Virus-specific proteins in the plasma membrane of cells lytically infected or transformed by pol-oma virus

Membrane-bound Merkel cell polyomavirus middle T protein constitutively activates PLCγ1 signaling through Src-family kinases

Merkel cell polyomavirus (MCV or MCPyV) is an alphapolyomavirus causing human Merkel cell carcinoma and encodes four tumor (T) antigen proteins: large T (LT), small tumor (sT), 57 kT, and middle T (MT)/alternate LT open reading frame proteins. We show that MCV MT is generated as multiple isoforms through internal methionine translational initiation that insert into membrane lipid rafts. The membrane-localized MCV MT oligomerizes and promiscuously binds to lipid raft-associated Src family kinases (SFKs). MCV MT-SFK interaction is mediated by a Src homology (SH) 3 recognition motif as determined by surface plasmon resonance, coimmunoprecipitation, and bimolecular fluorescence complementation assays. SFK recruitment by MT leads to tyrosine phosphorylation at a SH2 recognition motif (pMTY114), allowing interaction with phospholipase C gamma 1 (PLCγ1). The secondary recruitment of PLCγ1 to the SFK-MT membrane complex promotes PLCγ1 tyrosine phosphorylation on Y783 and activates the NF-κB inflammatory signaling pathway. Mutations at either the MCV MT SH2 or SH3 recognition sites abrogate PLCγ1-dependent activation of NF-κB signaling and increase viral replication after MCV genome transfection into 293 cells. These findings reveal a conserved viral targeting of the SFK-PLCγ1 pathway by both MCV and murine polyomavirus (MuPyV) MT proteins. The molecular steps in how SFK-PLCγ1 activation is achieved, however, differ between these two viruses.

The Human Polyomavirus Middle and Alternative T-Antigens; Thoughts on Roles and Relevance to Cancer

Approximately 15–20% of human cancer is related to infection, which renders them potentially preventable by antimicrobial or antiviral therapy. Human polyomaviruses (PyVs) are relevant in this regard, as illustrated by the involvement of Merkel cell polyomavirus (MCPyV) in the development of Merkel cell carcinoma. The polyomavirus Small and Large tumor antigen (ST and LT) have been extensively studied with respect to their role in oncogenesis. Recently it was shown that a number of human PyVs, including MCPyV and the trichodysplasia spinulosa polyomavirus (TSPyV), express additional T-antigens called Middle T (MT) and alternative T (ALT). ALT is encoded by ORF5, also known as the alternative T open reading frame (ALTO), which also encodes the second exon of MT, and overlaps out-of-frame with the second exon of LT. Previously, MT was considered unique for oncogenic rodent polyomaviruses, and ALT was still unknown. In this mini-review, we want to point out there are important reasons to explore the involvement of MT and ALT in human cellular transformation. First, just like their rodent equivalents, MT and ALT probably disrupt cellular pathways that control signaling and proliferation. Second, expression of the MT and ALT-encoding ORF5/ALTO characterizes a monophyletic polyomavirus clade that includes human and animal PyVs with known oncogenic potential. And third, ORF5/ALTO is subject to strong positive selection aimed specifically at a short linear motif within MT and ALT that overlaps completely with the RB-binding motif in LT. The latter suggests tight interplay between these T-antigens with possible consequences for cell transformation.

SV40 TAg mouse models of cancer

The discovery of a number of viruses with the ability to induce tumours in animals and transform human cells has vastly impacted cancer research. Much of what is known about tumorigenesis today regarding tumour drivers and tumour suppressors has been discovered through experiments using viruses. The SV40 virus has proven extremely successful in generating transgenic models of many human cancer types and this review provides an overview of these models and seeks to give evidence as to their relevance in this modern era of personalised medicine and technological advancements.

Infection, stem cells and cancer signals

The association of cancer with preceding parasitic infections has been observed for over 200 years. Some such cancers arise from infection of tissue stem cells by viruses with insertion of viral oncogenes into the host DNA (mouse polyoma virus, mouse mammary tumor virus). In other cases the virus does not insert its DNA into the host cells, but rather commandeers the metabolism of the infected cells, so that the cells continue to proliferate and do not differentiate (human papilloma virus and cervical cancer). Cytoplasmic Epstein Barr virus infection is associated with a specific gene translocation (Ig/c-myc) that activates proliferation of affected cells (Burkitt lymphoma). In chronic osteomyelitis an inflammatory reaction to the infection appears to act through production of inflammatory cytokines and oxygen radical formation to induce epithelial cancers. Infection with Helicobacter pylori leads to epigenetic changes in methylation and infection by a parasite. Clonorchis sinensis also acts as a promoter of cancer of the bile ducts of the liver (cholaniocarcinoma). The common thread among these diverse pathways is that the infections act to alter tissue stem cell signaling with continued proliferation of tumor transit amplifying cells.

Polyomavirus middle T-antigen is a transmembrane protein that binds signaling proteins in discrete subcellular membrane sites

Murine polyomavirus middle T-antigen (MT) induces tumors by mimicking an activated growth factor receptor. An essential component of this action is a 22-amino-acid hydrophobic region close to the C terminus which locates MT to cell membranes. Here, we demonstrate that this sequence is a transmembrane domain (TMD) by showing that a hemagglutinin (HA) tag added to the MT C terminus is exposed on the outside of the cells, with the N terminus inside. To determine whether this MT TMD is inserted into the endoplasmic reticulum (ER) membrane, we added the ER retention signal KDEL to the MT C terminus (MTKDEL). This mutant protein locates only in the ER, demonstrating that MT does insert into membranes solely at this location. In addition, this ER-located MT failed to transform. Examination of the binding proteins associated with the MTKDEL protein demonstrated that it associates with PP2A and c-Src but fails to interact with ShcA, phosphatidylinositol 3-kinase (PI3K), and phospholipase C-γ1 (PLC-γ1), despite being tyrosine phosphorylated. Additional mutant and antibody studies show that MT binding to PP2A is probably required for MT to efficiently exit the ER and migrate to the plasma membrane though the TMD also plays a role in this relocation. Overall, these data, together with previous publications, illustrate that MT associates with signaling proteins at different sites in its maturation pathway. MT binds to PP2A in the cytoplasm, to c-Src at the endoplasmic reticulum, and to ShcA, PI3K, and PLC-γ1 at subsequent locations en route to the plasma membrane.

