Biochemical characterization of nuclear and cytoplasmic forms of SV40 tumor antigens encoded by parental and transport-detective mutant SV40-adenovirus 7 hybrid viruses

A functional and structural comparative analysis of large tumor antigens reveals evolution of different importin α-dependent nuclear localization signals

Nucleocytoplasmic transport regulates the passage of proteins between the nucleus and cytoplasm. In the best characterized pathway, importin (IMP) α bridges cargoes bearing basic, classical nuclear localization signals (cNLSs) to IMPβ1, which mediates transport through the nuclear pore complex. IMPα recognizes three types of cNLSs via two binding sites: the major binding site accommodates monopartite cNLSs, the minor binding site recognizes atypical cNLSs, while bipartite cNLSs simultaneously interact with both major and minor sites. Despite the growing knowledge regarding IMPα-cNLS interactions, our understanding of the evolution of cNLSs is limited. We combined bioinformatic, biochemical, functional, and structural approaches to study this phenomenon, using polyomaviruses (PyVs) large tumor antigens (LTAs) as a model. We characterized functional cNLSs from all human (H)PyV LTAs, located between the LXCXE motif and origin binding domain. Surprisingly, the prototypical SV40 monopartite NLS is not well conserved; HPyV LTA NLSs are extremely heterogenous in terms of structural organization, IMPα isoform binding, and nuclear targeting abilities, thus influencing the nuclear accumulation properties of full-length proteins. While several LTAs possess bipartite cNLSs, merkel cell PyV contains a hybrid bipartite cNLS whose upstream stretch of basic amino acids can function as an atypical cNLS, specifically binding to the IMPα minor site upon deletion of the downstream amino acids after viral integration in the host genome. Therefore, duplication of a monopartite cNLS and subsequent accumulation of point mutations, optimizing interaction with distinct IMPα binding sites, led to the evolution of bipartite and atypical NLSs binding at the minor site.

The E1 replication protein of bovine papillomavirus type 1 contains an extended nuclear localization signal that includes a p34cdc2 phosphorylation site

Bovine papillomavirus (BPV) DNA replication occurs in the nucleus of infected cells. Most enzymatic activities are carried out by host cell proteins, with the viral E1 and E2 proteins required for the assembly of an initiation complex at the replication origin. In latently infected cells, viral DNA replication occurs in synchrony with the host cell chromosomes, maintaining a constant average copy number of BPV genomes per infected cell. By analyzing a series of mutants of the amino-terminal region of the E1 protein, we have identified the signal for transport of this protein to the cell nucleus. The E1 nuclear transport motif is highly conserved in the animal and human papillomaviruses and is encoded in a similar region in the related E1 genes. The signal is extended relative to the simple nuclear localization signals and contains two short amino acid sequences which contribute to nuclear transport, located between amino acids 85 and 108 of the BPV-1 E1 protein. Mutations in either basic region reduce nuclear transport of E1 protein and interfere with viral DNA replication. Mutations in both sequences simultaneously prevent any observable accumulation of the protein and reduce replication in transient assays to barely detectable levels. Surprisingly, these mutations had no effect on the ability of viral genomes to morphologically transform cells, although the plasmid DNA in the transformed cells was maintained at a very low copy number. Between these two basic amino acid blocks in the nuclear transport signal, at threonine 102, is a putative site for phosphorylation by the cell cycle regulated kinase p34cdc2. Utilizing an E1 protein purified from either a baculovirus vector system or Escherichia coli, we have shown that the E1 protein is a substrate for this kinase. An E1 gene mutant at threonine 102 encodes for a protein which is no longer a substrate for the p34cdc2 kinase. Mutation of this threonine to isoleucine had no observable effect on either nuclear localization of E1 or DNA replication of the intact viral genome.

