A DNA replication-positive mutant of simian virus 40 that is defective for transformation and the production of infectious virions

Update on human polyomaviruses and cancer

Over 50 years of polyomavirus research has produced a wealth of insights into not only general biologic processes in mammalian cells, but also, how conditions can be altered and signaling systems tweaked to produce transformation phenotypes. In the past few years three new members (KIV, WUV, and MCV) have joined two previously known (JCV and BKV) human polyomaviruses. In this review, we present updated information on general virologic features of these polyomaviruses in their natural host, concentrating on the association of MCV with human Merkel cell carcinoma. We further present a discussion on advances made in SV40 as the prototypic model, which has and will continue to inform our understanding about viruses and cancer.

Conformational rearrangements of SV40 large T antigen during early replication events

The Simian virus 40 (SV40) large tumor antigen (LTag) functions as the replicative helicase and initiator for viral DNA replication. For SV40 replication, the first essential step is the assembly of an LTag double hexamer at the origin DNA that will subsequently melt the origin DNA to initiate fork unwinding. In this study, we used three-dimensional cryo-electron microscopy to visualize early events in the activation of DNA replication in the SV40 model system. We obtained structures of wild-type double-hexamer complexes of LTag bound to SV40 origin DNA, to which atomic structures have been fitted. Wild-type LTag was observed in two distinct conformations: In one conformation, the central module containing the J-domains and the origin binding domains of both hexamers is a compact closed ring. In the other conformation, the central module is an open ring with a gap formed by rearrangement of the N-terminal regions of the two hexamers, potentially allowing for the passage of single-stranded DNA generated from the melted origin DNA. Double-hexamer complexes containing mutant LTag that lacks the N-terminal J-domain show the central module predominantly in the closed-ring state. Analyses of the LTag C-terminal regions reveal that the LTag hexamers bound to the A/T-rich tract origin of replication and early palindrome origin of replication elements are structurally distinct. Lastly, visualization of DNA density protruding from the LTag C-terminal domains suggests that oligomerization of the LTag complex takes place on double-stranded DNA.

SV40: Cell transformation and tumorigenesis

The story of SV40-induced tumorigenesis and cellular transformation is intimately entwined with the development of modern molecular biology. Because SV40 and other viruses have small genomes and are relatively easy to manipulate in the laboratory, they offered tractable systems for molecular analysis. Thus, many of the early efforts to understand how eukaryotes replicate their DNA, regulate expression of their genes, and translate mRNA were focused on viral systems. The discovery that SV40 induces tumors in certain laboratory animals and transforms many types of cultured cells offered the first opportunity to explore the molecular basis for cancer. The goal of this article is to highlight some of the experiments that have led to our current view of SV40-induced transformation and to provide some context as to how they contributed to basic research in molecular biology and to our understanding of cancer.

Small tumor antigen of polyomaviruses: role in viral life cycle and cell transformation

The regulatory proteins of polyomaviruses, including small and large T antigens, play important roles, not only in the viral life cycle but also in virus-induced cell transformation. Unlike many other tumor viruses, the transforming proteins of polyomaviruses have no cellular homologs but rather exert their effects mostly by interacting with cellular proteins that control fundamental processes in the regulation of cell proliferation and the cell cycle. Thus, they have proven to be valuable tools to identify specific signaling pathways involved in tumor progression. Elucidation of these pathways using polyomavirus transforming proteins as tools is critically important in understanding fundamental regulatory mechanisms and hence to develop effective therapeutic strategies against cancer. In this short review, we will focus on the structural and functional features of one polyomavirus transforming protein, that is, the small t-antigen of the human neurotropic JC virus (JCV) and the simian virus, SV40.

SV40 T antigen interacts with Nbs1 to disrupt DNA replication control

Nijmegen breakage syndrome (NBS) is characterized by radiation hypersensitivity, chromosomal instability, and predisposition to cancer. Nbs1, the NBS protein, forms a tight complex with Mre11 and Rad50, and these interactions contribute to proper double-strand break repair. The simian virus 40 (SV40) oncoprotein, large T antigen (T), also interacts with Nbs1, and T-containing cells experience chromosomal hyperreplication in a manner dependent on T/Nbs1 complex formation. A substantial fraction of NBS-deficient fibroblasts reinitiate DNA replication in discrete regions, and wild-type Nbs1 corrects this defect. Similarly, synthesis of an N-terminal Nbs1 fragment induced DNA rereplication and tetraploidy, in NBS-deficient but not NBS-proficient cells. Moreover, SV40 origin-containing DNA hyperreplicated in T-containing NBS-deficient cells by comparison with T-containing, Nbs1-reconstituted derivatives. Thus, Nbs1 suppresses rereplication of cellular DNA and SV40 origin-containing replicons, and T targets Nbs1, thereby enhancing the yield of new SV40 genomes during viral DNA replication.

