Monoclonal antibodies against simian virus 40 nuclear large T tumour antigen: epitope mapping, papova virus cross-reaction and cell surface staining

Gene transfer and expression studies in cultured avian neural crest cells differentiating into melanocytes

Neural crest cells obtained from explanted neural tubes take up, express, and retain exogenous DNA applied by the CaPO4 co-precipitation method during their differentiation into melanocytes. High efficiencies of gene transfer were obtained with both supercoiled DNA and intact phage particles; linear DNA or DNA from the phage yielded very low efficiencies. There is some evidence that transferred gene expression is differentiation dependent. The system should be useful for studies concerned with the analysis of cell developmental genes and their regulatory elements.

Synthetic oligopeptides define epitopes at the amino- and carboxy-terminus of simian virus 40 large tumor antigen which are recognized by monoclonal antibodies

Synthetic oligopeptides were used to define the epitopes recognized by monoclonal antibodies directed against the SV40 large tumor (T) antigen. The two monoclonal antibodies PAb 1622 and PAb 1626 reacted with the N-terminal undecapeptide Met(1)-Leu(11) as detected by inhibition of nuclear SV40 T antigen immunofluorescence. Antibody PAb 1605 recognized the hexapeptide Thr(701)-Pro(706) at the carboxy-terminus of the SV40 T antigen as detected by inhibition of an enzyme-linked immunosorbent assay. The specificities of these monoclonal antibodies differ from those of antibodies previously raised against the respective synthetic oligopeptides.

Fine mapping two distinct antigenic sites on simian virus 40 (SV40) T antigen reactive with SV40-specific cytotoxic T-cell clones by using SV40 deletion mutants

The existence of two distinct antigenic sites at the surface of simian virus 40 (SV40)-transformed H-2b cells has been previously demonstrated (A. E. Campbell, L. F. Foley, and S. S. Tevethia, J. Immunol. 130:490-492, 1983) by using two independently isolated SV40-specific cytotoxic T-lymphocyte (CTL) clones, K11 and K19. We identified amino acids in the amino-terminal half of SV40 T antigen that are essential for the recognition of antigenic sites by these CTL clones by using H-2b cells transformed by mutants that produce T antigen truncated from the amino-terminal or carboxy-terminal end or carrying overlapping internal deletions in the amino-terminal regions of SV40 T antigen. The results show that CTL clone K11 failed to recognize and lyse target cells missing SV40 T-antigen amino acids 189 to 211, whereas CTL clone K19 lysed these cells. The cell lines missing SV40 T-antigen amino acids 220 to 223 and 220 to 228 were not lysed by CTL clone K19 but were susceptible to lysis by CTL clone K11. Two other cell lines missing amino acids 189 to 223 and 189 to 228 of SV40 T antigen were not lysed by either of the CTL clones but were lysed by SV40-specific bulk-culture CTL if sufficient amounts of relevant restriction elements were expressed at the cell surface. The SV40 T-antigen amino acids critical for the recognition of an antigenic site by CTL clone K11 were identified to be 193 to 211; 220 to 223 were identified as critical for recognition by CTL clone K19. The deletion of these amino acids from the T antigen resulted in the loss of antigenic sites specific for CTL clones K11 and K19.

Recognition of simian virus 40 T antigen synthesized during viral lytic cycle in monkey kidney cells expressing mouse H-2Kb- and H-2Db-transfected genes by SV40-specific cytotoxic T lymphocytes leads to the abrogation of virus lytic cycle

Simian virus 40 (SV40)-encoded tumor or T antigen localizes in the membranes in addition to the nucleus of SV40-infected permissive monkey cells and SV40-transformed nonpermissive cells. The surface T antigen in SV40-transformed mouse cells provides a target for the cytotoxic T lymphocytes (CTL) which recognize SV40 T antigen in association with murine K/D, class I H-2 antigens. In order to demonstrate that SV40 T antigen synthesized in SV40-infected permissive monkey kidney cells (TC-7) may also function as a target for CTL, cloned murine H-2Db and H-2Kb genes were expressed in TC-7 cells by DNA transfection and TC-7 cell lines expressing high levels of either H-2Kb or H-Db antigens were established after cell sorting. SV40-infected TC-7/H-2Kb and TC-7/H-2Db cells became susceptible to lysis by SV40-specific H-2b restricted CTL. The susceptibility of these transfected SV40-infected monkey cells to anti-SV40 bulk culture CTL and SV40-specific H-2Db- and H-2Db-restricted CTL clones depended upon the synthesis of SV40 T antigen and the expression of the appropriate H-2Kb or H-2Db restriction elements. Treatment of SV40-infected TC-7/H-2Db and TC-7/H-2Kb with CTL clones abrogated the virus lytic cycle indicating that CTL may play an important role in limiting papovavirus infection in the natural host.

