Mutant carrying deletions in the two simian virus 40 early genes

Interaction of simian virus 40 large T-antigen with cellular DNA polymerase alpha: studies with various T-antigen mutants of SV40

Simian virus 40 (SV40) large T antigen binds to host cell DNA polymerase alpha and the T-antigen-DNA polymerase alpha complex is implicated in the initiation of viral DNA replication. We have examined various SV40 T-antigen mutants to test the correspondence between viral DNA replication and T-antigen-DNA polymerase alpha complex formation. The various SV40 T-antigen mutants were used to either infect or transfect African green monkey kidney cell line CV-1, and at different time intervals we measured the production of T-antigen and host cell DNA polymerase alpha by radioimmunoassay, complex formation by a sandwich radioimmunoassay and the amount of viral DNA synthesis by dot-blot hybridization analysis. There was a good correlation between complex formation and viral DNA synthesis in lytic mutants of SV40. Poor complex formation and correspondingly lower DNA synthesis were observed in the non-viable mutants of SV40, even though significant amounts of T-antigen and DNA polymerase alpha were present. Our results substantiate the earlier findings of T-antigen-DNA polymerase alpha complex formation and establish the need for formation of this complex in promoting viral DNA synthesis.

Molecular cloning and characterization of endogenous SV40 DNA from human HBL-100 cells

The human HBL-100 cell line harbours SV40 DNA integrated in tandem at a unique site. The SV40 T-antigen expressed in these cells is defective in a function essential to the replication of the viral genome. The integrated SV40 sequences were molecularly cloned in a bacteriophage, and a subclone (plasmid pSVHBI) containing a complete SV40 DNA was isolated. As compared to SV40 wild-type strain 776, sequence analysis of pSVHBI early region revealed the presence of several DNA alterations. Among these, a point mutation at position 3199, predicting a change at amino-acid 540 of arginine to isoleucine, was shown by marker rescue to be responsible for the deficiency of T-antigen. This novel mutation further delimits one of the T-antigen domains involved in SV40 DNA replication. Transfection experiments demonstrated that the transforming activity of the SV40 genome from HBL-100 cells is still preserved. Moreover, several transformed human cell clones thus obtained could be permanently established in culture.

Requirements for immortalization of primary mouse embryo fibroblasts probed with mutants bearing deletions in the 3' end of SV40 gene A

The influence of specific contiguous stretches of amino acids predominantly in the carboxy terminal third of the SV40 large T antigen on the immortalization of cells in culture was investigated. Mutants that bear either small in-phase or frameshift deletions in the large T antigen coding sequence were transfected into primary mouse embryo fibroblasts of C57Bl/6 origin (B6/MEF). The frequency of immortalization was determined as the number of colonies that developed from cells escaping senescence. The results indicated that the terminal 81 amino acids of large T antigen are not needed for efficient immortalization or tumorigenicity. In contrast removal of as few as three amino acids encoded in the vicinity of the Dde-1 site at 0.234 map units (m.u.) severely restricted immortalization, suggesting that this region of the coding sequence either structurally or functionally is essential to at least one parameter of the transformed cell phenotype. The T antigen produced by dlA2433 which bears a deletion of nine nucleotides at 0.234 m.u. fails to associate stably with the cellular protein p53. The results showed that the addition of long stretches of amino acids (96 or 97 residues) from the open reading frame at the 3' end of the early region inactivated immortalizing functions, although the addition of as many as 18 amino acids from other reading frames was not detrimental. The evidence presented also confirmed that wild-type levels of ATPase activity are not necessary for immortalization or tumorigenicity of B6/MEF. Finally, we show that one of the mutants that immortalized primary cells did not produce dense foci on a cell monolayer. This last result indicated that independent functions are required for these two parameters of the transformed cell phenotype.

