Different forms of simian virus 40 large tumor antigen varying in their affinities for DNA

Structure of the origin-binding domain of simian virus 40 large T antigen bound to DNA

The large T antigen (T-ag) protein binds to and activates DNA replication from the origin of DNA replication (ori) in simian virus 40 (SV40). Here, we determined the crystal structures of the T-ag origin-binding domain (OBD) in apo form, and bound to either a 17 bp palindrome (sites 1 and 3) or a 23 bp ori DNA palindrome comprising all four GAGGC binding sites for OBD. The T-ag OBDs were shown to interact with the DNA through a loop comprising Ser147-Thr155 (A1 loop), a combination of a DNA-binding helix and loop (His203-Asn210), and Asn227. The A1 loop traveled back-and-forth along the major groove and accounted for most of the sequence-determining contacts with the DNA. Unexpectedly, in both T-ag-DNA structures, the T-ag OBDs bound DNA independently and did not make direct protein-protein contacts. The T-ag OBD was also captured bound to a non-consensus site ATGGC even in the presence of its canonical site GAGGC. Our observations taken together with the known biochemical and structural features of the T-ag-origin interaction suggest a model for origin unwinding.

Polyomavirus large T antigen binds cooperatively to its multiple binding sites in the viral origin of DNA replication

Polyomavirus large T antigen binds to multiple 5'-G(A/G)GGC-3' pentanucleotide sequences in sites 1/2, A, B, and C within and adjacent to the origin of viral DNA replication on the polyomavirus genome. We asked whether the binding of large T antigen to one of these sites could influence binding to other sites. We discovered that binding to origin DNA is substantially stronger at pH 6 to 7 than at pH 7.4 to 7.8, a range often used in DNA binding assays. Large T antigen-DNA complexes formed at pH 6 to 7 were stable, but a fraction of these complexes dissociated at pH 7.6 and above upon dilution or during electrophoresis. Increased binding at low pH is therefore due at least in part to increased stability of protein-DNA complexes, and binding at higher pH values is reversible. Binding to fragments of origin DNA in which one or more sites were deleted or inactivated by point mutations was measured by nitrocellulose filter binding and DNase I footprinting. The results showed that large T antigen binds cooperatively to its four binding sites in viral DNA, suggesting that the binding of this protein to one of these sites stabilizes its binding to other sites via protein-protein contacts. Sites A, B, and C may therefore augment DNA replication by facilitating the binding of large T antigen to site 1/2 at the replication origin. ATP stabilized large T antigen-DNA complexes against dissociation in the presence, but not the absence, of site 1/2, and ATP specifically enhanced protection against DNase I digestion in the central 10 to 12 bp of site 1/2, at which hexamers are believed to form and begin unwinding DNA. We propose that large T antigen molecules bound to these multiple sites on origin DNA interact with each other to form a compact protein-DNA complex and, furthermore, that ATP stimulates their assembly into hexamers at site 1/2 by a "handover" mechanism mediated by these protein-protein contacts.

Assembly of T-antigen double hexamers on the simian virus 40 core origin requires only a subset of the available binding sites

Initiation of simian virus 40 (SV40) DNA replication is dependent upon the assembly of two T-antigen (T-ag) hexamers on the SV40 core origin. To further define the oligomerization mechanism, the pentanucleotide requirements for T-ag assembly were investigated. Here, we demonstrate that individual pentanucleotides support hexamer formation, while particular pairs of pentanucleotides suffice for the assembly of T-ag double hexamers. Related studies demonstrate that T-ag double hexamers formed on "active pairs" of pentanucleotides catalyze a set of previously described structural distortions within the core origin. For the four-pentanucleotide-containing wild-type SV40 core origin, footprinting experiments indicate that T-ag double hexamers prefer to bind to pentanucleotides 1 and 3. Collectively, these experiments demonstrate that only two of the four pentanucleotides in the core origin are necessary for T-ag assembly and the induction of structural changes in the core origin. Since all four pentanucleotides in the wild-type origin are necessary for extensive DNA unwinding, we concluded that the second pair of pentanucleotides is required at a step subsequent to the initial assembly process.

The initiation of simian virus 40 DNA replication in vitro

DNA replication is a complicated process that is largely regulated during stages of initiation. The Siman Virus 40 in vitro replication system has served as an excellent model for studies of the initiation of DNA replication, and its regulation, in eukaryotes. Initiation of SV40 replication requires a single viral protein termed T-antigen, all other proteins are supplied by the host. The recent determination of the solution structure of the T-antigen domain that recognizes the SV40 origin has provided significant insights into the initiation process. For example, it has afforded a clearer understanding of origin recognition, T-antigen oligomerization, and DNA unwinding. Furthermore, the Simian virus 40 in vitro replication system has been used to study nascent DNA formation in the vicinity of the viral origin of replication. Among the conclusions drawn from these experiments is that nascent DNA synthesis does not initiate in the core origin in vitro and that Okazaki fragment formation is complex. These and related studies demonstrate that significant progress has been made in understanding the initiation of DNA synthesis at the molecular level.

