Correlation between the conformational phenotype of p53 and its subcellular location

In order to obtain insight into the parameters determining the subcellular localization of mutant and wild-type forms of p53, we analysed the subcellular distribution of p53 in four Balb/c mouse-derived cell lines ranging in their cellular phenotypes from normal (3T3), via minimal transformant (T3T3), to maximally transformed (3T3tx, Meth A). Epitope mapping showed the p53 proteins in 3T3 and in T3T3 cells to be in a wild-type conformation, as they reacted with PAb246, whereas p53 in 3T3tx and in Meth A cells were PAb246 negative and thus displayed a mutant conformation. Despite its reactivity with PAb246, p53 in T3T3 cells had an extended half-life and accumulated to abnormally high levels. We show that the conformationally wild-type p53 in 3T3 and T3T3 cells predominantly localized to the cell nucleus, with about half of it being tightly associated with nuclear structures. In contrast, approximately 60% of mutant p53 in 3T3tx and Meth A cells localized to the cytoplasm, the rest residing in the cell nucleus; all the nuclear p53 in these cells appeared to be structurally bound. The cytoplasmic location of mutant p53 in 3T3tx and Meth A cells was not seen by immunofluorescence microscopic analysis, and required cell fractionation for its detection. Both cytoplasmic and nuclear p53 of the mutant phenotype bound to hsc proteins with a similar stoichiometry, suggesting that hsc binding is not directly related to the subcellular distribution of these proteins. We suggest that the conformational phenotype of p53 is a major determinant of its subcellular location.

Lytic infection of primary rhesus kidney cells by simian virus 40

In an attempt to analyze the persistent infection of rhesus monkey cells with Simian virus 40 (SV40) in vitro, as described previously (reviewed in L. C. Norkin, Microbiol. Rev. 46, 384-425, 1982), we infected primary rhesus cell cultures (PRK), derived from a SV40-free monkey colony with SV40. Surprisingly, SV40 infected PRK cell cultures released as much infectious virus as cultures of the permissive African green monkey kidney cell line TC7. Infected PRK cells exhibited typical symptoms of a lytic infection, and the bulk of infected PRK cells died within 8 days postinfection (p.i.). A considerable proportion of infected PRK cells exhibited distinct SV40-caused cytopathic effects (CPE), similar to CPE in infected TC7 cells. We conclude that the in vivo persistence of SV40 in rhesus monkeys is not determined by cellular host factors, but by the immune system of the infected animals.

Species-specific phosphorylation of mouse and rat p53 in simian virus 40-transformed cells

We have analyzed in detail the phosphorylation of p53 from normal (3T3) and simian virus 40 (SV40)-transformed (SV3T3) BALB/c mouse cells and from normal (F111) and SV40-transformed [FR(wt648)] rat cells by two-dimensional tryptic peptide mapping and phosphoamino acid analyses. To accommodate the different half-lives of p53 in normal (half-life, 15 min) and transformed (half-life, 20 h) cells and possible differences in the rates of turnover of phosphate at specific sites, cells were labeled for 2 h (short-term labeling) or 18 h (long-term labeling). Depending on the labeling conditions, either close similarities or marked differences were observed in the phosphorylation patterns of p53 from normal and transformed cells. After the 2-h labeling, the phosphorylation patterns of p53 from normal and transformed mouse cells were quite similar. In contrast, p53 from normal and transformed rat cells exhibited dramatic quantitative and qualitative differences under these labeling conditions. The reverse was found after an 18-h label leading to steady-state phosphorylation of p53 in transformed cells: while p53 in transformed mouse cells revealed a marked quantitative increase in phosphorylation compared with p53 from normal cells, the corresponding patterns of p53 from normal and transformed rat cells were similar. Our data thus indicate species-specific differences in the phosphorylation of mouse and rat p53 in SV40-transformed cells, reflected by (i) different turnover rates at specific sites in mouse and rat p53 and (ii) phosphorylation of nonhomologous serine and threonine residues in rat p53, as revealed by indirect assignment of phosphorylation sites to the phosphopeptides of rat p53. Analyses of p53 from the SV40 tsA58 mutant-transformed F111 cell lines FR(tsA58)A (N type) and FR(tsA58)57 (A type) yielded no conclusive evidence for a direct correlation between phosphorylation of p53, the metabolic stabilization of p53, and expression of the transformed phenotype.

