These assays yielded mainly the viral large T-antigen and, in particular, the associated cellular p53 as endogenous substrates. The nature of these substrates was confirmed by proteolysis techniques. Under specific conditions, casein could be used as an exogenous substrate as well. The kinase reactions showed preference for ATP and MgCl2 instead of GTP or MnCl2. Both phosphoserine and phosphothreonine, but in no case phosphotyrosine, were detected after an immune complex kinase reaction. Apparently, several in vivo phosphorylation sites were recognized in vitro in both large T-antigen and p53, but the presence of some artifactual sites could not be completely excluded. Although contaminating kinases were detectable in the immune complexes, at least the p53 molecules were phosphorylated in vitro in a more specific way. This followed from several characteristics of the immune complex kinase reactions and especially from the strong inhibition of p53 phosphorylation by two anti-large-T monoclonal antibodies. It was shown that large T-antigen showed associated kinase activity, although none of our results could unambiguously demonstrate an intrinsic kinase activity of this protein. Finally, anti-p53 monoclonal antibodies only slightly affected in vitro phosphorylation reactions, whereas a p53 molecule from a simian virus 40-free, chemically transformed human cell line was not phosphorylated in vitro under any condition tested. Thus, it is highly unlikely that the p53 molecule per se carries intrinsic or even associated kinase activities.

Protein kinase activities in immune complexes of simian virus 40 large T-antigen and transformation-associated cellular p53 protein

Immune complex kinase assays in the simian virus 40 system were performed by incubation of immunoprecipitates containing tumor antigens with [gamma-32P]ATP, followed by analysis of any phosphoacceptor proteins. These assays yielded mainly the viral large T-antigen and, in particular, the associated cellular p53 as endogenous substrates. The nature of these substrates was confirmed by proteolysis techniques. Under specific conditions, casein could be used as an exogenous substrate as well. The kinase reactions showed preference for ATP and MgCl2 instead of GTP or MnCl2. Both phosphoserine and phosphothreonine, but in no case phosphotyrosine, were detected after an immune complex kinase reaction. Apparently, several in vivo phosphorylation sites were recognized in vitro in both large T-antigen and p53, but the presence of some artifactual sites could not be completely excluded. Although contaminating kinases were detectable in the immune complexes, at least the p53 molecules were phosphorylated in vitro in a more specific way. This followed from several characteristics of the immune complex kinase reactions and especially from the strong inhibition of p53 phosphorylation by two anti-large-T monoclonal antibodies. It was shown that large T-antigen showed associated kinase activity, although none of our results could unambiguously demonstrate an intrinsic kinase activity of this protein. Finally, anti-p53 monoclonal antibodies only slightly affected in vitro phosphorylation reactions, whereas a p53 molecule from a simian virus 40-free, chemically transformed human cell line was not phosphorylated in vitro under any condition tested. Thus, it is highly unlikely that the p53 molecule per se carries intrinsic or even associated kinase activities.

Analysis of simian virus 40 chromosome-T-antigen complexes: T-antigen is preferentially associated with early replicating DNA intermediates