Biochemical characterization of phosphorylation site mutants of simian virus 40 large T antigen: evidence for interaction between amino- and carboxy-terminal domains

The simian virus 40 large T antigen is phosphorylated at eight or more sites that are clustered in an amino-terminal region and a carboxy-terminal region of the protein. Mutants carrying exchanges at these phosphorylation sites have been generated in vitro by bisulfite or oligonucleotide-directed mutagenesis and analyzed for their phosphorylation patterns. Two-dimensional phosphopeptide analyses of the mutant large T antigens confirmed most of the previously identified phosphorylation sites, namely, serine residues 106, 112, 123, 639, 677, and 679 and threonine residues 124 and 701. In addition, serine residue 120 was identified as a new site, whereas serines residues 111 and 676 were excluded. Interestingly, several of the mutants exhibited secondary effects in that a mutation in the amino-terminal region affected phosphorylation at distant and even carboxy-terminal sites and vice versa. Thus, the amino- and carboxy-terminal domains appear to be in close proximity in the three-dimensional structure of large T antigen. The possible consequences of the above findings and the role of phosphorylation are discussed.

The DNA-binding properties of polyomavirus large T antigen are altered by ATP and other nucleotides

We have examined the influence of ATP on the DNA-binding properties of polyomavirus large T antigen (Py TAg). Utilizing nitrocellulose filter binding, DNase I footprinting, and gel mobility shift assays, we observed that ATP increased Py TAg binding to DNA fragments containing either all Py TAg-binding sites (whole origin) or those sites within (core origin) or adjacent to (early) the origin of replication. Even nonspecific binding to DNA fragments lacking Py TAg-binding sites was increased somewhat by ATP. Binding to the core origin was increased to a greater extent than binding to other DNA fragments tested. Gel band mobility shift assays revealed that ATP increased the production of core origin-specific Py TAg-DNA complexes of high molecular weight. ATP stimulation depended on the presence of MgCl2. Other nucleotides and nonhydrolyzable ATP analogs also increased Py TAg binding to the core origin but to various degrees: ATP, dATP, 5'-adenylyl imidodiphosphate (AMPPNP) greater than 5'-adenylyl methylenediphosphate (AMPPCP) greater than dCTP greater than UTP greater than TTP. GTP and dGTP did not increase DNA binding by Py TAg. The rates of association and disassociation of Py TAg with all the DNA fragments were altered by the presence of ATP. DNase I footprinting showed that ATP extensively extended the region protected within the core origin and also produced a distinctive DNase I-hypersensitive site on the late strand at nucleotides 5255 to 5262 (TTACTATG).

Altered phosphorylation pattern of simian virus 40 T antigen expressed in insect cells by using a baculovirus vector

The phosphorylation pattern of simian virus 40 (SV40) large tumor (T) antigen purified from insect cells infected with a recombinant baculovirus was compared with that reported previously for T antigen from SV40-infected monkey cells. The specific activity of metabolic phosphate labeling of baculovirus T antigen was reduced, and the phosphopeptide map of the baculovirus protein, while qualitatively similar to that of lytic T, revealed several quantitative differences. The most striking difference was the prominence in the baculovirus map of peptides containing phosphothreonine 124. These peptides are known to arise from other phosphopeptides upon dephosphorylation of neighboring serines, suggesting that baculovirus T may be underphosphorylated at these serines and perhaps other sites. Functional assays used to further investigate the phosphorylation state of the baculovirus protein included SV40 DNA binding after enzymatic dephosphorylation with alkaline phosphatase and after phosphorylation by a murine homolog of cdc2 protein kinase. The results imply that baculovirus T antigen is underphosphorylated, in particular at those serine residues whose phosphorylation is responsible for down regulation of DNA-binding activity at site II in the core origin of DNA replication. In contrast, no evidence for a functionally significant underphosphorylation at threonine 124 could be found.

The retinoblastoma susceptibility gene product undergoes cell cycle-dependent dephosphorylation and binding to and release from SV40 large T

Synchronized monkey cells pulse-labeled with [35S]-methionine and chased for various lengths of time were extracted, and immunoprecipitations were performed using monoclonal antibodies directed against the retinoblastoma protein (RB) and SV40 T antigen (T). By following a discrete population of these two proteins through the cell cycle, the following information was obtained. RB, which is wholly unphosphorylated in G1, became phosphorylated at the beginning of S and remained phosphorylated through S and G2. RB was, then, completely dephosphorylated between the end of G2 and the beginning of G1. Second, while all of the detectable unphosphorylated RB can be found complexed with T, these complexes present during G1 dissociated in S and reformed again in M or early G1. Finally, T molecules appeared to oligomerize prior to binding RB. Thus, complex formation between T and RB may be regulated in part by the cell cycle-dependent phosphorylation and dephosphorylation of RB and by the quaternary structure of T.

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

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

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

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

SV40 T-antigen as a dual oncogene: structure and function of the plasma membrane-associated population

SV40 T-antigen (T-ag) is localized in both the nucleus (nT-ag) and plasma membrane (pmT-ag) of cells and provides multiple functions necessary for cell transformation. The pmT-ag population is structurally very similar to the nT-ag. Transport to the cell surface is by an unknown mechanism that does not involve the secretory pathway. The disposition of T-ag in the membrane exposes both the amino and the carboxyl terminus on the exterior of the cell. Nuclear-transport-defective mutants of T-ag can transform established cells in culture, but not primary cells, suggesting that non-nuclear forms of T-ag may mediate some transformation-related process(es). A non-cytolytic protein extraction technique utilizing 1-butanol solubilized from SV40-transformed cells a multimeric complex composed of pmT-ag and at least five cellular proteins ranging in size from 35,000 (35K) to 60K M. Both amino- and carboxylterminal T-ag-specific monoclonal antibodies co-precipitated T-ag and the 35-60K Mr proteins, but antibodies against the internal portion of T-ag precipitated only uncomplexed T-ag. The growth state of the cells markedly influenced the expression of the T-ag-containing surface complexes; more complexes were recovered from actively dividing cells than from confluent cell cultures, and suspension cells yielded more complexes than cells on a substratum. The complex exhibited a highly dynamic association with the cell membrane, as demonstrated by pulse-chase analysis. The characteristics of growth-dependent expression and rapid turnover rate suggest a functional role for the membrane complex. The identities of the cellular proteins in the complex with pmT-ag are unknown, although one member (56K) is recognized by p53-specific monoclonal antibodies.

