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

Phosphorylation of Human Polyomavirus Large and Small T Antigens: An Ignored Research Field

Protein phosphorylation and dephosphorylation are the most common post-translational modifications mediated by protein kinases and protein phosphatases, respectively. These reversible processes can modulate the function of the target protein, such as its activity, subcellular localization, stability, and interaction with other proteins. Phosphorylation of viral proteins plays an important role in the life cycle of a virus. In this review, we highlight biological implications of the phosphorylation of the monkey polyomavirus SV40 large T and small t antigens, summarize our current knowledge of the phosphorylation of these proteins of human polyomaviruses, and conclude with gaps in the knowledge and a proposal for future research directions.

Correlation between structure, protein composition, morphogenesis and cytopathology of Glossina pallidipes salivary gland hypertrophy virus

The Glossina pallidipes salivary gland hypertrophy virus (GpSGHV) is a dsDNA virus with rod-shaped, enveloped virions. Its 190 kb genome contains 160 putative protein-coding ORFs. Here, the structural components, protein composition and associated aspects of GpSGHV morphogenesis and cytopathology were investigated. Four morphologically distinct structures: the nucleocapsid, tegument, envelope and helical surface projections, were observed in purified GpSGHV virions by electron microscopy. Nucleocapsids were present in virogenic stroma within the nuclei of infected salivary gland cells, whereas enveloped virions were located in the cytoplasm. The cytoplasm of infected cells appeared disordered and the plasma membranes disintegrated. Treatment of virions with 1 % NP-40 efficiently partitioned the virions into envelope and nucleocapsid fractions. The fractions were separated by SDS-PAGE followed by in-gel trypsin digestion and analysis of the tryptic peptides by liquid chromatography coupled to electrospray and tandem mass spectrometry. Using the MaxQuant program with Andromeda as a database search engine, a total of 45 viral proteins were identified. Of these, ten and 15 were associated with the envelope and the nucleocapsid fractions, respectively, whilst 20 were detected in both fractions, most likely representing tegument proteins. In addition, 51 host-derived proteins were identified in the proteome of the virus particle, 13 of which were verified to be incorporated into the mature virion using a proteinase K protection assay. This study provides important information about GpSGHV biology and suggests options for the development of future anti-GpSGHV strategies by interfering with virus-host interactions.

Regulation of nucleocytoplasmic trafficking of viral proteins: an integral role in pathogenesis?

Signal-dependent targeting of proteins into and out of the nucleus is mediated by members of the importin (IMP) family of transport receptors, which recognise targeting signals within a cargo protein and mediate passage through the nuclear envelope-embedded nuclear pore complexes. Regulation of this process is paramount to processes such as cell division and differentiation, but is also critically important for viral replication and pathogenesis; phosphorylation appears to play a major role in regulating viral protein nucleocytoplasmic trafficking, along with other posttranslational modifications. This review focuses on viral proteins that utilise the host cell IMP machinery in order to traffic into/out of the nucleus, and in particular those where trafficking is critical to viral replication and/or pathogenesis, such as simian virus SV40 large tumour antigen (T-ag), human papilloma virus E1 protein, human cytomegalovirus processivity factor ppUL44, and various gene products from RNA viruses such as Rabies. Understanding of the mechanisms regulating viral protein nucleocytoplasmic trafficking is paramount to the future development of urgently needed specific and effective anti-viral therapeutics. This article was originally intended for the special issue "Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import". The Publisher apologizes for any inconvenience caused.

Binding of p110 retinoblastoma protein inhibits nuclear import of simian virus SV40 large tumor antigen

Nuclear import of the simian virus 40 large tumor antigen (T-ag) is dependent on its nuclear localization signal (NLS) within amino acids 126-132 that is recognized by the importin alpha/beta1 heterodimer, as well as a protein kinase CK2 site at serine 112 upstream of the NLS, which enhances the interaction approximately 50-fold. Here we show for the first time that T-ag nuclear import is negatively regulated by N-terminal sequences (amino acids 102-110), which represent the binding site (BS) for the retinoblastoma (Rb) tumor suppressor protein (p110(Rb)). Quantitative confocal laser scanning microscopic analysis of the transport properties of T-ag constructs with or without Rb binding site mutations in living transfected cells or in a reconstituted nuclear transport system indicates that the presence of the RbBS significantly reduces nuclear accumulation of T-ag. A number of approaches, including the analysis of T-ag nuclear import in an isogenic cell pair with and without functional p110(Rb) implicate p110(Rb) binding as being responsible for the reduced nuclear accumulation, with the Ser(106) phosphorylation site within the RbBS appearing to enhance the inhibitory effect. Immunoprecipitation experiments confirmed association of T-ag and p110(Rb) and dependence thereof on negative charge at Ser(106). The involvement of p110(Rb) in modulating T-ag nuclear transport has implications for the regulation of nuclear import of other proteins from viruses of medical significance that interact with p110(Rb), and how this may relate to transformation.