The fraction and DNA composition of simian virus 40 chromosomes that were complexed with large T-antigens (T-Ag) were determined at the peak of viral DNA replication. Simian virus 40 chromatin containing radiolabeled DNA was extracted by the hypotonic method of Su and DePamphilis (Proc. Natl. Acad. Sci. U.S.A. 73:3466-3470, 1976) and then fractionated by sucrose gradient sedimentation into replicating (90S) and mature (70S) chromosomes. Viral chromosomes containing T-Ag were isolated by immunoprecipitation with saturating amounts of either an anti-T-Ag monoclonal antibody or an anti-T-Ag hamster serum under conditions that specifically precipitated T-Ag protein from cytosol extracts. An average of 10% of the uniformly labeled DNA in the 90S pool and 7.5% in the 70S pool was specifically precipitated, demonstrating that under these conditions immunologically reactive T-Ag was tightly bound to only 8% of the total viral chromosomes. In contrast, simian virus 40 replicating intermediates (RI) represented only 1.2% of the viral DNA, but most of these molecules were associated with T-Ag. At the shortest pulse-labeling periods, an average of 72 +/- 18% of the radiolabeled DNA in 90S chromosomes could be immunoprecipitated, and this value rapidly decreased as the labeling period was increased. Electron microscopic analysis of the DNA before and after precipitation revealed that about 55% of the 90S chromosomal RI and 72% of the total RI from both pools were specifically bound to T-Ag. Comparison of the extent of replication with the fraction of RI precipitated revealed a strong selection for early replicating DNA intermediates. Essentially all of the RI in the 70S chromosomes were less than 30% replicated and were precipitated with anti-T-Ag monoclonal antibody or hamster antiserum. An average of 88% of the 90S chromosomal RI which were from 5 to 75% replicated were immunoprecipitated, but the proportion of RI associated with T-Ag rapidly decreased as replication proceeded beyond 70% completion. By the time sibling chromosomes had separated, only 3% of the newly replicated catenated dimers in the 90S pool (<1% of the dimers in both pools) were associated with T-Ag. Measurements of the fraction of radiolabeled DNA in each quarter of the genome confirmed that T-Ag was preferentially associated with newly initiated molecules in which the nascent DNA was nearest the origin of replication. These results are consistent with a specific requirement for the binding of T-Ag to viral chromosomes to initiate DNA replication, and they also demonstrate that T-Ag does not immediately dissociate from chromosomes once replication begins. The biphasic relationship between the fraction of T-Ag-containing RI and the extent of DNA replication suggests either that 1 or 2 molecules of T-Ag remain stably bound until replication is about 70% completed or that 4 to 6 molecules of T-Ag are randomly released from each RI at a uniform rate throughout replication.

Characterization of am404, an amber mutation in the simian virus 40 T antigen gene

We analyzed the biological activity of an amber mutation, am404, at map position 0.27 in the T antigen gene of simian virus 40. Immunoprecipitation of extracts from am404-infected cells demonstrated the presence of an amber protein fragment (am T antigen) of the expected molecular weight (67,000). Differential immunoprecipitation with monoclonal antibody demonstrated that am T antigen was missing the carboxy-terminal antigenic determinants. The amber mutant was shown to be defective for most of the functions associated with wild-type T antigen. The mutant did not replicate autonomously, but this defect could be complemented by a helper virus (D. R. Rawlins and N. Muzyczka, J. Virol. 36:611-616, 1980). The mutant failed to transform nonpermissive rodent cells and did not relieve the host range restriction of adenovirus 2 in monkey cells. However, stimulation of host cell DNA, whose functional region domain has been mapped within that portion of the protein synthesized by the mutant, could be demonstrated in am404-infected cells. A number of unexpected observations were made. First, the am T antigen was produced in unusually large amounts in a simian virus 40-transformed monkey cell line (COS-1), but overproduction was not seen in nontransformed monkey cells regardless of whether or not a helper virus was present. This feature of the mutant was presumably the result of the inability of am T antigen to autoregulate, the level of wild-type T antigen in COS-1 cells, and the unusually short half-life of am T antigen in vivo. Pulse-chase experiments indicated that am T antigen had an intracellular half-life of approximately 10 min. In addition, although the am T antigen retained the major phosphorylation site found in simian virus 40 T antigen, it was not phosphorylated. Thus, phosphorylation of simian virus 40 T antigen is not required for the stimulation of host cell DNA synthesis. Finally, fusion of am404-infected monkey cells with Escherichia coli protoplasts containing appropriate procaryotic suppressor tRNAs showed that am404 is a suppressible nonsense mutation.

Inhibition of SV40-induced cellular DNA synthesis by microinjection of monoclonal antibodies

The region of the SV40 large T-antigen molecule recognized by a panel of monoclonal antibodies has been determined using hybrid Adeno-SV40 viruses, and manual microinjection of cloned deletion mutants. In addition, an investigation was made of how monoclonal antibodies microinjected into the nucleus can affect the ability of the T-antigen coding gene to stimulate cell DNA synthesis. The monoclonal antibody Pab 14, that recognized the -COOH terminal half of large T, was comicroinjected into quiescent cells together with plasmid pCl-1. This plasmid contains only that part of the T-antigen coding gene that extends from nucleotide residue 120, counterclockwise to nucleotide residue 4002, and makes a truncated T antigen 33,000 in molecular weight and missing the last 435 amino acids on the -COOH terminal side. Monoclonal antibody Pab 14 did not inhibit the stimulation of cellular DNA synthesis caused by microinjection of pCl-1, although it did inhibit cell DNA synthesis induced by microinjection of pSV2G, a recombinant plasmid that contains the entire T-antigen coding gene of SV40.

