Chemical modification as a tool for analysis of messenger RNA secondary structure in ribonucleoprotein particles

Chemical modification of unpaired bases is demonstrated in this study to be a reliable method for determining the conformation of nucleotides in mRNA. The modified nucleotides are identified by primer extension using reverse transcriptase. We have used this procedure to compare the structure of limited regions of SV40 T-antigen mRNA in solution, in nonpolysome-bound cytoplasmic messenger ribonucleoprotein particles, and in nuclear ribonucleoprotein complexes. The results indicate that SV40 T-antigen mRNA adopts a specific structure both in solution and when complexed with cellular proteins. The structures adopted by the mRNA in solution and in native cellular protein particles are very similar.

A morphological analysis endocrine tumour genesis in pancreas and anterior pituitary of AVP/SV40 transgenic mice

Insertion into the mouse genome of the hybrid oncogene made up of bovine vasopressin gene derived 5' upstream sequences and the coding sequences of SV40 large T-antigen promoted tumours in anterior pituitary and endocrine pancreas of mice bearing this transgene. In order to investigate the morphology of the steps in the neoplastic process, we used light and electron microscopy to study these organs in 42 animals belonging to the 3rd, 4th and 5th generations, subdivided into 4 age groups from 20 days to 100 days of life. Antibodies to large T-antigen were used to identify sites of expression of the hybrid oncogene, thus monitoring the steps in neoplastic transformation. Large T-antigen immunoreactivity was identified in dysplastic lesions of younger animals and in both dysplastic lesions and tumours of older mice. Insulin (100% of cases) and pancreatic polypeptide (25% of cases) immunoreactivities were revealed in pancreatic lesions but no hormonal immunoreactivity was detected in the pituitary lesions. The ultrastructural study confirmed that the majority cell population of the pancreatic neoplasms was B-type and that the anterior pituitary tumours were poorly granulated. The subcellular localization of large T-antigen immunoreactivity was investigated by the immunogold method and was confined to the heterochromatin of tumour cell nuclei. These findings provide evidence for the dysplasia-neoplasia sequence in the genesis of endocrine tumours of pituitary and pancreas of transgenic mice. The vasopressin-SV40 large T-antigen transgenic mice may therefore be an useful model for the study of endocrine cell oncogenesis.

Driven by the same Ig enhancer and SV40 T promoter ras induced lung adenomatous tumors, myc induced pre-B cell lymphomas and SV40 large T gene a variety of tumors in transgenic mice

Different types of tumors developed in transgenic mice following the introduction of the entire coding region of ras, myc or SV40 large T gene (T) linked to the same regulatory unit, consisting of a human immunoglobulin gene enhancer (Ig) and SV40 early gene promoter (Tp) with a 21-bp repeat. All the 12 transgenic mice harboring the intact T gene developed a variety of tumors including choroid plexus tumor, B cell lymphoma, histiocytic lymphoma, thymoma and others. This suggests that the Ig/Tp regulatory unit has transcriptional activity in these heterologous tissues. With this regulatory unit, myc gene induced solely pre-B cell lymphomas (five out of nine mice). Contrary to our expectation, however, the mutated ras gene induced lung adenomatous tumors in six out of eight transgenic mice over the 10-month observation period; the tumors are histologically comparable to adenocarcinomas in man. The tumors developed as early as 4 weeks after birth and the introduced ras gene was as efficiently expressed in both normal and neoplastic bronchioloalveolar epithelial cells as in normal lymphoid cells. An unidentified secondary event thus appears to be necessary for these ras-expressing cells to become neoplastic, as observed for myc (Leder et al., 1986). In a variety of tumors induced by Ig/Tp-T, on the other hand, T gene was expressed only in the tumor cells, but not in normal cells. Thus, derepression of T gene in normal cells appears to be closely related to their malignant change as observed in development of pancreatic acinar cell tumors by the T gene (Ornitz et al., 1985). These results suggest that ras and myc oncogenes penetrate differentially specific types of cells, while the SV40 T gene is tumorigenic in a variety of cell types.