Development of quantitative and high-throughput assays of polyomavirus and papillomavirus DNA replication

Polyoma- and papillomaviruses genome replication is initiated by the binding of large T antigen (LT) and of E1 and E2, respectively, at the viral origin (ori). Replication of an ori-containing plasmid occurs in cells transiently expressing these viral proteins and is typically quantified by Southern blotting or PCR. To facilitate the study of SV40 and HPV31 DNA replication, we developed cellular assays in which transient replication of the ori-plasmid is quantified using a firefly luciferase gene located in cis to the ori. Under optimized conditions, replication of the SV40 and HPV31 ori-plasmids resulted in a 50- and 150-fold increase in firefly luciferase levels, respectively. These results were validated using replication-defective mutants of LT, E1 and E2 and with inhibitors of DNA replication and cell-cycle progression. These quantitative and high-throughput assays should greatly facilitate the study of SV40 and HPV31 DNA replication and the identification of small-molecule inhibitors of this process.

Quantitative analysis of the binding of simian virus 40 large T antigen to DNA

SV40 large T antigen (T-ag) is a multifunctional protein that successively binds to 5'-GAGGC-3' sequences in the viral origin of replication, melts the origin, unwinds DNA ahead of the replication fork, and interacts with host DNA replication factors to promote replication of the simian virus 40 genome. The transition of T-ag from a sequence-specific binding protein to a nonspecific helicase involves its assembly into a double hexamer whose formation is likely dictated by the propensity of T-ag to oligomerize and its relative affinities for the origin as well as for nonspecific double- and single-stranded DNA. In this study, we used a sensitive assay based on fluorescence anisotropy to measure the affinities of wild-type and mutant forms of the T-ag origin-binding domain (OBD), and of a larger fragment containing the N-terminal domain (N260), for different DNA substrates. We report that the N-terminal domain does not contribute to binding affinity but reduces the propensity of the OBD to self-associate. We found that the OBD binds with different affinities to its four sites in the origin and determined a consensus binding site by systematic mutagenesis of the 5'-GAGGC-3' sequence and of the residue downstream of it, which also contributes to affinity. Interestingly, the OBD also binds to single-stranded DNA with an approximately 10-fold higher affinity than to nonspecific duplex DNA and in a mutually exclusive manner. Finally, we provide evidence that the sequence specificity of full-length T-ag is lower than that of the OBD. These results provide a quantitative basis onto which to anchor our understanding of the interaction of T-ag with the origin and its assembly into a double hexamer.

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.

Phosphorylation of simian virus 40 T antigen on Thr 124 selectively promotes double-hexamer formation on subfragments of the viral core origin

Cell cycle-dependent phosphorylation of simian virus 40 (SV40) large tumor antigen (T-ag) on threonine 124 is essential for the initiation of viral DNA replication. A T-ag molecule containing a Thr-->Ala substitution at this position (T124A) was previously shown to bind to the SV40 core origin but to be defective in DNA unwinding and initiation of DNA replication. However, exactly what step in the initiation process is defective as a result of the T124A mutation has not been established. Therefore, to better understand the control of SV40 replication, we have reinvestigated the assembly of T124A molecules on the SV40 origin. Herein it is demonstrated that hexamer formation is unaffected by the phosphorylation state of Thr 124. In contrast, T124A molecules are defective in double-hexamer assembly on subfragments of the core origin containing single assembly units. We also report that T124A molecules are inhibitors of T-ag double hexamer formation. These and related studies indicate that phosphorylation of T-ag on Thr 124 is a necessary step for completing the assembly of functional double hexamers on the SV40 origin. The implications of these studies for the cell cycle control of SV40 DNA replication are discussed.

Two regions of simian virus 40 T antigen determine cooperativity of double-hexamer assembly on the viral origin of DNA replication and promote hexamer interactions during bidirectional origin DNA unwinding

Phosphorylation of simian virus 40 large tumor (T) antigen on threonine 124 is essential for viral DNA replication. A mutant T antigen (T124A), in which this threonine was replaced by alanine, has helicase activity, assembles double hexamers on viral-origin DNA, and locally distorts the origin DNA structure, but it cannot catalyze origin DNA unwinding. A class of T-antigen mutants with single-amino-acid substitutions in the DNA binding domain (class 4) has remarkably similar properties, although these proteins are phosphorylated on threonine 124, as we show here. By comparing the DNA binding properties of the T124A and class 4 mutant proteins with those of the wild type, we demonstrate that mutant double hexamers bind to viral origin DNA with reduced cooperativity. We report that T124A T-antigen subunits impair the ability of double hexamers containing the wild-type protein to unwind viral origin DNA, suggesting that interactions between hexamers are also required for unwinding. Moreover, the T124A and class 4 mutant T antigens display dominant-negative inhibition of the viral DNA replication activity of the wild-type protein. We propose that interactions between hexamers, mediated through the DNA binding domain and the N-terminal phosphorylated region of T antigen, play a role in double-hexamer assembly and origin DNA unwinding. We speculate that one surface of the DNA binding domain in each subunit of one hexamer may form a docking site that can interact with each subunit in the other hexamer, either directly with the N-terminal phosphorylated region or with another region that is regulated by phosphorylation.

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.