Properties of simian virus 40 mutants lacking the Asp4-Glu-Asp stretch at the carboxyl-terminus of large T antigen

The biological activity carried by the carboxy-terminal domain of SV40 large T antigen has been investigated by isolating mutants deleted for a stretch of six acidic residues which by analogy with polyoma middle T antigen might be essential for the activity of the protein. We have constructed an "in-phase" deletion of 37 residues that includes the complete acid residues cluster. In order to parallel the polyoma hr-t mutants genotype, the deletion was introduced in virus strains either competent or defective for the small t antigen. We conclude from these experiments that the deletion of this unusual sequence does not affect per se any of the known biological properties of the virus.

Influence of amino acids encoded in the 3' open reading frame of the SV40 early region on transformation and antigenicity of large T antigen

The mutant dlA2414 bears a frame-shift deletion of nucleotides 2936-2927 in the coding sequence for the simian virus 40 (SV40) large T antigen. Based on its nucleotide sequence, this mutant should produce a T antigen containing the first 627 authentic large T antigen amino acids followed by 97 amino acids encoded in the alternate open reading frame at the 3' end of the early region. This protein resembles the hypothetical T* protein that would be translated from an early SV40 mRNA if it were spliced to permit utilization of the open reading frame. We show that stable mouse cell lines can be generated that express the T antigen produced by dlA2414 and that this T antigen has an altered carboxy terminus. In addition, the expected tryptic peptides were missing from the large T antigen and replaced by more hydrophobic peptides. The T*-like protein produced by dlA2414 was apparently less stable than wild-type T antigen and did not stably complex with the cellular phosphoprotein p53. This protein retained the ability to immunize mice against a challenge of syngeneic SV40-tumor cells. The dlA2414 T antigen was expressed at the surface of cells as shown by in vitro lymphocyte-mediated cytotoxicity assay. The results presented here also showed that the expression of a T*-like protein at the cell surface is not likely to be essential for tumorigenesis of cells transformed by SV40.

Efficient and accurate in vitro processing of simian virus 40-associated small RNA

Nuclei were isolated from simian virus 40 (SV40)-infected cells with a hypotonic, detergent-free buffer and incubated in vitro in a high-ionic-strength buffer containing [alpha-32P]UTP. The labeled viral RNAs produced were analyzed by gel electrophoresis together with 3-h-labeled viral RNAs extracted from SV40-infected cells. The in vitro-synthesized RNA contained a major RNA species of 62 to 64 nucleotides that appeared on the gel at the same position as in vivo-synthesized SV40-associated small RNA (SAS-RNA). Analyses of the in vitro-synthesized 62- to 64-nucleotide RNA by hybridization to restriction fragments and by the use of an SAS-RNA deletion mutant clearly identified it as SAS-RNA. The intensity of the band of the in vitro-synthesized SAS-RNA increased with an increase in the labeling time or when a short pulse was followed by a chase. Moreover, the SAS-RNA band disappeared when ITP replaced GTP in the transcription reaction mixture. These results indicate that SAS-RNA is processed from a precursor molecule and that an RNA secondary structure could be an element recognized by the processing enzyme.

Synthesis of the Ad2+ND5-specified 42K protein is regulated posttranscriptionally in abortively infected monkey cells

In abortive infections of monkey cells by Ad2+ND5, the synthesis of the simian virus 40-specific 42,000-molecular-weight (42K) protein was reduced approximately 10-fold compared with a productive coinfection by Ad2+ND5 plus Ad2hr400 and about 20-fold compared with productive infections by Ad2+ND5 plus simian virus 40 or by Ad2+ND2 alone. However, the level of Ad2+ND5-specific mRNA was depressed twofold or less in abortive infections compared with productive infections. Moreover, the 42K mRNA isolated from abortive Ad2+ND5 infections translated in vitro with the same efficiency as the mRNA isolated from productive coinfections. This is analogous to the block to synthesis of the adenovirus fiber polypeptide in monkey cells (Anderson and Klessig, J. Mol. Appl. Genet. 2:31-43, 1983). Also like fiber protein, the increased level of the 42K protein found in productive infections was due to enhanced synthesis, not increased stability of the protein. Our results suggest that the synthesis of the Ad2+ND5-specified 42K protein and the adenovirus fiber protein are regulated in similar posttranscriptional manners.