Structure and function of SV40 large-T antigen

The small eukaryotic DNA tumour virus, SV40, has long provided a very useful model for the study of eukaryotic DNA replication and cellular transformation. The viral gene product, large-tumour (large-T) antigen, is essential for the initiation of viral DNA replication and the initiation and maintenance of SV40-virus-mediated cellular transformation. The large-T antigen is a complex multifunctional protein, and to delineate its activity more precisely in viral DNA replication and cellular transformation, small functional domains of the protein have been expressed in Escherichia coli and analysed by using a very extensive library of anti-T monoclonal antibodies.

The in vitro behavior of hemopoietic cells transformed by polyoma middle T antigen parallels that of primary human myeloid leukemic cells

A retrovirus encoding polyoma middle T antigen has been used to infect a murine hemopoietic cell line (FDC-P1) dependent on either granulocyte-macrophage colony-stimulating factor (GM-CSF) or multipotential colony-stimulating factor (Multi-CSF). A number of cell lines have been established on the basis of their initial ability to proliferate in the absence of added colony-stimulating factor (CSF). The transformed lines display one of three patterns of growth in vitro: those able to grow fully autonomously; those whose proliferation depends on cell density; and those displaying dependence on added CSF regardless cell density. This latter class of cells are reminiscent of the majority of primary myeloid leukemic cells. Unlike parental FDC-P1 cells, all three classes of transformed cells are leukemogenic in syngeneic mice; moreover, they produce variable amounts of GM-CSF which we believe underlies their neoplastic behavior.

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.

Polyomavirus early region alternative poly(A) site: 3'-end heterogeneity and altered splicing pattern

The position of an alternative polyadenylation [poly(A)] site at the 3' end of the polyomavirus middle T antigen (T-Ag) coding sequences suggests the possibility of a functional role for this site in early gene regulation. The fine structure of this alternative poly(A) site was determined by cDNA sequence and 3' S1 analyses. Cleavage-poly(A) was found to be heterogeneous, occurring at multiple CA dinucleotides downstream from the AATAAA signal sequence. About 50% of the alternative poly(A) takes place upstream from the middle T-Ag stop codon. In addition, the pattern of splicing of transcripts with the alternative poly(A) site differed from that with the major poly(A) site at the end of the early region. The ratio of the small and middle T-Ag splices to the large T-Ag splice for the alternative poly(A)+ mRNAs is about 2.5 times that found for mRNAs with the major poly(A) site. The altered splicing pattern and 3'-end heterogeneity of the alternative poly(A)+ mRNAs would result in preferential translation of small T-Ag (to a greater degree) and middle T-Ag over large T-Ag at later times in the polyomavirus lytic cycle.

Effects of deletion of the cytoplasmic domain upon surface expression and membrane stability of a viral envelope glycoprotein

The envelope protein (gp52) of Friend spleen focus-forming virus (F-SFFV) is defective in its intracellular transport and accumulates in the rough endoplasmic reticulum of F-SFFV-infected cells. This defect in transport has been attributed to the lack of a cytoplasmic domain, and possible loss of signals required for transport to the cell surface. The mature form of gp52, designated gp65, is also reported to be secreted from SFFV-infected cells. To determine the specific changes in the envelope protein which may lead to its lack of transport and to its lack of stability in associating with membranes, the 3' end of the F-SFFV envelope gene, which encodes the transmembrane domain, was inserted in place of the normal 3' end of the Friend murine leukemia virus genome. This chimeric envelope gene was expressed using the vaccinia virus expression system. The chimeric gp70/p15E glycoprotein molecule lacks the cytoplasmic tail residues and as a consequence is about 3300 daltons smaller. The chimeric PrEnv molecule was found to be cleaved efficiently as indicated by pulse-chase experiments. Immunofluorescence studies demonstrate that the chimeric molecule is efficiently transported to the surface of cells, unlike the SFFV gp52 glycoprotein. The chimeric molecule was found to be unstable in its membrane association and is released into the culture medium. These results indicate that the changes in the membrane spanning region and the lack of a cytoplasmic tail do not determine the defective transport of gp52, but may determine the stability of its association with membranes.