Simian virus 40 small-t antigen stimulates viral DNA replication in permissive monkey cells

The simian virus 40 (SV40) large-T antigen is essential for SV40 DNA replication and for late viral gene expression, but the role of the SV40 small-t antigen in these processes is still unclear. We have previously demonstrated that small t inhibits SV40 DNA replication in vitro. In this study, we investigated the effect of small t on SV40 replication in cultured cells. CV1 monkey cell infection experiments indicated that mutant viruses that lack small t replicate less efficiently than the wild-type virus. We next microinjected CV1 cells with SV40 DNA with and without purified small-t protein and analyzed viral DNA replication efficiency by Southern blotting. Replication of either wild-type SV40 or small-t deletion mutant DNA was increased three- to fivefold in cells coinjected with purified small t. Thus, in contrast to our in vitro observation, small t stimulated viral DNA replication in vivo. This result suggests that small t has cellular effects that are not detectable in a reconstituted in vitro replication system. We also found that small t stimulated progression of permissive monkey cells--but not of nonpermissive rodent cells--from G0-G1 to the S phase of the cell cycle, possibly leading to an optimal intracellular environment for viral replication.

Synchronous replication of SV 40 DNA in virus infected TC 7 cells induced by transient hypoxia

We transiently exposed SV 40 infected TC 7 cell cultures to a reduced O2 tension (4-8 h, about 200 ppm relative to 10(5) Pa total pressure). Under the hypoxic conditions, 'working' viral replication forks were greatly retarded or stopped, and initiation of daughter strand synthesis in further SV 40 DNA molecules was suppressed. Reoxygenation released an immediate burst of SV 40 replication which mainly consisted of a synchronous viral replication round. This synchronous in vivo replication began at the known origin of replication and proceeded at normal rates to the known termination region. Viral replicons seemed to accumulate under hypoxia in a state fully prepared to begin replication immediately after recovery of a normal pO2. The shut-down and sudden reactivation of DNA synthesis under hypoxia and reoxygenation, respectively, were not accompanied by changes of the phosphorylation state of large T antigen. The described synchronization procedure can be applied to optionally large SV 40 infected cell cultures.

Viral protein kinases and protein phosphatases

Certain large DNA viruses (e.g. herpesviruses and poxviruses) encode proteins related to cellular protein-serine/threonine kinases, and Hepatitis B virus and vesicular stomatitis virus may encode structurally different protein kinases. Other viruses activate cellular protein kinases, e.g. interferon-induced eukaryotic initiation factor-2 kinase, growth factor-induced kinases and protein kinases that regulate mitosis. Protein phosphatases are encoded by vaccinia virus and bacteriophage lambda and must also play a role in viral infection--as do cellular protein phosphatases. The functions of many of these viral enzymes remain to be determined, but they represent possible new targets for anti-viral therapy.

Mechanism of activation of simian virus 40 DNA replication by protein phosphatase 2A

The catalytic subunit of protein phosphatase 2A (PP2Ac) stimulates the initiation of replication of simian virus 40 DNA in vitro by dephosphorylating T antigen at specific phosphoserine residues (K. H. Scheidtmann, D. M. Virshup, and T. J. Kelly, J. Virol. 65:2098-2101, 1991). To better define the biochemical mechanism responsible for this stimulation, we investigated the effect of PP2Ac on the interaction of T antigen with wild-type and mutant origins of replication. Analysis of the binding of T antigen to the wild-type origin as a function of protein concentration revealed that binding occurs in two relatively discrete steps: the assembly of a T-antigen hexamer on one half-site of the origin, followed by the assembly of the second hexamer on the other half-site. The major effect of PP2Ac was to stimulate binding of the second hexamer, so that the binding reaction became much more cooperative. This observation suggests that dephosphorylation of T antigen by PP2Ac primarily affects interactions between the two hexamers bound to the origin. Pretreatment with PP2Ac increased the ability of the bound T antigen to unwind the origin of replication but had no effect on the intrinsic helicase activity of the protein. Thus, dephosphorylation of PP2Ac appears to increase the efficiency of the initial opening of the origin by T antigen. An insertion mutation at the dyad axis in the simian virus 40 origin, which altered the structural relationship of the two halves of the origin, abolished the effect of the phosphatase on the cooperativity of binding and completely prevented origin unwinding. These findings suggest that the ability of T antigen to open the viral origin of DNA replication is critically dependent on the appropriate functional interactions between T-antigen hexamers and that these interactions are regulated by the phosphorylation state of the viral initiator protein.

