Role of the simian virus 40 gene A product in regulation of DNA synthesis in transformed cells

Absence of a structural basis for intracellular recognition and differential localization of nuclear and plasma membrane-associated forms of simian virus 40 large tumor antigen

The simian virus 40 large tumor antigen (T-ag) is found in both the nuclei (nT-ag) and plasma membranes (mT-ag) of simian virus 40-infected or -transformed cells. It is not known how newly synthesized T-ag molecules are recognized, sorted, and transported to their ultimate subcellular destinations. One possibility is that these events depend upon structural differences between nT-ag and mT-ag. To test this possibility, we compared the structures of nT-ag and mT-ag from simian virus 40-infected cells. No differences between the two forms of T-ag were detected by migration in polyacrylamide gels, by Staphylococcus aureus V8 partial proteolytic mapping of methionine- or proline-containing peptides, or by two-dimensional tryptic peptide mapping of methionine-containing peptides. The carboxy-terminal, methionine-containing tryptic peptide was identified in the two-dimensional maps and was shown to be identical in nT-ag and mT-ag. Thus, a structural basis for the recognition and differential localization of T-ags could not be demonstrated. The carboxy terminus of the T-ag encoded by mutant dlA2413 is derived from the alternate open reading frame of the simian virus 40 early region, in analogy with the theoretical early gene product, T*-ag. We used this mutant to identify peptides unique to T*-ag. None of these peptides were detected in maps of mT-ag; only wild-type T-ag-specific peptides were found. These findings suggest that T*-ag does not represent the membrane-associated form of T-ag, but that mT-ag is encoded within the same reading frame used for nT-ag.

Absence of a structural basis for intracellular recognition and differential localization of nuclear and plasma membrane-associated forms of simian virus 40 large tumor antigen

The simian virus 40 large tumor antigen (T-ag) is found in both the nuclei (nT-ag) and plasma membranes (mT-ag) of simian virus 40-infected or -transformed cells. It is not known how newly synthesized T-ag molecules are recognized, sorted, and transported to their ultimate subcellular destinations. One possibility is that these events depend upon structural differences between nT-ag and mT-ag. To test this possibility, we compared the structures of nT-ag and mT-ag from simian virus 40-infected cells. No differences between the two forms of T-ag were detected by migration in polyacrylamide gels, by Staphylococcus aureus V8 partial proteolytic mapping of methionine- or proline-containing peptides, or by two-dimensional tryptic peptide mapping of methionine-containing peptides. The carboxy-terminal, methionine-containing tryptic peptide was identified in the two-dimensional maps and was shown to be identical in nT-ag and mT-ag. Thus, a structural basis for the recognition and differential localization of T-ags could not be demonstrated. The carboxy terminus of the T-ag encoded by mutant dlA2413 is derived from the alternate open reading frame of the simian virus 40 early region, in analogy with the theoretical early gene product, T*-ag. We used this mutant to identify peptides unique to T*-ag. None of these peptides were detected in maps of mT-ag; only wild-type T-ag-specific peptides were found. These findings suggest that T*-ag does not represent the membrane-associated form of T-ag, but that mT-ag is encoded within the same reading frame used for nT-ag.

Surface T-antigen expression in simian virus 40-transformed mouse cells: correlation with cell growth rate

Cell growth control appears to be drastically altered as a consequence of transformation. Because the cell surface appears to have a role in modulating cell growth and simian virus 40 (SV40)-transformed cells express large T antigen (T-Ag) in the plasma membrane, we investigated whether surface T-Ag expression varies according to cell growth rate. Different growth states were obtained by various combinations of seeding density, serum concentration, and temperature, and cell cycle distributions were determined by flow microcytofluorometry. Actively dividing SV40-transformed mouse cell cultures were consistently found to express higher levels of surface T-Ag and T-Ag/p53 complex than cultures in which cells were mostly resting. In addition, the T-Ag/p53 complex disappeared from the surface of tsA7-transformed cells cultured under restrictive conditions known to induce complete growth arrest (39.5 degrees C), although the surface complex did not disappear from other tsA transformants able to keep cycling at 39.5 degrees C. These results suggest that surface SV40 T-Ag or surface T-Ag/p53 complex, or both, are involved in determining the growth characteristics of SV40-transformed cells.

