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

Expression of two bovine rotavirus non-structural proteins (NSP2, NSP3) in the baculovirus system and production of monoclonal antibodies directed against the expressed proteins

Studies on rotavirus non-structural proteins have been hampered in the past by difficulties in obtaining monospecific reagents. To make such reagents available, we have expressed in the baculovirus system NSP2 and NSP3 (formerly called NS35 and NS34, respectively) of the bovine rotavirus RF and produced hybridomas against these proteins. Full-length DNA copies of RNA segments 7 (coding for NSP3) and 8 (coding for NSP2) of the virus strain RF were cloned and sequenced. Each cDNA was inserted in the transfer vector pVL941 and used to transfect Spodoptera frugiperda cells (Sf9). Recombinant baculoviruses encoding these proteins were obtained. Infection of Sf9 cells with these recombinant viruses resulted in a high level of expression of NSP2 and NSP3 (range of 1 microgram per 10(6) cells). Monoclonal antibodies (MAbs) were elicited by immunization of BALB/c mice with adjuvented, unpurified recombinant proteins in the rear foot pads. Fusion was performed using lymphocytes from popliteal lymph nodes with SP2/O-Ag14 myeloma line. Screening was by differential indirect immunofluorescent staining on monolayers of Sf9 cells infected with each recombinant virus. Two MAbs proved to be reactive against NSP3 and a single one against NSP2. They showed high specificity by immunofluorescence, immunoprecipitation and Western blot. The isotype of these MAbs was IgG1. Oligomeric forms of NSP3 and NSP2 proteins were detected and the existence of intra-chain disulfide bridge in NSP2 protein was suggested. The levels of synthesis and cellular localization of NSP3 and NSP2 proteins were different as shown by immunoprecipitation and immunofluorescence.

Two conditional tsA mutant simian virus 40 T antigens display marked differences in thermal inactivation

We have characterized the simian virus 40 (SV40) origin-containing DNA (ori-DNA) replication functions of two SV40 conditional mutant T antigens: tsA438 A-V (tsA58) and tsA357 R-K (tsA30). Both tsA mutant T antigens, immunopurified from recombinant baculovirus-infected insect cells, mediated replication of SV40 ori-DNA in vitro to similar extents as did wild-type T antigen in reactions at 33 degrees C. However, at 41 degrees C, the restrictive temperature, while tsA438 T antigen still generated substantial levels of replication products, tsA357 T antigen did not support any detectable DNA synthesis. Furthermore, preincubation for approximately fourfold-longer time periods at 41 degrees C was required to heat inactivate tsA438 T antigen than to heat inactivate tsA357 T antigen. Unexpectedly, results of analyses of the various DNA replication activities of the two mutant T antigens did not correlate with results from ori-DNA replication reactions. In particular, although tsA357 T antigen was incapable of mediating replication at 41 degrees C at all protein concentrations examined, it displayed either wild-type levels or only partial reductions of the several T-antigen replication-associated activities. These data suggest either that tsA357 T antigen is defective in an as yet unidentified replication function of T antigen or that the combination of its partial defects result in a protein that is unable to support replication. The data also show that two conditional mutant T antigens can be markedly different with respect to thermal sensitivity.

Thermally inactivated simian virus 40 tsA58 mutant T antigen cannot initiate viral DNA replication in vitro

The mutation in the temperature-sensitive tsA58 mutant T antigen (Ala-438----Val) lies within the presumptive ATP-binding fold. We have constructed a recombinant baculovirus that expresses large quantities of the tsA58 T antigen in infected insect cells. The mutant T antigen mediated simian virus 40 origin-containing DNA (ori-DNA) synthesis in vitro to nearly the same extent as similar quantities of wild-type T antigen at 33 degrees C. However, if wild-type and tsA58 T antigens were heated at 41 degrees C in replication extracts prior to addition of template DNA, the tsA58 T antigen but not the wild type was completely inactivated. The mutant protein displayed greater thermosensitivity for many of the DNA replication activities of T antigen than did the wild-type protein. Some of the replication functions of tsA58 T antigen were differentially affected depending on the presence or absence of ATP during the preheating period. When tsA58 T antigen was preheated in the presence of ATP at 41 degrees C for a time sufficient to completely inactivate its ability to replicate ori-DNA in vitro, it displayed substantial ATPase and normal DNA helicase activities. Conversely, when preheated in the absence of nucleotide, it completely lost both ATPase and helicase activities. Preheating tsA58 T antigen, even in the presence of ATP, led to drastic reductions in its ability to bind to and unwind DNA containing the replication origin. The mutant T antigen also displayed thermosensitivity for binding to and unwinding nonspecific double-stranded DNA in the presence of ATP. Our results suggest that the interactions of T antigen with ATP that are involved in T-antigen DNA binding and DNA helicase activities are different. Moreover, we conclude, consistent with its phenotype in vivo, that the tsA58 T antigen is defective in the initiation but not in the putative elongation functions of T antigen in vitro.