Mechanism of activation of simian virus 40 DNA replication by protein phosphatase 2A

The catalytic subunit of protein phosphatase 2A (PP2Ac) stimulates the initiation of replication of simian virus 40 DNA in vitro by dephosphorylating T antigen at specific phosphoserine residues (K. H. Scheidtmann, D. M. Virshup, and T. J. Kelly, J. Virol. 65:2098-2101, 1991). To better define the biochemical mechanism responsible for this stimulation, we investigated the effect of PP2Ac on the interaction of T antigen with wild-type and mutant origins of replication. Analysis of the binding of T antigen to the wild-type origin as a function of protein concentration revealed that binding occurs in two relatively discrete steps: the assembly of a T-antigen hexamer on one half-site of the origin, followed by the assembly of the second hexamer on the other half-site. The major effect of PP2Ac was to stimulate binding of the second hexamer, so that the binding reaction became much more cooperative. This observation suggests that dephosphorylation of T antigen by PP2Ac primarily affects interactions between the two hexamers bound to the origin. Pretreatment with PP2Ac increased the ability of the bound T antigen to unwind the origin of replication but had no effect on the intrinsic helicase activity of the protein. Thus, dephosphorylation of PP2Ac appears to increase the efficiency of the initial opening of the origin by T antigen. An insertion mutation at the dyad axis in the simian virus 40 origin, which altered the structural relationship of the two halves of the origin, abolished the effect of the phosphatase on the cooperativity of binding and completely prevented origin unwinding. These findings suggest that the ability of T antigen to open the viral origin of DNA replication is critically dependent on the appropriate functional interactions between T-antigen hexamers and that these interactions are regulated by the phosphorylation state of the viral initiator protein.

Phosphorylation of SV40 large T antigen at threonine residues results in conversion to a lower apparent molecular weight form

Simian virus 40 large T antigen (large T) was dephosphorylated with potato acid phosphatase at pH 5.5. Phosphate residues bound to serine were more susceptible to potato acid phosphatase than threonine-bound phosphate residues. Dephosphorylation of the threonine residues with potato acid phosphatase resulted in a reduced gel-electrophoretic mobility, while removal of the remaining phosphate groups had no effect on the mobility of large T. Pulse-chase experiments revealed a slower migrating form of newly synthesized large T which was converted by phosphorylation to a faster migrating form and had a half-life of approximately 1 h.

Altered phosphorylation of free and bound forms of monkey p53 and simian virus 40 large T antigen during lytic infection

We have identified the phosphorylation sites in monkey p53 as well as specific changes in the phosphorylation state of free and complexed forms of simian virus 40 (SV40) large T antigen (T) and monkey p53 isolate from SV40 lytically infected CV1 cells. Phosphopeptide analyses of free T and p53 (To and p53o) and complexed T and p53 (T+ and p53+) fractions indicated several quantitative increases in the specific phosphorylation of complexed forms of both proteins. The N terminus of monkey p53+ is phosphorylated at Ser-9, Ser-15, Ser-20, either Ser-33 or Ser-37, and at least one of Ser-90 to Ser-99. The C-terminal sites are Ser-315 and Ser-392. On comparing p53+ with p53o, we found that labeling of the two N-terminal phosphotryptic peptides encompassing residues 1 to 20 and 33 to 101 was increased fivefold and that Ser-315 was sevenfold more labeled than was Ser-392. When T+ was compared with To, the N-terminal peptide containing phosphorylation sites Ser-106 through Thr-124 was twofold more labeled, the peptide containing Ser-657 through Ser-679 was sixfold more labeled and contained up to four phosphorylated serine residues, and Ser-639 and Thr-701 appeared unchanged. Overall, T+ molecules appeared to contain 3.5 mol more of labeled phosphate than did To, with the N-terminal peptide appearing fully phosphorylated. The phosphopeptide patterns obtained for lytic T+ and To fractions were nearly identical to those found for wild-type SV40 T (stably complexed with mouse p53) and mutant 5080 T (defective for p53 binding) expressed in transformed C3H10T1/2 cells (L. Tack, C. Cartwright, J. Wright, A. Srinivasan, W. Eckhart, K. Peden, and J. Pipas, J. Virol. 63:3362-3367, 1989). These results indicate that increases in specific phosphorylation sites in both T+ and p53+ correlate with the association of T with p53. The enhanced phosphorylation state may be a consequence of complex formation between T and p53 or reflect an increased affinity of p53 for highly phosphorylated forms of T.

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.

Phenotype-specific phosphorylation of simian virus 40 tsA mutant large T antigens in tsA N-type and A-type transformants

To identify molecular differences between simian virus 40 (SV40) tsA58 mutant large tumor antigen (large T) in cells of tsA58 N-type transformants [FR(tsA58)A cells], which revert to the normal phenotype after the cells are shifted to the nonpermissive growth temperature, and mutant large T in tsA58 A-type transformants [FR(tsA58)57 cells], which maintain their transformed phenotype after the temperature shift, we asked whether the biological activity of these mutant large T antigens at the nonpermissive growth temperature might correlate with phosphorylation at specific sites. At the permissive growth temperature, the phosphorylation patterns of the mutant large T proteins in FR(tsA58)A (N-type) cells and in FR(tsA58)57 (A-type) cells were largely indistinguishable from that of wild-type large T in FR(wt648) cells. After a shift to the nonpermissive growth temperature, no significant changes in the phosphorylation patterns of wild-type large T in FR(wt648) or of mutant large T in FR(tsA58)57 (A-type) cells were observed. In contrast, the phosphorylation pattern of mutant large T in FR(tsA58)A (N-type) cells changed in a characteristic manner, leading to an apparent underphosphorylation at specific sites. Phosphorylation of the cellular protein p53 was analyzed in parallel. Characteristic differences in the phosphorylation pattern of p53 were observed when cells of N-type and A-type transformants were kept at 39 degrees C as opposed to 32 degrees C. However, these differences did not relate to the different phenotypes of FR(tsA58)A (N-type) and FR(tsA58)57 (A-type) cells at the nonpermissive growth temperature. Our results, therefore, suggest that phosphorylation of large T at specific sites correlates with the transforming activity of tsA mutant large T in SV40 N-type and A-type transformants. This conclusion was substantiated by demonstrating that the biological properties as well as the phosphorylation patterns of SV40 tsA28 mutant large T in cells of SV40 tsA28 N-type and A-type transformants were similar to those in FR(tsA58)A (N-type) and in FR(tsA58)57 (A-type) cells, respectively. The phenotype-specific phosphorylation of tsA mutant large T in tsA A-type transformants probably is a cellular process induced during establishment of SV40 tsA A-type transformants, since tsA28 A-type transformant cells could be obtained by a large-T-dependent in vitro progression of cells of the tsA28 N-type transformant tsA28.3 (M. Osborn and K. Weber, J. Virol. 15:636-644, 1975).