Surface T-antigen expression in simian virus 40-transformed mouse cells: correlation with cell growth rate

Cell growth control appears to be drastically altered as a consequence of transformation. Because the cell surface appears to have a role in modulating cell growth and simian virus 40 (SV40)-transformed cells express large T antigen (T-Ag) in the plasma membrane, we investigated whether surface T-Ag expression varies according to cell growth rate. Different growth states were obtained by various combinations of seeding density, serum concentration, and temperature, and cell cycle distributions were determined by flow microcytofluorometry. Actively dividing SV40-transformed mouse cell cultures were consistently found to express higher levels of surface T-Ag and T-Ag/p53 complex than cultures in which cells were mostly resting. In addition, the T-Ag/p53 complex disappeared from the surface of tsA7-transformed cells cultured under restrictive conditions known to induce complete growth arrest (39.5 degrees C), although the surface complex did not disappear from other tsA transformants able to keep cycling at 39.5 degrees C. These results suggest that surface SV40 T-Ag or surface T-Ag/p53 complex, or both, are involved in determining the growth characteristics of SV40-transformed cells.

Antigenic structure of simian virus 40 large tumor antigen and association with cellular protein p53 on the surfaces of simian virus 40-infected and -transformed cells

The antigenic structure of simian virus 40 (SV40) large tumor antigen (T-ag) in the plasma membranes of SV40-transformed mouse cells and SV40-infected monkey cells was characterized as a step toward defining possible biological function(s). Wild-type SV40, as well as a deletion mutant of SV40 (dl1263) which codes for a truncated T-ag with an altered carboxy terminus, was used to infect permissive cells. Members of a series of monoclonal antibodies directed against antigenic determinants on either the amino or the carboxy terminus of the T-ag polypeptide were able to precipitate surface T-ag (as well as nuclear T-ag) from both SV40-transformed and SV40-infected cells. Cellular protein p53 was coprecipitated with T-ag by all T-ag-reactive reagents from the surface and nucleus of SV40-transformed cells. In contrast, T-ag, but not T-ag-p53 complex, was recovered from the surface of SV40-infected cells. These results confirm that nuclear T-ag and surface T-ag are highly related molecules and that a complex of SV40 T-ag and p53 is present at the surface of SV40-transformed cells. Detectable levels of such a complex do not appear to be present on SV40-infected cells. Both the carboxy and amino termini of T-ag are exposed on the surfaces of SV40-transformed and -infected cells. The possible relevance of the presence of a T-ag-p53 complex on the surface of SV40-transformed cells and its absence from SV40-infected cells is considered.

Structural comparisons of wild-type and nuclear transport-defective simian virus 40 large tumor antigens

PARA(nT) is a defective SV40-adenovirus 7 hybrid virus which contains the entire early region of the SV40 genome and codes for the synthesis of SV40 large tumor antigen (T-ag). A transport-defective variant of this hybrid, PARA(cT), encodes T-ag that is not transported to the nucleus, but accumulates in the cytoplasm. The structures of T-ags extracted from wild-type (WT) SV40-, PARA(nT)-, and PARA(cT)-infected cells were compared by peptide mapping. All three types of T-ag underwent considerable degradation when extracted using Tris-buffered Nonidet P-40 at pH 8.0. The addition of 200 microM leupeptin to the extraction buffer significantly inhibited this degradation. Comparison of methionine-containing tryptic peptides revealed no differences among the T-ags, suggesting that their primary structures are similar or identical. Phosphopeptide mapping revealed no differences between SV40- and PARA(nT)-encoded T-ags. In contrast, PARA(cT)-encoded T-ag lacked a prominent phosphopeptide that was present in both of the others. The possible relevance of this difference in phosphorylation to the transport defect is discussed.