Lessons in signaling and tumorigenesis from polyomavirus middle T antigen

The small DNA tumor viruses have provided a very long-lived source of insights into many aspects of the life cycle of eukaryotic cells. In recent years, the emphasis has been on cancer-related signaling. Here we review murine polyomavirus middle T antigen, its mechanisms, and its downstream pathways of transformation. We concentrate on the MMTV-PyMT transgenic mouse, one of the most studied models of breast cancer, which permits the examination of in situ tumor progression from hyperplasia to metastasis.

Lessons from polyoma middle T antigen on signaling and transformation: A DNA tumor virus contribution to the war on cancer

Middle T antigen (MT) is the principal oncogene of murine polyomavirus. Its study has led to the discovery of the roles of tyrosine kinase and phosphoinositide 3-kinase (PI3K) signaling in mammalian growth control and transformation. MT is necessary for viral transformation in tissue culture cells and tumorigenesis in animals. When expressed alone as a transgene, MT causes tumors in a wide variety of tissues. It has no known catalytic activity, but rather acts by assembling cellular signal transduction molecules. Protein phosphatase 2A, protein tyrosine kinases of the src family, PI3K, phospholipase Cgamma1 as well as the Shc/Grb2 adaptors are all assembled on MT. Their activation sets off a series of signaling cascades. Analyses of virus mutants as well as transgenic animals have demonstrated that the effects of a given signal depend not only tissue type, but on the genetic background of the host animal. There remain many opportunities as we seek a full molecular understanding of MT and apply some of its lessons to human cancer.

Polyoma virus middle T antigen and its role in identifying cancer-related molecules

Most cancer researchers take for granted some of the basic concepts about the molecular changes that underlie tumorigenesis. These include the principles that tyrosine kinases and the phosphorylation of phosphatidylinositol by phosphatidylinositol 3-kinases are important in the signalling pathways that control proliferation and apoptosis, and hence cancer formation. However, how many know that a small DNA mouse virus was crucial in establishing both of these tenets?

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

Mechanisms of cell transformation induced by polyomavirus

Polyomavirus is a DNA tumor virus that induces a variety of tumors in mice. Its genome encodes three proteins, namely large T (LT), middle T (MT), and small T (ST) antigens, that have been implicated in cell transformation and tumorigenesis. LT is associated with cell immortalization, whereas MT plays an essential role in cell transformation by binding to and activating several cytoplasmic proteins that participate in growth factor-induced mitogenic signal transduction to the nucleus. The use of different MT mutants has led to the identification of MT-binding proteins as well as analysis of their importance during cell transformation. Studying the molecular mechanisms of cell transformation by MT has contributed to a better understanding of cell cycle regulation and growth control.

Approaches to study interactions between small DNA viruses and differentiated tissue

Polyomavirus (Py) derives its name from the early observation of multiple tumors that develop in newborn mice following inoculation with this family of viruses. In nature, however, tumor development is rare in the virus life cycle, rather a two-phase infection occurs, acute and persistent, resulting in a final latent infection in the kidneys. The acute phase induces an antiviral immune response, although no recognizable inflammation, which can last the lifetime of the mouse, even passing on antibodies to its offspring. The structure, replication, and expression of the Py viral genome in permissive and nonpermissive infections has been studied extensively in various cell culture systems. However, the nature of Py expression, replication, and immunopathogenesis in mice has not been thoroughly researched.

Evidence that the middle T antigen of polyomavirus interacts with the membrane skeleton

The transforming protein of polyomavirus, middle T antigen, is associated with cellular membranes. We have examined the subcellular location of the middle T antigen in two different cell types by fractionation and detergent phase partitioning. Middle T antigen expressed in human cells by a recombinant adenovirus was detected primarily in the membrane skeleton. Sucrose gradient fractionation revealed that the middle T antigen was associated with complexes with molecular weights of 500,000 to 1,000,000. Several markers for cytoskeleton cofractionate with these complexes, including actin, tubulin, and vimentin. Electron micrographs of membrane skeleton prepared from cells expressing middle T antigen demonstrated that this material contained primarily fibrous structures and was clearly devoid of bilayer membranes. These structures were distinct from the filamentous structures observed in fractions enriched for cytoskeleton. Consistent with a role for membrane skeleton localization in transformation, middle T antigen was detected exclusively in fractions enriched for membrane skeleton in middle T antigen-transformed Rat-2 cells. Our results may resolve the apparent difference between middle T antigen localization as determined by immunomicroscopy and that determined by subcellular fractionation.

Evidence that the middle T antigen of polyomavirus interacts with the membrane skeleton

The transforming protein of polyomavirus, middle T antigen, is associated with cellular membranes. We have examined the subcellular location of the middle T antigen in two different cell types by fractionation and detergent phase partitioning. Middle T antigen expressed in human cells by a recombinant adenovirus was detected primarily in the membrane skeleton. Sucrose gradient fractionation revealed that the middle T antigen was associated with complexes with molecular weights of 500,000 to 1,000,000. Several markers for cytoskeleton cofractionate with these complexes, including actin, tubulin, and vimentin. Electron micrographs of membrane skeleton prepared from cells expressing middle T antigen demonstrated that this material contained primarily fibrous structures and was clearly devoid of bilayer membranes. These structures were distinct from the filamentous structures observed in fractions enriched for cytoskeleton. Consistent with a role for membrane skeleton localization in transformation, middle T antigen was detected exclusively in fractions enriched for membrane skeleton in middle T antigen-transformed Rat-2 cells. Our results may resolve the apparent difference between middle T antigen localization as determined by immunomicroscopy and that determined by subcellular fractionation.

Immunological elimination of infected cells as the candidate mechanism for tumor protection in polyomavirus-infected mice

The uniformly lethal development of mammary tumors in polyomavirus-infected adult female nude mice was prevented by adoptive cell transfer of polyomavirus-immune splenocytes or peritoneal cells. Transferred immune cells also lowered the growth rate of emerging tumors. The induction of other relatively less frequent tumors of the skin and bone was decreased as well. Using in situ hybridization of whole-body sections as well as hybridization of nucleic acids from the mammary glands, we show for the first time that transferred immune cells, but not normal cells, virtually eliminated virus signal in the whole mouse and in the mammary glands. Since infected and tumorous mammary glands produce very little infectious virus, it appears that a major mechanism mediating the prevention of polyomavirus oncogenesis involves the immunological elimination of nonproductively and persistently infected cells.