Simian virus 40 T antigens and J domains: analysis of Hsp40 cochaperone functions in Escherichia coli

The N-terminal exon of DNA tumor virus T antigens represents a J domain that can direct interaction with the host-encoded Hsp70 chaperones. We have taken advantage of rapid Hsp40 cochaperone assays with Escherichia coli to assess simian virus 40 (SV40)-encoded J-domain loss of function. We found a strong correlation between loss of cochaperone function in E. coli and defective SV40 growth, suggesting that the major role of the J domain in DNA tumor viruses is to provide cochaperone function. We also report the expression of native SV40 virus T antigens in E. coli. Our results show that small t antigen, but not large T antigen (LT) or LT truncation TN125 or TN136, can functionally replace under limited growth conditions DnaJ (Hsp40) function in vivo. In addition, purified small t antigen can efficiently stimulate E. coli DnaK's (Hsp70) ATPase in vitro, thus behaving like a bona fide cochaperone. Furthermore, small t amino acids 83 to 174, which are adjacent to the viral J domain, can replace the E. coli DnaJ J-domain glycine-phenylalanine-rich domain, immediately adjacent to the J-domain sequences, even in the absence of significant amino acid similarity to their DnaJ counterpart. Taken together, our studies demonstrate that functionally related Hsp40 proteins from mammalian viral systems can be rapidly studied in bacteria and exploited to probe the universally conserved Hsp70 chaperone machine mechanism.

Simian virus 40 T antigens and J domains: analysis of Hsp40 cochaperone functions in Escherichia coli

The N-terminal exon of DNA tumor virus T antigens represents a J domain that can direct interaction with the host-encoded Hsp70 chaperones. We have taken advantage of rapid Hsp40 cochaperone assays with Escherichia coli to assess simian virus 40 (SV40)-encoded J-domain loss of function. We found a strong correlation between loss of cochaperone function in E. coli and defective SV40 growth, suggesting that the major role of the J domain in DNA tumor viruses is to provide cochaperone function. We also report the expression of native SV40 virus T antigens in E. coli. Our results show that small t antigen, but not large T antigen (LT) or LT truncation TN125 or TN136, can functionally replace under limited growth conditions DnaJ (Hsp40) function in vivo. In addition, purified small t antigen can efficiently stimulate E. coli DnaK's (Hsp70) ATPase in vitro, thus behaving like a bona fide cochaperone. Furthermore, small t amino acids 83 to 174, which are adjacent to the viral J domain, can replace the E. coli DnaJ J-domain glycine-phenylalanine-rich domain, immediately adjacent to the J-domain sequences, even in the absence of significant amino acid similarity to their DnaJ counterpart. Taken together, our studies demonstrate that functionally related Hsp40 proteins from mammalian viral systems can be rapidly studied in bacteria and exploited to probe the universally conserved Hsp70 chaperone machine mechanism.

T antigens of simian virus 40: molecular chaperones for viral replication and tumorigenesis

Simian virus 40 (SV40) is a small DNA tumor virus that has been extensively characterized due to its relatively simple genetic organization and the ease with which its genome is manipulated. The large and small tumor antigens (T antigens) are the major regulatory proteins encoded by SV40. Large T antigen is responsible for both viral and cellular transcriptional regulation, virion assembly, viral DNA replication, and alteration of the cell cycle. Deciphering how a single protein can perform such numerous and diverse functions has remained elusive. Recently it was established that the SV40 T antigens, including large T antigen, are molecular chaperones, each with a functioning DnaJ domain. The molecular chaperones were originally identified as bacterial genes essential for bacteriophage growth and have since been shown to be conserved in eukaryotes, participating in an array of both viral and cellular processes. This review discusses the mechanisms of DnaJ/Hsc70 interactions and how they are used by T antigen to control viral replication and tumorigenesis. The use of the DnaJ/Hsc70 system by SV40 and other viruses suggests an important role for these molecular chaperones in the regulation of the mammalian cell cycle and sheds light on the enigmatic SV40 T antigen-a most amazing molecule.