Cell growth and differentiation in vitro in mouse macrophages transformed by a tsA mutant of simian virus 40. I. Cellular response in proliferative and phagocytic activities to the shift of temperature differs depending on the culture state in mouse bone marrow cells transformed by the tsA640 mutant of simian virus 40

It was shown previously that mouse bone marrow cells transformed by simian virus 40 (SV40) show a reversible cell density-dependent phenotypic transition between the nonmacrophage (rapidly growing) and the macrophage (stationary) states; cells in low-density cultures are in the growing phase, express SV40 T antigen strongly as revealed by immunofluorescence, and lose typical macrophage properties such as immune phagocytosis; whereas cells in high-density cultures are in the stationary (nongrowing) phase, express SV40 T antigen weakly, and recover their macrophage properties (Takayama, 1980). In the hope of clarifying the relationship between T antigen, cell growth, and macrophage-specific cellular function, we examined the behavior at 33 and 39 degrees C of mouse bone marrow cells transformed by an SV40 gene A mutant (tsA640) whose mutation renders the molecular weight of 90K (large) T antigen temperature sensitive. The results presented in this paper suggest that functional large T antigen is required for cells in the stationary phase to initiate multiplication when transferred at lower density and is not necessary for a majority of them to maintain the nongrowing state (viability) at both high and lower cell densities, whereas it is required for cells in the growing phase to keep multiplying without losing their viability. The results also suggest that the functional large T antigen does not play a direct role in maintaining the cells as either phagocytic or nonphagocytic. It is also suggested that the physiological or tsA mutation-mediated arrest of growth may or may not be accompanied by induction and/or maintenance of cellular phagocytic activity depending on the culture state.

Effects of large and small T antigens on DNA synthesis and cell division in simian virus 40-transformed BALB/c 3T3 cells

The roles of the large T and small t antigens of simian virus 40 in cellular DNA synthesis and cell division were analyzed in BALB/c 3T3 mouse cells transformed by wild-type, temperature-sensitive A (tsA), or tsA-deletion (tsA/dl) double mutants. Assessment of DNA replication and cell cycle distribution by radioautography of [3H]thymidine-labeled nuclei and by flow microfluorimetry indicate that tsA transformants do not synthesize DNA or divide at the restrictive temperature to the same extent as they do at the permissive temperature or as wild-type transformants do at the restrictive temperature. This confirms earlier studies suggesting that large T induces DNA synthesis and mitosis in transformed cells. Inhibition of replication in tsA transformants at the restrictive temperature, however, is not complete. Some residual cell division does occur but is in large part offset by cell detachment and death. This failure to revert completely to the parental 3T3 phenotype, as indicated by residual cell cycling at the restrictive temperature, was also observed in cells transformed by tsA/dl double mutants which, in addition to producing a ts large T, make no small t protein. Small t, therefore, does not appear to be responsible for the residual cell cycling and plays no demonstrable role in the induction of DNA synthesis or cell division in stably transformed BALB/c 3T3 cells. Comparison of cell cycling in tsA and tsA/dl transformants, normal 3T3 cells, and a transformation revertant suggests that the failure of tsA transformants to revert completely may be due to leakiness of the tsA mutation as well as to a permanent cellular alteration induced during viral transformation. Finally, analysis of cells transformed by tsA/dl double mutants indicates that small t is not required for full expression of growth properties characteristic of transformed cells.

Simian virus 40 A gene function: further characterization and growth of tsA transformed chinese hamster cells

Chinese hamster embryo cells transformed with the tsA 58 mutant of Simian virus 40 express the transformed phenotype at the permissive temperature (33 degrees C or 37 degrees C) and a "normal" phenotype at the nonpermissive temperature (40.5 degrees C). Immunofluorescence and immunoprecipitation of T antigens demonstrated that the "T" antigen (100 K) has an increase rate of synthesis and degradation at 40.5 degrees C. However, the cells continue to replicate at the nonpermissive temperature when assayed by flow cytometry and autoradiography. This DNA synthesis was cellular, not viral, and not owing to an increase in DNA repair. When the cell cycle distributions of G1, S, and G2 + M were assayed by the fraction labeled mitoses method, no differences were evident at the permissive and nonpermissive temperature; however, the doubling time was lengthened at 40.5 degrees C (13 hours vs. 100 hours). These results suggest that at 40.5 degrees C, the tsA transformed cells are cycling and dying. However, if the transformed cells are seeded onto monolayers of normal Chinese hamster cells at 40.5 degrees C, the cells are growth arrested when measured by growth assays, flow cytometry, autoradiography, and immunofluorescence for T antigen. Therefore, growth arrest can be obtained in tsA 58 transformed Chinese hamster cells when cocultured with normal Chinese hamster cells.