Interaction of human embryonal carcinoma cells and differentiated derivatives in vitro with simian virus 40, human adenovirus type 7, or PARA

Polyclonal antibodies were used to assay human embryonal carcinoma (EC), differentiating EC, yolk sac carcinoma, and teratoma cells for expression of viral early antigen (T-Ag) after infection with simian virus 40 (SV40). Cells of four EC lines were induced to differentiate by cultivation at low density or by exposure to retinoic acid or dimethyl sulfoxide. After infection with SV40, T-Ag was expressed by 1%, or less, of the cells (presumed to be differentiated derivatives) in only some EC cultures whereas the antigen was synthesized by a significant percentage of the yolk sac carcinoma, teratoma, and differentiating EC cells. Also, viral late proteins were produced by EC cells infected with human adenovirus type 7 (Ad7), and SV40 T-Ag was expressed by EC cells after infection with PARA, which is an Ad7-SV40 hybrid virus containing the SV40 T-Ag sequence controlled by Ad7 late regulatory sequences. Thus, T-Ag is not synthesized by the parental EC cells infected with SV40, but it is expressed in cultures of infected differentiated derivatives. The EC cells produce T-Ag, however, when expression of the viral protein is controlled by the Ad7 regulatory sequences in PARA particles. These results demonstrate that expression of T-Ag after infection with SV40 is an indicator of EC cell differentiation and also raise the possibility that, as in mouse EC cells infected with the virus, the SV40 regulatory sequences controlling T-Ag synthesis are not active in human EC cells.

Transformation of precrisis human cells by the simian virus 40 cytoplasmic-localization mutant pSVCT3 is accompanied by nuclear T antigen

pSVCT3 is a cytoplasmic-localization mutant of simian virus 40 (SV40) isolated from the SV40 adenovirus 7 hybrid virus (PARA) and cloned into plasmid PBR. The large T antigen of pSVCT3 accumulates in the cytoplasm of infected monkey cells instead of being transported to the nucleus. The sole change in CT3 large T antigen is amino acid residue 128 (Lys----Asn). Transformation of precrisis rodent cells by pSVCT3 is negligible, whereas the frequency of transformation of established rodent cell lines by pSVCT3 is comparable to that of wild-type SV40. According to the model, in which transformation of precrisis cells involves the combined oncogenic action of both nuclear and cytoplasmic gene products, we predicted that pSVCT3 would localize in the cytoplasm of human cells and would therefore at most only partially and rarely transform precrisis human cells. We have found that pSVCT3 is able to transform precrisis human cells at high frequency. Furthermore, pSVCT3-transformed human precrisis cells relocalized T antigen to their nuclei. The relocalization of large T antigen was not dependent on cell growth. Wild-type and pSVCT3-transformed human cell lines both have about five copies of integrated SV40 DNA. SV40 virus-specific proteins, including the 100,000-molecular-weight super large T antigen, were expressed in pSVCT3-transformed human cells. Our results suggest that molecules in precrisis human cells, but not cells of other species, are able to complement the cytoplasmic-localization defect of the CT3 mutant large T antigen.

Synthetic peptides containing a region of SV 40 large T-antigen involved in nuclear localization direct the transport of proteins into the nucleus

We studied the mechanism of transport of proteins into the nucleus using synthetic peptides containing the nuclear location signal sequence of Simian virus 40 (SV 40) large T-antigen. When chick erythrocytes containing a synthetic large T-antigen nuclear translocation signal were fused with SV 40-transformed human fibroblasts, the migration of native large T-antigen into the chick nuclei was suppressed. Migration of proteins detected by human specific antinuclear autoimmune antibody was not blocked. An analog of the nuclear location signal peptide did not inhibit entry of large T-antigen into the chick nuclei. This result suggests that the peptide blocked the migration of only native large T-antigen into the nucleus, and that the signal of the majority of nuclear proteins for nuclear transport is not the same as that of the large T-antigen. The synthetic peptide was conjugated chemically with bovine serum albumin (BSA) and introduced into the cytoplasm of cultured human cells by red blood cell ghost-mediated microinjection. The BSA-synthetic peptide conjugate was found predominantly in the nucleus within 2 h after its introduction into the cells. BSA conjugated with the cross-linking reagent alone was not transported into the nucleus. Acetylated synthetic peptide was not effective in promoting nuclear localization of BSA. Mild trypsin treatment of the BSA-synthetic peptide conjugate suppressed nuclear localization. Conjugates of the synthetic peptide with phycoerythrin (Mr about 150 kD) and with secretory IgA (Mr about 380 kD) were both found in the nucleus very shortly after their introduction into the cytoplasm. These results suggest that the synthetic peptide containing the nuclear location signal sequence provides exogenous proteins with the ability to migrate into the nucleus. But, since a conjugate of the synthetic peptide with IgM (Mr about 940 kD) did not migrate into the nucleus after its microinjection, there may be a size limit in nuclear transport of conjugated proteins.