The cellular chromatin is an important target for SV40 large T antigen in maintaining the transformed phenotype

To identify cellular targets of simian virus 40 large T antigen (SV40 large T) important for the maintenance of cellular transformation, we have compared biological properties of SV40 tsA58 mutant large T antigens expressed in cells of a matched pair of SV40 tsA58 N-type (temperature-sensitive) and A-type (temperature-insensitive) transformants of the normal rat fibroblast line F111 (D. Pintel et al., J. Virol. 38, 518-528, 1981). Characterization of the selected cell lines demonstrated that cells of the N-type transformant [FR(tsA58)A] exhibited properties similar to those of the corresponding SV40 wild-type transformant [FR(wt648)] at the permissive growth temperature (32 degrees ), but reverted to a phenotype indistinguishable from the parental F111 cells at the nonpermissive growth temperature (39 degrees). At both growth temperatures, cells of the A-type transformant [FR(tsA58)57] were very similar to FR(wt648) cells in all properties analyzed. Both mutant-transformed cell lines expressed authentic tsA58 mutant large T antigens at comparable steady-state levels. Analysis of the subnuclear distribution of large T antigens in wild-type and in mutant-transformed cells kept at permissive or at nonpermissive growth temperature, respectively, revealed an important biological difference between the mutant T antigens in N- and A-type transformants: Whereas the subnuclear distribution of wild-type large T in FR(wt648) cells remained unchanged at both growth temperatures, mutant large T in FR(tsA58)A cells (N-type transformant) already 1 day after the shift to the nonpermissive growth temperature no longer stably associated with nuclear substructures, notably the cellular chromatin. In contrast, mutant large T in FR(tsA58)57 cells (A-type transformant) retained this ability. The ability (or inability) of the mutant T antigens to associate with the cellular chromatin in vivo was paralleled by different DNA binding properties of the mutant large T antigens in vitro. Large T in FR(tsA58)A cells no longer bound to the SV40 ORI in vitro after the shift to the nonpermissive growth temperature, whereas large T in FR(tsA58)57 cells at the elevated growth temperature had preserved this activity to a degree similar to its ability to associate with the cellular chromatin. We suggest that in the system of matched pairs of N- and A-type transformants analyzed in this study, expression of the transformed phenotype in FR(tsA58)57 (A-type) cells at the nonpermissive growth temperature is due to the preservation of a biologically active conformation of the mutant large T, allowing it to maintain its interaction with specific targets at the cellular chromatin.

Properties of a simian virus 40 mutant T antigen substituted in the hydrophobic region: defective ATPase and oligomerization activities and altered phosphorylation accompany an inability to complex with cellular p53

We have analyzed the biochemical properties of a nonviable simian virus 40 (SV40) mutant encoding a large T antigen (T) bearing an amino acid substitution (Pro-584-Leu) in its hydrophobic region. Mutant 5080 has an altered cell type specificity for transformation (transforming mouse C3H10T1/2 but not rat REF52 cells), is defective for viral DNA replication, and encodes a T that is unable to form a complex with the cellular p53 protein (K. Peden, A. Srinivasan, J. Farber, and J. Pipas, Virology 168:13-21, 1989). In this article, we show that 5080-transformed C3H10T1/2 cell lines express an altered T that is synthesized at a significantly higher rate but with a shorter half-life than normal T from wild-type SV40-transformed cells. 5080 T did not oligomerize beyond 5 to 10S in size compared with normal T, which oligomerized predominantly to 14 to 20S species. In addition, the 5080 T complex had significantly decreased ATPase activity and had a 10-fold-lower level of in vivo phosphorylation compared with that of normal T. Two-dimensional phosphopeptide analysis indicated several changes in the specific 32P labeling pattern, with altered phosphorylation occurring at both termini of the mutant protein compared with the wild-type T. Loss of p53 binding is therefore concomitant with changes in ATPase activity, oligomerization, stability, and in vivo phosphorylation of T and can be correlated with defective replication and restricted transformation functions. That so many biochemical changes are associated with a single substitution in the hydrophobic region of T is consistent with its importance in regulating higher-order structural and functional relationships in SV40 T.