Detection of simian virus 40 surface-associated large tumor antigen by enzyme-catalyzed radioiodination

To facilitate detection of SV40 surface-associated tumor antigen (T-ag), conditions were established to surface label T-ag on intact cells by lactoperoxidase-catalyzed radioiodination (125I/LPO). SDS-PAGE analysis of anti-T immunoprecipitates of SV40-transformed and -infected cells labelled with 125I/LPO revealed the presence of iodinated T-ag. Several types of control experiments were employed to guarantee the surface specificity of the 125I/LPO labelling technique. When SV40-transformed mouse cells were surface labelled with lactoperoxidase and glucose oxidase immobilized on insoluble beads, a preparation less readily internalized than soluble enzymes, T-ag was iodinated. Selective immunoprecipitation of surface antigens demonstrated that lactoperoxidase did not iodinate internally localized T-ag. A reconstruction experiment in which an extract of SV40-infected cells was added to uninfected cells prior to surface labelling suggested that T-ag released from lysed cells did not adhere significantly to monolayer surfaces and become iodinated. Finally, systematic omission of reactants from the iodination reaction revealed that exogenous addition of lactoperoxidase and H2O2 was necessary to generate an iodinated T-ag, indicating that endogenous host cell reactants do not contribute significantly to the iodination of T-ag. 125I-labelled T-ag was detectable on the surface of SV40 tsA-infected cells at the nonpermissive temperature 24 h post infection, indicating that the tsA lesion does not prevent the interaction of T-ag with the cell surface. When 125I/LPO-labelled transformed or infected cells were chased for 2.5 h after labelling, iodinated T-ag was no longer associated with the cell monolayer but was immunoprecipitable from culture supernatants. Cultures from which labelled T-ag had been shed could then be relabelled with 125I/LPO and surface-associated T-ag was again detectable. These data suggest that surface-associated T-ag is continuously shed from the cell surface and is rapidly replaced in the membrane by intracellular T-ag.

Alterations to controls of cellular DNA synthesis by adenovirus infection

Human adenovirus type 5 and temperature-sensitive mutants ts36, ts37, and ts125 induced cellular DNA synthesis in quiescent rodent cells at both permissive and nonpermissive temperatures. Cellular DNA synthesis induced by adenovirus type 5 or by serum required protein synthesis for both initiation and continuation, whereas viral DNA synthesis was not dependent upon continued protein synthesis once it was initiated. Both cellular and viral DNA replication was induced in adenovirus type 5-infected cells in the presence of dibutyryl cyclic AMP at concentrations which inhibited induction by serum which suggested that some of the controls of DNA synthesis in serum-treated and virus-infected cells are different. After adenovirus infection of quiescent cells, there was a decrease in the number of cells with G1 DNA content and an increase in cells with G2 diploid and greater DNA contents. Thus, adenovirus type 5 induces a complete round of cellular DNA replication, but in some cells, it induces a second round without completion of a normal mitosis. These results suggest that adenovirus type 5 is able to alter cell growth cycle controls in a way which may be related to its ability to transform cells.

Stimulation of DNA polymerase alpha by a nuclear DNA/protein complex

A nuclear DNA complex containing DNA polymerase and SV40 T-antigen was isolated from nuclei of SV40-transformed mouse fibroblasts. DNA polymerase could be separated from the complex. The remaining DNA/T-antigen-containing complex stimulated DNA polymerase alpha activity about 10-fold. The complex contained 4 major proteins with molecular weights of 46, 54, 76, and 94 kilo-dalton (KD). The stimulation activity was retained by protein A-Sepharose loaded with specific IgG from SV40-tumor bearer serum, or from antisera against the 94 KD and 76 KD components and was partially inhibited in the presence of these antisera. The stimulation activity was completely abolished by treatment of the complex with trypsin or DNase I.

