Evidence for free and metabolically stable p53 protein in nuclear subfractions of simian virus 40-transformed cells

Overexpression of p53 protein in human tumors

According to the current concept of carcinogenesis, neoplastic transformation consists of multistep accumulations of adverse genetic and epigenetic events. p53 is a transcription factor that regulates cellular response to diverse forms of stress through a complex network which monitors genome integrity and cell homeostasis. Mutant p53 loss-of-function, dominant-negative, and gain-of-function properties have been implicated in the development of a wide variety of human cancers, and it is generally accepted that p53 is a component in biochemical pathways central to human carcinogenesis. Study of p53 has come to the forefront of cancer research, and detection of its abnormalities during the development of tumors may have diagnostic, prognostic, and therapeutic implications. In this review, we focus on recent research on overexpression of mutant p53 in human cancer, with an emphasis on mutant p53 regulation, gain of function of mutant p53 in transcriptional effects, and the diagnostic, prognostic, and predictive value of p53 overexpression in human cancer.

Viral oncoprotein LMP1 disrupts p53-induced cell cycle arrest and apoptosis through modulating K63-linked ubiquitination of p53

Disruption of the gatekeeper p53 tumor suppressor is involved in various virus-associated tumorigeneses, with aberrant ubiquitination as the major cause of p53 abnormalities in virus-associated tumors. Of note, wild-type p53 is accumulated in Epstein-Barr virus (EBV)-associated tumors, especially in nasopharyngeal carcinoma (NPC). We have previously identified that p53 is accumulated and phosphorylated by EBV oncoprotein latent membrane protein 1 (LMP1) in NPC. Here, we further found that LMP1 promoted p53 accumulation via two distinct ubiquitin modifications. LMP1 promoted p53 stability and accumulation by suppressing K48-linked ubiquitination of p53 mediated by E3 ligase MDM2, which is associated with its phosphorylation at Ser20, while increasing the levels of total cellular ubiquitinated p53. LMP1 also induced K63-linked ubiquitination of p53 by interacting with tumor necrosis factor receptor-associated factor 2 (TRAF2), thus contributing to p53 accumulation. Furthermore, LMP1 rescued tumor cell apoptosis and cell cycle arrest mediated by K63-linked ubiquitination of p53. Collectively, these results demonstrate aberrant ubiquitin modifications of p53 and its biological functions by viral protein LMP1, which has broad implications to the pathogenesis of multiple EBV-associated tumors.

Update on human polyomaviruses and cancer

Over 50 years of polyomavirus research has produced a wealth of insights into not only general biologic processes in mammalian cells, but also, how conditions can be altered and signaling systems tweaked to produce transformation phenotypes. In the past few years three new members (KIV, WUV, and MCV) have joined two previously known (JCV and BKV) human polyomaviruses. In this review, we present updated information on general virologic features of these polyomaviruses in their natural host, concentrating on the association of MCV with human Merkel cell carcinoma. We further present a discussion on advances made in SV40 as the prototypic model, which has and will continue to inform our understanding about viruses and cancer.

The ING1b tumor suppressor facilitates nucleotide excision repair by promoting chromatin accessibility to XPA

ING1b is the most studied ING family protein and perhaps the most ubiquitously and abundantly expressed. This protein is involved in the regulation of various biological functions ranging from senescence, cell cycle arrest, apoptosis, to DNA repair. ING1b is upregulated by UV irradiation and enhances the removal of bulky nucleic acid photoproducts. In this study, we provide evidence that ING1b mediates nucleotide excision repair by facilitating the access to damaged nucleosomal DNA. We demonstrate that ING1b is not recruited to UV-induced DNA lesions but enhances nucleotide excision repair only in XPC-proficient cells, implying an essential role in early steps of the 'access, repair, restore' model. We also find that ING1b alters histone acetylation dynamics upon exposure to UV radiation and induces chromatin relaxation in microccocal nuclease digestion assay, revealing that ING1b may allow better access to nucleotide excision repair machinery. More importantly, ING1b associates with chromatin in a UV-inducible manner and facilitates DNA access to nucleotide excision repair factor XPA. Furthermore, depletion of the endogenous ING1b results to the sensitization of cells at S-phase to UV irradiation. Taken together, these observations establish a role of ING1b acting as a chromatin accessibility factor for DNA damage recognition proteins upon genotoxic injury.