Different functions are required for initiation and maintenance of immortalization of rat embryo fibroblasts by SV40 large T antigen

We have used two different, but complementary assays to characterize functions of SV40 T antigen that are necessary for its ability to immortalize rat embryo fibroblasts. In accordance with previous work, we found that several functions were required. These include activities that map to the p53 binding domain and the amino terminal 176 amino acids which contain the J domain as well as the CR1 and CR2 domain required for binding and sequestering the RB family of pocket proteins. Moreover, we found that even though activities dependent only upon the amino terminus were sufficient for immortalization they were unable to maintain it. This suggests that immortalization by these amino terminal functions requires either additional events or immortalization of a subset of cells within the heterogeneous rat embryo fibroblast population. We further found that an activity dependent upon amino acids 17 - 27 which remove a portion of the CR1 domain and the predicted alpha-1 helix of the J domain was not necessary to maintain growth but was required for direct immortalization suggesting that at least one of the functions required initially was not required to maintain the immortal state. This represents the first demonstration that some of the functions required for maintenance of the immortal state differ from those required for initiation of immortalization.

Induction of tumor antigen-specific immunity in vivo by a novel vaccinia vector encoding safety-modified simian virus 40 T antigen

BACKGROUND

Evidence that simian virus 40 (SV40) is associated with human mesotheliomas, osteosarcomas, and brain tumors suggests that a recombinant vaccine directed against lethal cancers expressing SV40 T antigen (Tag) could have clinical utility. To address this potential need, we designed a novel vaccinia virus construct that encodes an SV40 Tag in which oncogenic domains were excluded and immunogenic domains were preserved. We named this recombinant construct vaccinia-encoding safety-modified SV40 Tag (vac-mTag).

METHODS

Purified vac-mTag was characterized by DNA sequencing, reverse transcription-coupled polymerase chain reaction, western blot analysis, and immunocytochemical techniques. Induction of Tag-specific immunity was examined by cytolytic T-cell assays, and the efficacy of vac-mTag in protecting animals against Tag-expressing tumors and in treating pre-established microscopic tumors was evaluated in vac-mTag-immunized BALB/c mice.

RESULTS

The immune response elicited by vac-mTag in C57BL/6 and BALB/c mice included an SV40 Tag-specific cytolytic T-lymphocyte activity against syngeneic (identical genetic background) SV40 Tag-expressing tumor targets. Immunization of mice with a single dose of vac-mTag resulted in potent protection against subsequent challenge with a lethal mouse cancer expressing SV40 Tag. In addition, single-dose vac-mTag immunization coadministered with interleukin 2 produced a possible therapeutic effect against a preadministered microscopic (but lethal) burden of Tag-expressing tumor cells in vivo.

CONCLUSION

vac-mTag induces an effective immune response in mice that is specific for a tumor-associated antigen. This response protects against a lethal tumor challenge and results in a possible therapeutic effect against Tag-expressing tumors in vivo. Thus, vac-mTag provides a new avenue for the development of therapies for human cancers thought to be associated with SV40.

Effects of mutations within the SV40 large T antigen ATPase/p53 binding domain on viral replication and transformation

The simian virus 40 (SV40) large T antigen is a 708 amino-acid protein possessing multiple biochemical activities that play distinct roles in productive infection or virus-induced cell transformation. The carboxy-terminal portion of T antigen includes a domain that carries the nucleotide binding and ATPase activities of the protein, as well as sequences required for T antigen to associate with the cellular tumor suppressor p53. Consequently this domain functions both in viral DNA replication and cellular transformation. We have generated a collection of SV40 mutants with amino-acid deletions, insertions or substitutions in specific domains of the protein. Here we report the properties of nine mutants with single or multiple substitutions between amino acids 402 and 430, a region thought to be important for both the p53 binding and ATPase functions. The mutants were examined for the ability to produce infectious progeny virions, replicate viral DNA in vivo, perform in trans complementation tests, and transform established cell lines. Two of the mutants exhibited a wild-type phenotype in all these tests. The remaining seven mutants were defective for plaque formation and viral DNA replication, but in each case these defects could be complemented by a wild-type T antigen supplied in trans. One of these replication-defective mutants efficiently transformed the REF52 and C3H10T1/2 cell lines as assessed by the dense-focus assay. The remaining six mutants were defective for transforming REF52 cells and transformed the C3H10T1/2 line with a reduced efficiency. The ability of mutant T antigen to transform REF52 cells correlated with their ability to induce increased levels of p53.

