Phosphorylation pattern of large T antigens in mouse cells infected by simian virus 40 wild type or deletion mutants

The carboxyl-terminal domain of large T antigen rescues SV40 host range activity in trans independent of acetylation

The host range activity of SV40 has been described as the inability of mutant viruses with deletions in the C terminal region of large T Ag to replicate in certain types of African green monkey kidney cells. We constructed new mutant viruses expressing truncated T Ag proteins and found that these mutant viruses exhibited the host range phenotype. The host range phenotype was independent of acetylation of T Ag at lysine 697. Co-expression of the C terminal domain of T Ag (aa 627-708) in trans increased both T Ag and VP1 mRNA as well as protein levels for host range mutant viruses in the restrictive cell type. In addition, the T Ag 627-708 fragment promoted the productive lytic infection of host range mutant viruses in the nonpermissive cell type. The carboxyl-terminal region of T Ag contains a biological function essential for the SV40 viral life cycle.

Nuclear protein phosphorylation and growth control

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The human DNA-activated protein kinase phosphorylates simian virus 40 T antigen at amino- and carboxy-terminal sites

Protein phosphorylation modulates the functions of simian virus 40 large T antigen (TAg) in productive and transforming infections. We recently described a DNA-activated protein kinase (DNA-PK) that efficiently phosphorylates TAg and several other nuclear, DNA-binding proteins in vitro (S.P. Lees-Miller, Y.-R. Chen, and C. W. Anderson, Mol. Cell. Biol. 10:6472-6481, 1990). In this report, we show by direct amino acid sequence analysis that DNA-PK phosphorylates TAg strongly at Ser-677, a residue known to be important for TAg interaction with origin site I and for transformation. We propose that DNA-PK may modulate the role of TAg in repressing early viral transcription and cell transformation, but a role for DNA-PK in regulating simian virus 40 DNA synthesis is not excluded. DNA-PK also phosphorylates Ser-665, and Ser-667, and one or more serines between amino acids 110 and 131. At least six serines, Ser-111, Ser-112, Ser-120, Ser-665, Ser-667, and Ser-677, are phosphorylated in TAg purified from baculovirus vector-infected insect cells.

Dephosphorylation of simian virus 40 large-T antigen and p53 protein by protein phosphatase 2A: inhibition by small-t antigen

Simian virus 40 (SV40) large-T antigen and the cellular protein p53 were phosphorylated in vivo by growing cells in the presence of 32Pi. The large-T/p53 complex was isolated by immunoprecipitation and used as a substrate for protein phosphatase 2A (PP2A) consisting of the catalytic subunit (C) and the two regulatory subunits, A and B. Three different purified forms of PP2A, including free C, the AC form, and the ABC form, could readily dephosphorylate both proteins. With both large-T and p53, the C subunit was most active, followed by the AC form, which was more active than the ABC form. The activity of all three forms of PP2A toward these proteins was strongly stimulated by manganese ions and to a lesser extent by magnesium ions. The presence of complexed p53 did not affect the dephosphorylation of large-T antigen by PP2A. The dephosphorylation of individual phosphorylation sites of large-T and p53 were determined by two-dimensional peptide mapping. Individual sites within large-T and p53 were dephosphorylated at different rates by all three forms of PP2A. The phosphates at Ser-120 and Ser-123 of large-T, which affect binding to the origin of SV40 DNA, were removed most rapidly. Three of the six major phosphopeptides of p53 were readily dephosphorylated, while the remaining three were relatively resistant to PP2A. Dephosphorylation of most of the sites in large-T and p53 by the AC form was inhibited by SV40 small-t antigen. The inhibition was most apparent for those sites which were preferentially dephosphorylated. Inhibition was specific for the AC form; no effect was observed on the dephosphorylation of either protein by the free C subunit or the ABC form. The inhibitory effect of small-t on dephosphorylation by PP2A could explain its role in transformation.

Protein phosphatase 2A dephosphorylates simian virus 40 large T antigen specifically at residues involved in regulation of DNA-binding activity

Treatment of purified simian virus 40 large T antigen (LT) with protein phosphatase 2A stimulates LT-dependent DNA unwinding and replication (D. M. Virshup, M. G. Kauffman, and T. J. Kelly, EMBO J. 8: 3891-3898, 1989). The specificity of the catalytic subunit of protein phosphatase 2A toward LT was investigated by two-dimensional peptide mapping. Increasing amounts of phosphatase sequentially removed the phosphates from serine residues 120, 123, 677, and perhaps 679, residues which have been implicated in regulating the DNA-binding activity of LT.

Dephosphorylation of simian virus 40 large-T antigen and p53 protein by protein phosphatase 2A: inhibition by small-t antigen

Simian virus 40 (SV40) large-T antigen and the cellular protein p53 were phosphorylated in vivo by growing cells in the presence of 32Pi. The large-T/p53 complex was isolated by immunoprecipitation and used as a substrate for protein phosphatase 2A (PP2A) consisting of the catalytic subunit (C) and the two regulatory subunits, A and B. Three different purified forms of PP2A, including free C, the AC form, and the ABC form, could readily dephosphorylate both proteins. With both large-T and p53, the C subunit was most active, followed by the AC form, which was more active than the ABC form. The activity of all three forms of PP2A toward these proteins was strongly stimulated by manganese ions and to a lesser extent by magnesium ions. The presence of complexed p53 did not affect the dephosphorylation of large-T antigen by PP2A. The dephosphorylation of individual phosphorylation sites of large-T and p53 were determined by two-dimensional peptide mapping. Individual sites within large-T and p53 were dephosphorylated at different rates by all three forms of PP2A. The phosphates at Ser-120 and Ser-123 of large-T, which affect binding to the origin of SV40 DNA, were removed most rapidly. Three of the six major phosphopeptides of p53 were readily dephosphorylated, while the remaining three were relatively resistant to PP2A. Dephosphorylation of most of the sites in large-T and p53 by the AC form was inhibited by SV40 small-t antigen. The inhibition was most apparent for those sites which were preferentially dephosphorylated. Inhibition was specific for the AC form; no effect was observed on the dephosphorylation of either protein by the free C subunit or the ABC form. The inhibitory effect of small-t on dephosphorylation by PP2A could explain its role in transformation.