Conformational analysis of p53 in resting and concanavalin A-stimulated mouse lymphocytes

We report that p53 in resting and concanavalin A-stimulated Balb/c mouse lymphocytes cannot be distinguished on the basis of different reactivity with various epitope-specific monoclonal antibodies, regardless of whether the lymphocytes are stimulated with concanavalin A in the presence or absence of serum. Our results thus question the 'conformational hypothesis' put forward by Milner [Milner, J. (1991). Curr. Op. Cell Biol., 3, 282-286], according to which wild-type p53, depending on its conformational status, can act as a negative or a positive growth regulator.

Immunization with soluble simian virus 40 large T antigen induces a specific response of CD3+ CD4- CD8+ cytotoxic T lymphocytes in mice

C57BL/6 (B6) mice (H-2b) were immunized with the large tumor antigen (T Ag) of simian virus 40 (SV40). Intraperitoneal or subcutaneous sensitization with soluble T Ag specifically primed cytotoxic lymphocyte precursors (CTLp). T Ag-specific cytotoxic T lymphocytes (CTL) were detected in a cytotoxicity assay after specific in vitro restimulation of effector cell populations from mice immunized with 2-10 micrograms purified, soluble T Ag and boosted with an injection of 2 micrograms T Ag 2-4 weeks after priming. Cells used for in vitro restimulation and as targets in cytotoxicity assays were syngeneic (B6-derived) RBL5 lymphoma cells expressing SV40 T Ag after transfection with a T Ag-encoding expression vector. Effector cells of this response were H-2 class I-restricted CD3+ CD4-CD8+ CTL. The magnitude of the anti-T Ag CTL response of B6 mice stimulated by soluble virus protein was comparable to the anti-T Ag CTL response of SV40-infected B6 mice. Injections of denatured or native T Ag protein primed CTLp equally well, but immunization with an equal dose of antigen emulsified in incomplete Freund's adjuvants inefficiently stimulated CTLp.

Altered phosphorylation at specific sites confers a mutant phenotype to SV40 wild-type large T antigen in a flat revertant of SV40-transformed cells

The Rev2 cell line is a cellular revertant of the SV40 wild-type transformed rat cell line SV-52 [Bauer, M., Guhl, E., Graessmann, M. & Graessmann, A. (1987). J. Virol., 61, 1821-1827]. To characterize the level of cellular interference with the SV40 large T antigen (large T)-induced transformation pathway in Rev2 cells, we analysed the biological and biochemical properties of large T expressed in Rev2 cells. We found that Rev2 cells encoded an authentic wild-type large T, with regard to its sequence and its transforming functions. No differences were found in the metabolic stability of large T, or in complex formation with the cellular p53 protein, or in p53 metabolic stabilization. In contrast to SV-52 cells, Rev2 cells showed no association of large T with the chromatin fraction of isolated nuclei. This difference correlated with a reduced affinity of the Rev2 large T to SV40 DNA in vitro. The T proteins from both cell lines were phosphorylated at the same multiple sites. However, in Rev2 cells the phosphorylation of large T at specific serine -residues was significantly reduced. Thus the revertant phenotype of Rev2 cells may be due to an altered phosphorylation state of its large T protein, leading to altered nuclear localization and reduced transforming activity. The alterations of Rev2 large T properties and phosphorylation were very similar to the changes observed with mutant large T in FR(tsA58)A cells, an SV40 tsA58 N-type transformant, when the cells had reverted to the normal phenotype at the non-permissive growth temperature. Thus altered phosphorylation might provide a common structural basis for the biological inactivation of the large T proteins in these 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).