Preferential binding of simian virus 40 T-antigen dimers to origin region I

The sequence components that direct high-affinity binding of simian virus 40 (SV40) T antigen to SV40 origin region I are composed of two recognition pentanucleotides separated by a spacer. This region has binding sites for two T-antigen monomeric units. We extended the tripartite region I sequence by one and two sets of spacers and pentanucleotides and also shortened the region by one pentanucleotide. Our T-antigen-binding studies with these constructs show that the protein has a strong preference for binding to an even rather than an odd number of pentanucleotides separated by spacer sequences. Gel retardation assays reveal that the size of the complex formed between the 17-base-pair region I sequence and T antigen did not increase when the sequence was extended with one spacer-pentanucleotide sequence but did increase with two such units. DNase I footprinting and fragment assay experiments indicate that the protein did not protect a pentanucleotide that was not paired with another pentanucleotide. The unpaired pentanucleotide resumed its binding activity when it was paired with a spacer and another pentanucleotide sequence. We propose that T antigen binds to region I as a preformed dimer.

The murine p53 protein blocks replication of SV40 DNA in vitro by inhibiting the initiation functions of SV40 large T antigen

We have characterized the effect of murine p53 on SV40 DNA replication in vitro. Purified wild-type murine p53 dramatically inhibited the ability of SV40 T antigen to mediate the replication of a plasmid bearing the viral origin (ori-DNA) in vitro. In contrast, polyoma ori-DNA replication in vitro was unaffected by p53. Surprisingly, both unbound p53 and SV40 T antigen-bound p53 were equally detrimental to SV40 ori-DNA replication. Thus, p53 interferes with interactions between T antigen molecules that are required for DNA synthesis. p53 inhibited the binding to and subsequent unwinding of the SV40 origin by T antigen and thus selectively blocked the initial stages of ori-DNA replication. In contrast to the nononcogenic wild-type murine p53, high concentrations of a mutant transforming p53 failed to block SV40 ori-DNA replication in vitro. These observations may provide insight into a possible role for p53 in the cell.

ATP-dependent assembly of double hexamers of SV40 T antigen at the viral origin of DNA replication

Simian virus 40 (SV40) replicates in nuclei of human and monkey cells. One viral protein, large tumour (T) antigen, is required for the initiation of DNA replication. The development of in vitro replication systems which retain this property has facilitated the identification of the cellular components required for replication. T antigen recognizes the pentanucleotide 5'-GAGGC-3' which is present in four copies within the 64 base-pairs (bp) of the core origin. In the presence of ATP it binds with increased affinity forming a distinctive, bilobed structure visible in electron micrographs. As a helicase, it unwinds SV40 DNA bidirectionally from the origin. We report here that in vitro and in the presence of ATP, T antigen assembles a double hexamer, centred on the core origin and extending beyond it by 12 bp in each direction. The assembly of this dodecamer initiates an untwisting of the duplex by 2-3 turns. In the absence of ATP, a tetrameric structure is the largest found at the core origin. In the absence of DNA, but in the presence of ATP or its non-hydrolysable analogues, T antigen assembles into hexamers. This suggests that ATP effects an allosteric change in the monomer. The change alters protein-protein interactions and allows the assembly of a double hexamer, which initiates replication at the core origin.

Analysis of mechanisms controlling the interactions of SV40 large T antigen with the SV40 ORI region

We have characterized the interactions of simian virus 40 (SV40) large tumor antigen (large T) with the control region of the SV40 genome, the SV40 ORI, by analyzing the specific binding of large T antigen to SV40 wild-type origin DNA and to isolated binding sites I and II, respectively. DNA binding affinities of large T antigen were determined under standardized conditions and DNA excess, using a target-bound DNA binding assay (M. Hinzpeter, E. Fanning, and W. Deppert, 1986, Virology 148, 159-167). Our results show that large T antigen exhibits similar affinities for isolated binding sites I and II and for combined sites I and II on wild-type ORI DNA. When the fraction of large T antigen molecules (calculated per large T antigen monomers) able to bind specifically to these sites was determined (DNA binding activity of large T antigen) we found that only 2% of large T antigen molecules present in extracts of lytically infected cells were able to bind to isolated site II, whereas about 50% bound to isolated site I. However, only about 10% of large T antigen molecules bound to the complete wild-type ORI, containing combined binding sites I and II. Thus, a much larger proportion of large T antigen molecules is capable of binding specifically to site I as is suggested by analysis of large T antigen binding to combined sites I and II on the SV40 wild-type ORI. These findings indicate that the interaction of large T antigen with the SV40 wild-type ORI is restricted on one hand by the ability of large T antigen to bind to site II, and on the other hand by the spatial arrangement of binding sites I and II on the SV40 wild-type ORI.

Only one of the origin binding forms of SV40 T antigen has helicase activity

SV40 T antigen exists in monomeric and multimeric forms. We have separated the individual components by glycerol gradient centrifugation. Helicase activity is found to be associated with monomeric forms only. Dimers and other multimeric forms have no discernable helicase activity. However, results obtained from DNA binding experiments carried out with separated forms of T antigen indicate that both monomers and dimers bind to region I and region II of SV40 origin of replication. Possibly monomeric T antigen unwinds DNA at the replication fork while both monomeric and dimeric forms are utilized for positioning of T antigen at the origin of replication.