Extensive mutagenesis of the nuclear location signal of simian virus 40 large-T antigen

Site-directed mutagenesis was used to change Lys-128 of the simian virus 40 large-T nuclear location signal to Met, Ile, Arg, Gln, Asn, Leu, or His. Except for the large-T antigen of the Arg mutation, which was present in cytoplasmic and nuclear compartments, the resultant proteins were unable to enter the nucleus. By contrast, mutations at other sites within the signal were generally less severe in their effect. In some cases (Lys-128 to Gln, Asn, and His), the apparently cytoplasmic variants were able to support limited plasmid DNA replication, suggesting that low levels of large-T antigen undetectable by immunofluorescence were present in the nucleus. Such mutants did not support viral DNA replication. We conclude that there is a strong requirement for a basic residue at position 128 in the large-T nuclear location signal, with Lys the preferred residue.

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.

Genomic organization of the simian virus 40-adenovirus 7 hybrid virus, PARA(cT), that encodes a nuclear transport defective simian virus 40 T antigen

The genomic organization of the simian virus 40 (SV40)-adenovirus (Ad)7 hybrid virus, PARA(cT), was examined. A deletion of approximately 5529 bp of Ad7 DNA extends from 78.8 map units to 94.0 map units and is replaced by an SV40 DNA insert of 3809 bp. The left-hand end of the insertion begins at SV40 nucleotide 5168, 5 bp upstream of the ATG initiation codon for T-ag synthesis. The sequence extends counterclockwise through the T-ag encoding sequences and into SV40 late region DNA. Most of the late region DNA has been removed in a deletion between nucleotides 2464 and 301. One of the 72-bp repeats has also been deleted. The right-hand end of the SV40 DNA insert is at nucleotide 4366. Thus, a portion of the SV40 DNA early region is repeated at both ends of the insert (nucleotides 5168-4366).

Extensive mutagenesis of the nuclear location signal of simian virus 40 large-T antigen

Site-directed mutagenesis was used to change Lys-128 of the simian virus 40 large-T nuclear location signal to Met, Ile, Arg, Gln, Asn, Leu, or His. Except for the large-T antigen of the Arg mutation, which was present in cytoplasmic and nuclear compartments, the resultant proteins were unable to enter the nucleus. By contrast, mutations at other sites within the signal were generally less severe in their effect. In some cases (Lys-128 to Gln, Asn, and His), the apparently cytoplasmic variants were able to support limited plasmid DNA replication, suggesting that low levels of large-T antigen undetectable by immunofluorescence were present in the nucleus. Such mutants did not support viral DNA replication. We conclude that there is a strong requirement for a basic residue at position 128 in the large-T nuclear location signal, with Lys the preferred residue.

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.

Kinetics of nuclear transport and oligomerization of simian virus 40 large T antigen

The kinetics of nuclear transport and of oligomerization of simian virus 40 (SV40) large T antigen in lytically infected cells were investigated by pulse-chase experiments, cell fractionation, and sedimentation analyses in sucrose gradients. After synthesis, large T was rapidly translocated to the nucleus. Within 10 min, half of the pulse-labeled molecules had entered the nucleus and after an additional 30 min, nuclear accumulation of large T reached a constant plateau of about 95%. Within that time, the majority of large T was in monomeric form suggesting that nuclear transport takes place in this state. In the nucleus, conversion to tetramers proceeded slowly and steadily. By 60 min half of the molecules had formed tetramers and by 6 hr a steady-state ratio between tetramers and monomers of 4:1 was observed. A small fraction of large T remaining in the cytoplasm oligomerized considerably faster than large T in the nuclear fraction. This phenomenon of accelerated oligomerization was also observed with a mutant of large T defective for nuclear transport. Perhaps, the nuclear envelope is a barrier for the complex forms of large T which prevents premature oligomers in the cytoplasm from entering the nucleus and oligomers in the nucleus from migrating back to the cytoplasm.