Structural topography of simian virus 40 DNA replication

Applying an in situ cell fractionation procedure, we analyzed structural systems of the cell nucleus for the presence of mature and replicating simian virus 40 (SV40) DNA. Replicating SV40 DNA intermediates were tightly and quantitatively associated with the nuclear matrix, indicating that elongation processes of SV40 DNA replication proceed at this structure. Isolated nuclei as well as nuclear matrices were able to continue SV40 DNA elongation under replication conditions in situ, arguing for a coordinated and functional association of SV40 DNA and large T molecules at nuclear structures. SV40 DNA replication also was terminated at the nuclear matrix. While the bulk of newly synthesized, mature SV40 DNA molecules then remained at this structure, some left the nuclear matrix and accumulated at the chromatin.

Cell cycle control by p53 in normal (3T3) and chemically transformed (Meth A) mouse cells. I. Regulation of p53 expression

In addition to controlling the transition of resting normal cells from the G0-state of the cell cycle into S-phase, expression of the cellular protein p53 also seems to be necessary for the proliferation of cycling normal cells in an as yet undefined manner. To further elaborate the role of p53 in growing cells, we analysed p53 expression and its regulation in cells going into, and after release from, growth arrest at the restriction point (R-point) in the G1-phase of the cell cycle, induced by isoleucine depletion. Since growth arrest at the R-point is subject to internal control mechanisms of the cell cycle, this approach allowed us to include in our analyses normal Balb/c 3T3 fibroblasts, as well as cells of the chemically induced Balb/c fibrosarcoma cell line Meth A, expressing mutated p53. Isoleucine depletion induced a viable growth arrest at the R-point in cells of both cell lines, marked by a synchronous shut-down of DNA synthesis when the cells went into growth arrest, and a synchronous resumption of DNA synthesis after a lag period of about 2-4 h when the cells were released from growth arrest, as well as a shift to a G1 DNA content at the R-point. p53 expression in both cell lines showed a phenotypically similar regulation, as its synthesis was specifically reduced at the R-point. At the molecular level, however, p53 expression in growth arrested 3T3 cells was controlled at the transcriptional/post-transcriptional level, whereas control in growth arrested Meth A cells seemed to be at the level of mRNA translation. After release from growth arrest, p53 synthesis in both types of cells was rapidly restored, preceding resumption of total protein synthesis, and exhibiting a p53-specific profile.

Cell cycle control of p53 in normal (3T3) and chemically transformed (Meth A) mouse cells. II. Requirement for cell cycle progression

To further characterize the role of p53 in growing normal Balb/c 3T3 fibroblasts, as well as of p53 in cells of the methylcholanthrene induced fibrosarcoma cell line Meth A, we analysed the effect of inhibition of p53 synthesis by microinjection of p53-specific monoclonal antibody PAb 122 into the nuclei of these cells after release from growth arrest induced by isoleucine starvation (see preceding paper [Steinmeyer et al., this issue] ). We show that microinjection of PAb 122, but not of control immunoglobulins, into the nuclei of both types of cells effectively blocked their re-entry into the S-phase of the cell cycle. Since isoleucine depletion of these cells was shown to lead to a growth arrest at the restriction point (R-point) in the G1-phase of the cell cycle, our results (i) define more precisely the role of p53 in growing cells as a protein controlling transition of the cells through this restriction point, and (ii) demonstrate that mutated p53 in Meth A cells still is functional with regard to cell cycle control at this restriction point. We suggest that p53 acts as a 'gate-keeping' protein at restriction points in the cell cycle, exerting a positive effect on the transition of cells through the cell cycle.