Variable expression of SV40 large T antigen in CV1 cell clones

Using immunofluorescence and immunoadsorption, CV1 cell clones MA2, V4, USA3, TR7 and P3 infected with SV40 were found to express variably SV40 large T antigen. The monoclonal antibody used was Pab 419. The results indicate that P3 cells express T antigen to a considerable level as early as 10 h post-infection, while that of TR7 and USA3 cells is minute as judged from their positive nuclei. MA2 and V4 cells did not show any positive nuclei over this period of infection. At 20 h post-infection MA2, V4 and USA3 cells developed a considerable amount of fluorescence in their nuclei while TR7 and P3 cells produced high values. By immunoadsorption of cell extracts for the same periods of infection, similar results were obtained on the electrophoretograms. We also relate these findings with those from induction of heatshock proteins by SV40 infection.

Oligomerization and origin DNA-binding activity of simian virus 40 large T antigen

Simian virus 40 (SV40) large tumor antigen (T antigen) exists in multiple molecular forms, some of which are separable by zone velocity sedimentation of soluble extracts from infected monkey cells. Three subclasses of this antigen from SV40-infected monkey cells have been separated and characterized: the 5S, 7S, and 14S forms. Newly synthesized T antigen occurs primarily in the 5S form. Chemical cross-linking provided evidence that the 14S form is primarily a tetramer, whereas the 5S and 7S forms could not be cross-linked into oligomers. The DNA-binding properties of each subclass were investigated after immunopurification. The affinities of the three forms for SV40 DNA and for a synthetic 19-base-pair sequence from binding site I are very similar (equilibrium dissociation constant [KD], 0.3 to 0.4 nM). The specific activity of DNA binding was greatest for the 5S and 7S subclasses and least for the 14S subclass. Moreover, the specific activity of the 5S and 7S subclasses increased sharply at about 40 h after infection, whereas the activity of the 14S subclass was maintained at a constant low level throughout infection. A model relating oligomerization and DNA binding of T antigen in infected cells is presented.

trans-dominant defective mutants of simian virus 40 T antigen

We constructed a collection of linker insertion mutants in the simian virus 40 (SV40) genome and studied several of these with changes limited to a part of the large T antigen gene corresponding to an amino acid sequence shared with other ATPases. Two of these mutants were found to have a novel phenotype in that they could not be complemented for plaque formation by a late-region deletion mutant. These two mutants, in contrast to other mutants in this region, were able to transform rat cells in culture at a frequency close to that of the wild-type gene. The noncomplementing mutants were found to be potent inhibitors of SV40 DNA replication despite the presence of wild-type T antigen in the transfected cells. This inhibition was shown to be the result of the introduced mutations in the large T antigen gene. We conclude that the large T antigens of the noncomplementing mutants can act as inhibitors of SV40 DNA replication.

Phosphorylation downregulates the DNA-binding activity of simian virus 40 T antigen

Proteolytic fragments of simian virus 40 tumor (T) antigen and T antigen that was dephosphorylated with alkaline phosphatase bound between 1.5 to 2 times more origin-containing simian virus 40 DNA than did intact T antigen in DNA saturation experiments. Kinetic experiments showed that these treatments also enhanced the rate at which T antigen bound to the DNA. The enhanced binding of T-antigen fragments correlated with the generation of DNA-binding fragments that lacked the NH2-terminal region. Dephosphorylation of T antigen in vitro resulted in the removal of phosphate groups from the NH2-terminal region as well as from the COOH-terminal region. To test the effects of dephosphorylation on the size of the protein, immunoaffinity-purified T antigen was subjected to sedimentation with and without prior treatment with alkaline phosphatase. Most of the purified protein sedimented as a monomer and no significant effect was observed after dephosphorylation, indicating that the enhanced DNA-binding activity was probably not due to the uncovering of additional binding sites buried specifically in oligomerized T antigen. Taken together, these results indicate that in vivo phosphorylation of the NH2-terminal region (residues 106 to 124) decreases the binding of the protein to the DNA origin. The effect is reversed by in vitro dephosphorylation or by proteolysis which removes the highly phosphorylated NH2-terminal arm of the polypeptide. We suggest that phosphorylation inactivates one of two distinct DNA-binding activities on the polypeptide chain perhaps corresponding to two separate regions in T antigen.

Biochemical properties of a transforming nonkaryophilic T antigen of SV40

We reconstructed into wt SV40 DNA a previously described deletion of the A gene, eliminating amino acids 110 through 152 of the large T (L. Fischer-Fantuzzi and C. Vesco (1985) Proc. Natl. Acad. Sci. USA 82, 1891-1895); the gene product of the new recombinant pACTSV2, like the previous product, has a cytoplasmic instead of a nuclear localization and efficiently transforms NIH3T3 cells. Three main functions of this nonkaryophilic large T (NKLT) were examined, and the results obtained were the following: the NKLT does not bind to the SV40 origin DNA under conditions where the normal large T shows specific binding; the NKLT has conserved the ability to form high molecular weight aggregates; the NKLT becomes phosphorylated in vivo at only two residues: serine 639 and threonine 701. This indicates that the NH2-terminal phosphorylation of the large T is unnecessary for established-cell transformation. In addition, this and previous evidence (K. H. Scheidtmann et al. (1984) J. Virol. 50, 636-640) suggest that the lack of phosphorylation in serines 106, 676, 677, and 679 may constitute a characteristic of the large T molecules with extranuclear localization.