Two forms of the nuclear matrix-bound p53 protein in HEK293 cells

Like many other transcription factors, the tumor suppressor protein p53 is bound to the nuclear matrix (NM). To study the interaction of p53 with the NM in more detail, we used alkaline and acidic extractions of NM proteins. It was found that there are two forms of p53, alkali- and acid-soluble, in NM of HEK293 cells and only one alkali-soluble form in NM of actinomycin D-treated MCF-7 cells. We suggest that distinct forms of p53 differ either in interactions with NM proteins or in their charges.

The interaction of p53 with the nuclear matrix is mediated by F-actin and modulated by DNA damage

The tumour suppressor protein p53 is localized in the cell nucleus where it serves to initiate cellular responses to a variety of stresses, particularly DNA damage and has the capacity to transactivate stress response genes. An emerging body of evidence indicates that its action is also exerted through direct protein-protein interactions. An approach to understanding p53 function has been to analyse its positioning in relation to nuclear structures and we have shown that p53 can associate with the nuclear matrix. A potential nuclear matrix component for this association is actin. Here we show that p53 interacts with nuclear F-actin and we map the domains involved in this interaction. Using fluorescence resonance energy transfer, we demonstrate that the partition of p53 between F-actin bound and unbound forms is not constant, but is modulated by the presence of DNA damage, which increases binding. Our results indicate that the dynamic interaction of p53 with the nuclear matrix has to be considered for a full understanding of the mechanisms of the p53-mediated cellular response to DNA damage.

A subset of cells expressing SV40 large T antigen contain elevated p53 levels and have an altered cell cycle phenotype

Cells transformed by the simian virus 40 (SV40) large T antigen (Tag) contain elevated levels of cellular p53 protein. To quantify this relationship, levels of p53 were measured in NIH 3T3 cells that expressed different concentrations of Tag. Using immunoblotting, average p53 levels were shown to increase linearly with Tag concentrations in these cell lines. Single-cell measurements were also performed using flow cytometry to measure p53 immunofluorescence. Surprisingly, the flow cytometry experiments showed that two distinct cell populations, based on p53 content, were present in cells expressing high levels of Tag. One cell population contained elevated p53 levels. A second population did not contain elevated p53, even though high concentrations of Tag were present in the cells. This latter cell population did not appear to arise because of mutations in either Tag or p53. The two cell populations also had phenotypic differences. In exponentially growing cells, Tag alters the cell cycle distribution (decreases the percentage of G1 phase cells and increases the percentages of S and G2 + M phase cells). This phenotype was maximum in the cell population containing elevated p53. A lesser phenotype was found in the cell population that did not contain elevated p53. These data show, firstly, that cells can express significant levels of Tag and not contain elevated levels of p53 and, secondly, that elevated p53 correlates with the altered cell cycle distribution produced by Tag in growing cells.

p53 binds the nuclear matrix in normal cells: binding involves the proline-rich domain of p53 and increases following genotoxic stress

The tumour suppressor p53 is a multifunctional protein important for the maintenance of genomic integrity. It is able to form molecular complexes with different DNA targets and also with cellular proteins involved in DNA transcription and DNA repair. In mammalian cells the biochemical processing of DNA occurs on a nuclear sub-structure termed the nuclear matrix. Previously Deppert and co-workers have identified p53 in association with the nuclear matrix in viral- and non-viral transformed cell lines. In the present study we demonstrate, for the first time, that p53 is bound to the nuclear matrix in primary cultures of normal mammalian cells and that this binding increases following DNA damage. Analysis of cell lines expressing structural mutants of p53 revealed that association with the nuclear matrix is independent of the tertiary and quaternary structure of p53. However, the proline-rich domain towards the N-terminus of p53 (residues 67 to 98) appeared important for binding to the nuclear matrix. This was demonstrated by TET-ON regulated expression of p53-derived constructs in p53(-/-) murine embryonic fibroblasts (MEF p53(-/-)). The proline-rich domain of p53 has potential for SH3 protein-protein interaction, and has a role in p53-mediated apoptosis and possibly base excision repair of DNA damage. We discuss our observations in relation to the ability of p53 to facilitate DNA repair and also review evidence indicating that matrix-bound p53 in SV40-transformed cells may facilitate the transforming potential of SV40 large T antigen.