Time-dependent maturation of the simian virus 40 large T antigen-p53 complex studied by using monoclonal antibodies

Newly synthesized simian virus 40 large tumor antigen (T Ag) slowly forms a stable complex with the host tumor antigen, "p53." By the use of immunological and temporal separations and inhibition of aggregation and processing by A locus mutation, we have distinguished specific steps in the reaction sequence leading to formation of the rapidly sedimenting oligomeric complex. The monoclonal antibody PAb101 bound only a fraction of the total soluble pulse-labeled T Ag bound by antitumor serum. After a chase, all T Ag had matured to the form recognized by PAb101. All p53 in the mouse line SVA31E7 was precipitated by the T Ag-specific monoclonal antibody PAb101, even after a short pulse, and is therefore entirely bound to mature T Ag. The p53-specific monoclonal antibody PAb122 precipitates nearly all of the mature T Ag recognized by PAb101, except A locus mutant T Ag, synthesized at the nonpermissive temperature. A locus mutation inhibited entry of newly synthesized T Ag into the oligomeric greater than 28S complex of T Ag and p53.

A small subclass of SV40 T antigen binds to the viral origin of replication

We examined the affinities of SV40 large T antigen for unique viral DNA sequences by binding SV40 Bst NI DNA fragments in extracts of infected or transformed cells, and then immunoprecipitating the T antigen-DNA complex. The G fragment, which spans the viral origin of replication (ori) was quantitatively bound to T antigen. A T-antigen-specific monoclonal antibody (McI 7), which recognized only 5%-10% of the T antigen from infected or transformed cells, immunoprecipitated the majority of the ori-binding activity. This suggests that only a minor subclass of wild-type T antigen is active in binding to the origin. C6 cells contain a replication-defective mutant T antigen that when tested in the DNA-binding immunoassay, showed no affinity for the ori fragment. McI 7 not only failed to immunoprecipitate ori binding in C6 cells, but also did not detect any labeled C6 T antigen whatever. Thus McI 7 recognizes an immunologically distinct subset of wild-type 7 antigen that comprises the origin-binding form of the viral protein, which is absent in the C6 T antigen population. McI 122, which recognizes a 53 kilodalton host protein that complexes with T antigen, immunoprecipitated ori-binding activity from extracts of infected or transformed cells, but not from C6 cells. Thus wild-type T antigen can bind ori sequences even when complexed to the host protein. These data suggest that T antigen consists of different subpopulations with different functions.

Kinetics of the immune response of tumor-bearing hamsters to two simian virus 40 coded non-structural polypeptides present in simian virus 40 tumor cells

The kinetics of the immune response of hamsters transplanted with virus-free SV40 tumor cells (TSV5) to two SV40-coded non-structural proteins of 94,000 daltons (T antigen) and 17,000 daltons (t antigen) was studied in order to determine if the variation in the immune reactivity of serum from tumor-bearing animals towards these proteins can be explained on the basis of differential immune response during various stages of tumor growth. The results demonstrate that individual animals vary in their capacity to respond immunologically to T and t antigens in SV40-transformed cells and that, further, the immune response to T and t antigens is influenced by the stage of tumor development.

Detection of a complex of SV40 large tumor antigen and 53K cellular protein on the surface of SV40-transformed mouse cells

The possible interaction between simian virus 40 (SV40) large tumor antigen (T-ag) and cellular proteins in the plasma membrane of SV40-transformed mouse cells was investigated. The presence of SV40 T-ag, 53,000 (53K) cellular protein, and histocompatibility (H-2) antigens on the surface of SV40-transformed cells was demonstrated by immunofluorescence. The use of lactoperoxidase-catalyzed cell surface iodination and a differential immunoprecipitation technique established that large T-ag is associated with the 53K host-coded protein on the surface of the transformed cells. In contrast, no detergent-stable complex between large t-ag and H-2 antigens was detected. Both labeled T-ag and 53K protein were coprecipitated from surface-iodinated SV40-transformed cells by monoclonal antibodies directed against either the viral or the cellular protein. Based on the unique antigenic sites recognized by the anti-T monoclonal antibodies, it appears that both the carboxy and amino termini of the T-ag polypeptide are exposed on the surface of SV40-transformed mouse cells. The nature of the association between surface T-ag and 53K protein, as well as that between the molecular complex and the plasma membrane, remains to be determined. The possible effect of the surface-associated T-ag/53K complex on cellular proliferation is considered.

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.

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.