Unwinding of duplex DNA from the SV40 origin of replication by T antigen

The T antigen specified by SV40 virus is the only viral-encoded protein required for replication of SV40 DNA. T antigen has two activities that appear to be essential for viral DNA replication: specific binding to duplex DNA at the origin of replication and helicase activity that unwinds the two DNA strands. As judged by electron microscopy, DNA unwinding is initiated at the origin of replication and proceeds bidirectionally. Either linear or circular DNA molecules containing the origin of replication are effective substrates; with closed circular DNA, a topoisomerase capable of removing positive superhelical turns is required for an efficient reaction. Presence of an origin sequence on duplex DNA and a single-strand DNA-binding protein appear to be the only requirements for T antigen to catalyze unwinding. This reaction mediated by T antigen defines a likely pathway to precise initiation of DNA replication: (i) the sequence-specific binding activity locates the origin sequence, (ii) the duplex DNA is unwound at this site, and (iii) the DNA polymerase and primase begin DNA replication. A similar pathway has been inferred for the localized initiation of DNA replication by bacteriophage lambda and by Escherichia coli in which a sequence-specific binding protein locates the origin and directs the DnaB helicase to this site. Observations with the SV40 system indicate that localized initiation of duplex DNA replication may be similar for prokaryotes and eukaryotes.

Removal of serine phosphates from simian virus 40 large T antigen increases its ability to stimulate DNA replication in vitro but has no effect on ATPase and DNA binding

The effect of phosphorylation on the ability of simian virus 40 large T antigen to stimulate DNA synthesis in vitro was tested. Treatment of affinity-purified large T antigen with calf intestinal alkaline phosphatase resulted in the removal of 70 to 80% of the phosphate residues. Only serine-bound phosphate residues were affected. Phosphatase-treated large T antigen stimulated in vitro DNA synthesis fourfold over the untreated control. The stimulation was strongest at early times of DNA replication. At later times, DNA replication proceeded at equal rates with dephosphorylated and untreated large T antigen. The ATPase activity of large T antigen was not affected by phosphatase treatment. The origin-binding activity of large T antigen was tested over a wide range of large T antigen to DNA ratios, including DNA excess, and in the presence and absence of carrier DNA. Under no condition was an effect of dephosphorylation of large T antigen on its DNA-binding activity observed. These findings might indicate that phosphorylation at serine residues modulates the interaction of large T antigen with cellular factors. During DNA synthesis large T antigen was substantially rephosphorylated by kinases in the HeLa cell extract. As shown by two-dimensional peptide mapping, this phosphorylation occurred at all known in vivo sites. No phosphatase and protease activities were detectable in the HeLa cell extract.

Transformation of precrisis human cells by the simian virus 40 cytoplasmic-localization mutant pSVCT3 is accompanied by nuclear T antigen

pSVCT3 is a cytoplasmic-localization mutant of simian virus 40 (SV40) isolated from the SV40 adenovirus 7 hybrid virus (PARA) and cloned into plasmid PBR. The large T antigen of pSVCT3 accumulates in the cytoplasm of infected monkey cells instead of being transported to the nucleus. The sole change in CT3 large T antigen is amino acid residue 128 (Lys----Asn). Transformation of precrisis rodent cells by pSVCT3 is negligible, whereas the frequency of transformation of established rodent cell lines by pSVCT3 is comparable to that of wild-type SV40. According to the model, in which transformation of precrisis cells involves the combined oncogenic action of both nuclear and cytoplasmic gene products, we predicted that pSVCT3 would localize in the cytoplasm of human cells and would therefore at most only partially and rarely transform precrisis human cells. We have found that pSVCT3 is able to transform precrisis human cells at high frequency. Furthermore, pSVCT3-transformed human precrisis cells relocalized T antigen to their nuclei. The relocalization of large T antigen was not dependent on cell growth. Wild-type and pSVCT3-transformed human cell lines both have about five copies of integrated SV40 DNA. SV40 virus-specific proteins, including the 100,000-molecular-weight super large T antigen, were expressed in pSVCT3-transformed human cells. Our results suggest that molecules in precrisis human cells, but not cells of other species, are able to complement the cytoplasmic-localization defect of the CT3 mutant large T antigen.