p53/T-antigen complex disruption in T-antigen transformed NIH3T3 fibroblasts exposed to oxidative stress: correlation with the appearance of a Fas/APO-1/CD95 dependent, caspase independent, necrotic pathway

Simian Virus 40 Large T-antigen expressed in NIH3T3 cells increases p53 level and interacts with this tumor suppressor to form large nuclear complexes. We show here that T-antigen sensitizes NIH3T3 cells to low doses of the oxidative stress inducer menadione. This oxidant increased p53 accumulation and disrupted p53/T-antigen interaction, but not T-antigen/pRb, T-antigen/Hsc70 and p53/Hsc70 complexes; a phenomenon inhibited by the anti-oxidant N-acetyl-cysteine. Analysis of several p53 downstream gene products revealed that the level of Fas receptor, which was sharply reduced by T-antigen expression, was drastically increased in response to menadione treatment. Menadione also induced a T-antigen dependent cleavage of Fas ligand. Analysis performed with Fas receptor antagonist antibody and metalloproteinases inhibitor revealed that menadione triggers a Fas-dependent death of a fraction of T-antigen expressing cells. This Fas pathway does not activate caspase 8 or 3, probably because of the inhibition induced by T-antigen, and leads to a necrotic cell death which contributes at least in part to the hypersensitivity of T-antigen transformed cells to oxidative stress.

Maintenance of genomic integrity by p53: complementary roles for activated and non-activated p53

In this review we describe the multiple functions of p53 in response to DNA damage, with an emphasis on p53's role in DNA repair. We summarize data demonstrating that p53, through its various biochemical activities and via its ability to interact with components of the repair and recombination machinery, actively participates in various processes of DNA repair and DNA recombination. An important aspect in evaluating p53 functions arises from the finding that the p53 core domain harbors two mutually exclusive biochemical activities, sequence-specific DNA binding, required for its transactivation function, and 3'->5' exonuclease activity, possibly involved in various aspects of DNA repair. As modifications of p53 that lead to activation of its sequence-specific DNA-binding activity result in inactivation of its 3'-> 5' exonuclease activity, we propose that p53 exerts its functions as a 'guardian of the genome' at various levels: in its non-induced state, p53 should not be regarded as a non-functional protein, but might be actively involved in prevention and repair of endogenous DNA damage, for example via its exonuclease activity. Upon induction through exogenous DNA damage, p53 will exert its well-documented functions as a superior response element in various types of cellular stress. The dual role model for p53 in maintaining genomic integrity significantly enhances p53's possibilities as a guardian of the genome.

Two different forms of p53 localized differently within cells of urogenital tumours

We analyzed the subcellular localization of p53 in prostate and bladder carcinoma cells. Using laser scanning microscopy and PAb1620, a monoclonal antibody recognizing the wildtype conformation of p53, and another monoclonal antibody directed against the mutant conformation of the protein (PAb240), we found two different subsets of p53 within the same cell. The wildtype subgroup was found in the nucleolus, whereas the mutant protein was confined to the nucleus. The results obtained by immunofluorescence were verified by Western blot analysis and immunoprecipitation. Thus, our findings demonstrate an unusual subcellular localization pattern of p53 in prostate and bladder cancer cells which may indicate another mechanism of inactivation of p53.

Nuclear import of p53 during Xenopus laevis early development in relation to DNA replication and DNA repair

The role of p53 in transcriptional activation of genes involved in cell cycle progression is well established. However, the wide range of functions attributed to this gene suggests that some of them might be unrelated to transcription. Here we investigated p53 localization and recruitment to chromatin during Xenopus early development when 12 rapid cell cycles occur without transcription of the genome. We show that after fertilization, part of the large store of p53 previously stored in the cytoplasm of the oocyte is imported into the nucleus. This import was further analyzed in relation with DNA replication and DNA repair using cell-free systems from Xenopus eggs. Formation of a nuclear lamina envelope is necessary for the import of p53 into the nucleus. p53 associates both with decondensed DNA and the nuclear lamina envelope, but no colocalization with prereplication or replication complexes is observed. We show that UV- or gamma-damaged nuclei recruit p53 as well as replication protein A (RPA) in large common foci. Together, these data suggest that p53 plays a role in the regulation of the accelerated S phases that occur during Xenopus early development, in a manner that does not rely on its transcription-mediated activity.