Phosphorylation of p53 in primary, immortalised and transformed Balb/c mouse cells

To address the question whether phosphorylation of p53 might be functionally involved in its metabolic stabilisation and in cellular transformation processes, we have analysed the phosphorylation of the cellular protein p53 in normal and transformed cells of Balb/c mouse origin. Two-dimensional tryptic peptide maps of metabolically unstable p53 from normal Balb/c 3T3 cells and of metabolically stable p53 from 3T3 cells transformed by Simian virus 40 (SV3T3 cells) revealed no qualitative differences between their phosphorylation sites. Except for the unique lack of one or possibly two sites, the phosphopeptide map of p53 from cells transformed by the chemical carcinogen methylcholanthrene (MethA cells), expressing two different, metabolically stable mutant forms of p53, was identical to that of wild-type p53 from normal and SV40-transformed 3T3 cells. These results suggest that no direct relationship exists between phosphorylation of p53, its metabolic stabilisation, and cellular transformation processes. We have included in our analyses p53 from primary Balb/c mouse embryo fibroblast (MEF) cells and the immortalised MEFP27 cell line, established from primary cells after 27 passages. The phosphorylation sites of p53 from these cells were found to be identical to that of p53 from 3T3 cells. In addition, primary and established (immortalised) Balb/c mouse cells revealed no major differences in abundance of p53 mRNA and p53 protein, as well as stability of p53. These results indicate that immortalisation of normal cells involves neither gross alterations of p53 expression nor changes in specific phosphorylation of p53.

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.

Functional interaction of nuclear transport-defective simian virus 40 large T antigen with chromatin and nuclear matrix

We analyzed the subcellular distribution of nuclear transport-defective simian virus 40 Lys-128-mutant (cT-3 [R. E. Lanford and J. S. Butel, Cell 37:801-813, 1984] and d10 [D. Kalderon, W. D. Richardson, A. F. Markham, and A. E. Smith, Nature (London) 311:33-38, 1984]) large T antigens in various Lys-128-mutant-transformed rodent cells and in Lys-128-mutant d10-infected TC7 cells. Small but significant amounts of the mutant large T antigens were found in association with nuclear substructures, both in mutant-transformed and in mutant-infected cells. Experiments with TC7 cells made incompetent for cell division by 60Co irradiation supported the assumption that Lys-128-mutant large T antigen did not associate with nuclear components during mitosis but most likely was transported into the nucleus because the Lys-128 mutation was leaky for nuclear transport. Low-level simian virus 40 DNA replication and production of infectious mutant virus progeny in TC7 cells indicated that the association of Lys-128-mutant large T antigen with nuclear substructures is functional.

Cooperation of SV40 large T antigen and the cellular protein p53 in maintenance of cell transformation

We analysed large T antigen expression and metabolic stabilisation of the cellular protein p53 in cells of a matched pair of SV40 tsA mutant (tsA58) N-type or A-type transformants, respectively. At the permissive growth temperature (32 degrees C), cells of both transformants, like SV40 wild-type transformed cells, were phenotypically transformed and expressed large T antigen, as well as metabolically stable p53 (both complexed and free p53). At the nonpermissive growth temperature (39 degrees C), cells of the N-type transformant reverted to a normal phenotype, whereas cells of the A-type transformant still displayed a transformed phenotype. Under these growth conditions, the mutant large T antigens in both cell types were no longer able to complex p53 (both in vivo and in vitro), but the metabolic stabilities of the free p53 in these cells correlated with their phenotypes: p53 in cells of the N-type transformant was rapidly degraded, whereas it was metabolically stable in cells of the A-type transformant. This difference in p53 stability correlated with an in vivo functional difference between the mutant large T antigens at the nonpermissive growth temperature: large T antigen in cells of the N-type transformant no longer stably associated with the cellular chromatin and the nuclear matrix, but accumulated in the nucleoplasm. In contrast, large T antigen in cells of the A-type transformant at least partially had retained this ability. Maintenance of SV40 cell transformation thus seems to require both a functional large T antigen and a metabolically stabilised p53.