Vaccination against polyoma virus (PyV) tumors using vaccinia-PyV recombinants: a major tumor-specific transplantation antigen (TSTA) epitope resides within the C-terminal segment of middle-T protein

We previously reported that inoculation of rats with live vaccinia virus (VV) recombinants VVpyMT, VVpyLT expressing either the middle-T (MT) or large-T (LT) proteins of polyomavirus (PyV) can elicit immunity to challenge with syngeneic PyV-tumor cells. We now report the results of cross-vaccination studies. VVpyMT was ineffective against cells expressing LT protein but prevented development of MT-expressing cells. Conversely, the VVpyLT was ineffective against MT-expressing cells. In the two experiments performed, tumor growth enhancement rather than retardation was observed in VVpyLT-vaccinated animals receiving PyV-LT (FRLTI) challenge tumor cells. To determine the location of the major TSTA within MT, a further VV recombinant (VVpyMT/Cfr) was constructed that expresses only the unique C-terminal segment of MT. VVpyMT-Cfr and VVpyMT were equally effective in eliciting tumor immunity, indicating the presence of a major TSTA epitope within the unique C-terminal region of MT.

The bcl-2 candidate proto-oncogene product is a 24-kilodalton integral-membrane protein highly expressed in lymphoid cell lines and lymphomas carrying the t(14;18) translocation

We have identified a 24-kilodalton protein that is the product of the human bcl-2 gene, implicated as an oncogene because of its presence at the site of t(14;18) translocation breakpoints. The Bcl-2 protein was detected by specific, highly sensitive rabbit antibodies and was shown to be present in a number of human lymphoid cell lines and tissues, as well as in mouse B cells transfected with a bcl-2 cDNA construct. Characterization of the Bcl-2 protein demonstrated that it has a lipophilic nature and is associated with membrane structures, probably by means of its hydrophobic carboxy-terminal membrane-spanning domain. In t(14;18)-carrying cell lines, the protein is predominantly localized to the perinuclear endoplasmic reticulum, with a minor fraction in the plasma membrane. These properties, together with the observations that Bcl-2 does not have a characteristic signal peptide and is not glycosylated, suggest that it is an integral-membrane protein that spans the bilayer at its C-terminal hydrophobic region but is exposed only at the cytoplasmic surface. The relative abundance of the Bcl-2 protein in various human lymphoid cell lines correlated with transcription of the bcl-2 gene. The protein was abundant in all t(14;18)-carrying cell lines and lymphomas and was also found at lower levels in pre-B-cell lines and nonmalignant lymphoid tissues that do not carry t(14;18) translocations. These results suggest that the Bcl-2 protein is functional in normal B lymphocytes and that a quantitative difference in its expression may play a role in the pathogenesis of lymphomas carrying the t(14;18) translocation.

Expression of biologically active middle T antigen of polyoma virus from recombinant baculoviruses

Two different recombinant baculoviruses have been generated for expressing the middle T antigen (MT) of polyoma virus in insect (Sf9) cells. One (pAcI-PyMT) produces moderate levels of MT and the other (pVL-PyMT) high levels. Indirect immunofluorescence and cellular fractionation studies with pAcI-PyMT infected Sf9 cells give results similar to those observed with wild type polyoma virus infected mouse cells, and show MT to be mainly associated with cytoplasmic membranes in the insect cell. In the latter, a sub-population of MT is phosphorylated in in vitro protein kinase assays. The yields of MT from pVL-PyMT infected cells are high enough to suggest that this protein can now be produced by this method in sufficient amounts for definitive biochemical and crystallographic analyses.

The bcl-2 candidate proto-oncogene product is a 24-kilodalton integral-membrane protein highly expressed in lymphoid cell lines and lymphomas carrying the t(14;18) translocation

We have identified a 24-kilodalton protein that is the product of the human bcl-2 gene, implicated as an oncogene because of its presence at the site of t(14;18) translocation breakpoints. The Bcl-2 protein was detected by specific, highly sensitive rabbit antibodies and was shown to be present in a number of human lymphoid cell lines and tissues, as well as in mouse B cells transfected with a bcl-2 cDNA construct. Characterization of the Bcl-2 protein demonstrated that it has a lipophilic nature and is associated with membrane structures, probably by means of its hydrophobic carboxy-terminal membrane-spanning domain. In t(14;18)-carrying cell lines, the protein is predominantly localized to the perinuclear endoplasmic reticulum, with a minor fraction in the plasma membrane. These properties, together with the observations that Bcl-2 does not have a characteristic signal peptide and is not glycosylated, suggest that it is an integral-membrane protein that spans the bilayer at its C-terminal hydrophobic region but is exposed only at the cytoplasmic surface. The relative abundance of the Bcl-2 protein in various human lymphoid cell lines correlated with transcription of the bcl-2 gene. The protein was abundant in all t(14;18)-carrying cell lines and lymphomas and was also found at lower levels in pre-B-cell lines and nonmalignant lymphoid tissues that do not carry t(14;18) translocations. These results suggest that the Bcl-2 protein is functional in normal B lymphocytes and that a quantitative difference in its expression may play a role in the pathogenesis of lymphomas carrying the t(14;18) translocation.

Analysis of middle tumor antigen and pp60c-src interactions in polyomavirus-transformed rat cells

The relative abundance of pp60c-src molecules associated with polyomavirus (Py) middle tumor antigen (MTAg) and the relative abundance of MTAg associated with pp60c-src in a variety of Py-transformed rat cells was determined by quantitative immunoblot analyses which detect pp60c-src or Py MTAg. The results demonstrate that approximately 5 to 10% of the total immunoprecipitable pp60c-src molecules in Py-transformed rat cells are stably associated with MTAg and have elevated protein kinase activities. In these same cells, it was found that approximately 10 to 15% of the detectable MTAg molecules are stably associated with pp60c-src. Other results presented in this report demonstrate that approximately 50 to 75% of the total MTAg-associated cellular tyrosine kinase activity potentially represents the enzymatic activity of pp60c-src, while the remaining 25 to 50% represents the activity of other cellular tyrosine kinases. Our results also show that most pp60c-src molecules associated with Py MTAg do not possess electrophoretic mobilities that are altered from those of pp60c-src molecules not associated with MTAg or pp60c-src molecules obtained from normal rodent cells.