p53 is associated with the nuclear envelope in mouse testis

p53 has been postulated to play a role in meiosis as well as in the regulation of germ cell numbers by apoptosis. This study investigated the subcellular localization of p53 in the testis, including conditions known to induce germ cell apoptosis. Western blot analysis showed that p53 was enriched in the nuclear envelope fraction, and confocal microscopy confirmed that p53 was associated with the nuclear envelope of germ cells. Exposure of the testis to heat stress induced translocation of p53 into the nucleus. Nuclear envelope binding provides an optimal site for rapid entry of p53 into the nucleus, where it may act as a DNA-binding protein to induce apoptosis or cell cycle arrest in response to appropriate stimuli. The nuclear envelope sequestration of p53 also provides a framework to understand how mitosis and meiosis in the testis may proceed despite high intracellular concentration of p53.

Simian virus 40 small t antigen activates the carboxyl-terminal transforming p53-binding domain of large T antigen

Expression of the simian virus 40 large T antigen (large T) in F111 rat fibroblasts generated only minimal transformants (e.g., F5 cells). Interestingly, F111-derived cells expressing only an amino-terminal fragment of large T spanning amino acids 1 to 147 (e.g., FR3 cells), revealed the same minimal transformed phenotype as F111 cells expressing full-length large T. This suggested that in F5 cells the transforming domain of large T contained within the C-terminal half of the large T molecule, and spanning the p53 binding domain, was not active. Progression to a more transformed phenotype by coexpression of small t antigen (small t) could be achieved in F5 cells but not in FR3 cells. Small-t-induced progression of F5 cells correlated with metabolic stabilization of p53 in complex with large T: whereas in F5 cells the half-life of p53 in complex with large T was only slightly elevated compared with that of (uncomplexed) p53 in parental F111 cells or that in FR3 cells, coexpression of small t in F5 cells led to metabolic stabilization and to high-level accumulation of p53 complexed to large T. In contrast, coexpression of small t had no effect on p53 stabilization or accumulation in FR3 cells. This finding strongly supports the assumption that the mere physical interaction of large T with p53, and thus p53 inactivation, in F5 cells expressing large T only does not reflect the main transforming activity of the C-terminal transforming domain of large T. In contrast, we assume that the transforming potential of this domain requires activation by a cellular function(s) which is mediated by small t and correlates with metabolic stabilization of p53.

Binding of MAR-DNA elements by mutant p53: possible implications for its oncogenic functions

The tumor suppressor p53 is a multifunctional protein whose main duty is to preserve the integrity of the genome. This function of wild-type p53 as "guardian of the genome" is achieved at different levels, as a cell cycle checkpoint protein, halting the cell cycle upon DNA damage, and via a direct involvement in processes of DNA repair. Alternatively, p53 can induce apoptosis. Mutations in the p53 gene occur in about 50% of all human tumors and eliminate the tumor suppressor functions of p53. However, many mutant p53 proteins have not simply lost tumor suppressor functions but have gained oncogenic properties which contribute to the progression of tumor cells to a more malignant phenotype. The molecular basis for this gain of function of mutant p53 is still unknown. However, mutant (mut) p53 specifically binds to nuclear matrix attachment region (MAR) DNA elements. MAR elements constitute important higher order regulatory elements of chromatin structure and function. By binding to these elements, mut p53 could modulate important cellular processes, like gene expression, replication, and recombination, resulting in phenotypic alterations of the tumor cells. Mut p53 thus could be the first representative of a new class of oncogenes, which exert their functions via long-range alterations or perturbation of chromatin structure and function.

Los1p, involved in yeast pre-tRNA splicing, positively regulates members of the SOL gene family

To understand the role of Los1p in pre-tRNA splicing, we sought los1 multicopy suppressors. We found SOL1 that suppresses both point and null LOS1 mutations. Since, when fused to the Ga14p DNA-binding domain, Los1p activates transcription, we tested whether Los1p regulates SOL1. We found that las1 mutants have depleted levels of SOL1 mRNA and Sol1p. Thus, LOS1 appears to positively regulate SOL1. SOL1 belongs to a multigene family with at least two additional members, SOL2 and SOL3. Sol proteins have extensive similarity to an unusual group of glucose-6-phosphate dehydrogenases. As the similarities are restricted to areas separate from the catalytic domain, these G6PDs may have more than one function. The SOL family appears to be unessential since cells with a triple disruption of all three SOL genes are viable. SOL gene disruptions negatively affect tRNA-mediated nonsense suppression and the severity increases with the number of mutant SOL genes. However, tRNA levels do not vary with either multicopy SOL genes or with SOL disruptions. Therefore, the Sol proteins affect tRNA expression/ function at steps other than transcription or splicing. We propose that LOS1 regulates gene products involved in tRNA expression/function as well as pre-tRNA splicing.