Immunological characterization of the Epstein-Barr virus phosphoprotein PP58 and deoxyribonuclease expressed in the baculovirus expression system

The open reading frames of the phosphoprotein pp58 (BMRFI) and the deoxyribonuclease (BGLF5) of the Epstein-Barr-virus (EBV) strain M-ABA were cloned in the baculovirus expression vectors pAc373 and pAc360 and expressed in the Spodoptera frugiperda (SF158) insect cells. The recombinant phosphoprotein pp58 expressed in SF158 cells was recognized by the anti-pp58 rabbit anti-sera which were generated by immunizing rabbits with a TrpE-BMRFI fusion protein expressed in E. coli. DNA-cellulose chromatography showed that the recombinant pp58 exhibited DNA-binding activities. Immunofluorescence, immunoblot and ELISA analysis indicated that sera from patients with nasopharyngeal carcinoma (NPC) contained antibodies against pp58. The recombinant EBV DNase expressed in SF158 cells was recognized by the anti-EBV DNase rabbit anti-sera which were generated by immunizing rabbits with a TrpE-C-terminal part of BGLF5 fusion protein expressed in E. coli. The anti-EBV DNase rabbit anti-sera recognized also a protein of about 52 kDa in the EBV-harboring human B-cell lines Raji, Jijoye, B95-8, M-ABA and BL74 induced by TPA and n-butyrate. The recombinant EBV DNase exhibited exonuclease and endonuclease activities, a requirement for magnesium, and a high pH optimum (8.0). Its enzyme activities could be inhibited by sera from NPC patients and anti-EBV DNase rabbit anti-sera. Comparable studies of Raji EBV-DNase and recombinant EBV-DNase implied that recombinant EBV-DNase could also be used in the enzyme activity assay for the detection of NPC. In contrast to the enzyme inhibition test, immunofluorescence and immunoblot analysis demonstrated that the recombinant EBV DNase exhibited only a weak immunological reaction with NPC sera.

Biochemical characterization of Epstein-Barr virus nuclear antigen 2A

The Epstein-Barr virus nuclear antigen 2A (EBNA-2A) was immunoprecipitated from latently Epstein-Barr virus-infected lymphocytes with a polyclonal serum raised against the EBNA-2A C terminus. The nucleus contained three subfractions of EBNA-2A which could be distinguished by their resistance to salt extraction: (i) a nucleoplasmatic fraction that was solubilized at 50 mM NaCl, (ii) a chromatin-associated fraction extractable at 1.5 M NaCl, and (iii) a nuclear matrix-associated fraction solubilized only by boiling with buffer containing 2% sodium dodecyl sulfate. The three subfractions were phosphorylated; it was demonstrated that the nucleoplasmatic and the chromatin-associated fractions were phosphorylated at serine and threonine residues. The half-life of the EBNA-2A protein was determined by cycloheximide treatment and by pulse-chase experiments and was found to be at least 24 h. The turnover of the phosphate residues bound to the two salt-soluble subfractions was determined to be approximately 6 to 9 h, suggesting a possible role of the phosphorylation in the regulation of the biological activity of EBNA-2A. Dephosphorylation of EBNA-2A resulted in an increased mobility of the protein during sodium dodecyl sulfate-polyacrylamide gel electrophoresis and indicated the presence of differentially phosphorylated subclasses of the protein. Analysis of EBNA-2A by sucrose gradient centrifugation revealed the existence of two subclasses of complexed molecules which exhibited sedimentation coefficients of approximately 13S and 34S.

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.

The finger domain of simian virus 40 large T antigen controls DNA-binding specificity

The specificity and regulation of protein-DNA interactions play a crucial role in all aspects of communication between genotype and phenotype in a cell. The large T antigen of simian virus 40 binds to identical, yet quite differently arranged, pentanucleotide motifs in the simian virus 40 control region, sites I and II. Wild-type T antigen preferentially binds site I. We demonstrate that a bacterial peptide encoding residues 1 to 259 (T260) includes the essential amino acids required for binding to both DNA sites but predominantly binds site II. However, a longer peptide (residues 1 to 369; T370) binds almost exclusively to site I. Thus, the addition of amino acids 260 to 369 to the T260 peptide results in the loss of site II binding. This region includes a single putative metal-binding region, and mutation of T370 at either conserved cysteine of the finger results in equal but inefficient binding to both sites. While no metal binding has been shown to be directly associated with this sequence, these results suggest a novel, perhaps structural, function for such a finger motif, since this domain of T antigen appears to play a crucial role in modulating the DNA-binding behavior of T-antigen peptides.