DnaJ/hsp40 chaperone domain of SV40 large T antigen promotes efficient viral DNA replication

The amino-terminal domain of SV40 large tumor antigen (TAg) is required for efficient viral DNA replication. However, the biochemical activity associated with this domain has remained obscure. We show here that the amino-terminal domain of TAg shares functional homology with the J-domain of DnaJ/hsp40 molecular chaperones. DnaJ proteins function as cofactors by regulating the activity of a member of the 70-kD heat shock protein family. Genetic analyses demonstrated that amino-terminal sequences of TAg comprise a novel J-domain that mediates a specific interaction with the constitutively expressed hsc70 and show that the J-domain is also required for efficient viral DNA replication in vivo. Furthermore, we demonstrated that the J-domain of two human DnaJ homologs, HSJ1 or DNAJ2, could substitute functionally for the amino-terminus of TAg in promoting viral DNA replication. Together, our findings suggest that TAg uses its J-domain to support SV40 DNA replication in a manner that is strikingly similar to the use of Escherichia coli DnaJ by bacteriophage lambda in DNA replication. However, TAg has evolved a more efficient strategy of DNA replication through an intrinsic J-domain to associate directly with a partner chaperone protein. Our observations provide evidence of a role for chaperone proteins in the process of eukaryotic DNA replication.

Adding an Rb-binding site to an N-terminally truncated simian virus 40 T antigen restores growth to high cell density, and the T common region in trans provides anchorage-independent growth and rapid growth in low serum concentrations

The simian virus 40 large T antigen is sufficient to confer on cells multiple transformed cell growth characteristics, including growth to a high cell density, rapid growth in medium containing low serum concentrations, and anchorage-independent growth. We showed previously that distinct regions of the protein were involved in conferring these properties and that removal of the first 127 amino acids of T antigen abrogated all three activities. At least three large-T-antigen transformation-related activities have been localized to that region: binding of the tumor suppressor gene product Rb and two independent activities contained within the common region shared by large T and small t antigens. The experiments described here were directed toward determining whether these were the only activities from the N terminus that were needed. To do so we reintroduced an Rb-binding region into the N-terminally truncated T antigen (T128-708) and examined the growth properties of cells immortalized by it in the presence and absence of small t antigen, which can provide the T-common-region transformation-related activities in trans. We show that an Rb-binding region consisting of amino acids 101 to 118, when introduced into a heterologous site in T128-708, is capable of physically binding Rb and that binding is sufficient for cells expressing the protein to acquire the ability to grow to a high saturation density. However, in low-serum medium, the growth rate of the cells and maximal cell density are reduced relative to those of wild-type-T-antigen-expressing cells, and the cells cannot divide without anchorage. This result suggests that although Rb binding is sufficient in the context of T128-708 to confer growth to a high density, one or more other N-terminally located T-antigen activities are needed for cells to acquire the additional growth properties. Small t antigen in trans supplied those activities. These results indicate that the T-common-region activities and Rb binding are the only activities from the T-antigen N terminus needed to restore full transforming activity to the N-terminally truncated T antigen.

An N-terminal deletion mutant of simian virus 40 (SV40) large T antigen oligomerizes incorrectly on SV40 DNA but retains the ability to bind to DNA polymerase alpha and replicate SV40 DNA in vitro

A peptide encompassing the N-terminal 82 amino acids of simian virus 40 (SV40) large T antigen was previously shown to bind to the large subunit of DNA polymerase alpha-primase (I. Dornreiter, A. Höss, A. K. Arthur, and E. Fanning, EMBO J. 9:3329-3336, 1990). We report here that a mutant T antigen, T83-708, lacking residues 2 to 82 retained the ability to bind to DNA polymerase alpha-primase, implying that it carries a second binding site for DNA polymerase alpha-primase. The mutant protein also retained ATPase, helicase, and SV40 origin DNA-binding activity. However, its SV40 DNA replication activity in vitro was reduced compared with that of wild-type protein. The reduction in replication activity was accompanied by a lower DNA-binding affinity to SV40 origin sequences and aberrant oligomerization on viral origin DNA. Thus, the first 82 residues of SV40 T antigen are not strictly required for its interaction with DNA polymerase alpha-primase or for DNA replication function but may play a role in correct hexamer assembly and efficient DNA binding at the origin.