SV40-infected muntjac cells: cell cycle kinetics, cell ploidy and T antigen concentration

Muntjac cells in which the SV40 virus neither readily causes transformation nor replicates were used to study the effect of SV40 infection on cell ploidy and the influence of ploidy on the concentration of T antigen, which is required for the initiation of viral DNA synthesis. Both the DNA content, as measured by the flow microfluorometry of propidium iodide-DNA fluorescence, and the average number of chromosomes per cell indicated that infection with SV40 did not alter the ploidy of the host cell. SV40 infection had no effect on the ploidy distribution of muntjac cells. After immunofluorescence staining with anti-T serum and fluorescein-labeled anti-gamma G, infected and uninfected cultures were compared. In uninfected cells incubated with a 1:20 dilution of anti-T serum no fluorescence could be observed by fluorescence microscopy, but when examined by flow microfluorometry, fluorescence was detected after staining with as little as 1000-fold diluted antiserum. Determination of the amount of T antigen and DNA content in the same cell by simultaneous measurement of fluorescein isothiocyanate-conjugate and propidium iodide fluorescence, indicated that the cellular concentration of T antigen did not vary with the ploidy of the genome or the number of nuclei per cell. These results suggest that gene dosage is not a factor which determines the permissive environment for SV40 replication.

Temperature dependency for maintenance of transformation in mouse cells transformed by simian virus 40 tsA mutants

Mouse embryo fibroblasts and 3T3 cells were transformed by wild-type, tsB4, tsA7, tsA58, and tsA209 simian virus 40. Clones of transformants were generated both in soft agar and in liquid medium by focus formation and at both high and relatively low multiplicities of infection. All transformants were assayed for three phenotypes of transformation: (i) the ability to form highly multinucleated cells in cytochalasin B-supplemented medium, i.e., uncontrolled nuclear division; (ii) the capacity to continue DNA synthesis at increasing cell density; and (iii) the ability to form colonies in soft agar. The great majority of mouse embryo fibroblast transformants generated with tsA mutant virus were temperature sensitive for transformation in all three assays, regardless of the input multiplicity or whether they were generated in liquid medium or soft agar. These transformants exhibited a normal or near-normal phenotype at the nonpermissive temperature of 40 degrees C. All but one of the transformants which appeared transformed at both temperatures were in the A209 group. In contrast to mouse embryo fibroblasts, transformants generated with 3T3 cells and tsA virus were often not temperature sensitive, exhibiting the transformation phenotypes at both temperatures. This phenomenon was more often observed when 3T3 transformants were generated in soft agar. These results, along with other published data, suggest that uncontrolled nuclear division and uncontrolled DNA synthesis are a function of the simian virus 40 A gene. Finally, with the 3T3 transformants, there was often discordance in the expression of transformation among the three phenotypes. Some tsA transformants were temperature sensitive in one of two assays but were transformed at both 33 and 40 degrees C in the remaining assay(s). Other transformants exhibited a normal cytochalasin B response at either temperature but were temperature sensitive in the other assays.

Replication of Chinese hamster embryo cells transformed by temperature-sensitive T-antigen mutants of simian virus 40

Chinese hamster embryo cells transformed by simian virus 40 temperature-sensitive T-antigen mutants replicated when confluent at 40.5 degrees C, regardless of the selection method, selection temperature, or virus strain used.

Antigenic and immunogenic characteristics of nuclear and membrane-associated simian virus 40 tumor antigen