BK virus large T antigen: interactions with the retinoblastoma family of tumor suppressor proteins and effects on cellular growth control

BK virus (BKV) is a polyomavirus which infects a large percentage of the human population. It is a potent transforming agent and is tumorigenic in rodents. BKV DNA has also been found in human brain, pancreatic islet, and urinary tract tumors, implicating this virus in neoplastic processes. BKV T antigen (TAg) is highly homologous to simian virus 40 TAg, particularly in regions required for mitogenic stimulation and binding to tumor suppressor proteins, The experiments presented in this report show that BKV TAg can bind the tumor suppressor protein p53. BKV TAg also has the ability to bind to members of the retinoblastoma (pRb) family of tumor suppressor proteins both in vivo and in vitro. However, these interactions are detected only when large amounts of total protein are used, because the levels of BKV TAg normally produced from viral promoter-enhancer elements are too low to bind a significant amount of the pRb family proteins in the cell. The low levels of BKV TAg produced by the viral promoter elements are sufficient to affect the levels and the phosphorylation patterns of these proteins and to induce serum-independent growth in these cells. Additional events, however, are required for full transformation. These data further support the notion that BKV TAg can affect cellular growth control mechanisms and may in fact be involved in neoplastic processes.

Association of p53 binding and immortalization of primary C57BL/6 mouse embryo fibroblasts by using simian virus 40 T-antigen mutants bearing internal overlapping deletion mutations

To more precisely map the immortalization and p53 binding domains of T antigen, a large series of overlapping deletion mutations were created between codons 251 to 651 by utilizing a combination of Bal 31 deletion and oligonucleotide-directed mutagenesis. Immortalization assay results indicated that amino acids (aa) 252 to 350, 400, and 451 to 532 could be removed without seriously compromising immortalization, although the appearance of immortal colonies was delayed in some cases. Western immunoblotting experiments indicated that the p53 binding capacities of T antigen produced by mutants missing aa 252 to 300, 301 to 350, 400, or 451 to 532 were only slightly reduced relative to that of wild-type T antigen. Within the limits of this deletion analysis, the immortalization and p53 binding domains appear to be colinear and, in fact, may represent two aspects of the same domain. This deletion analysis eliminates the entire zinc finger domain (aa 302 to 320), a small portion of the leucine-rich region (aa 345 to 350), and a large portion of the ATP binding domain (aa 451 to 528) as participants in p53 binding or in the immortalization process. The results also show that removal of T antigen amino acids within the region 451 to 532 appears to alter the capacity of newly synthesized but not older T antigen and p53 molecules to form complexes.

High-frequency recombination mediated by polyomavirus large T antigen defective in replication

We investigated the mechanism by which the large T antigen (T-Ag) of both polyomavirus and simian virus 40 (SV40) promotes homologous recombination in mammalian cells. To this end, we constructed a rat cell line, designated Hy5, that carries two mutated copies of the polyomavirus middle-T-Ag (pmt) oncogene lying as direct repeats on the same chromosome. The structure of the viral insert was devised so that intrachromosomal recombination between the pmt repeats reconstitutes wild-type pmt and yields cell populations amenable to selection for the transformed phenotype. Correction of pmt by gene conversion occurred spontaneously at a rate of ca. 1.7 x 10(-7) per cell generation and was masked by another recombination event that also led to the transformation of the Hy5 cell line. This event was identified as chromosomal inversion and overexpression of the upstream pmt copy as a result of homologous recombination between adjacent pBR322 sequences. Both events were promoted by the polyomavirus large T-Ag by several orders of magnitude, as well as by mutants defective in the initiation of viral DNA synthesis. Large T-Ag also promoted reconstitution of wild-type pmt by unequal exchange between sister chromatids, yielding structures compatible with some of the chromosomal aberrations commonly observed in transformed cells. Our data indicate that large T-Ag has a recombination-promoting activity that can be dissociated from its replicative function.