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.

Characterization of purified pp60c-src protein tyrosine kinase from human platelets

Intact pp60c-src, the cellular homologue of the transforming protein of Rous sarcoma virus, was purified from human platelets. The purified fractions also contained small amounts of a 54-kDa proteolytic degradation product of pp60c-src. We investigated some of the biochemical and kinetic properties of pp60c-src protein tyrosine kinase. Maximum kinase activity occurred at pH 6.5 and required a mixture of 2 mM Mn2+/Mg2+ as divalent cations. The enzyme most strongly phosphorylated casein, followed by enolase and alcohol dehydrogenase. The Km value for ATP was 4 microM for substrate phosphorylation and for autophosphorylation. Using casein, we determined a Vmax for substrate phosphorylation by pp60c-src in the range of 1.9-3.4 nmol.min-1.mg-1. Since the Vmax value for the purified 54-kDa fragment of pp60c-src was also included in this value, we conclude that proteolytic degradation of a 6-kDa fragment from the N-terminus of pp60c-src did not affect its kinase activity. Tryptic phosphopeptide analysis identified Tyr-416 as the major autophosphorylation site. Preincubation of purified pp60c-src with ATP increased the amount of autophosphorylation accompanied by an increase in Vmax, whereas the Km values were not altered. Our data directly demonstrate that autophosphorylation at Tyr-416 exerts, in contrast to phosphorylation at Tyr-527, a positive regulatory effect on the pp60c-src kinase activity.

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.

Identification and characterization of an Epstein-Barr virus early antigen that is encoded by the NotI repeats

The Epstein-Barr virus (EBV) genome is characterized by two regions carrying partially homologous clusters of short tandem repeats (NotI and PstI repeats) flanked by 1,044 and 1,045 base pairs with almost perfect homology (DL and DR, left and right duplications, respectively). Both repetitive regions are transcribed into poly(A)+ mRNA after induction of the productive EBV cycle with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate and contain open reading frames. To identify the potential protein encoded by the NotI repeat open reading frame (BHLF1), two repeat units of EBV strain M-ABA were expressed using the tryptophan-regulated Escherichia coli expression vector pATH11. Rabbit antisera generated against the resulting fusion protein reacted specifically with a protein varying in molecular size between 70,000 and 90,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, found after 12-O-tetradecanoyl-phorbol-13-acetate or n-butyrate induction in various cell lines harboring EBV. In immunofluorescence tests with the BHLF1-specific antiserum, an immunofluorescence with EA-D specificity could be observed. In addition, the BHLF1 protein is exhibiting polyanion-binding activity with a maximum for single-stranded DNA. Furthermore, the fusion protein is recognized by a number of human EBV-positive sera.

Linker insertion mutants of simian virus 40 large T antigen that show trans-dominant interference with wild-type large T antigen map to multiple sites within the T-antigen gene

Linker insertion mutants affecting the simian virus 40 (SV40) large tumor (T) antigen were constructed by inserting a 12-base-pair oligonucleotide linker into restriction endonuclease cleavage sites located within the early region of SV40. One mutant, with the insertion at amino acid 5, was viable in CV-1p and BSC-1 cells, indicating that sequences very close to the amino terminus of large T could be altered without affecting the lytic infection cycle of SV40. All other mutants affecting large T were not viable. In complementation assays between the linker insertion mutants and either a late-gene mutant, dlBC865, or a host range/helper function (hr/hf) mutant, dlA2475, delayed complementation was seen with the 6 of the 10 nonviable mutants. Of these 10 mutants, 5 formed plaques 3 to 4 days later than in control complementations, while complementation by one of the mutants, inA2827, with an insertion at amino acid 520, was delayed more than 1 week. Most mutants which showed delayed complementation replicated less well in Cos-1 cells than did a control mutant, dlA1209, which produced no T antigen. The replication of inA2827(aa520) was reduced by more than 90%. Similar interference with viral DNA replication was seen when CV-1, HeLa, or 293 cells were cotransfected with an origin-defective plasmid encoding wild-type large T antigen and with inA2827(aa520). Only one of the mutant T antigens, inA2807(aa303), was unstable. These results indicate that some of the mutant T antigens interfered with functions of wild-type T required for viral DNA replication. However, not all of the mutants which showed delayed complementation also showed interference with viral DNA replication. This indicates that mutant large T antigens may interfere trans dominantly with multiple activities of wild-type large T antigen.