Only a minor fraction of plasma membrane-associated large T antigen in simian virus 40-transformed mouse tumor cells (mKSA) is exposed on the cell surface

The bulk of simian virus 40 (SV40) large T antigen in SV40-infected and -transformed cells localizes within the cell nucleus, while a minor fraction specifically associates with the plasma membrane (PM) and is exposed on the cell surface. PM-associated large T seems to span the lipid bilayer but, on the other hand, does not display typical features of a transmembrane protein. To further characterize the postulated transmembrane orientation of large T, we asked whether all large T molecules associated with the plasma membrane indeed are exposed on the cell surface. We compared the amount of cell surface-exposed large T, determined on living cells by a sensitive 3H-protein A-binding assay and by external immunoprecipitation, with that of total PM-associated large T extracted from isolated PM. We demonstrate that in mKSA cells (SV40-transformed BALB/c mouse fibroblasts), total PM-associated large T accounted for a substantial portion (ca. 2%) of total cellular large T. However, only 0.1 to 0.2% of it could be detected on the cell surface. Thus, only a minor fraction of PM-associated large T (less than 10%) is exposed on the surface of these cells. Interior PM-associated large T is stably associated with the plasma membrane, while the small fraction of surface-exposed large T is rapidly released from the cell surface.

Quantitative analysis of cell surface-associated SV40 large T antigen using a newly developed 3H-protein A binding assay

We have established a sensitive assay for the quantitative determination of large T antigen determinants on the surface of living simian virus 40 (SV40)-transformed cells (mKSA). Cells in suspension culture were incubated with monoclonal antibodies specific for large T antigen (KT3, directed against the carboxyterminus of large T antigen, and PAb 108, directed against an aminoterminal determinant on large T antigen). After incubation with secondary antibody (rabbit anti-mouse IgG), followed by incubation with 3H-protein A, the cells were sequentially extracted first with the nonionic detergent NP-40, followed by ultrasonication and extraction with the zwitterionic detergent Empigen BB. NP-40 solubilized large T antigen associated with NP-40-soluble constituents of the plasma membrane, whereas Empigen BB solubilized the plasma membrane lamina-associated subclass of large T antigen (U. Klockmann and W. Deppert, 1983, EMBO J., 7, 1151-1157). The amount of cell surface-bound 3H-protein A in the NP-40 and Empigen BB extracts was determined by liquid scintillation counting. In agreement with earlier reports, cell surface large T antigen was mainly found in association with the plasma membrane lamina (PML). Since the specific activity of 3H-protein A was known, it was possible to calculate the number of surface-bound 3H-protein A molecules, and thus to estimate the average number of surface-exposed amino- and carboxyterminal determinants of large T antigen per cell. KT3 recognized about 450-900 carboxyterminal determinants, while PAb 108 bound to about 1200-2400 aminoterminal determinants on the surface of a single mKSA cell. The cellular protein p53 also was detected on the surface of mKSA cells and was found to be present in amounts comparable to cell surface large T antigen.

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.

A novel mechanism for covalent attachment of fatty acid to SV40 large T antigen

The plasma membrane associated subclass of simian virus 40 large T antigen is specifically acylated with palmitic acid in vivo. To further analyze possible biological functions of fatty acid acylation, we developed a target-bound cell free in vitro acylation assay, in which immunopurified large T, bound to protein A-sepharose, was incubated with [3H]fatty acid. In this assay, large T was efficiently labeled with [3H]palmitic acid, but not with [3H]myristic acid. Thus the specificity of the in vivo labeling was preserved in vitro, too. The specific acylation of large T in vitro seemed to occur by an autocatalytic reaction, since it was found to be independent of added acyltransferases and exogenous energy. The energy for this reaction must be provided by the large T molecule itself, probably by an energy-rich internal ester bond. Our results provide evidence for a novel mechanism for the covalent attachment of fatty acids to proteins, which might also operate in vivo.

DNA binding properties of murine p53

We analysed the in vitro binding of p53 from normal (3T3) and from chemically transformed (Meth A) Balb/c mouse cells to double-stranded (ds-) DNA and to single-stranded (ss-) DNA by DNA-cellulose chromatography. We confirm previous findings that p53 in cellular extracts exhibits ds-DNA-binding activity (Lane and Gannon, 1983). In addition, we demonstrate that such p53 also binds to ss-DNA. Analyses with immunopurified p53 protein provide evidence that this DNA-binding activity is intrinsic to p53. DNA binding of p53 could not be inhibited by a monoclonal antibody specific for the C-terminal region. An N-terminal deletion mutant of p53 (Rovinski et al., 1987) exhibited similar DNA-binding properties as wild-type p53, indicating that the N-terminus also is dispensable for DNA binding. We further show a close correlation between the DNA-binding activity of p53 from 3T3 cells and its association with nuclear substructures.