Cooperation of middle and small T antigens of polyomavirus in transformation of established fibroblast and epithelial-like cell lines

We have reported recently that small T antigen of polyomavirus stimulates the growth of NIH 3T3 cells beyond their saturation density and induces weak anchorage-independent growth (T. Noda, M. Satake, T. Robins, and Y. Ito, J. Virol. 60:105-113, 1986). We examined whether small T antigen would cooperate with middle T antigen in the in vitro transformation of NIH 3T3 (fibroblasts) and NRK-52E (epitheliallike) cells. The small-T-antigen gene, when cotransfected with the middle-T-antigen gene, had no additional effect on the efficiency or size of dense foci formation induced by the middle-T-antigen gene on a monolayer of NIH 3T3 cells. However, the small-T-antigen gene dramatically increased the rate of growth of NIH 3T3 cells transformed by middle T antigen in semisolid medium. Introduction of the small-T-antigen gene into middle-T-antigen-transformed cells did not disturb the integrated middle-T gene, alter expression of the middle-T gene, or enhance middle-T-antigen-associated tyrosine protein kinase activity. For NRK-52E cells, the expression of middle T antigen alone resulted in small, slow-growing foci on a monolayer. These cells did not show anchorage-independent growth, despite the fact that middle-T-antigen-associated tyrosine protein kinase activity was clearly detected in these cells. NRK-52E cells expressing both middle and small T antigens formed faster growing foci on a monolayer than middle-T-antigen-expressing cells did and grew in semisolid medium, even when the amounts of middle T antigen and its associated kinase activities were lower than those of middle-T-antigen-expressing cells. We conclude that small T antigen cooperates with middle T antigen in the in vitro transformation of established cell lines of fibroblast and epitheliallike cells, that it does not share the middle-T-antigen function even though they are structurally related, and that it has a significantly more important role in the transformation of NRK-52E cells than that of NIH 3T3 cells.

Tumour prevention and rejection with recombinant vaccinia

Tumour-specific antigens (TSA; ref. 1) have been exploited in the diagnosis and imaging of human cancer and anti-TSA antibodies have therapeutic potential. Vaccination with TSA or anti-idiotypic (TSA) antibodies has also been used to control tumour growth in model systems. An effective immune response nevertheless demands copresentation of antigen with host histocompatibility determinants. We therefore examined whether live vaccinia virus recombinants expressing TSA in cells of the vaccinated host might better elicit tumour immunity. Polyoma virus (PY) is tumorigenic in rodents; because killed PY-transformed cells can elicit tumour immunity, a PY-specific TSA has been postulated. Tumorigenesis involves expression of three early PY proteins, large-T (LT), middle-T (MT) and small-T (ST), but their role as TSAs is unclear. We therefore expressed the three T proteins in separate vaccinia recombinants. Rejection of PY tumours was observed in rats immunized with recombinants expressing either LT or MT. Further, tumour-bearing animals could be induced to reject their tumours by inoculation of recombinants.

Overproduction of polyomavirus middle T antigen in mammalian cells through the use of an adenovirus vector

To overproduce biologically active polyomavirus middle T antigen, we used an adenovirus vector and human 293 cells as hosts. Two helper-independent recombinant adenoviruses were isolated that contain a hybrid transcription unit, in differing orientations, at a site in the adenovirus genome from which the E1a and most of the E1b transcription units have been deleted. The hybrid transcription unit consists of the adenovirus type 2 major late promoter and tripartite leader and a cDNA segment capable of encoding polyomavirus middle T antigen and accompanying 3' RNA-processing signals. Both recombinant viruses were stable and replicated to high titers in human 293 cells. The polyomavirus sequences were expressed, predominantly at late times after infection of 293 cells, to yield mRNAs that encoded middle T antigen. One of the recombinant viruses also expressed a middle T antigen-related protein in 293 cells. The latter was translated from one of several novel mRNA species that resulted from aberrant splicing and incomplete RNA processing of precursor RNA transcripts. Comparison of the amount of middle T antigen produced in 3T6 cells infected with polyomavirus with that in 293 cells infected with either of the recombinant adenoviruses, under optimal conditions for each system, revealed at least a 10-fold greater yield of the protein on a per-cell basis in the latter system than in the former. The recombinant-virus-encoded middle T antigen was biologically active, as evidenced by its ability to associate with and serve as a substrate for human pp60c-src. The functionality of the middle T antigen was further confirmed by demonstrating that both recombinant viruses efficiently transformed Rat-1 cells. These recombinant viruses will be useful to overproduce middle T antigen and to introduce the polyomavirus oncogene into a wide variety of mammalian cells.

Multicopy plasmid with a structure related to the polyoma virus genome

In a subclone derived from mouse L(tk-) cells, we found a plasmid present in a high copy number (greater than 5000 copies per cell) that was stably maintained extrachromosomally without any cytopathic effect to the host cells. This plasmid, termed L factor, has two forms: 5.3 and 5.5 kilobase pairs. DNA sequencing and restriction enzyme mapping showed that, although the structure contains DNA sequences common to polyoma virus, plasmid sequences belonging to the regulatory region (the enhancer region) and other regions are quite different from those in polyoma. In cells bearing the plasmid, we detected a low level of material that cross-reacts with antibody to polyoma tumor antigens, suggesting that the plasmids replicate and are maintained in the cells by a mechanism different from that functioning during propagation following infection of papovaviruses.

Regulation of viral and cellular promoter activity by polyomavirus early proteins

The chloramphenicol-acetyl-transferase (CAT) expression system has been utilized to study the ability of the polyomavirus (Py) early proteins, the 100K large T, the 55K middle T and 22K small T-antigens, to activate a variety of eukaryotic promoters (the SV40 early, the alpha 2(1) collagen, the rabbit beta-globin, the polyomavirus early and the H-2 class I) in both transient and stable expression assays. We have found that either the complete polyomavirus early region or a plasmid expressing only the 55K middle T-antigen are capable of stimulating the expression of all the promoter-CAT plasmids in transient co-transfection experiments in both NIH-3T3 and Rat-2 cells. Conversely, the Py early proteins do not stimulate the transcription of most of the promoter-CAT genes stably introduced in the cell chromosomes, with the exception of H-2 class I promoter, when stimulation of transcription has been observed upon infection with recombinant retrovirus encoding the Py middle T-antigen.