Monoclonal antibodies as probes for a function of large T antigen during the elongation process of simian virus 40 DNA replication

Various monoclonal antibodies specific for simian virus 40 large tumor antigen (T antigen) inhibit the elongation process of viral DNA replication in an in vitro system. The results provide strong evidence for a function intrinsic to T antigen during ongoing replicative-chain elongation. The antibody inhibition studies were further used to establish a correlation between the known biochemical activities of T antigen and its function during the elongation phase. The data demonstrate that, in addition to DNA binding and ATPase, a third function of T antigen is required for replicative chain elongation. This function is most probably related to the recently described DNA helicase activity of T antigen. This conclusion is based on the following results: aphidicolin treatment of actively replicating simian virus 40 minichromosomes causes a partial uncoupling of parental DNA strand separation and DNA synthesis; the strand separation reaction is blocked by the same monoclonal antibodies which strongly inhibit the elongation process. DNA helicase activity of isolated T antigen is equally well inhibited by the same set of monoclonal antibodies that affect minichromosome replication in vitro.

Polyomavirus small t antigen: overproduction in bacteria, purification, and utilization for monoclonal and polyclonal antibody production

Polyomavirus small t antigen was purified from genetically engineered Escherichia coli and used as the immunogen for the production of polyclonal and monoclonal antibodies. A new series of plasmids for increased expression of polyomavirus T antigens or a T antigen-beta-galactosidase fusion protein was constructed by replacing sequences coding for the ribosome-binding site of previously published plasmids with a chemically synthesized sequence that has a higher degree of complementarity to the 3' end of the 16S rRNA. Cells expressing the fusion protein from the plasmid with the synthetic sequence contained 5- to 10-fold more fusion protein after a 3-h induction than did control cells. Pulse-labeling of cells bearing the new plasmids revealed that the T antigens were synthesized at high levels after induction: 10% of total synthesis for small t; 15% for Py-1387T middle T, a truncated mutant of middle T; and probably 1 to 5% for middle T. Small t and Py-1387T middle T, but not wild-type middle T, were seen as minor bands in total cell protein analyzed on sodium dodecyl sulfate-polyacrylamide gels stained with Coomassie blue. A simple, rapid procedure for purification of bacterial small t from the pellet of sonicated bacteria yielded 1 to 2 mg of small t per liter of bacterial culture at 80 to 90% homogeneity. High-titer polyclonal rabbit antisera raised against purified small t recognized all three T antigens and were suitable for immunoaffinity purification of middle T. Mouse monoclonal antibodies raised against bacterial small t were of four classes, immunoprecipitating either all three polyomavirus T antigens, small t and middle T only, primarily small t, or middle T and large T in preference to small t. One of the latter monoclonal antibodies also immunoprecipitated large T but not small t of simian virus 40, suggesting that the site recognized by this antibody may be functionally important. None of the monoclonal antibodies yielded an immunoprecipitate active in phosphorylating middle T in vitro.

DNA helicase activity of SV40 large tumor antigen

Large tumor antigen (T antigen) was extracted from SV40-infected African Green Monkey cells and purified to homogeneity by immunoaffinity chromatography. The purified T antigen preparations unwind DNA duplices of greater than 120 bp in a reaction which is dependent on magnesium ions and ATP hydrolysis. Based on these and other properties of the reaction we classify this newly discovered enzymatic activity as a eukaryotic DNA helicase. The helicase and the known ATPase function of T antigen cosediment with the mono- or dimeric 4-6 S form of T antigen, but not with higher T antigen aggregates. The helicase activity seems to be an intrinsic function of SV40 T antigen. First, several different T antigen-specific monoclonal antibodies interfere with the DNA unwinding activity; monoclonals which are known to reduce the T antigen-specific ATPase most strongly inhibited the helicase reaction. Second, mutant T antigens with impaired ATPase function also showed a reduced DNA unwinding activity.