Temperature-sensitive phenotype of the human parainfluenza virus type 3 candidate vaccine strain (cp45) correlates with a defect in the L gene

We have previously demonstrated that the temperature sensitivity of a human parainfluenza virus type 3 (HPIV-3) candidate vaccine strain (cp45), which is currently under evaluation in humans, is associated with poor transcriptional activity of the virus at the nonpermissive temperature (R. Ray, K. Meyer, F. Newman, and R. B. Belshe, J. Virol. 69:1959-1963, 1995). In this study, the temperature sensitivity of cp45 virus was further investigated by the complementation of a specific gene function. CV-1 cells were transfected with cloned genes from wild-type HPIV-3 encoding the large protein (L), phosphoprotein (P), and nucleocapsid protein (NP), alone or together, for the expression of biologically active proteins. Only cells expressing the L gene were able to rescue cp45 replication when incubated at the nonpermissive temperature (39.5 degrees C), whereas cells transiently expressing NP or P were incapable of rescuing the virus. The virus titers obtained following complementation of the L protein were 190 to 2,300 PFU/ml of culture medium, compared with the undetectable growth of the cp45 temperature-sensitive mutant at the nonpermissive temperature. Rescued progeny virus still maintained the temperature-sensitive phenotype. Results from this study suggest that the temperature sensitivity of the cp45 candidate vaccine strain is associated primarily with L-protein function and that the defect can be complemented by transient expression of the wild-type protein. This study underscores the importance of the L protein in RNA polymerase activity and its critical role in virus replication.

T antigens encoded by replication-defective simian virus 40 mutants dl1135 and 5080

We present a preliminary biochemical characterization of two simian virus 40 mutants that affect different T antigen replication functions. SV40 T antigen mutants dl1135 (delta 17-27 amino acids) and 5080 (P-L) have been studied extensively with regard to their ability to transform cells in culture and induce tumors in transgenic mice. Both mutants are defective for viral DNA replication in vivo. In order to assess in more detail the molecular basis for the in vivo replication defects of 5080 and dl1135, we expressed the mutant proteins using the baculovirus system and purified them by immunoaffinity chromatography. With each of the purified proteins, we examined some of the biochemical activities of T antigen required for replication, viz. ATPase, binding to the origin of replication (ori) and assembly on ori, DNA helicase and unwinding, and replication in in vitro assays. Consistent with previous studies, we found that the 5080 protein is defective for multiple biochemical activities including ATPase, helicase, ori-specific unwinding, and ATP-induced hexamerization. However, this mutant retains some sequence-specific DNA binding activity. In contrast, the dl1135 protein exhibited significant levels of activity in all assays, including the ability to drive SV40 DNA replication in vitro. Thus, dl1135 is one of several mutants with an altered amino-terminal domain which can replicate DNA in vitro, but not in vivo. Thus, while the 5080 mutation affects a T antigen enzymatic function directly required for viral DNA synthesis, dl1135 may alter an activity required to prepare the cell for viral replication.

A TEF-1-independent mechanism for activation of the simian virus 40 (SV40) late promoter by mutant SV40 large T antigens

Simian virus 40 (SV40) large tumor antigen (T antigen) stimulates the activity of the SV40 late promoter and a number of cellular and other viral promoters. We have characterized the ability of T antigens with mutations in the DNA-binding domain and within the N-terminal 85 residues to activate the SV40 late promoter. T antigens lacking both nonspecific and sequence-specific DNA-binding activities were able to induce the late promoter. Mutations within the N-terminal 85 residues of T antigen diminished activation by less than twofold. Activation by wild-type and most of the mutant T antigens required intact binding sites for the cellular transcription factor TEF-1 in the late promoter. Curiously, two mutants altered in the N-terminal region and an additional mutant altered in the DNA-binding domain activated a late promoter derivative lacking TEF-1 binding sites, indicating the existence of a TEF-1-independent pathway for activation of the late promoter. A consensus binding site for the TATA binding protein, TBP, was created in variants of late promoters either containing or lacking TEF-1 binding sites. Basal expression was increased by the consensus TBP binding site only when TEF-1 binding sites were present, leading to a reduction in the degree of activation by T antigen. However, activation by a mutant T antigen of the promoter lacking TEF-1 sites was unchanged or slightly enhanced by the consensus TBP binding site. These results suggest that some mutant T antigens can stabilize an interaction between TBP and additional factors bound to the late promoter.