Immunoenzymatic labelling of JC papovavirus T antigen in brains of patients with progressive multifocal leukoencephalopathy

Formalin-fixed, paraffin-embedded autopsy sections of brains from two patients with progressive multifocal leukoencephalopathy (PML) were stained by peroxidase-antiperoxidase methods for human papovavirus T antigen, a nonstructural protein expressed in cells lytically infected or transformed by JC, BK, and SV40 viruses. Adjacent sections were stained for papovavirus common structural antigen, a component of JC, BK, and SV40 virions which is synthesized in productively infected but not transformed cells. Intense immunoperoxidase labelling specific for T antigen was detected in large numbers of oligodendrocytes at the edges of demyelinated areas and in occasional oligodendrocytes within otherwise normal brain. Occasional morphologically normal astrocytic cells exhibited similar specific staining, but only rate atypical astrocytic cells contained detectable amounts of T antigen. Examination of adjacent sections stained with antisera to common structural antigen revealed an identical pattern of immunoenzymatic labelling, indicating that most of the cells expressing T antigen were also expressing viral structural proteins. The present study demonstrates that T antigen can be identified by immunoperoxidase methods in routinely processed autopsy material from cases of PML, but that detectable amounts of antigen are found almost exclusively in cells undergoing lytic infection.

Affinity-labeling steroids as biologically active probes of antiglucocorticoid hormone action

The role of the glucocorticoid receptor in the expression of antiglucocorticoid action has been investigated with a chemically-reactive derivative of three glucocorticoid steroids with differing biological potencies, i.e. the C-21 mesylates of cortisol, dexamethasone and deacylcortivazol. Dexamethasone 21-mesylate (Dex-Mes) was the most useful derivative due to its favorable balance of high receptor affinity and predominantly irreversible antiglucocorticoid activity. A number of criteria have been used to conclude that [3H]Dex-Mes covalently labels glucocorticoid receptors in the steroid-binding cavity. The available data indicate that covalent Dex-Mes-labeled receptors (mol. wt approximately equal to 98,000) are responsible for the irreversible antiglucocorticoid activity while the partial agonist activity of Dex-Mes is due to non-covalent Dex-Mes-bound receptors. Further support for this hypothesis comes from the observations that deacylcortivazol 21-mesylate was a full glucocorticoid and did not affinity label receptors (and marginally labeled cytosol proteins) although it was capable of covalently-labeling bovine serum albumin. Several mechanisms for the expression of irreversible antiglucocorticoid activity by covalent Dex-Mes-labeled receptors were examined and can be eliminated. Covalent receptor-Dex-Mes complexes formed in whole HTC cells were found to have a decreased capacity for nuclear binding. This decreased nuclear-binding capacity could be responsible for the whole-cell irreversible antiglucocorticoid activity of Dex-Mes.

Replicative functions of the SV40(cT)-3 mutant defective for nuclear transport of T antigen

The SV40(cT)-3 mutant is defective in transport of SV40 large tumor antigen (T-ag) to the nucleus. Several properties of T-ag associated with SV40 lytic infection and attributed to its nuclear localization were examined to determine whether biologically significant levels of the mutant T-ag (cT-ag) that were immunologically undetectable were transported to the nucleus in SV40(cT)-3-infected TC-7 cells. SV40(cT)-3 was defective in regulation of T-ag synthesis and initiation of viral DNA synthesis. These defects were presumably due to the lack of nuclear transport of cT-ag, since cT-ag was capable of interacting with the SV40 origin of viral DNA synthesis in a solution binding assay. The level of fatty acid acylation, a modification specific for the cell surface associated T-ag, was not affected by the cT mutation. The cT mutation sufficiently suppressed the nuclear transport of wild-type (WT) T-ag in SV40(cT)-3-infected COS-1 cells to result in the cessation of WT-T-ag-stimulated SV40(cT)-3 viral DNA synthesis. These results are discussed with respect to the recent findings that SV40(cT)-3 is fully competent for the transformation of established cell lines and the induction of cellular DNA synthesis in quiescent cells.