Purification and functional properties of simian virus 40 large and small T antigens overproduced in insect cells

The insect baculovirus Autographa californica nuclear polyhedrosis virus was used as an expression vector for the simian virus 40 (SV40) small t (t) and large T (T) antigens. Spodoptera frugiperda (SF9) cells infected with recombinant viruses encoding these proteins produced approximately 1 to 2 micrograms of t and up to 30 micrograms of T per 3 X 10(6) cells. The former was highly soluble after Nonidet P-40 extraction of the infected cells, unlike its Escherichia coli-produced counterpart. Both SF9-produced proteins were of authentic size and could be readily immunoprecipitated by specific antibodies. Single-step immunoaffinity chromatography was used to purify the two proteins to near homogeneity, with yields averaging 70% in each case. Experiments to test the biological activity of the baculovirus SV40 proteins showed that SF9 t was capable of associating with two of the cellular proteins reported to bind to t in SV40-infected mammalian cells. Moreover, SF9 T had ATPase activity comparable to that of T produced in monkey cells, exhibited helicase activity and SV40 origin-specific DNA binding, and was active in the SV40 DNA replication assay in vitro. Thus, the SV40 T antigens produced in insect cells can be used in future studies of their biochemical roles in vitro and in vivo.

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.

The AT-rich sequence of the SV40 control region influences the binding of SV40 T antigen to binding sites II and III

During lytic infection SV40 T antigen binds specifically to three different regions of the SV40 DNA to initiate DNA replication and to regulate early and late transcription. We constructed plasmids containing either 23-bp synthetic oligonucleotides representing site I or II or SV40 DNA fragments with combinations of binding sites II and III with or without SV40 specific flanking regions. These plasmids were used to determine which sequences are sufficient for specific binding to isolated regions II and III. Under identical conditions T antigen bound in a sensitive in vitro binding assay efficiently to site I but not to the corresponding oligonucleotide of site II. Binding to site II could only be observed in the presence of the adjacent 17-bp AT-rich region of the SV40 DNA. On account of the markedly low affinity for binding site II, T antigen concentrations were required which exceeded those necessary to achieve saturation of binding to site I. The very low affinity for isolated site III could be slightly raised by the same AT-rich region. An increased binding to site II at 37 degrees compared to 0 degree in the presence of this region points to an indirect influence on the DNA structure of the binding site.

Enhanced protein phosphorylation in SV40-transformed and -infected cells

We have studied the phosphorylation of cellular phosphoproteins and, in more detail, of SV40 T antigen and the cellular protein p53 in SV40 tsA-transformed cells. As detected by radiolabeling cold-sensitive tsA1499- or heat-sensitive tsA58-transformed rat fibroblasts with [32P]orthophosphate or by in vitro labeling extracts with [gamma-32P]ATP the hyperphosphorylation of certain cellular phosphoproteins including p53 and also of free SV40 large T antigen and T antigen complexed with p53 is strictly correlated with the expression of the transformed phenotype. This hyperphosphorylation can be observed as early as 30 min after shifting to the temperature where the cells expressed the transformed phenotype and, furthermore, it is dependent on protein synthesis. To evaluate the influence of a functional T antigen and to exclude properties of individual transformants we 32P labeled in vitro cellular proteins from rat F111, mouse NIH 3T3, and monkey TC-7 cells infected with tsA58 or tsA1499. In tsA58-infected cells we found a heat-sensitive enhancement of protein phosphorylation just as in tsA58 transformants. In tsA1499-infected monkey cells we observed a heat-sensitive and in abortively infected rat or mouse cells a cold-sensitive hyperphosphorylation of proteins. Thus in tsA-transformants and in various tsA-infected cells we found a strong correlation among the transformed phenotype, functions of T antigen, and the phosphorylation of various cellular proteins and in particular T antigen and p53.