Human parainfluenza virus type 3: analysis of the cytoplasmic tail and transmembrane anchor of the hemagglutinin-neuraminidase protein in promoting cell fusion

The role of the cytoplasmic tail and transmembrane anchor of the human parainfluenza virus type 3 (HPIV3) hemagglutinin-neuraminidase (HN) glycoprotein in promoting cell fusion was investigated. A series of amino terminal deletion mutants (d10, d20, d27, d31, d40, d44, and d73) were compared for processing, cell surface expression, and maintenance of their biological attributes by recombinant expression of mutant genes using a plasmid vector (pcDL-SR alpha-296) in CV-1 and HeLa cells. To determine the fusion promoting activity (FPA) of the various mutant proteins, a simple assay was developed which quantified the fusion of two different HeLa cell types. One of the cell types, HeLa-tat, constitutively expressed the human immunodeficiency virus type I (HIV-1) tat protein from a Moloney murine leukemia virus long terminal repeat (LTR), while the second cell type, HeLa beta-gal, contained a reporter gene, beta-galactosidase, under the control of an HIV1-LTR. Fusion of mixed HeLa cell monolayers (50:50, HeLa-tat: HeLa beta-gal), following transfection with appropriate plasmids, resulted in transactivation of the reporter gene which was then measured by direct staining of cells or using cell lysates with appropriate substrates. Cell fusion was observed only when both the HPIV3 F and functional HN proteins were both co-transfected into cells. Of the seven deletion mutants examined, only d10, d20, d27 and d31 were expressed to significant levels on the cell surface and only these four mutant proteins maintained FPA. Compared with the wt HN at 48 h post transfection, d10 and d20 had enhanced FPA (119% and 158%, respectively), while d27 and d31 were diminished (74% and > 4%, respectively). Analysis of protein expression suggested that the reason for the increase in FPA of the mutant proteins was that the levels of protein expressed at the cell surface was twofold or threefold higher for d10 and d20, respectively, compared to the wt HN.

The kinetics of simian virus 40-induced progression of quiescent cells into S phase depend on four independent functions of large T antigen

Microinjection of purified simian virus 40 large-T-antigen protein or DNA encoding T antigen into serum-starved cells stimulates them to re-enter the cell cycle and progress through G1 into the S phase. Genetic analysis of T antigen indicated that neither its Rb/p107-binding activity nor its p53-binding activity is essential to induce DNA synthesis in CV1P cells. However, T antigens bearing missense mutations that inactivate either activity induced slower progression of the cells into the S phase than did wild-type T antigen. Inactivation of both activities resulted in a T antigen essentially unable to induce DNA synthesis. Missense mutations in either the DNA-binding region of the N terminus also impaired the ability of full-length T antigen to stimulate DNA synthesis in CV1P cells. The wild-type kinetics of cell cycle progression were restored by genetic complementation after coinjection of plasmid DNAs encoding different mutant T antigens or coinjection of purified mutant T-antigen proteins, suggesting that the four mitogenic functions of T antigen are independent. The maximal rate of induction of DNA synthesis in secondary primate cells and established rodent cell lines required the same four functions of T antigen. A model to explain how four independent activities could cooperate to stimulate cell cycle progression is presented.

An amino-terminal fragment of SV40 T antigen transforms REF52 cells

An SV40 mutant, T147D, encodes only the amino-terminal 147 amino acids of large T antigen and does not make small t antigen. We show here that a retrovirus which expresses this mutant T antigen transforms rat REF52 cells as efficiently as a retrovirus that expresses both the wild-type large and small T antigens. This cell line had previously been refractory to transformation by mutants that make short, amino-terminal fragments of T antigen.