Differential ability of a T-antigen transport-defective mutant of simian virus 40 to transform primary and established rodent cells

The transforming potential and oncogenicity of a simian virus 40 (SV40) mutant affecting T-antigen (T-ag), SV40(cT)-3, was examined in an effort to dissect T-ag functions in transformation. SV40(cT)-3 has a point mutation at nucleotide 4434 that abolishes the transport of T-ag to the nucleus but does not affect its association with the cell surface. Transfection-transformation assays were performed with primary cells and established cell lines of mouse and rat origin. The efficiency of transformation for established cell lines by SV40(cT)-3 was comparable to that of wild-type SV40, indicating that transformation of established cell lines can occur in the absence of detectable amounts of nuclear T-ag. Transformation of primary mouse embryo fibroblasts by SV40(cT)-3 was markedly influenced by culture conditions; the relative transforming frequency was dramatically reduced in assays involving focus formation in low serum concentrations or anchorage-independent growth. Immunofluorescence tests revealed that the transformed mouse embryo fibroblasts partially transport the mutant cT-ag to the cell nucleus. Transformed cell lines induced by SV40(cT)-3 did not differ in growth properties from wild-type transformants. SV40(cT)-3 was completely defective for the transformation of primary baby rat kidney cells, a primary cell type unable to transport the mutant T-ag to the nucleus. The intracellular localization of cellular protein p53 was found to mimic T-ag distribution in all the transformants analyzed. The mutant virus was weakly oncogenic in vivo: the induction of tumors in newborn hamsters by SV40(cT)-3 was reduced in incidence and delayed in appearance in comparison to wild-type SV40. These observations suggest that cellular transformation is regulated by both nuclear and surface-associated forms of SV40 T-ag.

Differential ability of a T-antigen transport-defective mutant of simian virus 40 to transform primary and established rodent cells

The transforming potential and oncogenicity of a simian virus 40 (SV40) mutant affecting T-antigen (T-ag), SV40(cT)-3, was examined in an effort to dissect T-ag functions in transformation. SV40(cT)-3 has a point mutation at nucleotide 4434 that abolishes the transport of T-ag to the nucleus but does not affect its association with the cell surface. Transfection-transformation assays were performed with primary cells and established cell lines of mouse and rat origin. The efficiency of transformation for established cell lines by SV40(cT)-3 was comparable to that of wild-type SV40, indicating that transformation of established cell lines can occur in the absence of detectable amounts of nuclear T-ag. Transformation of primary mouse embryo fibroblasts by SV40(cT)-3 was markedly influenced by culture conditions; the relative transforming frequency was dramatically reduced in assays involving focus formation in low serum concentrations or anchorage-independent growth. Immunofluorescence tests revealed that the transformed mouse embryo fibroblasts partially transport the mutant cT-ag to the cell nucleus. Transformed cell lines induced by SV40(cT)-3 did not differ in growth properties from wild-type transformants. SV40(cT)-3 was completely defective for the transformation of primary baby rat kidney cells, a primary cell type unable to transport the mutant T-ag to the nucleus. The intracellular localization of cellular protein p53 was found to mimic T-ag distribution in all the transformants analyzed. The mutant virus was weakly oncogenic in vivo: the induction of tumors in newborn hamsters by SV40(cT)-3 was reduced in incidence and delayed in appearance in comparison to wild-type SV40. These observations suggest that cellular transformation is regulated by both nuclear and surface-associated forms of SV40 T-ag.