Biological properties of simian virus 40 host range mutants lacking the COOH-terminus of large T antigen

Three mutants of simian virus 40 (SV40), with deletions near the 3' end of the A gene, displayed a host range phenotype for growth and virus production in various African green monkey kidney cell lines. The mutants formed plaques in CV-1P cells at 40.5 degrees, in BSC-1 cells at 37 and 40.5 degrees, and in Vero cells at 32, 37, and 40.5 degrees. Virus yields in these three lines were cold sensitive: the burst size was greatest at 40.5 degrees and least at 32 degrees, but some progeny was produced under all conditions examined. Mutant yields never exceeded 10% of wild-type yields under the most permissive conditions (Vero cells at 37 or 40 degrees) and were less than 1% of wild type under the most restrictive conditions (CV-1P cells at 32 degrees). These mutants can be complemented by any SV40 mutant which produces a large T antigen containing a normal COOH-terminus. Mutants whose T antigens could not be transported to the nucleus were most efficient at complementation. Mutant virus production in a line of rhesus monkey kidney cells and in primary cultures of African green and rhesus monkey kidney cells was also substantially below wild type. These mutants were also completely defective for adenovirus helper function. Our data suggest that the host range property and adenovirus helper function represent the same activities of large T antigen.

Transfection of neonatal rat Schwann cells with SV-40 large T antigen gene under control of the metallothionein promoter

Secondary cultures of Schwann cells were transfected with a plasmid containing the SV-40 T antigen gene expressed under the control of the mouse metallothionein-I promoter. We used the calcium phosphate method for transfection and obtained a transfection efficiency of 0.01%. The colonies were cloned by limited dilution, and these cloned cell lines were carried in medium containing zinc chloride (100 microM). One cloned cell line, which has now been carried for 180 doublings, appears to have a transformed phenotype with a doubling time of 20 h. These cells express SV-40 T antigen while maintaining established Schwann cell properties (positive staining for 217c, Ran-2, A5E3, glial fibrillary acidic protein, presence of 2',3'-cyclic nucleotide phosphohydrolase [CNPase] activity, and the ability to synthesize sulfogalactosylceramide and mRNA for the myelin protein, P0). Removal of zinc chloride from the medium resulted in reduced expression of T antigen and a change in the appearance of the cells to a more bipolar shape, although they still did not exhibit contact inhibition and maintained a doubling time of 20 h. These cells now became Ran-2-negative and showed increases in CNPase activity and in their ability to synthesize sulfogalactosylceramide. The amount of P0 mRNA remained unchanged. Transfected Schwann cells, however, stopped dividing when they contacted either basal lamina or neurites and became bipolar in appearance. The Schwann cells in contact with the neurites then extended processes to wrap around bundles of neurites. Transfection with the SV-40 T antigen gene therefore provides a method for obtaining Schwann cell lines that continue to express properties associated with untransfected cells in culture and may be used to study axon-Schwann cell interaction.

Pancreatic neoplasia induced by SV40 T-antigen expression in acinar cells of transgenic mice

Three lines of transgenic mice were produced that develop pancreatic neoplasms as a consequence of expression of an elastase I-SV40 T-antigen fusion gene in the acinar cells. A developmental analysis suggests at least a two-stage process in the ontogeny of this disease. The first stage is a T antigen-induced, preneoplastic state characterized by a progression from hyperplasia to dysplasia of the exocrine pancreas, by an increased percentage of tetraploid cells, and by an arrest in acinar cell differentiation. The second stage is characterized by the formation of tumor nodules that appear to be monoclonal, because they have discrete aneuploid DNA contents. The cells within the nodules as compared to normal pancreatic tissue have less total RNA by a factor of 5, less pancreas-specific messenger RNA by a factor of about 50, and increased levels of T-antigen messenger RNA. A tumor cell line has been derived that retains both pancreatic and neoplastic properties.

Inhibition of SV40 replicon function by engineered antisense RNA transcribed by RNA polymerase III

Promoters recognized by RNA polymerase III were used to direct synthesis of RNAs of opposite polarity to the 5' end of the mRNA for the large T-antigen of SV40. A construct was made utilizing the adenovirus (human type II) VA1 gene promoter linked to 163 bp of SV40 DNA sequences cloned in antisense orientation relative to the promoter. The SV40 sequence corresponds to the 5' end of the large T-antigen gene. In addition to the antisense constructs control plasmids were utilized which either lacked both promoter and SV40 elements, lacked RNA polymerase III promoter elements but contained SV40 sequences or contained the VA1 gene promoter fused to SV40 sequences in the sense orientation. The function of the various gene fusions was demonstrated in an in vitro transcription system and in vivo by S1 nuclease 5' end mapping following transfection into COS1 cells. Cotransfection of COS1 cells with the 'antisense' gene and a plasmid containing an SV40 origin of replication resulted in a substantial transient inhibition of SV40-replicon function when compared to control determinations (50% to nearly complete inhibition of large T-antigen dependent DNA replication for 18-36 h). These results show that an antisense RNA generated by RNA polymerase III can effectively block expression of a chromosomally located gene.