Antisera were prepared in syngeneic hosts against subcellular fractions of simian virus 40 (SV40)-transformed cells (MoalphaPM, MoalphaNuc), glutaraldehydefixed SV40-transformed cells (HaalphaH-50-G, MoalphaVLM-G), and electrophoretically purified denatured SV40 tumor antigen (T-ag) (RaalphaT). Immune sera were also collected from animals bearing tumors induced by SV40-transformed cells (HaalphaT, MoalphaT, HAF) and from SV40-immunized animals that had rejected a transplant of SV40-transformed cells (HaalphaS, MoalphaS). Immunological reagents prepared against cell surface (MoalphaPM, HaalphaS, MoalphaS, HaalphaH-50-G, MoalphaVLM-G) reacted exclusively with the surface of SV40-transformed cells by indirect immunofluorescence or protein A surface antigen radioimmunoassay. Immunological reagents prepared against the nuclear fraction (MoalphaNuc) or whole-cell determinants (HaalphaT, MoalphaT, HAF, RaalphaT) reacted with both the nuclei and surface of SV40-transformed or -infected cells. All reagents were capable of immunoprecipitating 96,000-molecular weight large T-ag from solubilized whole cell extracts of SV40-transformed cells. The exclusive surface reactivity of HaalphaS exhibited in immunofluorescence tests was abolished by solubilization of subcellular fractions, which then allowed immunoprecipitation of T-ag by HaalphaS from both nuclear and plasma membrane preparations. Specificity was established by the fact that all T-reactive reagents failed to react in serological tests against chemically transformed mouse cells, and sera from mice bearing transplants chemically transformed mouse cells (MoalphaDMBA-2) failed to react with SV40-transformed mouse or hamster cells. Reagents demonstrating positive surface immunofluorescence and protein A radioimmunoassay reactions against SV40-transformed cells were capable of blocking the surface binding of RaalphaT to SV40-transformed cells in a double-antibody surface antigen radioimmunoassay. This blocking ability demonstrated directly that a component specificity of each surface-reactive reagent is directed against SV40 T-ag. A model is presented which postulates that the differential detection of T-ag by the various serological reagents is a reflection of immunogenic and antigenic differences between T-ag polypeptides localized in nuclei and plasma membranes.

Density dependent inhibition of both growth and T-antigen expression in revertants isolated from simian virus 40-transformed mouse SVT2 cells

Phenotypic revertants were isolated from simian virus 40-transformed cells in order to examine the relationship between simian virus 40 T-antigen expression and G1 arrest of growth. Revertant clones with increased adherence were selected from cultures of SVT2, a simian virus 40-transformed BALB/c mouse cell line, and screened to find arrestable revertant clones which inhibited DNA synthesis when crowded. The clones selected from untreated SVT2 were unstable and showed little or no inhibition of DNA synthesis when crowded. Stable revertants were found after treatment of SVT2 with Colcemid to increase ploidy. The stable revertants all lost most transformed growth properties tested, including tumorigenicity, but only a few showed the same degree of inhibition of DNA synthesis at high cell density as BALB/3T3. All revertant clones expressed T antigen at low cell density. Three revertants showed coordinate inhibition of DNA synthesis and apparent loss of T antigen at high cell density. We suggest that changes in gene dosage rather than mutations caused the altered properties of the new revertants and that continued DNA synthesis in confluent cultures may be the transformed phenotype that requires the least simian virus 40 T antigen.

T-antigen expression in proliferating and non-proliferating simian virus 40-transformed mouse cells

Previous studies with simian virus 40-transformed mouse 3T3 cells which are temperature sensitive for the expression of the transformed phenotype (ts SV3T3 cells) have shown that T-antigen expression and viral DNA transcription are under cell cycle control. Using these ts SV3T3 cells, we studied the expression of the viral genome under proliferating and non-proliferating conditions, in the presence and absence of inhibitors of macromolecular synthesis and of the tumor promoter phorbol myristate acetate. ts SV3TE cells which are growth arrested at 39 degrees C by low serum concentration or saturation density accumulated in G1 and did not express T-antigen. When these cells were induced to proliferate, at either 32 or 39 degrees C, T-antigen synthesis preceded the entry of the cells into the S-phase and was not coupled to DNA replication. G1-arrested ts SV3T3 cells were induced to synthesize T-antigen by phorbol myristate acetate treatment, but T-antigen alone was not sufficient to induce cellular DNA synthesis. Isoleucine deprivation arrested growth of ts SV3T3 cells, but these cells, as well as normal 3T3, did not accumulate in G1 and continued to express T-antigen. The temperature-sensitive expression of the transformed phenotype in the ts SV3T3 cells does not appear to be due to a lack of transcription of specific regions of the integrated simian virus 40 genome at 39 degrees C.