Transformation of chicken embryo fibroblasts and tumor induction by the middle T antigen of polyomavirus carried in an avian retroviral vector

The middle T antigen of polyomavirus transformed primary chicken embryo fibroblasts when expressed from a replication-competent avian retrovirus. This in vitro-constructed retrovirus, SRMT1, is a variant of Rous sarcoma virus that encodes the middle T antigen in place of v-src. Inoculation of SRMT1 into 1-week-old chickens rapidly induced hemangiomas and hemangiosarcomas. As shown with mammalian cells infected with polyomavirus, polyomavirus middle T antigen appears to be associated with p60c-src in chicken cells infected with SRMT1. When lysates of SRMT1-infected cells immunoprecipitated with either a monoclonal antibody against p60src or anti-T serum were assayed in an in vitro kinase reaction, the middle T antigen was heavily phosphorylated. To see whether an excess of p60c-src could alter the extent of phosphorylation of the middle T protein or the process of cell transformation by middle T, cells were doubly infected with SRMT1 and NY501, a virus which overexpresses p60c-src. Doubly infected chicken embryo fibroblasts transformed with the same kinetics and were morphologically indistinguishable from chicken embryo fibroblasts infected with SRMT1 alone. Phosphorylation of the middle T antigen was elevated two- to fivefold relative to cells infected only with SRMT1.

Isolation and characterization of NIH 3T3 cells expressing polyomavirus small T antigen

The polyomavirus small T-antigen gene, together with the polyomavirus promoter, was inserted into a retrovirus vector pGV16 which contains the Moloney sarcoma virus long terminal repeat and neomycin resistance gene driven by the simian virus 40 promoter. This expression vector, pGVST, was packaged into retrovirus particles by transfection of psi 2 cells which harbor packaging-defective murine retrovirus genome. NIH 3T3 cells were infected by this replication-defective retrovirus containing pGVST. Of the 15 G418-resistant cell clones, 8 express small T antigen at various levels as revealed by immunoprecipitation. A cellular protein with an apparent molecular weight of about 32,000 coprecipitates with small T antigen. Immunofluorescent staining shows that small T antigen is mainly present in the nuclei. Morphologically, cells expressing small T antigen are indistinguishable from parental NIH 3T3 cells and have a microfilament pattern similar to that in parental NIH 3T3 cells. Cells expressing small T antigen form a flat monolayer but continue to grow beyond the saturation density observed for parental NIH 3T3 cells and eventually come off the culture plate as a result of overconfluency. There is some correlation between the level of expression of small T antigen and the growth rate of the cells. Small T-antigen-expressing cells form small colonies in soft agar. However, the proportion of cells which form these small colonies is rather small. A clone of these cells tested did not form tumors in nude mice within 3 months after inoculation of 10(6) cells per animal. Thus, present studies establish that the small T antigen of polyomavirus is a second nucleus-localized transforming gene product of the virus (the first one being large T antigen) and by itself has a function which is to stimulate the growth of NIH 3T3 cells beyond their saturation density in monolayer culture.

Ability of a T-antigen transport-defective mutant of simian virus 40 to immortalize primary cells and to complement polyomavirus middle T in tumorigenesis

The oncogenic potential of polyomavirus in newborn rats could not be expressed by a genome encoding only the middle T antigen but required the presence of one of the other two viral early genes, small T or large T. The tumorigenicity defect could also be complemented by other viral or cellular genes that are known to be implicated in immortalization and establishment functions. The simian virus 40(cT)-3 mutant (R. E. Lanford and J. S. Butel, Cell 37:801-813, 1984), which fails to localize to the nucleus, has the capacity to complement polyomavirus middle T in tumorigenesis and to immortalize primary rat embryo fibroblasts when it was cotransfected in the presence of pSV2-neo. Our data suggested that under the conditions of DNA-mediated tumor induction and cotransfection with a dominant selection marker, the cellular alterations achieved by nonnuclear oncogenes such as polyomavirus small T and simian virus 40(cT)-3 were sufficient to complement polyomavirus middle T in transformation and tumorigenesis.

Transformation of chicken embryo fibroblasts and tumor induction by the middle T antigen of polyomavirus carried in an avian retroviral vector

The middle T antigen of polyomavirus transformed primary chicken embryo fibroblasts when expressed from a replication-competent avian retrovirus. This in vitro-constructed retrovirus, SRMT1, is a variant of Rous sarcoma virus that encodes the middle T antigen in place of v-src. Inoculation of SRMT1 into 1-week-old chickens rapidly induced hemangiomas and hemangiosarcomas. As shown with mammalian cells infected with polyomavirus, polyomavirus middle T antigen appears to be associated with p60c-src in chicken cells infected with SRMT1. When lysates of SRMT1-infected cells immunoprecipitated with either a monoclonal antibody against p60src or anti-T serum were assayed in an in vitro kinase reaction, the middle T antigen was heavily phosphorylated. To see whether an excess of p60c-src could alter the extent of phosphorylation of the middle T protein or the process of cell transformation by middle T, cells were doubly infected with SRMT1 and NY501, a virus which overexpresses p60c-src. Doubly infected chicken embryo fibroblasts transformed with the same kinetics and were morphologically indistinguishable from chicken embryo fibroblasts infected with SRMT1 alone. Phosphorylation of the middle T antigen was elevated two- to fivefold relative to cells infected only with SRMT1.

Expression of the polyoma middle-size T antigen in Escherichia coli

We constructed a plasmid encoding a hybrid protein, consisting of the N-terminal signal sequence of the major outer membrane lipoprotein (lpp) of Serratia marcescens joined to the polyoma middle-size T antigen (mT antigen). The hybrid protein expressed under the control of a lpp-lac hybrid promoter was synthesized at levels up to 5% of newly synthesized protein and could be accumulated in Escherichia coli strains carrying the Cap R mutation. The mT antigen produced in E. coli was precipitated by polyoma antitumor serum, and by serum directed against a synthetic peptide corresponding to the C terminus of the authentic mT antigen. The protein was secreted into the periplasmic space, from which it could be released by osmotic shock. The bacterial mT antigen had no detectable associated protein kinase activity.