Replication and transformation functions of in vitro-generated simian virus 40 large T antigen mutants

We used sodium bisulfite mutagenesis to introduce point mutations within the early region of the simian virus 40 genome. Seventeen mutants which contained amino acid changes in the amino-terminal half of the large T antigen coding sequence were assayed for their ability to replicate viral DNA and to induce transformation in the established rodent cell line Rat-3. The mutants fell into four basic classes with respect to these two biological functions. Five mutants had wild-type replication and transformation activities, six were totally defective, three were replication deficient and transformation competent, and two were replication competent and transformation deficient. Within these classes were mutants which displayed intermediate phenotypes, such as four mutants which were not totally deficient in viral replication or cellular transformation but instead showed reduced large T antigen function relative to wild type. Three large T mutants displayed transforming activity that was greater than that of wild type and are called supertransforming mutants. Of the most interest are mutants differentially defective in replication and transformation activities. These results both support and extend previous findings that two important biological functions of large T antigen can be genetically separated.

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.

SV40 large T-antigen and transformation related protein p53 are associated in situ with nuclear RNP structures containing hnRNA of transformed cells

The localization of SV40 large T-antigen (T-Ag) and the cellular protein p53 in the nuclei of mouse and human SV40-transformed cells and of a methylcholanthrene-transformed mouse cell line, was studied. Their detection by ultrastructural immunocytochemistry with specific monoclonal antibodies employed two complementary methods used in parallel. These consisted of indirect immunoperoxidase labelling carried out before embedment on Triton-permeabilized cells, or indirect immunogold labelling applied to thin sections of cells embedded in Lowicryl K4M. The results indicate that in SV40-transformed cells both proteins are chiefly localized on peri- and interchromatin RNP fibrils. This shows that they occur in structures involved in the synthesis and processing of hnRNA. The nucleoli and chromatin did not appear to be labelled. In methylcholanthrene-transformed cells the protein p53 (in the absence of large T-Ag) was also detected on peri- and interchomatin fibrils. Taken together with recent results which demonstrated that, during lytic infection, T-Ag was associated chiefly with cellular chromatin (Harper, F, Florentin, Y & Puvion, E, Exp cell res 161 (1985) 434) [33], our experiments provide evidence that the transforming function of SV40 large T-Ag is dissociable from its function in SV40 lytic infection in terms of its subnuclear distribution.

A subclass of polyomavirus middle tumor antigen binds to DNA cellulose

We examined the binding of polyomavirus large (L-T)-, middle (M-T)-, and small-tumor antigens to DNA cellulose. At pH 6.0, the majority of L-T bound to calf thymus DNA cellulose, while little or no small tumor antigen was retained under these conditions. Unexpectedly, a small but reproducible proportion of M-T bound to both native and denatured DNA cellulose. M-T encoded by polyomavirus mutant dl 8, which expressed shortened L-T and M-T, bound to DNA, indicating that the deleted sequences are not required for DNA binding. Also, M-T from transformed BMT-1 rat cells, which synthesize exclusively this polyomavirus tumor antigen, bound to DNA, indicating that its binding is not due to association with other polyomavirus-encoded proteins. Using the DNA fragment immunoassay, we found that, under conditions in which L-T bound specifically to DNA fragments containing viral regulatory sequences, no viral DNA fragments were bound by M-T. The existence of distinct subpopulations of M-T that differ in their DNA-binding properties was indicated by rebinding experiments in which M-T that had bound to DNA cellulose rebound very efficiently, while that which had not been originally retained by DNA cellulose rebound poorly. Furthermore, the M-T-pp60 c-src complex did not bind to DNA cellulose. These data suggest that polyomavirus M-T is heterogeneous, consisting of populations of molecules that differ in their interactions with DNA cellulose.

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.

Simian virus 40 origin DNA-binding domain on large T antigen

Fifty variant forms of simian virus 40 (SV40) large T antigen bearing point, multiple point, deletion, or termination mutations within a region of the protein thought to be involved in DNA binding were tested for their ability to bind to SV40 origin DNA. A number of the mutant large T species including some with point mutations were unable to bind, whereas many were wild type in this activity. The clustering of the mutations that are defective in origin DNA binding both reported here and by others suggests a DNA-binding domain on large T maps between residues 139 and approximately 220, with a particularly sensitive sequence between amino acids 147 and 166. The results indicate that the domain is involved in binding to both site I and site II on SV40 DNA, but it remains unclear whether it is responsible for binding to cellular DNA. Since all the mutants retain the ability to transform Rat-1 cells, we conclude that the ability of large T to bind to SV40 origin DNA is not a prerequisite for its transforming activity.

Simian virus 40 large T antigen oligomers: analysis of electrophoresis in the absence of detergent

Large T antigen of simian virus 40 is found as monomeric and oligomeric species in transformed cells. These can be demonstrated in cell extracts by velocity centrifugation in sucrose gradients. We analyzed them further in a transformed human line cell (SV80) and a transformed mouse line cell (SVT2). Individual fractions from sucrose gradients were subjected to polyacrylamide gel electrophoresis in the absence of detergent. T-antigen species were then detected by protein blotting and antibody overlay with polyclonal anti-D2 T antibody or monoclonal Pab419, Pab101, or Pb1700 antibody. The rapidly sedimenting species (14S and larger) of large T antigen from both cell lines reproducibly showed two major bands with estimated molecular weights of 670,000 and 850,000. A third band of 1,200,000 was more prominent in SVT2 cells than in SV80 cells. In SV80 cells the slowly sedimenting species of large T antigen (5S to 11S) contained two reproducible bands. A band with a molecular weight of 95,000 was the predominant one in all fractions between 5S and 11S. A relatively minor band with a molecular weight of 230,000 was found in fractions between 9S and 11S. The low-molecular-weight forms were seen in SVT2 cells only when a prominent peak at 5S to 7S was present, that is, when extracts were stored before analysis. In fresh extracts, the low-molecular-weight bands and slowly sedimenting forms were absent.