Simian virus 40 gene A regulation of cellular DNA synthesis. I. In permissive cells

The kinetics of host cellular DNA stimulation by simian virus 40 (SV40) tsA58 infection was studied by flow microfluorometry and autoradiography in two types of productively infected monkey kidney cells (AGMK, secondary passage, and the TC-7 cell line). Prior to infection, the cell populations were maintained predominantly in G0-G1 hase of the cell cycle by low (0.25%) serum concentration. Infection of TC-7 or AGMK cells by wild-type SV40, viable deletion mutant dl890, or by SV40 tsA58 at 33 degrees C induced cells through S phase after which they were blocked with a 4N DNA content in the G2 phase. The infection of TC-7 cells by tsA58 at 41 degrees C, which was a nonpermissive temperature for viral DNA replication, induced a round of cell DNA synthesis in approximately 30% of the cell population. These cells proceeded through S phase but then re-entered the G1 resting state. In contrast, infection of AGMK cells by tsA58 at 41 degrees C induced DNA synthesis in approximately 50% of the cells, but this population remained blocked in the G2 phase. These results indicate that the mitogenic effect of the A gene product upon cellular DNA is more heat resistant than its regulating activity on viral DNA synthesis and that the extent of induction of cell DNA synthesis by the A gene product may be influenced by the host cell.

Subcellular Localization of simian virus 40 large tumor antigen

The distribution of simian virus 40 large tumor antigen in subcellular fractions from simian virus 40-transformed hamster (H-50) and mouse (VLM) cells and from simian virus 40-infected monkey cells was determined. Solubilized [(35)S]-methionine- or (32)P(i)-labeled surface membrane and nuclear fractions were prepared, immunoprecipitated with hamster anti-T serum, and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Tumor antigen with an apparent molecular weight of approximately 96,000 was detected in both subcellular fractions. Minor components of approximately 68,000 and approximately 56,000 with anti-T reactivity which labeled with [(35)S]methionine were also detected in both fractions from H-50 cells, as were components of approximately 140,000 and approximately 56,000 from VLM cells. The 56,000 component appeared to be greatly reduced in (32)P(i)-labeled surface membrane fractions. Normal cells or cells transformed with a heterologous agent, such as polyoma virus or a chemical carcinogen, lacked immunoprecipitable tumor antigen. Cell fractionation was monitored by [(3)H]thymidine labeling, NADH-diaphorase activity, and Na(+)-K(+)-dependent ATPase activity. These analyses revealed only trace contamination of surface membranes by nuclei, extremely low levels of nuclear rupture during homogenization, and an approximate 10-fold enrichment of surface membrane. Reconstruction experiments demonstrated that soluble tumor antigen failed to associate or copurify with surface membranes during fractionation procedures. These results indicate the presence of a protein in the plasma membrane of cells transformed or infected by simian virus 40 that is immunologically indistinguishable from nuclear tumor antigen.

Adenovirus type 12 gene 401 function and maintenance of transformation

Rat (3Y1) and hamster embryo brain cells were transformed by wild-type adenovirus type 12 or the DNA-minus temperature-sensitive mutant ts401. The ts401-transformed 3Y1 cells, but not the wild-type transformants, displayed a temperature-sensitive response with respect to the following characteristics of the transformed phenotype: morphology, saturation density, growth rate, cloning in soft agar, colony formation on plastic at low cell densities in 1% serum medium, and the T antigen(s). Temperature shift-down experiments showed that the density-dependent inhibition of growth of the ts401-transformed cells was reversible, as was, to some extent, the low efficiency of colony formation at low cell densities in 1% serum. Examination of hamster transformants for their ability to clone in soft agar at permissive and nonpermissive temperatures showed that this property was temperature dependent, again only in the ts401 transformants and not in the wild-type transformants. Alteration in uptake of 2-deoxyglucose or in intracellular cyclic AMP content was not a characteristic of the adenovirus-transformed phenotype in the 3Y1 cells. The findings suggest that an active 401 function is required for maintenance of the adenovirus-transformed cell pheno-type.