Complexes of polyoma virus medium T antigen and cellular proteins

Antibodies against synthetic peptides corresponding to the carboxyl-terminal six amino acids, Lys-Arg-Ser-Arg-His-Phe (KF), and an internal region, Glu-Glu-Glu-Glu-Tyr-Met-Pro-Met-Glu (EE), of polyoma virus medium T antigen were used successively to purify medium T antigen by affinity chromatography. Medium T antigen from cell extracts was first bound to anti-KF antibodies and released from the immune complex with excess KF peptide; then it was bound to anti-EE antibodies and released with excess EE peptide. Two proteins, pp60c-src and a new protein of approximately equal to 61,000 Da (61-kDa protein), were copurified because they formed complexes with medium T antigen. The 61-kDa protein-medium T antigen complex was detected in extracts from wild-type-infected and transformed cells but not from cells infected with NG59 virus, which has a mutation in the medium T gene and is transformation defective. Instead, NG59 medium T antigen formed a complex with another cellular protein of approximately equal to 72,000 Da.

Differential ability of a T-antigen transport-defective mutant of simian virus 40 to transform primary and established rodent cells

The transforming potential and oncogenicity of a simian virus 40 (SV40) mutant affecting T-antigen (T-ag), SV40(cT)-3, was examined in an effort to dissect T-ag functions in transformation. SV40(cT)-3 has a point mutation at nucleotide 4434 that abolishes the transport of T-ag to the nucleus but does not affect its association with the cell surface. Transfection-transformation assays were performed with primary cells and established cell lines of mouse and rat origin. The efficiency of transformation for established cell lines by SV40(cT)-3 was comparable to that of wild-type SV40, indicating that transformation of established cell lines can occur in the absence of detectable amounts of nuclear T-ag. Transformation of primary mouse embryo fibroblasts by SV40(cT)-3 was markedly influenced by culture conditions; the relative transforming frequency was dramatically reduced in assays involving focus formation in low serum concentrations or anchorage-independent growth. Immunofluorescence tests revealed that the transformed mouse embryo fibroblasts partially transport the mutant cT-ag to the cell nucleus. Transformed cell lines induced by SV40(cT)-3 did not differ in growth properties from wild-type transformants. SV40(cT)-3 was completely defective for the transformation of primary baby rat kidney cells, a primary cell type unable to transport the mutant T-ag to the nucleus. The intracellular localization of cellular protein p53 was found to mimic T-ag distribution in all the transformants analyzed. The mutant virus was weakly oncogenic in vivo: the induction of tumors in newborn hamsters by SV40(cT)-3 was reduced in incidence and delayed in appearance in comparison to wild-type SV40. These observations suggest that cellular transformation is regulated by both nuclear and surface-associated forms of SV40 T-ag.

Tyrosine phosphorylation within the amino-terminal domain of pp60c-src molecules associated with polyoma virus middle-sized tumor antigen

We have examined the in vitro phosphorylation of cellular src protein (pp60c-src) molecules associated with the polyoma virus middle-sized tumor antigen in polyoma virus-transformed cells. These pp60c-src molecules possessed an enhanced tyrosyl kinase activity, migrated aberrantly on NaDodSO4/polyacrylamide gels, and contained a novel site of tyrosine phosphorylation within the amino-terminal region of the molecule. The pp60c-src molecules not associated with the middle-sized tumor antigen were phosphorylated exclusively on a tyrosine residue within the carboxyl-terminal domain of pp60c-src. A similar modified form of the middle-sized tumor antigen-associated pp60c-src protein was detected in lysates from polyoma virus-transformed cells labeled in vivo with [32P]orthophosphate in the presence of sodium orthovanadate, an inhibitor of phosphotyrosyl phosphatases.

Differential ability of a T-antigen transport-defective mutant of simian virus 40 to transform primary and established rodent cells

The transforming potential and oncogenicity of a simian virus 40 (SV40) mutant affecting T-antigen (T-ag), SV40(cT)-3, was examined in an effort to dissect T-ag functions in transformation. SV40(cT)-3 has a point mutation at nucleotide 4434 that abolishes the transport of T-ag to the nucleus but does not affect its association with the cell surface. Transfection-transformation assays were performed with primary cells and established cell lines of mouse and rat origin. The efficiency of transformation for established cell lines by SV40(cT)-3 was comparable to that of wild-type SV40, indicating that transformation of established cell lines can occur in the absence of detectable amounts of nuclear T-ag. Transformation of primary mouse embryo fibroblasts by SV40(cT)-3 was markedly influenced by culture conditions; the relative transforming frequency was dramatically reduced in assays involving focus formation in low serum concentrations or anchorage-independent growth. Immunofluorescence tests revealed that the transformed mouse embryo fibroblasts partially transport the mutant cT-ag to the cell nucleus. Transformed cell lines induced by SV40(cT)-3 did not differ in growth properties from wild-type transformants. SV40(cT)-3 was completely defective for the transformation of primary baby rat kidney cells, a primary cell type unable to transport the mutant T-ag to the nucleus. The intracellular localization of cellular protein p53 was found to mimic T-ag distribution in all the transformants analyzed. The mutant virus was weakly oncogenic in vivo: the induction of tumors in newborn hamsters by SV40(cT)-3 was reduced in incidence and delayed in appearance in comparison to wild-type SV40. These observations suggest that cellular transformation is regulated by both nuclear and surface-associated forms of SV40 T-ag.

Acylation of adenovirus type 12 early region 1b 18-kDa protein. Further evidence for its localisation in the cell membrane

The 18-kDa E1b protein in Ad 12-transformed rat cells and in Ad 12-infected human cells binds lipid strongly. The lipid is not removed by boiling in the presence of SDS or by extraction with methanol/chloroform. It is, however, dissociated from the protein by treatment with methanolic KOH suggesting that attachment is through an ester linkage. The acylated 18-kDa protein is detected only in the membrane fraction. Labelling cell surface proteins on Ad 12-transformed cells with [125I]iodosulphanilic acid shows that some of the Ad 12 18-kDa E1b protein is present on the outside of the cell. It is concluded that this protein is responsible for cell surface T-antigen activity.