Functional implications of oligomerization of simian virus 40 large T antigen during lytic virus infection

The formation of oligomers of simian virus 40 (SV40) large T antigen in SV40-infected and -transformed monkey cells was analyzed by sucrose density gradient centrifugation. The overall distribution of total T antigen during lytic infection showed mainly low-molecular-weight forms (monomers and dimers) in the early phase (10 h postinfection) and an increase in the number of oligomers in the late phase of the lytic cycle (36 h postinfection), indicating an accumulation of these final products. In contrast, studying the conversion of newly synthesized T antigen into oligomers by appropriate pulse-chase radiolabeling of infected cells revealed that this processing decelerates considerably during the late phase of infection. This mechanism can be reaccelerated by blocking DNA replication with aphidicolin. Since none of these results could be obtained by using synchronized SV40-transformed monkey cells (COS-1), these observations are compatible with the idea that the process of T antigen oligomerization may be involved in viral, but not in cellular, DNA synthesis.

Oligomerization of simian virus 40 tumor antigen may be involved in viral DNA replication

Biological implications of the oligomerization of simian virus 40 (SV40) large T antigen for viral DNA replication were studied by using two temperature-sensitive SV40 A-gene mutants, tsA 58 and tsA 1499. Both mutants are defective at elevated temperature for viral DNA replication whereas tsA 58 is like most other tsA mutants additionally heat sensitive for cell transformation. We found that in contrast to tsA 58 encoded T antigen, tsA 1499 T antigen is thermostable in the ability to bind specifically to the origin of replication of SV40 DNA. Detailed structural analysis of tsA 1499 T antigen by sucrose density gradient centrifugation revealed that it is strictly temperature sensitive for the formation of homologous oligomers but, as we reported previously (M. Montenarh, M. Kohler, and R. Henning, 1984, J. Virol, 49, 658-664), not for the association with the cellular phosphoprotein p53. These observations are compatible with the idea that, in addition to the specific origin-binding ability as well as other functional features, the oligomerization of T antigen may be essential for viral DNA replication.

Monomers through trimers of large tumor antigen bind in region I and monomers through tetramers bind in region II of simian virus 40 origin of replication DNA as stable structures in solution

Large tumor (T) antigen and its bound multimeric states are positioned by scanning transmission electron microscopy (STEM) within a few base pairs at control sequences of the simian virus 40 DNA origin of replication region. Proximal and distal edge positions for each multimer group match the end positions of previously mapped fragments protected from DNase cleavage. Since chance correspondence is shown to be extremely unlikely, STEM mass measurements, obtained concurrently with STEM map positions, indicate that the DNase fragments arise from bound monomers, dimers, trimers, and tetramers in binding region II and monomers, dimers, and trimers in binding region I. Simultaneous binding of seven monomer-equivalent masses is observed, three in region I and four in region II, with an ordered and interpretable mass distribution in the plane of the foil. Although this observation does not prove that the six G-A-G-G-C and one T-A-G-G-C sequences, similarly distributed, function as recognition sequences for T-antigen monomer, it provides strong support for such a model. The stable existence in solution of low-and intermediate-mass structures, observed at lower T-antigen concentrations, suggests a role as assembly intermediates.

DNA-binding properties of phosphorylated and dephosphorylated D2-T antigen, a simian-virus-40 T-antigen-related protein

I have examined the role of phosphorylation of D2-T antigen in its DNA-binding properties and ATPase activity. Treatment of partially purified D2-T antigen with alkaline phosphatase resulted in removal of maximally 90% of the phosphate label associated with the radio-labeled protein. The specific and nonspecific DNA-binding properties of partially dephosphorylated D2-T antigen were identical to those of the untreated control. In contrast, acid phosphatase was able to dephosphorylate D2-T antigen quantitatively. The general affinity for DNA of the completely dephosphorylated protein was unchanged or eventually slightly increased. However, its specific affinity for a restriction fragment containing the canonical T-antigen-binding sites was drastically reduced as shown by competition with unlabeled salmon sperm DNA. The results imply that nonspecific DNA binding of D2-T antigen is unaffected by phosphorylation whereas a specific phosphorylation site seems to be involved in the formation and/or stabilization of the specific protein-DNA complex. On the other hand, the ATPase activity of D2-T antigen seems to be unaffected by the degree of phosphorylation.

Modification of simian virus 40 large tumor antigen by glycosylation

The SV40-encoded transforming protein, large tumor antigen (T-ag), is multifunctional. Chemical modifications of the T-ag polypeptide may be important for its multifunctional capacity. T-ag is additionally modified by glycosylation. T-ag was metabolically labeled in SV40-infected cells with tritiated galactose or glucosamine, but not with mannose or fucose. The identity of glycosylated T-ag was established by immunoprecipitation with a variety of T-ag-specific antisera, including monoclonal antibodies. Incorporation of labeled sugar into T-ag was inhibited in the presence of excess unlabeled sugars, but not in the presence of excess unlabeled amino acids. Labeled monosaccharides could be preferentially removed from T-ag with a mixture of glycosidic enzymes. In addition, galactose was removed from purified T-ag by acid hydrolysis and identified as such by thin-layer chromatography. T-ag oligosaccharides were resistant to treatment with EndoH, and glycosylation was not inhibited by tunicamycin. Together, these data strongly suggest that T-ag is glycosylated. Several characteristics, including lack of mannose labeling, EndoH resistance, and tunicamycin resistance, suggest that T-ag is not an N-linked glycoprotein. Rather, these properties are more consistent with the identification of T-ag as an O-linked glycoprotein.