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.

A subclass of the adenovirus 72K DNA binding protein specifically associating with the cytoskeletal framework of the plasma membrane

We have analyzed by immunofluorescence microscopy and by biochemical cell fractionation the subcellular distribution of the adenovirus type 2 72K DNA binding protein (DBP) during the course of infection in HeLa cells. Early in infection, the 72K DBP was strictly localized in the cell nucleus. However, as infection progressed, the 72K DBP was additionally found in other subcellular fractions, notably in association with the cytoskeletal framework of the plasma membrane, the plasma membrane lamina. Pulse-chase experiments demonstrated that this association was specific. Control experiments excluded the possibility of an artificial redistribution of the 72K DBP during cell fractionation. Our data, therefore, demonstrate that a significant portion of the 72K DBP during late times of infection associates specifically with the cytoskeletal framework of plasma membranes of infected cells.

Modulation of p53 protein expression during cellular transformation with simian virus 40

We analyzed the relation of metabolic stabilization of the p53 protein during cellular transformation by simian virus 40 (SV40) to (i) expression of the transformed phenotype and (ii) expression of the large tumor antigen (large T). Analysis of SV40-tsA28-mutant-transformed rat cells (tsA28.3 cells) showed that both p53 complexed to large T and free p53 (W. Deppert and M. Haug, Mol. Cell. Biol. 6:2233-2240, 1986) were metabolically stable when the cells were cultured at 32 degrees C and expressed large T and the transformed phenotype. At the nonpermissive temperature (39 degrees C), large-T expression is shut off in these cells and they revert to the normal phenotype. In such cells, p53 was metabolically unstable, like p53 in untransformed cells. To determine whether metabolic stabilization of p53 is directly controlled by large T, we next analyzed the metabolic stability of complexed and free p53 in SV40 abortively infected normal BALB/c mouse 3T3 cells. We found that neither p53 in complex with large T nor free p53 was metabolically stable. However, both forms of p53 were stabilized in SV40-transformed cells which had been developed in parallel from SV40 abortively infected cultures. Our results indicate that neither formation of a complex of p53 with large T nor large-T expression as such is sufficient for a significant metabolic stabilization of p53. Therefore, we suggest that metabolic stabilization of p53 during cellular transformation with SV40 is mediated by a cellular process and probably is the consequence of the large-T-induced transformed phenotype.

Specific interaction of simian virus 40 large T antigen with cellular chromatin and nuclear matrix during the course of infection

We analyzed the subnuclear distribution of the simian virus 40 (SV40) large tumor (large T) antigen during the course of viral infection. Three distinct nuclear subclasses were detected in SV40 lytically infected TC7 cells (large T antigen in the nucleoplasm, at the cellular chromatin, and at the nuclear matrix). During the course of infection the relative subnuclear distribution of large T antigen changed significantly at about the switch from the early to late phase of infection: at early times postinfection, large T antigen was present mainly in the nucleoplasm and at the cellular chromatin, and nuclear-matrix-associated large T antigen was barely detectable. Concomitant with the onset of viral DNA replication, the amount of nuclear-matrix-associated large T antigen increased drastically. During the further course of infection large T antigen accumulated at the cellular chromatin and nuclear matrix, paralleling the increase in viral DNA synthesis. The biological significance of this correlation was corroborated by analysis of cells infected with the SV40 mutant tsA58 at permissive (32 degrees C) and restrictive (39 degrees C) temperatures. tsA58 large T antigen failed to initiate viral DNA replication in infected cells kept at the restrictive temperature and also failed to associate with the cellular chromatin and nuclear matrix. By blocking viral DNA synthesis with aphidicolin, an inhibitor of DNA polymerase alpha, we were able to show that the accumulation of large T antigen at these structures does not result from the binding of large T antigen to viral chromatin but reflects an association with cellular components of the chromatin and nuclear matrix of infected cells.