Coordinate expression of the endogenous p53 gene in beta cells of transgenic mice expressing hybrid insulin-SV40 T antigen genes

The expression of p53 has been evaluated during oncogenesis of the pancreatic beta cells in transgenic mice harboring hybrid insulin-SV40 T antigen genes. Significant levels of p53 are detected in all cells expressing large T antigen. In contrast, the protein is undetectable in normal beta cells. There is a complete correspondence between the onset of expression of T antigen and the appearance of the endogenous p53 protein. In tumors, the two proteins are found in a complex. In addition, free uncomplexed T antigen is detected in every cell which expresses the transgene. These results are consistent with the participation of p53 in T antigen-induced tumorigenesis in vivo. The early appearance of p53 in all beta cells expressing large T cannot readily explain the progression of a small fraction of these cells into solid tumors.

A nonkaryophilic T antigen of SV40 can either immortalize or transform rodent cells, and cooperates better with cytoplasmic than with nuclear oncoproteins

We investigated the cell growth alterations brought about by a mutant nonkaryophilic large T antigen (NKLT) of SV40, alone and in combination with other oncogenes. NKLT by itself exhibited bivalent functional competence: it induced immortalization in early-passage rat embryo fibroblasts (REFs) and transformation in established NIH3T3 cells, although it was totally unable to transform nonestablished REFs. The absence of a normal small T reduced but did not suppress such effects. Coexpression of NKLT with the nuclear oncoprotein Polyoma large T significantly increased the efficiency of the same activities sustained by NKLT alone, but did not confer the ability to transform nonestablished REFs. Similar results were also observed in cells cotransfected with NKLT and another nuclear oncoprotein, E1A of adenovirus. In contrast, NKLT coexpression with either of the cytoplasmic oncoproteins, Polyoma middle T or activated Ha-ras, produced full transformation of early passage REFs. Thus the NKLT stimulus has the following properties: (i) it straddles the immortalizing/transforming distinction of oncogenic competence, (ii) it potentiates the effects of some nuclear oncoproteins, and (iii) it complements certain cytoplasmic oncoproteins to promote transformation of nonestablished cells.

Electroporation: parameters affecting transfer of DNA into mammalian cells

Electroporation, the reversible breakdown of cell membranes caused by a high-voltage discharge, is a rapid, simple, and efficient method for introducing DNA into mammalian cells. An instrument for electroporation which permits the high-voltage discharge waveform to be varied with respect to rise time, peak voltage, and fall time is described. The uptake and expression of SV40 DNA following electroporation of two cell types, a human carcinoma-derived cell line, HEp-2, and a human lymphoblastoid cell line, 721, depended on the peak voltage and the fall time of the voltage discharge. The electronic parameters which produced optimum DNA transfer, however, differed for the two cell types. DNA as large as 150 kb was introduced into cells by electroporation. Cells can be electroporated in either phosphate-buffered saline or culture medium containing fetal bovine serum, and the efficiency of DNA transfer does not vary with cell densities from 10(6) to 2 X 10(7)/0.5 ml. Exposing the cells to multiple voltage discharges did not improve DNA transfer. DNA has been introduced by electroporation into all cell types tested, including human carcinoma-derived cell lines, human lymphoblastoid cell lines, human fibroblast strains, and primary human lymphocytes. To obtain maximal DNA transfer by this method, however, one must optimize the peak voltage and fall time of the discharge waveform for each cell type.