Middle tumor antigen of polyomavirus transformation-defective mutant NG59 is associated with pp60c-src

We have found that lysis of mouse embryo cells infected with the polyomavirus host range transformation-defective (hr-t) mutant NG59 under gentle conditions that avoid ionic detergents results in detectable NG59-encoded middle tumor antigen (MTAg) associated with pp60c-src. This MTAg-pp60c-src complex could be immunoprecipitated from NG59-infected cell lysates by either sera from animals bearing polyomavirus-induced tumors or by monoclonal antibodies directed against MTAg. Immune complex kinase assays revealed that, whereas the pp60c-src associated with NG59 MTAg possessed tyrosyl kinase activity, the NG59 MTAg in this complex was not phosphorylated in these in vitro reactions. These results demonstrate that the point insertion mutation found in this transformation-deficient strain of polyomavirus encodes MTAg molecules capable of associating with pp60c-src and defines a limited region within MTAg which appears to be critical for stable MTAg-pp60c-src interactions.

The major site of tyrosine phosphorylation in polyomavirus middle T antigen is not required for transformation

The induction of tumors and cellular transformation mediated by polyomavirus requires the action of middle T antigen. Accordingly, we have begun to define the domains of the viral protein important for these processes to learn more about its site and mechanism of action. One of the domains of middle T antigen which is thought to be important for its function includes a stretch of acidic amino acids and a vicinal tyrosine residue (tyrosine 315), the major site of tyrosine phosphorylation in vitro. To determine whether these acidic amino acids and tyrosine 315 are required to maintain the transforming activity of middle T antigen, we constructed deletions within the DNA sequences encoding these amino acids and measured the capacity of the resulting mutants to transform Rat-1 cells in culture. This was accomplished by using in vitro mutagenesis techniques with molecularly cloned polyomavirus DNA. Seven mutants were isolated. Five of these proved incapable of transforming Rat-1 cells and were found to contain deletions which altered the reading frame for middle T antigen. However, two mutants, pPdl1-4 and pPdl2-7, retained the capacity to transform Rat-1 cells at high frequencies. The middle T antigen encoded by one of these mutants, pPdl1-4, lacks part of the acidic string of amino acids but not tyrosine 315 (amino acids 304 through 310 are deleted), whereas the middle T antigen encoded by the other mutant, pPdl2-7, lacks the entire acidic amino acid stretch as well as tyrosine 315 (amino acids 285 through 323 are deleted). Rat-1 cells transformed by one or the other mutant DNA displayed a fully transformed phenotype, including the capacity to form tumors in animals. These results prove that the major site of tyrosine phosphorylation in middle T antigen and the acidic amino acids which precede it are not essential for its transforming activity.

N-terminal amino acid sequences of the polyoma middle-size T antigen are important for protein kinase activity and cell transformation

We constructed deletion mutations which removed N-terminal coding sequences of various lengths from a cloned polyoma middle-size T antigen (MT antigen) gene. We introduced the MT antigen genes into a simian virus 40 expression vector so that they were expressed at high levels under the control of the simian virus 40 late promoter in COS-1 cells. The deletion mutant genes synthesized truncated MT antigens whose size was consistent with the deletion of either 70 or 106 amino acids from N termini, owing to initiation of translation at internal methionine codons in the MT antigen-coding region. The truncated MT antigens were found in cell membrane fractions but failed to show MT antigen-associated protein kinase activity. The cloned deletion mutant DNAs failed to transform rat F2408 or mouse NIH 3T3 cells. Therefore, N-terminal amino acid sequences of the polyoma MT antigen, as well as C-terminal sequences, are important for protein kinase activity and cell transformation.

N-terminal amino acid sequences of the polyoma middle-size T antigen are important for protein kinase activity and cell transformation

We constructed deletion mutations which removed N-terminal coding sequences of various lengths from a cloned polyoma middle-size T antigen (MT antigen) gene. We introduced the MT antigen genes into a simian virus 40 expression vector so that they were expressed at high levels under the control of the simian virus 40 late promoter in COS-1 cells. The deletion mutant genes synthesized truncated MT antigens whose size was consistent with the deletion of either 70 or 106 amino acids from N termini, owing to initiation of translation at internal methionine codons in the MT antigen-coding region. The truncated MT antigens were found in cell membrane fractions but failed to show MT antigen-associated protein kinase activity. The cloned deletion mutant DNAs failed to transform rat F2408 or mouse NIH 3T3 cells. Therefore, N-terminal amino acid sequences of the polyoma MT antigen, as well as C-terminal sequences, are important for protein kinase activity and cell transformation.

Construction and expression of a recombinant DNA gene encoding a polyomavirus middle-size tumor antigen with the carboxyl terminus of the vesicular stomatitis virus glycoprotein G

We constructed a molecular clone encoding the N-terminal 379 amino acids of the polyomavirus middle-size tumor antigen, followed by the C-terminal 60 amino acids of the vesicular stomatitis virus glycoprotein G. This hybrid gene contained the coding region for the C-terminal hydrophobic membrane-spanning domain of the G protein in place of the C-terminal hydrophobic domain of the middle-size tumor antigen. The hybrid gene was expressed in COS-1 cells under the control of the simian virus 40 late promoter. The hybrid protein was located in cell membranes and was associated with a tyrosine-specific protein kinase activity, as was the middle-size tumor antigen. Plasmids encoding the hybrid protein failed to transform mouse NIH 3T3 or rat F2408 cells.

Transmission of the polyoma virus middle T gene as the oncogene of a murine retrovirus

Polyoma virus is a papovavirus that productively infects mouse cells. In cells of other species, such as rat cells, polyoma virus is virtually unable to replicate, and a small proportion of infected cells become stably transformed. The ability of polyoma virus to transform infected cells is determined by genes that encode the large, middle and small T antigens and which are found in the early region of the virus genome. We have inserted the transforming region of polyoma virus into a murine leukaemia virus (MLV) vector, to generate a replication-defective transforming retrovirus which for the first time allows efficient transformation of mouse cells by the polyoma virus middle T gene. During the life cycle of this recombinant virus the intervening sequence present in the original polyoma virus middle T gene was removed. The recombinant virus that we have constructed is analogous to other acutely transforming retroviruses, and demonstrates that the polyoma middle T gene is a dominant transforming oncogene.