The phosphorylation at Thr 124 of simian virus 40 large T antigen is crucial for its oligomerization

SV40 large T antigen is phosphorylated at up to ten different amino acids clustered in an N-terminal and a C-terminal part of the polypeptide chain. The N-terminal phosphorylated residues include Ser 123 and Thr 124. We have analyzed the oligomerization, the complex formation with the cellular oncoprotein p53 and the DNA-binding properties of T antigen from two different SV40 transformed cell lines which have either an amino acid exchange at Ser 123 to Phe (W7) or Thr 124 to Ile (D29). In comparison to wild-type T antigen both mutant T antigens have a slightly reduced binding affinity for both binding sites, I and II, of SV40 DNA. Phosphorylation at both residues of T antigen is not essential for formation of the complex with p53. Only the phosphorylation at Thr 124 seems to be critical for the formation of high molecular mass oligomers. Our data support the hypothesis that the oligomerization of T antigen seems to be implicated in viral DNA replication.

Resolution of a polyomavirus-mouse hybrid replicon: viral function required for recombination

RmI, a circular chimera made of the polyomavirus (Py) genome with an insertion of mouse DNA (Ins), effectively undergoes intramolecular recombination in normal mouse cells, as indicated by the conversion of cloned RmI (RmIc) into unit-length Py DNA in transfected cultures. To follow the fate of the cellular component of RmI after recombination, the origin of simian virus 40 (SV40) DNA was inserted into the Ins region of RmIc, generating a new molecular species designated SV-RmIc. The recombination of SV-RmIc in simian cells synthesizing SV40 large T antigen gave rise to a molecule containing the SV40 origin, the reciprocal of unit-length Py DNA. However, SV-RmIc failed to yield unit-length Py DNA in murine cells unless Py large T antigen was provided in trans. In murine cells synthesizing SV40 large T antigen, the only detectable product from SV-RmIc contained only Py sequences, but was heterogeneous in size. These results and others also reported here strongly suggest that Py large T antigen plays a direct role in the resolution of RmI in murine cells.

Dimers and complexes with p53 are the prevalent oligomeric forms of a transforming nonkaryophilic T antigen of simian virus 40

The oligomers formed by a mutant nonkaryophilic large T antigen of simian virus 40, which lacks residues 110 through 152 of normal large T antigen and transforms only established cells (L. Fischer-Fantuzzi and C. Vesco, Proc. Natl. Acad. Sci. USA 82:1891-1895, 1985), were found to consist predominantly of dimers. Anti-p53 antibodies precipitated 14 to 16S complexes containing the mutant nonkaryophilic large T antigen and p53 from extracts of transformed cells, and anti-p53 indirect immunofluorescence stained these cells in the cytoplasm.

Regions of SV40 large T antigen necessary for oligomerization and complex formation with the cellular oncoprotein p53

The simian virus 40 (SV40) T antigen is composed of 708 amino acids and forms monomers and various oligomers and, in small amounts, heterologous complexes with the cellular oncoprotein p53 (T-p53). Using SV40 mutants coding for T antigen fragments which are either deleted in the N-terminal half or truncated by various lengths at the C-terminal end, we found that a region between amino acids 114 and 152 and a C-terminal region up to amino acid 669 are essential for the formation of high Mr oligomers of T antigen. Furthermore, only the C-terminal end up to amino acid 669 is essential for T-p53 complex formation but not the N-terminus up to amino acid 152.

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.

Chemical crosslinking of different forms of the simian virus 40 large T antigen using bifunctional reagents

Chemical crosslinking was used for a direct analysis of the different forms of large tumor (T) antigen, the simian virus 40 A gene product. The first subclass, sedimenting at 14-16S, is composed of monomeric to tetrameric units, whereas the second, sedimenting at 5-6S, only contains dimers and monomers of T. The occurrence of oligomeric structures of T in solution which are higher than dimers suggests the possibility of direct binding of such trimers or tetramers to the origin of replication of the viral DNA as an alternative to the formation of these structures by aggregation of bound dimers or monomers after their sliding along the DNA.