Elevated levels of a specific class of nuclear phosphoproteins in cells transformed with v-ras and v-mos oncogenes and by cotransfection with c-myc and polyoma middle T genes

Transformation of a rat thyroid epithelial cell line (FRTL5-C12) with Kirsten and Harvey murine sarcoma viruses (carrying the ras oncogenes) results in elevated levels of three perchloric acid-soluble nuclear phosphoproteins. These three proteins are also induced to high levels in the PC-C13 thyroid epithelial cell line when transformed by the myeloproliferative sarcoma virus (carrying the v-mos oncogene) and when transformed by transfection with the c-myc proto-oncogene followed by infection with the polyoma leukaemia virus (PyMuLV) carry the polyoma middle T antigen gene. Neither c-myc or PyMuLV alone induced high levels of the three nuclear proteins. Untransformed thyroid fibroblasts have high levels of two of the three proteins and can be transformed by PyMuLV alone resulting in the appearance of the third protein. Transformation with Harvey sarcoma virus also results in the induction of the third protein. The three phosphoproteins have been purified by h.p.l.c. and shown to be related to the HeLa protein HMGI already described. The results of these studies indicate that elevated levels of these HMGI-like proteins are associated with neoplastic transformation and/or with an undifferentiated phenotype.

Integration of Ecogpt and SV40 early region sequences into human chromosome 17: a dominant selection system in whole cell and microcell human-mouse hybrids

The dominant selectable gene, Ecogpt, has been introduced, by the calcium phosphate precipitation technique, into normal human fibroblasts, along with the SV40 early region genes. In one transfectant clone, integration of these sequences into human chromosome 17 was demonstrated by the construction of human-mouse somatic cell hybrids, selected for by growth in medium containing mycophenolic acid and xanthine. A whole cell hybrid, made between the human transfectant and a mouse L cell, was used as donor of the Ecogpt-carrying human chromosome 17 to 'tribrids' growing in suspension, made by whole cell fusion between a mouse thymoma cell line, and to microcell hybrids made with a mouse teratocarcinoma cell line. Two tribrids contained karyotypically normal human chromosomes 17 and a small number of other human chromosomes, while a third tribrid had a portion of the long arm of chromosome 17 translocated to mouse as its only human genetic material. Two independent microcell hybrids contained a normal chromosome 17 and no other human chromosome on a mouse teratocarcinoma background. These experiments demonstrate the ability to construct human-mouse somatic cell hybrids using a dominant selection system. By applying this approach it should be possible to select for a wide range of different human chromosomes in whole cell and microcell hybrids. In particular, transfer of single human chromosomes to mouse teratocarcinoma cells will allow examination of developmentally regulated human gene sequences after differentiation of such hybrids.

T antigen repression of SV40 early transcription from two promoters

The SV40 early mRNAs encode large (T) and small (t) tumor antigens. During the lytic cycle, the 5' termini of the early mRNAs undergo a shift: shortly after infection only an initiation site downstream from the TATA box is utilized; later an upstream initiation site becomes prominent. Both initiation sites are utilized in an in vitro transcription extract. D2T, a T antigen analog, specifically represses transcription in vitro from both initiation sites, but at different concentrations. Binding of D2T to site I suppresses initiation from the site downstream of the TATA box; binding to sites II and I suppresses initiation from the upstream site. The role of T antigen binding in repression of transcription and in the shift of initiation sites on the early strand is discussed.

Post-translational regulation of the 54K cellular tumor antigen in normal and transformed cells

The 54K cellular tumor antigen has been translated in vitro, using messenger ribonucleic acids from simian virus 40 (SV40)-transformed cells or 3T3 cells. The in vitro 54K product could be immunoprecipitated with SV40 tumor serum and had a peptide map that was similar, but not identical, to the in vivo product. The levels of this 54K protein in SV3T3 cells were significantly higher than those detected in 3T3 cells (D. I. H. Linzer, W. Maltzman, and A. J. Levine, Virology 98:308-318, 1979). In spite of this, the levels of translatable 54K messenger ribonucleic acid from 3T3 and SV3T3 cells were roughly equivalent or often greater in 3T3 cells. Pulse-chase experiments with the 54K protein from 3T3 or SV3T3 cells demonstrated that this protein, once synthesized, was rapidly degraded in 3T3 cells but was extremely stable in SV3T3 cells. Similarly, in an SV40 tsA-transformed cell line, temperature sensitive for the SV40 T-antigen, the 54K protein was rapidly turned over at the nonpermissive temperature and stable at the permissive temperature, whereas the levels of translatable 54K messenger ribonucleic acid at each temperature were roughly equal. These results demonstrate a post-translational regulation of the 54K cellular tumor antigen and suggest that this control is mediated by the SV40 large T-antigen.