Mutations near the carboxyl terminus of the simian virus 40 large tumor antigen alter viral host range

We report the characterization of three mutants of simian virus 40 with mutations that delete sequences near the 3' end of the gene encoding large tumor antigen (T antigen). Two of these mutants, dl1066 and dl1140, exhibit an altered viral host range. Wild-type simian virus 40 is capable of undergoing a complete productive infection on several types of established African green monkey kidney lines, including BSC40 and CV1P. dl1066 and dl1140 grow on BSC40 cells at 37 degrees C. However, both mutants fail to form plaques on BSC40 cells at 32 degrees C or on CV1P cells at any temperature. These mutants are capable of replicating viral DNA in the nonpermissive cell type, indicating a defect in an activity of T antigen not related to its replication function. Furthermore this defect can be complemented in trans by the wild type or by a variety of DNA replication-negative T antigen mutants, so long as they produce a normal carboxyl-terminal region of the molecule. Our data are consistent with the hypothesis that the C-terminal region of T antigen constitutes a functional domain. We propose that this domain encodes an activity that is required for simian virus 40 productive infection on the CV1P cell line, but not on BSC40.

Two separable functional domains of simian virus 40 large T antigen: carboxyl-terminal region of simian virus 40 large T antigen is required for efficient capsid protein synthesis

The carboxyl-terminal portion of simian virus 40 large T antigen is essential for productive infection of CV-1 and CV-1p green monkey kidney cells. Mutant dlA2459, lacking 14 base pairs at 0.193 map units, was positive for viral DNA replication, but unable to form plaques in CV-1p cells (J. Tornow and C.N. Cole, J. Virol. 47:487-494, 1983). In this report, the defect of dlA2459 is further defined. Simian virus 40 late mRNAs were transcribed, polyadenylated, spliced, and transported in dlA2459-infected cells, but the level of capsid proteins produced in infected CV-1 green monkey kidney cells was extremely low. dlA2459 large T antigen lacks those residues known to be required for adenovirus helper function, and the block to productive infection by dlA2459 occurs at the same stage of infection as the block to productive adenovirus infection of CV-1 cells. These results suggest that the adenovirus helper function is required for productive infection by simian virus 40. Mutant dlA2459 was able to grow on the Vero and BSC-1 lines of African green monkey kidney cells. Additional mutants affecting the carboxyl-terminal portion of large T were prepared. Mutant inv2408 contains an inversion of the DNA between the BamHI and BclI sites (0.144 to 0.189 map units). This inversion causes transposition of the carboxyl-terminal 26 amino acids of large T antigen and the carboxyl-terminal 18 amino acids of VP1. This mutant was viable, even though the essential information absent from dlA2459 large T antigen has been transferred to the carboxyl terminus of VP1 of inv2408. The VP1 polypeptide carrying this carboxyl-terminal portion of large T could overcome the defect of dlA2459. This indicates that the carboxyl terminus of large T antigen is a separate and separable functional domain.

Genetic and biochemical analysis of transformation-competent, replication-defective simian virus 40 large T antigen mutants