Relationship of phosphorylation to the oligomerization of SV40 T antigen and its association with p53

The potential significance of the phosphorylation of SV40 large T antigen for oligomers and complexes with the cellular protein p53 was investigated. We observed that T antigen oligomers remain stable after enzymatic dephosphorylation by alkaline phosphatase up to 80%. Separate analysis of free and p53-bound T antigen revealed a considerably lower phosphorylation of the p53-bound subclass. Therefore, a simple correlation between the overall phosphorylation of T antigen and the formation of oligomers and T-p53 complexes is highly unlikely.

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

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

Two classes of transformation-deficient, immortalization-positive simian virus 40 mutants constructed by making three-base insertions in the T antigen gene

We constructed two mutants of simian virus 40 (SV40) by introducing a three-base duplication at AvaII cutting sites within the large T antigen coding region, and we examined these mutants for their abilities to replicate in monkey GC7 cells, to transform rat cell line 3Y1 cells, and to transform and immortalize primary cells from newborn rats. Neither of the mutants could replicate in GC7 cells. One mutant with the duplication at 0.335 SV40 map units (m.u.) (inA942) could transform 3Y1 cells, but the other mutant with the duplication at 0.636 m.u. (inA941) could not. The two mutants could not transform primary rat cells but retained immortalization activity. The results suggest that transformation of primary cells by SV40 requires at least two distinct activities of the large T antigen, one of which can be replaced by a cellular function(s) expressed in immortalized 3Y1 cells.

DNA-binding proteins of human placenta: purification and characterization of an endonuclease

DNA binding proteins present in the cytoplasm and nuclei of term placenta were isolated by DNA-cellulose chromatography and analysed by electrophoresis in high resolution polyacrylamide gradient gels. A denatured DNA specific protein of approximate molecular weight 34 000 daltons was the predominant DNA binding protein of the cytoplasm; this protein consisted of over 65% of the total DNA binding proteins of the 0.15 M NaCl eluate of the cytoplasm. The cytoplasmic extracts contained two additional DNA binding proteins of molecular weight 24 000 and 18 000 daltons and these proteins bound preferentially to ds DNA. All the three DNA binding proteins were also present in the nuclei and electrophoresis of histones in adjacent lanes indicated that they are not histones. The 34 000-dalton DNA binding protein has been purified by ammonium sulphate fractionation followed by phosphocellulose (PC) chromatography. The DBP eluted from the PC column between 0.125-0.15 M potassium phosphate. PC fractions containing electrophoretically pure 34 KD DBP showed an endonuclease activity capable of converting plasmid pBR 322 DNA to the linear form. Maximum endonucleolytic activity was observed in the presence of 3-5 mM Mg2+ and the enzyme activity was completely inhibited by 3 mM ethylenediamine tetraacetate.

In vitro mutagenesis of a putative DNA binding domain of SV40 large-T

A large number of deletion and point mutations were introduced into a small region of the SV40 large-T gene that was believed to encode part of a DNA-binding domain. The majority of mutant proteins constructed were unable to stimulate viral DNA replication, but all retained at least some transforming activity. Those replication-defective mutants with lesions affecting amino acid residues between 144 and 156 were postulated also to be defective in the autoregulation function of large-T to account for their ability to transform Rat-1 cells more avidly than wild-type. Two mutants (Glu 107----Lys and Ser 189----Asn) were isolated which exhibited severely reduced transforming activity but which supported normal rates of virus and viral DNA replication. Mutation of individual serine and threonine phosphorylation sites within the amino-terminal half of large-T had little effect on the protein's transforming activity. These and other mutations that affected amino acid residues either side of the region from 127 to 133, previously shown to be essential to the nuclear localisation of large-T [D. Kalderon, W. D. Richardson, A. F. Markham, and A. E. Smith (1984) Nature (London) 311, 33-38] did not discernibly impair nuclear accumulation.

Immunoprecipitation of some forms of simian virus 40 large-T antigen by antibodies to synthetic peptides

Antibodies were raised against six synthetic peptides corresponding to overlapping amino acid sequences (106 through 145) from a putative DNA binding domain in simian virus 40 (SV40) large-T antigens. All six antipeptide sera immunoprecipitated large-T from crude extracts of SV40-transformed cells, but the efficiency varied widely; in general, antibodies to the longer peptides produced the strongest anti-large-T activity. Antisera were purified by immunoaffinity chromatography on immobilized peptide. The purified antisera recognized only some forms of large-T; full-sized large-T from transformed cells, super-T from SV3T3 C120 cells, and 70,000-dalton T-antigen from Taq-BamHI cells were immunoprecipitated, whereas large-T from productively infected cells reacted irreproducibly, and the full-sized protein, synthesized in vitro or eluted from sodium dodecyl sulfate-containing gels, and the 33,000- and 22,000-dalton truncated large-Ts from Swiss SV3T3 and MES2006 cells, respectively, were not immunoprecipitated. This pattern of reactivity was explained when extracts were fractionated by sucrose density centrifugation, and it was found that only rapidly sedimenting forms of large-T were immunoprecipitated by the antipeptide sera; that is, large-T complexed with nonviral T antigen was detected, whereas lighter forms were not detected. Cascade immunoprecipitations did not support the view that this result was caused by the low affinity of the peptide antisera for large-T, and Western blotting experiments confirmed that the peptide antisera react directly with immobilized, monomeric large-T but not with nonviral T antigen. Immunoprecipitation assays to detect large-T:nonviral T antigen complexes bound specifically to fragments of SV40 DNA showed that under conditions of apparent antibody excess, DNA still bound to the complex.