Synthesis of functional bovine leukemia virus (BLV) p34tax protein by recombinant baculoviruses

The bovine leukemia virus (BLV) p34tax (also called tat, p34, XLOR gene product) is a 34-kDa polypeptide encoded in the 3'-terminal region of the virus. This protein is responsible for positive transcriptional trans-activation of promoter elements located within the BLV long-terminal repeat. We introduced the protein-coding region of BLV p34tax into the genome of the baculovirus Autographa californica nuclear polyhedrosis virus. After infection of the insect Spodoptera frugiperda (SF9) cell line, this recombinant strain of baculovirus produced approximately 100 to 150 mg of p34tax per 2 X 10(9) cells. This protein, when introduced into mammalian fibroblasts by using a cell-to-cell fusion technique, functionally trans-activated the BLV long-terminal repeat. Analysis of 32P-labeled proteins of SF9 cells expressing BLV tax by two-dimensional gel electrophoresis indicated that the BLV p34tax was phosphorylated.

Production of simian virus 40 large tumor antigen in bacteria: altered DNA-binding specificity and dna-replication activity of underphosphorylated large tumor antigen

A bacterial expression system was used to produce simian virus 40 large tumor antigen (T antigen) in the absence of the extensive posttranslational modifications that occur in mammalian cells. Wild-type T antigen produced in bacteria retained a specific subset of the biochemical activities displayed by its mammalian counterpart. Escherichia coli T antigen functioned as a helicase and bound to DNA fragments containing either site I or the wild-type origin of replication in a manner identical to mammalian T antigen. However, T antigen purified from E. coli did not efficiently bind to site II, an essential cis element within the simian virus 40 origin of replication. It therefore could not unwind origin-containing plasmids or efficiently replicate simian virus 40 DNA in vitro. The ability of protein phosphorylation to modulate the intrinsic preference of full-length T antigen for either site I or site II is discussed.

Analysis of a large-T-antigen variant expressed in simian virus 40-transformed mouse cell line mKS-A

Earlier reports had suggested that the large T antigen expressed in simian virus 40 (SV40)-transformed mKS-A cells may be replication defective. Our experiments support these earlier observations showing that the mKS-A T antigen has a reduced DNA-unwinding activity in vitro. To investigate the molecular basis for this defect, we have isolated from an mKS-A genomic library an EMBL-3 bacteriophage clone carrying in its insert a full-length SV40 DNA element that most likely encodes the expressed T-antigen variant. DNA sequencing revealed only one nonconservative amino acid exchange, Asp to Asn at residue 636. Surprisingly, when a plasmid clone carrying the mKS-A T-antigen-coding sequence was transfected into monkey cells, we found that it replicated quite efficiently, probably suggesting that a high nuclear concentration of the variant T-antigen form compensates for the partial biochemical defect. However, a high nuclear concentration of T antigen was also found in mKS-A T-antigen-transformed mouse cells, yet a fusion of these cells to permissive monkey cells failed to induce in situ replication and excision of integrated SV40 DNA. We discuss possible reasons for the different behavior of T antigen in monkey cells and in mouse cells and suggest that one possibility for the replication-negative phenotype in transformed cells may be related to the fact that T antigen forms a tight complex with the cellular p53 protein in mouse cells but not in monkey cells.

Transcriptional activation is not responsible for increased levels of autonomously expressed simian virus 40 T-antigen in herpes simplex virus-infected cells

Herpes simplex virus type 1 (HSV-1) superinfection of CV-1 cells weakly transactivated a plasmid-borne metallothionein 1 (MT-1) promoter, but activated the expression of a marker gene controlled by an authentic HSV-1 promoter to a high level. In contrast, CMT-3 cells, which are CV-1 cells stably transformed with the simian virus 40 (SV40) large T-antigen (T-Ag) gene controlled by the MT-1 promoter, contained high levels of T-Ag following HSV-1 superinfection, but only if cells were preincubated in the presence of heavy-metal ions. This T-Ag was functional in that it could mediate the increase in copy number of a marker plasmid containing the SV40 origin of DNA replication. Pulse and continuous labeling of preinduced CMT-3 cells showed that T-Ag expression was not induced by HSV-1; but rather, HSV-1 superinfection resulted in the stabilization of pre-existing protein.

Isolation and characterization of a viable adenovirus mutant defective in nuclear transport of the DNA-binding protein

The isolation and characterization of an adenovirus mutant, Ad5dl802r1, containing two independent deletions in the 72-kilodalton (kDa) DNA-binding protein (DBP) gene is described. The two deletions remove amino acids 23 through 105 of DBP, resulting in the production of a 50-kDa product. Expression of this truncated DBP was delayed 12 to 24 h compared with that of the 72-kDa protein produced by wild-type adenovirus type 5. The DBP was located primarily in the cytoplasm of infected cells, whereas the wild-type product was predominantly nuclear. Therefore, DBP appears to contain a nuclear localization signal within the deleted region. Ad5dl802r1 DNA synthesis, viral late gene expression, and virus production were all delayed 12 to 24 h and were approximately 10-fold lower than with wild-type adenovirus type 5. These phenotypic properties can be accounted for by the delay in synthesis and the inefficient accumulation of the 50-kDa DBP within the nucleus of infected cells. The truncated DBP also lacks the majority of amino acids which are phosphorylated in the normal protein. The loss of these phosphorylation sites does not appear to seriously impair the ability of the protein to carry out its functions.