Identification of the sequence responsible for the nuclear accumulation of the influenza virus nucleoprotein in Xenopus oocytes

Influenza virus nucleoprotein (NP), synthesized in Xenopus oocytes after injection of cloned NP cDNA, enters and accumulates in the nucleus. We have used in vitro mutagenesis of this cDNA to study the cellular distribution of mutated NP polypeptides. Mutants lacking amino acids 327-345 of wild-type NP enter the nucleus but do not accumulate there to the same extent as the wild-type protein, suggesting that this region has a role in nuclear accumulation. This possibility is further strengthened by similar studies involving the production of fusion proteins in which various amino-terminal sequences of the NP gene are fused to the complete chimpanzee alpha 1-globin sequence: when globin cDNA was injected into and expressed in oocytes the protein remains exclusively in the cytosol; however, when the globin cDNA is fused to a portion of NP cDNA that includes the region encoding amino acids 327-345, the resulting fusion protein enters and accumulates in the nucleus. Fusion proteins lacking this region of the NP enter but do not accumulate in the nucleus.

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.

A short amino acid sequence able to specify nuclear location

A short sequence of amino acids including Lys-128 is required for the normal nuclear accumulation of wild-type and deleted forms of SV40 large T antigen. A cytoplasmic large T mutant that lacks sequences from around Lys-128 localizes to the nucleus if the missing sequence is attached to its amino terminus. The implication that the sequence element around Lys-128 acts as an autonomous signal capable of specifying nuclear location was tested directly by transferring it to the amino termini of beta-galactosidase and of pyruvate kinase, normally a cytoplasmic protein. Sequences that included the putative signal induced each of the fusion proteins to accumulate completely in the nucleus but had no discernible effect when Lys-128 was replaced by Thr. By reducing the size of the transposed sequence we conclude that Pro-Lys-Lys-Lys-Arg-Lys-Val can act as a nuclear location signal. The sequence may represent a prototype of similar sequences in other nuclear proteins.

Construction and characterization of an SV40 mutant defective in nuclear transport of T antigen

An SV40-adenovirus 7 hybrid virus, PARA(cT), has been described that is defective for the nuclear transport of SV40 large tumor antigen. An SV40(cT) mutant was constructed using SV40 early and late region DNA fragments derived from PARA(cT) and wild-type SV40 respectively. The SV40(cT)-3 construct is defective for viral replication, but can be propagated in COS-1 cells. T antigen induced by SV40(cT)-3 is localized in the cytoplasm of infected cells. The cT mutation also inhibits the transport of wild-type T antigen; COS-1 cells lose their constitutive expression of nuclear T antigen after infection with SV40(cT)-3. Sequence analysis revealed that the cT mutation results in the replacement of a positively charged lysine in wild-type T antigen with a neutral asparagine at amino acid number 128, demonstrating that the alteration of a single amino acid is sufficient to abolish nuclear transport. Implications of the cT mutation on possible mechanisms for the transport of proteins to the nucleus are discussed.

Differential phosphorylation of cytoplasmic and nuclear variants of simian virus 40 large T antigen encoded by simian virus 40-adenovirus 7 hybrid viruses

The phosphorylation patterns of cytoplasmic and nuclear forms of simian virus 40 large T antigen encoded by simian virus 40-adenovirus 7 hybrid viruses were analyzed by two-dimensional peptide mapping. The PARA(cT) mutant which encodes a large T antigen defective for nuclear transport was used as source for cytoplasmic large T antigen. The data suggest that the large T antigen is phosphorylated in a sequential manner at a subset of sites in the cytoplasm and at additional sites in the nucleus.

Structural comparisons of wild-type and nuclear transport-defective simian virus 40 large tumor antigens

PARA(nT) is a defective SV40-adenovirus 7 hybrid virus which contains the entire early region of the SV40 genome and codes for the synthesis of SV40 large tumor antigen (T-ag). A transport-defective variant of this hybrid, PARA(cT), encodes T-ag that is not transported to the nucleus, but accumulates in the cytoplasm. The structures of T-ags extracted from wild-type (WT) SV40-, PARA(nT)-, and PARA(cT)-infected cells were compared by peptide mapping. All three types of T-ag underwent considerable degradation when extracted using Tris-buffered Nonidet P-40 at pH 8.0. The addition of 200 microM leupeptin to the extraction buffer significantly inhibited this degradation. Comparison of methionine-containing tryptic peptides revealed no differences among the T-ags, suggesting that their primary structures are similar or identical. Phosphopeptide mapping revealed no differences between SV40- and PARA(nT)-encoded T-ags. In contrast, PARA(cT)-encoded T-ag lacked a prominent phosphopeptide that was present in both of the others. The possible relevance of this difference in phosphorylation to the transport defect is discussed.