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

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

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.

DNA-binding activity of simian virus 40 large T antigen correlates with a distinct phosphorylation state

The state of phosphorylation and the relationship of various subclasses of simian virus 40 large T antigen (large T) differing in DNA-binding activity, degree of oligomerization, age, and subcellular distribution were investigated. Young large T (continuously labeled for 4 h late in infection) comprised about 20% of the total cellular large T. It was phosphorylated to a low degree and existed primarily in a monomeric form, sedimenting at 5S. More than 50% of this fraction bound to simian virus 40 DNA, preferentially to origin-containing sequences. Old large T (continuously labeled for 17 h, followed by a 4-h chase) represented the majority of the population. It was highly phosphorylated and predominantly in an oligomeric form, sedimenting at 15S to 23S. Only 10 to 20% of this fraction bound to simian virus 40 DNA. Another subclass of large T which was extracted from nuclei with 0.5 M salt resembled newly synthesized molecules in all properties tested; it was phosphorylated to a low degree, sedimented at 5S, and bound to viral DNA with high efficiency (greater than 70%). Two-dimensional phosphopeptide analysis of the individual subclasses revealed two distinct phosphorylation patterns, one characteristic for young, monomeric, and DNA-binding large T, the other for old, oligomeric, and non-DNA-binding large T. All sites previously identified in unfractionated large T (K.H. Scheidtmann et al., J. Virol. 44:116-133, 1982) were also phosphorylated in the various subclasses, but to different degrees. Peptide maps of the DNA-binding fraction, the 5S form, and the nuclear high-salt fraction showed two prominent phosphopeptides not previously characterized. Both peptides were derived from the amino-terminal region of large T, presumably involved in origin binding, and probably represent partially phosphorylated intermediates of known phosphopeptides. Our data show that the DNA-binding activity, age, and oligomerization of large T correlate with distinct states of phosphorylation. We propose that differential phosphorylation might play a role in the interaction of large T with DNA.

Polyomavirus and simian virus 40 large T antigens bind to common DNA sequences

The large T antigens of polyomavirus and simian virus 40 (SV40) recognize and bind to specific, noncoding DNA sequences which are located between the beginning of the early and late transcription units in their respective genomes. Each large T antigen binds to multiple sites within this intergenic DNA stretch. Polyomavirus large T antigen binds to at least two sites within its DNA, and SV40 large T antigen binds to three sites within SV40 DNA. Comparison of the DNA sequences which comprise the binding sites in polyomavirus DNA or those which make up the binding sites in SV40 DNA has led to recognition of a common sequence, -GAGGC-, which is repeated within each large-T-antigen-binding site. We tested the hypothesis that repeats of this pentanucleotide form the recognition-binding site for polyomavirus and SV40 large T antigen. This was accomplished by measuring the binding of each large T antigen to both polyomavirus and SV40 DNA and to synthetic DNA substrates which did or did not contain repeats of the -GAGGC- sequence. Polyomavirus large T antigen bound to specific fragments of SV40 DNA, and SV40 large T antigen bound with specificity to polyomavirus DNA. In each case, the DNA fragments bound by the heterologous large T antigen were the same as those bound by the homologous large T antigen. Moreover, polyomavirus and SV40 large T antigen only bound to synthetic DNA substrates which contained repeats of the pentameric sequence. This synthetic DNA also competed effectively with native polyomavirus or SV40 DNA as a substrate in binding reactions with one or the other large T antigen. These results led us to conclude that repeats of the -GAGGC- sequence form the recognition-binding site for both polyomavirus and SV40 large T antigen.

Oligomerization of simian virus 40 large T antigen is not necessarily repressed by temperature-sensitive A gene lesions

Simian virus 40 large T antigen is a multifunctional protein which exists in different molecular weight forms. According to several reports, T antigen encoded by temperature-sensitive simian virus 40 A locus mutants (tsA) is unable to oligomerize into high-molecular-weight species. To try to correlate structural and functional properties, we selected tsA58 and tsA1499, both of which are heat sensitive for lytic growth, but only tsA58 is heat sensitive for transformation. Here we report that at permissive and nonpermissive temperatures, T antigen from tsA1499-infected monkey cells retained the ability to oligomerize, whereas reported previously, tsA58 T antigen failed to oligomerize at the nonpermissive temperature. Furthermore, we studied the formation of complexes between T antigen and the cellular p53 protein (T-p53) late in infection. Corresponding to its heat-stable oligomerization properties, T antigen encoded by tsA1499 formed T-p53 complexes regardless of temperature. In contrast, tsA58 encoded T-p53 complexes, preformed at the permissive temperature, remained heat stable after shifting up to the nonpermissive temperature; but at this temperature no new T-p53 complexes arose. The mutants did not replicate viral DNA at the nonpermissive temperature, suggesting that neither the oligomerization of T antigen nor the formation of T-p53 complexes seems to be sufficient for viral DNA replication or for the expression of late viral proteins.