Common and unique features of T antigens encoded by the polyomavirus group

Although 12 different members of the polyomavirus group have now been identified, only SV40 and PyV have been studied extensively. Whereas each member of the group shows a restricted host range, viruses infecting species from birds to humans have been reported. Although little is known concerning the biology of natural infections in the wild, it is apparent that these viruses exhibit various cell-type tropisms. Some viruses, such as LPV (B lymphocytes) or KV (pulmonary endothelium), are tightly restricted to specific cell types, while others, such as PyV, infect a variety of tissues in the animal. Despite these differences, all polyomaviruses share a common strategy of productive infection, expressing T antigens which act both on cellular targets, preparing cellular metabolism for supporting optimal viral replication, and then on targets within the viral genome, to regulate viral DNA replication, transcription, and assembly. Presumably, this common replication strategy restricts the degree to which the sequences of these viruses can diverge. Thus, sequence motifs conserved among these different viruses may indicate key structural elements essential for biochemical function. In this article I have compared the sequences of all polyomavirus-encoded large and small T antigens sequenced to date. This has led to the following conclusions and speculations. (i) Comparison of the domain organization of different large T antigens reveals that these proteins fall into two structural classes. Members of the SV40 class, which include SV40, JCV, BKV, and SA12, possess a carboxyl-terminal domain, which in SV40 has been shown to be dispensable for viral DNA replication but essential for virion assembly. The PyV class lacks the carboxyl-terminal domain and carries additional amino acids within the amino-terminal domain. When total amino acid identity is examined, members of the SV40 class show the highest degree of conservation (65 to 85%), while sequence identity among the remaining viruses varies from 18 to 55%. (ii) The DNA binding domains of most large T antigens are closely related, with amino acid identities ranging from 35 to 86%. Several residues within this domain are invariant among all T antigens. All of these viruses have multiple copies of the consensus T-antigen-binding pentanucleotide (GAGGC) in their ori region, suggesting that all T antigens recognize this sequence. The single exception is the large T antigen encoded by the avian virus BFDV. The putative DNA binding domain of this protein shows little or no sequence relation to that of other T antigens. Furthermore, the GAGGC motif is not found in the ori region of this virus.(ABSTRACT TRUNCATED AT 400 WORDS)

Simian virus 40 mutants with amino-acid substitutions near the amino terminus of large T antigen

A series of amino-acid substitution mutants has been made with changes in the region of simian virus 40 large tumor antigen (T antigen) that is shared with the small tumor antigen (t antigen). Both single and multiple amino-acid replacements were obtained using the heteroduplex deletion loop method and sodium bisulfite as the mutagen. The mutants could be divided into five phenotypic classes on the basis of their biological properties: a) mutants whose changes did not affect their ability to propagate on permissive monkey cells, nor to transform nonpermissive rodent cells; b) mutants that were not viable, replicated their DNA to 5% or less of wild type, but were positive for transformation; c) mutants that were not viable, replicated their DNA to 5% or less of wild type, and were defective for transformation; and d) mutants that completely lost all three activities coordinately. In addition, one mutant with changes in this region, 5002, replicated its DNA to about 50% of wild type, had an impaired transformation activity, and produced virions at a level of about 4% that of wild type.

Simian virus 40 large T antigen stably complexes with a 185-kilodalton host protein

Stable interactions between simian virus 40 large T antigen and host proteins are believed to play a major role in the ability of the viral protein to transform cells in culture and induce tumors in vivo. Two of these host proteins, the retinoblastoma susceptibility protein (pRB) and p53, are products of tumor suppressor genes, suggesting that T antigen exerts at least a portion of its transforming activity by complexing with and inactivating the function of these proteins. While analyzing T antigen-host protein complexes in mouse cells, we noted a protein of 185 kDa (p185) which specifically coimmunoprecipitates with T antigen. Coimmunoprecipitation results from the formation of stable complexes between T antigen and p185. Complex formation is independent of the interactions of T antigen with pRB, p120, and p53. Furthermore, analysis of T-antigen mutants suggests that T antigen-p185 complex formation may be important in transformation by simian virus 40.

The amino-terminal 147 amino acids of SV40 large T antigen transform secondary rat embryo fibroblasts

T147D is an SV40 mutant that encodes only the amino-terminal 147 amino acids of large T antigen and does not make small t antigen. We have constructed a recombinant retrovirus that expresses the T147D mutant protein. We show here that this virus can transform the established rat cell line, F111, in an agar assay with high efficiency. More importantly, we demonstrate that this retrovirus transforms secondary rat embryo fibroblasts to anchorage independence as efficiently as a recombinant retrovirus that expresses both wild-type large and small T antigens. These data indicate that in rat cells, the amino-terminal 147 amino acids of T antigen are sufficient for transformation. Further, since the T147D protein does not bind p53, we conclude that the association between T antigen and p53 is not required for the transformation of rat cells to anchorage-independent growth.