To study the role of the biochemical and physiological activities of simian virus 40 (SV40) large T antigen in the lytic and transformation processes, we have analyzed DNA replication-defective, transformation-competent T-antigen mutants. Here we describe two such mutants, C8/SV40 and T22/SV40, and also summarize the properties of all of the mutants in this collection. C8/SV40 and T22/SV40 were isolated from C8 and T22 cells (simian cell lines transformed with UV-irradiated SV40). Early regions encoding the defective T antigens were cloned into a plasmid vector to generate pC8 and pT22. The mutations responsible for the defects in viral DNA replication were localized by marker rescue, and subsequent DNA sequencing revealed missense and one nonsense mutation. The T22 mutation predicts a change of histidine to glutamine at residue 203. C8 has two mutations, one predicts lysine224 to glutamamic acid and the other changes the codon for glutamic acid660 to a stop codon; therefore, C8 T antigen lacks the 49 carboxy-terminal amino acids. pC8A and pC8B were constructed to contain the C8 mutations separately. Plasmids pT22, pC8, pC8A, and pC8B were able to transform primary rodent cell cultures. T22 T antigen is defective in binding to the SV40 origin. C8B (49-amino-acid truncation) is a host-range mutant defective in a late function in CV-1 but not BSC cells. Analysis of T antigens in mutant SV40-transformed mouse cells suggests that the replicative function of T antigen is important in generating SV40 DNA rearrangements that allow the expression of "100K" variant T antigens in the transformants.

A frameshift mutation affecting the carboxyl terminus of the simian virus 40 large tumor antigen results in a replication- and transformation-defective virus

We have constructed a frameshift mutation in the simian virus 40 early region using a novel method of oligonucleotide-directed mutagenesis. The mutated DNA specifies an 84,000-dalton large tumor antigen that consists of approximately equal to 75,000 daltons encoded by the wild-type reading frame and 9,000 daltons, by the alternative reading frame (wild-type large tumor antigen is approximately equal to 82,000 daltons). The frameshifted carboxyl terminus of the protein bears a strong similarity to the same region of polyoma virus middle-sized tumor antigen. We have found that the mutant DNA is unable to replicate when introduced into permissive monkey cells and incapable of transforming nonpermissive mouse cells.

Nonviable mutants of simian virus 40 with deletions near the 3' end of gene A define a function for large T antigen required after onset of viral DNA replication

Deletion mutants of simian virus 40 (SV40) with lesions at the three DdeI sites near the 3' end of the early region were constructed. Mutants with deletions at 0.203 and 0.219 map units (mu) which did not change the large T antigen reading frame were viable. This extends slightly the upstream boundary for the location of viable mutants with deletions in the 3' end of the A gene. Mutants with frameshift deletions at 0.193 and 0.219 mu were nonviable. These are the first nonviable mutants with deletions in this portion of the A gene. None of the three nonviable mutants with deletions at 0.219 mu produced progeny viral DNA. These three mutants all used the alternate reading frame located in this portion of the SV40 early region. The mutant with a deletion at 0.193 mu, dlA2459, was positive for viral DNA replication and was defective for adenovirus helper function. All of these mutations were located in the portion of the SV40 large T antigen which has no homology to the polyoma T antigens. These results indicate that this portion of large T antigen is required for some late step in the viral growth cycle and suggest that adenovirus helper function is required for productive infection by SV40.

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.

Improved localization of phosphorylation sites in simian virus 40 large T antigen

The location of phosphorylation sites in the large T antigen of simian virus 40 has been studied both by partial chemical cleavage and by partial proteolysis of various forms of large T. These included the full-size wild-type molecule with an apparent molecular weight of 88,000, deleted molecules coded for by the mutants dl1265 and dl1263, and several shortened derivatives generated by the action of a cellular protease. These molecules differed from each other by variations in the carboxy-terminal end. In contrast, a ubiquitous but minor large T form with a molecular weight of 91,000 was found to be modified in the amino-terminal half of the molecule. In addition to the phosphorylation of threonine at position 701 (K.-H. Scheidtmann et al., J. Virol. 38:59-69, 1981), two other discrete domains of phosphorylation were recognized, one at either side of the molecule. The amino-terminal region was located between positions 81 and 124 and contained both phosphothreonine and phosphoserine residues. The carboxy-terminal region was located between approximate positions 500 and 640 and contained at least one phosphoserine residue but no phosphothreonine. The presence in the phosphorylated domains of large T of known recognition sequences for different types of protein kinases is discussed, together with possible functions of large T associated with these domains.

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.