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.

Phosphorylation and dephosphorylation alter the structure of D2 hybrid T antigen

D2 hybrid T antigen is a protein closely related to simian virus 40 large T antigen and is synthesized in large quantities in cells infected with Ad2+D2, an adenovirus-simian virus 40 hybrid. We have analyzed the effects of phosphorylation on the structure and DNA binding of this protein. On nondenaturing pore-gradient gels, the purified protein migrated with an apparent molecular weight of 135,000, with a minor band at 330,000 molecular weight. In vitro phosphorylation catalyzed by the protein kinase activity associated with the protein resulted in a structural change so that most of it migrated with an apparent molecular weight of 740,000. Treatment of the phosphorylated form of the protein with alkaline phosphatase (which removed 95% of the phosphate) caused the disappearance of the 740,000-molecular-weight form and reappearance of the smaller forms. Partial tryptic digestion showed that D2 T antigen has two major regions of phosphorylation, only one of which was phosphorylated in vitro. The region phosphorylated in vitro was responsible for the aggregation of D2 T antigen and was tentatively assigned to the N-terminal part of the protein. As shown by protein blotting onto nitrocellulose filters, it was mainly the form of 740,000 molecular weight that bound to simian virus 40 DNA. However, sucrose gradient analyses showed that only a fraction of the in vitro-phosphorylated protein bound to DNA, suggesting that aggregation alone is not sufficient for binding.

Simian virus 40 large T antigen is phosphorylated at multiple sites clustered in two separate regions

The phosphorylation sites of simian virus 40 large T antigen were determined within the primary structure of the molecule. Exhaustive digestion of (32)P-labeled large T antigen with trypsin generated six major phosphopeptides which could be separated in a newly developed isobutyric acid-containing chromatography system. By partial tryptic digestion, large T antigen was cleaved into an amino-terminal fragment of 17,000 daltons and overlapping fragments from the carboxy-terminal region ranging in size between 71,000 and 13,000 daltons. The location of the phosphopeptides was then determined by fingerprint analyses of individual fragments. Their physical properties were analyzed by sizing on polyacrylamide gels and by sequential digestion and peptide mapping; their amino acid composition was determined by differential labeling with various amino acids. The amino-terminal 17,000-dalton fragment gave rise to only one phosphopeptide (phosphopeptide 3) that contained half of the phosphate label incorporated into large T antigen. It contained phosphoserine and phosphothreonine sites, all of which were clustered within a small segment between Cys(105) and Lys(127). This segment contained five serines and two threonines. Among these, Ser(106), Ser(123), and Thr(124) were identified as phosphorylated residues; in addition, either one or both of Ser(111) and Ser(112) were phosphorylated. The neighboring residues, Ser(123) and Thr(124), were found in three different phosphorylation states in that either Ser(123) or Thr(124) or both were phosphorylated. Phosphopeptides 1, 2, 4, 5, and 6 were all derived from a single fragment extending 26,000 daltons upstream from the carboxy terminus of large T antigen. Phosphopeptide 6 was identical with the previously determined phosphothreonine peptide phosphorylated at Thr(701). Phosphopeptides 1, 2, 4, and 5 contained only serine-bound phosphate. Phosphopeptides 1, 2, and 4 represented overlapping peptides, all of which were phosphorylated at Ser(639) located next to a cluster of six acidic residues. In phosphopeptide 5, a large peptide ranging from Asn(653) to Arg(691), at least two of seven serines were phosphorylated. Thus, large T antigen contains at least eight phosphorylation sites. Their clustering within two separate regions might correlate with structural and functional domains of this protein.

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.