Construction and expression of a recombinant DNA gene encoding a polyomavirus middle-size tumor antigen with the carboxyl terminus of the vesicular stomatitis virus glycoprotein G

We constructed a molecular clone encoding the N-terminal 379 amino acids of the polyomavirus middle-size tumor antigen, followed by the C-terminal 60 amino acids of the vesicular stomatitis virus glycoprotein G. This hybrid gene contained the coding region for the C-terminal hydrophobic membrane-spanning domain of the G protein in place of the C-terminal hydrophobic domain of the middle-size tumor antigen. The hybrid gene was expressed in COS-1 cells under the control of the simian virus 40 late promoter. The hybrid protein was located in cell membranes and was associated with a tyrosine-specific protein kinase activity, as was the middle-size tumor antigen. Plasmids encoding the hybrid protein failed to transform mouse NIH 3T3 or rat F2408 cells.

On the role of protein phosphorylation in the ATP-dependent permeabilization of transformed cells

Incubation of transformed mouse fibroblasts with external ATP in alkaline medium low in divalent cations causes an increase in the permeability of the plasma membrane to nucleotides and other small molecules. Previous suggestions that the phosphorylation of a 44,000 dalton membrane protein is involved in this permeabilization process have been pursued. Fractionation of cells that had been incubated with [gamma-32P]ATP revealed that the labeled 44K phosphoprotein was found in both the membrane and mitochondrial fractions. Incubation of fractions isolated from unlabeled cells with [gamma-32P]ATP resulted in substantial formation of 32P-44K in the mitochondrial fraction and less incorporation in the membrane fraction. The 44,000 dalton protein was identified as the alpha-subunit of mitochondrial pyruvate dehydrogenase by partial proteolytic mapping and immunological cross-reactivity with antibodies prepared against bovine pyruvate dehydrogenase. The phosphorylation of this protein in whole cells by externally added ATP is suppressed by inclusion in the incubation medium of carboxyatractyloside (CAT) and EDTA. These substances have no effect on ATP-dependent permeabilization, indicating that the phosphorylation of pyruvate dehydrogenase is not involved in this process.

Decreased adenylate cyclase responsiveness of transformed cells correlates with the presence of a viral transforming protein

The adenylate cyclase responsiveness of transformed fibroblastic and epithelial cell lines to forskolin, fluoride, guanine nucleotides and cholera toxin was reduced compared to their parental counterparts. This phenomenon was observed in lines transformed by either RNA or DNA tumor viruses, and in the case of polyoma virus, coincided with the expression of middle T antigen. The data suggest that decreased responsiveness of adenylate cyclase to non-hormone activators is a general consequence of viral transformation and may be related to viral regulation of protein kinase activity.

Oncogenes: growth regulation and the papovaviruses polyoma and SV40

Cellular oncogenes and their activated and retrovirus-coded counterparts play an important role in cellular regulation. Here the relationship between such oncogenes and the genes coding for the transforming proteins of the papovaviruses, polyoma viruses, and simian virus 40 (SV40) is discussed. It is concluded that polyoma virus may transform established cells by a mechanism involving activation of a cellular oncogene product, whereas SV40 may transform by a mechanism involving a previously little studied cytoplasmic form of the transforming protein.

Membrane antigens associated with infection, transformation, and tumorigenesis by polyoma virus

Infection, transformation, or tumorigenesis by Py virus leads inter alia to modifications in the membrane antigens of the affected cells. The modifications include antigenic gains or losses or quantitative changes in both directions. Although there is a pronounced common denominator in the antigenic alterations in the three distinct Py-induced biological processes, it is nontheless possible that each of them is characterized by specific antigenic modifications. This possibility has yet to be analyzed. Specific antigenic modifications, if they occur, are probably the result of different selective processes and adaptions to these pressures.

In this brief review, we have attempted to survey the literature pertinent to this aspect. While doing so, we discovered that most researchers have not considered the possibility that differences could exist between antigens of cells infected by Py, cells transformed by this virus, and Py-induced tumor cells. We feel that a comprehensive antigenic comparison between these cells utilizing well-defined reagents is an essential prerequisite to understanding of the successful immunological surveillance against Py-induced malignancy.

Antibodies against a nonapeptide of polyomavirus middle T antigen: cross-reaction with a cellular protein(s)

Antibodies were raised against the sequence Glu-Glu-Glu-Glu-Tyr-Met-Pro-Met -Glu, which represents a part of the middle T antigen of polyomavirus that is considered to be important in inducing the phenotype of transformed cells. The antibodies reacted with native as well as denatured middle T antigens. In addition, the antibodies immunoprecipitated a cellular protein with an apparent molecular weight of 130,000 (130K) from mouse and rat cells. In some cases, a 33K protein was also immunoprecipitated. Immunoprecipitation of middle T antigen as well as 130K and 33K proteins was blocked by the peptide. The antibodies labeled microfilaments of untransformed mouse, rat, human, and chicken cells by immunofluorescence. This labeling was also blocked by the peptide. The labeling pattern and distribution under a variety of conditions were indistinguishable from those of anti-actin antibodies, although no evidence has been obtained to indicate that the anti-peptide antibodies react with actin. The 130K protein migrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis slightly slower than chicken gizzard vinculin (130K) and slightly faster than myosin light-chain kinase of chicken smooth muscle (130K). Neither of these proteins absorbed the anti-peptide antibodies. The 33K protein does not seem to be tropomyosin (32K to 40K).

A frameshift mutation affecting the carboxyl terminus of the simian virus 40 large tumor antigen results in a replication- and transformation-defective virus

We have constructed a frameshift mutation in the simian virus 40 early region using a novel method of oligonucleotide-directed mutagenesis. The mutated DNA specifies an 84,000-dalton large tumor antigen that consists of approximately equal to 75,000 daltons encoded by the wild-type reading frame and 9,000 daltons, by the alternative reading frame (wild-type large tumor antigen is approximately equal to 82,000 daltons). The frameshifted carboxyl terminus of the protein bears a strong similarity to the same region of polyoma virus middle-sized tumor antigen. We have found that the mutant DNA is unable to replicate when introduced into permissive monkey cells and incapable of transforming nonpermissive mouse cells.