SV40 T antigen and the exocytotic pathway

A chimeric gene consisting of DNA coding for the 15-amino acid signal peptide of influenza virus hemagglutinin and the C-terminal 694 amino acids of SV40 large T antigen was inserted into a bovine papilloma virus (BPV) expression vector and introduced into NIH-3T3 cells. Cell lines were obtained that express high levels (approximately 5 X 10(6) molecules/cell) of the chimeric protein (HA-T antigen). The biochemical properties and intracellular localization of HA-T antigens were compared with those of wild-type T antigen. Wild-type T antigen. Wild-type T antigen is located chiefly in the cell nucleus, although a small fraction is detected on the cell surface. By contrast, HA-T antigen is found exclusively in the endoplasmic reticulum (ER). During biosynthesis, HA-T antigen is co-translationally translocated across the membrane of the ER, the signal peptide is cleaved and a mannose-rich oligosaccharide is attached to the polypeptide (T antigen contains one potential N-linked glycosylation site at Asn154). HA-T antigen does not become terminally glycosylated or acylated and little or none reaches the cell surface. These results suggest that T antigen is incapable of being transported along the exocytotic pathway. To explain the presence of wild-type T antigen on the surface of SV40-transformed cells, an alternative route is proposed involving transport of T antigen from the nucleus to the cell surface.

Use of simian virus 40 large T-beta-galactosidase fusion proteins in an immunochemical analysis of simian virus 40 large T antigen

Simian virus 40 large T antigen is a multifunctional protein that is encoded by the early region of the viral genome. We constructed fusion proteins between simian virus 40 large T antigen and beta-galactosidase by cloning HindIII fragments A and D of the virus into the HindIII sites of expression vectors pUR290, pUR291, and pUR292. Large amounts of the fusion protein were synthesized when the DNA fragment encoding part of simian virus 40 large T antigen was in frame with the lacZ gene of the expression vector. Using Western blotting and a competition radioimmunoassay, we assessed the binding of existing anti-T monoclonal and polyclonal antibodies to the two fusion proteins. Several monoclonal antibodies reacted with the protein encoded by the fragment A construction, but none reacted with the protein encoded by the fragment D construction. However, mice immunized with pure beta-galactosidase-HindIII fragment D fusion protein produced good levels of anti-T antibodies, which immunoprecipitated simian virus 40 large T antigen from lytically infected cells, enabling derivation of monoclonal antibodies to this region of large T antigen. Therefore, the fusion proteins allowed novel epitopes to be discovered on large T antigen and permitted the precise localization of epitopes recognized by existing antibodies. The same approach can also be used to produce antibodies against defined regions of any gene.

A large-tumor-antigen-specific monoclonal antibody inhibits DNA replication of simian virus 40 minichromosomes in an in vitro elongation system

In productively infected cells, a fraction of large-tumor antigen (T antigen) is tightly bound to replicating simian virus 40 (SV40) minichromosomes and does not dissociate at salt concentrations of greater than 1 M NaCl. We present electronmicrograms demonstrating the presence of T antigen on the replicated sections of replicating SV40 minichromosomes. We also show that the fraction of tightly bound T antigen is recognized by antibodies from mouse tumor serum and, more specifically, by a particular T-antigen-specific monoclonal antibody, PAb 1630. A second T-antigen-specific monoclonal antibody, PAb 101, does not react with the T-antigen fraction remaining on replicating SV40 chromatin at high salt concentrations. We used an in vitro replication system which allows, via semiconservative DNA replication, the completion of in vivo-initiated replicative intermediate DNA molecules. We show that monoclonal antibody PAb 1630, but not monoclonal antibody PAb 101, inhibits viral DNA replication. We discuss the possibility that SV40 T antigen may play a role in chain elongation during SV40 chromatin replication.

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

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

Plasma membrane orientation of simian virus 40 T antigen in three transformed cell lines mapped with monoclonal antibodies

Simian virus 40 large T antigen transforms cells from several species. Recent studies show that it is present on the cell surface. As in other tumor virus systems, this may be important for transformation. We have used a radioimmunoassay to map antigenic determinants on living and formaldehyde-fixed transformed cells with six different monoclonal antibodies to T antigen. Nonrelevant monoclonal antibodies of the same subclasses served as controls. With the transformed mouse line SVT2, antibody PAb 101, which reacts with the C-terminal region of T antigen, and PAb 1700, which is directed against an internal region of T, reacted with both formaldehyde-fixed and living cells. Antibodies PAb 402 (C terminus) and 419 (N terminus) reacted only with living cells, their determinants being destroyed upon formaldehyde fixation. Antibodies PAb 405 (C terminus) and 100 (internal) fail to react on either fixed or living cells. Similar results were obtained on the simian virus 40-transformed human line SV80 and the fixed hamster line CHLwt23, although all antibodies failed to react with living CHLwt23 cells. The data suggest that T antigen is inserted into the plasma membrane of transformed cells in a specific, nonrandom manner, with the C and N termini exposed on the cell surface and the midportion either buried in the lipid bilayer, hidden by the tertiary structure of T antigen, or masked by a post-translational modification such as fatty acid acylation.