Nuclear subcompartmentalization of simian virus 40 large T antigen: evidence for in vivo regulation of biochemical activities

Simian virus 40 large T antigen (large T) in the early and the late phases of infection differs significantly in its sequence-specific DNA-binding and ATPase activities, indicating that different large-T populations participate in virus-specific events at various stages of the infectious cycle. To further characterize these large-T populations, we have analyzed nuclear subclasses of large T, isolated from their in vivo location, for their biochemical activities. We show that chromatin- and nuclear matrix-associated large-T molecules exhibit different simian virus 40 control region (ORI) DNA-binding and ATPase activities. The association of large T with a certain nuclear substructure, therefore, subcompartmentalizes large-T molecules exerting different biochemical activities. Nuclear subcompartmentalization thus may provide a higher-order level for the regulation of biochemical activities of large T in vivo.

Alterations in the structure of new and old forms of simian virus 40 large T antigen (T) defined by age-dependent epitope changes: new T is the same as ATPase-active T

The reactivity of simian virus 40 large T antigen labeled under pulse-chase conditions towards 22 antibodies was measured. Changes in epitope reactivity occurred in several domains of T as it matured, defining major structural alterations that distinguished mature from new molecules. New T reacted best with the same antibodies that bind and inhibit ATPase-active T. These antibodies thus can distinguish new T as a distinct structural and functional form.

Expression and complex formation of simian virus 40 large T antigen and mouse p53 in insect cells

Recombinant baculoviruses were constructed which express simian virus 40 large T antigen (SVT-Ag) or murine p53 to high levels in infected insect cells. Characterization of the expressed proteins revealed that they display many properties of the corresponding mammalian-derived proteins. Both proteins are of wild-type size, localize to the nucleus, are recognized by several SVT-Ag- or p53-specific monoclonal antibodies, and are phosphorylated in this system. Complexes are formed between baculovirus-derived SVT-Ag and p53 after coinfection of insect cells with both recombinant viruses. After infection of insect cells with either virus individually, each protein can self-associate to form a variety of oligomeric species. Pulse-chase experiments indicated that both SVT-Ag and p53 are highly stable in insect cells, even in the absence of complex formation.

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.

In vitro phosphorylation of SV40 large T antigen

Phosphorylation of simian virus 40 large T antigen (large T) was investigated in vitro. "Autophosphorylation" of large T resulted in the modification of Ser106, Ser112, Ser123, Thr124, either Ser676, Ser677, or Ser679, and Thr701. All of these residues were also found to be phosphorylated in vivo. Reaction of large T with purified casein kinase I resulted in phosphorylation of Ser123, possibly Thr124, and either Ser676, Ser677, or Ser679, while purified casein kinase II phosphorylated Ser106 and possibly Ser112. Submolar amounts of phosphate were transferred to large T indicating that only a fraction of large T served as substrate for the casein kinases. Removal of serine-bound phosphate did not affect the subsequent autophosphorylation or phosphorylation by casein kinase I and II. No phosphorylation at in vivo sites was observed with the cAMP-, cGMP-, or Ca2+/phospholipid-dependent protein kinases, or with the protease-activated kinase I and II.

Simian virus 40 (SV40) T antigen binds specifically to double-stranded DNA but not to single-stranded DNA or DNA/RNA hybrids containing the SV40 regulatory sequences

Simian virus 40 T antigen has been shown previously to bind specifically with high affinity to sites within the regulatory region of double-stranded simian virus 40 DNA. Using competition filter binding and the DNA-binding immunoassay, we show that T antigen did not bind specifically to either early or late single-stranded DNA containing these binding sites. Moreover, T antigen did not bind these sequences present in single-stranded RNA, RNA/RNA duplexes, or RNA/DNA hybrids. T antigen did, however, bind as efficiently to single-stranded DNA-cellulose as to double-stranded DNA-cellulose. This binding was nonspecific because it was independent of the presence of T-antigen-binding sites. The implications of these observations are discussed.

SV40 T-antigen binding to site II is functionally separated from binding to site I

During lytic infection SV40 T antigen binds specifically to three different regions of the SV40 DNA to initiate viral DNA replication and to regulate early and late transcription. We have used the recently described plasmids pKB1, containing a 23-bp oligonucleotide coding for site I, pdl1085 containing sites II and III together with SV40 specific flanking sequences, and as a control pATC, a plasmid which contains all three binding sites (D. Müller et al. (1987), Virology 161, 81-91) to analyze the differential binding of T antigen to these individual binding sites in the course of an SV40 infection. We found that shortly after infection the amount of bound DNA increased with the concentration of T antigen reaching a steady-state level at about 20 hr after infection. In comparison to binding at site I, binding to site II appeared with a delay of about 8-9 hr corresponding to the onset of viral DNA replication. The correlation between binding of T antigen to site II and the SV40 DNA replication could be further corroborated by using T antigen from the heat-sensitive mutant tsA58 which completely failed to bind to site II at nonpermissive temperature but exhibited a residual binding to site I. This reduced binding to site I proved insufficient for the proper functioning of autorepression. Our results support the hypothesis that distinctly different subclasses of T-antigen binding to site I or site II may exist.