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.

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.

Mutations in the phosphorylation sites of simian virus 40 (SV40) T antigen alter its origin DNA-binding specificity for sites I or II and affect SV40 DNA replication activity

A series of mutants of simian virus 40 was constructed by oligonucleotide-directed mutagenesis to study the role of phosphorylation in the functions of large T antigen. Each of the previously mapped phosphorylated serine and threonine residues in large T antigen was replaced by an alanine or cysteine residue or, in one case, by glutamic acid. Mutant DNAs were assayed for plaque-forming activity, viral DNA replication, expression of T antigen, and morphological transformation of rat cells. Viable mutants were isolated, suggesting that modification of some residues is not essential for the biological functions of T antigen. Two of these mutants replicated more efficiently than did the wild type. Seven mutants were partially or completely deficient in viral DNA replication but retained cell transformation activity comparable with that of the wild-type protein. Biochemical analysis of the mutant T antigens demonstrated novel origin DNA-binding properties of several mutant proteins. The results are consistent with the idea that differential phosphorylation defines several functional subclasses of T-antigen molecules.

Characterization of the in vitro interaction between SV40 T antigen and p53: mapping the p53 binding site

An efficient in vitro system for generating soluble complexes between simian virus 40 T antigen and the cellular protein p53 was developed. A p53 cDNA was inserted 3' to the SP6 promoter in pGEM-1 (Promega-Biotec) and transcribed by SP6 polymerase. In vitro translation of the cRNA generated p53 which was immunoprecipitable with all five monoclonal antibodies tested (PAb122, PAb421, PAb242, PAb246, and PAb248). The p53 sedimented at about 8-10 S in sucrose gradients, possibly corresponding to a tetramer. T-antigen-p53 complexes were produced by the addition of immunoaffinity-purified T antigen to p53-containing translation lysates. Equivalent amounts of p53 were immunoprecipitated with the anti-T-antigen antibodies PAb416, PAb419, and PAb101, suggesting that in vitro made p53 complexed mostly to a population of T-antigen molecules that had matured at least 15 min in the cell. The complexes sedimented at 18-20 S in sucrose gradients. In order to map the p53 binding site on T antigen, p53 was complexed in vitro to labeled proteolytic fragments of T antigen. A 46K fragment, spanning residues 131-517, was immunoprecipitated with the anti-p53 monoclonal PAb122 and therefore is likely to contain the p53 binding site. This region contains T-antigen sequences necessary for the efficient transformation of nonpermissive cells and for the induction of cellular rRNA synthesis. It also contains the binding sites for DNA polymerase alpha and ATP. We suggest a possible role for T-p53 complexes in T-antigen-associated functions.

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.

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

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

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

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

Binding sites for monoclonal antibodies and for mRNPs on SV40 large T-antigen determined with a cleavage map

Immune complexes of simian virus 40 large T-antigen with monoclonal papovavirus protein antibodies PAb 416, PAb 402, or PAb 423 were bound to protein-A-Sepharose and then cleaved into discrete fragments by limited tryptic proteolysis. PAb 402 protected a specific cleavage site, located approximately within amino acid residues 450-500, from tryptic proteolysis; PAb 423 protected another site within residues 675-699. As shown by immunoblotting, 125I-labeled PAb 416 was bound to a 17-kDa N-terminal fragment of large T-antigen (amino acid residues 1-130), and PAb 423 was bound to several overlapping fragments derived from the C terminus of large T-antigen. These monoclonal antibodies were then used as accessibility probes to study the interaction of mRNPs with cytoplasmic large T-antigen. Whereas small T-antigen and nuclear large T-antigen were fully immunoreactive, cytoplasmic large T-antigen reacted poorly with PAb 402 or polyclonal antibodies unless the mRNP moiety was removed by treatment with EDTA/RNase A. In contrast, mRNP/T-antigen complexes were fully immunoreactive with PAb 416 or PAb 423 and did not require treatment with EDTA/RNase A. The results suggest that the binding site of PAb 402 is blocked due to the interaction with mRNPs whereas the N-terminal binding site of PAb 416 and the C-terminal binding site of PAb 423 remain accessible to antibodies.

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

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

Simian virus 40 tumor antigen: isolation of the origin-specific DNA-binding domain

To localize the origin-specific DNA-binding domain on the simian virus 40 tumor (T) antigen molecule, we used limited proteolysis with trypsin to generate fractional peptides for analysis. A 17,000-Mr peptide was found to be capable of binding not only to calf thymus DNA, but also specifically to the simian virus 40 origin of DNA replication. This approximately 130-amino-acid peptide was derived from the extreme N-terminus of the T antigen and represented less than one-fifth of the entire molecule. The coding sequence for this tryptic peptide was located approximately between 0.51 and 0.67 map units (excluding the intron, which maps between 0.54 and 0.59). Since the first 82 amino acids are shared between large T and small t antigens, and since the latter does not bind DNA, it can be concluded that the sequence between isoleucine 83 and approximately arginine 130 is necessary for origin-specific binding by the T antigen. We also observed that in vivo phosphorylation of the T antigen within this region completely abolished the ability of the 17,000-Mr peptide to bind DNA. This observation is consistent with the idea that DNA binding by the T antigen is regulated by posttranslational modifications.