Selection against large T-antigen expression in cells transformed by lymphotropic papova virus

Large T-antigen (T-Ag) in hamster cells transformed by the lymphotropic papova virus (LPV) exhibits similar properties as the T-Ag of simian virus 40 (SV40) with regard to its interaction with cellular targets. However, in contrast to SV40-transformed cells, LPV-transformed cells in cell culture select against high expression of LPV T-Ag. Southern analysis revealed that this selection process was accompanied by drastic changes at the DNA level, involving the loss of most of the integrated viral DNA copies. These changes probably were responsible for an approximately 100-fold downregulation of LPV T-Ag transcription. To elucidate the biological significance of this phenomenon, we studied the effects of the expression of LPV and SV40 T-Ag, respectively, in a variety of cells. Our data suggest that LPV T-Ag, like SV40 T-Ag, acts as an immortalizing and transforming protein. However, in contrast to SV40 T-Ag, high-level expression of LPV T-Ag seems to be detrimental to the establishment and maintenance of LPV-transformed cells in vitro.

Simian virus 40 can overcome the antiproliferative effect of wild-type p53 in the absence of stable large T antigen-p53 binding

In simian virus 40 (SV40)-transformed cells, a tight complex is formed between the viral large T antigen (large T) and p53. It has been proposed that this complex interferes with the antiproliferative activity of p53. This notion was tested in primary rat fibroblasts by assessing the ability of SV40-mediated transformation to be spared from the inhibitory effect of wild-type (wt) p53. The data indicate that relative to transformation induced by myc plus ras, SV40-plus-ras-mediated focus formation was indeed much less suppressed by p53 plasmids. A majority of the resultant cell lines made a p53 protein with properties characteristic of a wt conformation. Furthermore, cell lines expressing stably both SV40 large T and a temperature-sensitive p53 mutant continued to proliferate at a temperature at which this p53 assumes wt-like properties and normally causes a growth arrest. Surprisingly, at least partial resistance to the growth-inhibitory effect of wt p53 was also evident when transformation was mediated by an SV40 deletion mutant, encoding a large T which does not bind p53 detectably. In addition to supporting the idea that SV40 can overcome the growth-restrictive activity of wt p53, these findings strongly suggest that at least part of this effect does not require a stable association between p53 and large T.

Structural topography of simian virus 40 DNA replication

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

Correlation of metabolic stability and altered quaternary structure of oncoprotein p53 with cell transformation

The phosphoprotein p53 seems to be implicated in various processes connected with cell transformation and in particular with the regulation of cell cycle and probably DNA replication. In the present paper we have analyzed two sets of closely related cell lines expressing the same p53 which exhibited either a nontransformed or a transformed phenotype. These cell lines were used to study biochemical properties of the p53 protein which might be correlated with cell transformation. We found a positive correlation among an elevated stability of p53, the formation of high-molecular-weight forms of p53, and the transformed phenotype of the corresponding cell lines. Furthermore, these data indicate that self-aggregation prevents p53 from rapid degradation. By a comparative analysis of the stability and oligomerization properties of mutant p53 and wild-type p53, we could demonstrate that elevated stability and self-aggregation of p53 are correlated with the transformed phenotype of the cells and independent of a particular mutation in the p53 gene.

Mutation of the serine 312 phosphorylation site does not alter the ability of mouse p53 to inhibit simian virus 40 DNA replication in vivo

Two mutations were introduced into the wild-type mouse p53 gene by oligonucleotide-directed mutagenesis. These mutations substituted alanine or aspartic acid for serine at position 312, which is constitutively phosphorylated. Phosphopeptide mapping of the mutant proteins, expressed in COS cells, confirmed the loss of phosphorylation at position 312. There were no changes in the ability of the mutant p53s to express the conformation-dependent epitope for monoclonal antibody PAb246 or to participate in complexes with the simian virus 40 (SV40) large T antigen. Replication of a plasmid containing the SV40 origin of replication was inhibited in COS cells by wild-type p53 and both of the phosphorylation site mutants with equal efficiency. A transforming mutant of p53, encoding valine at position 135, did not inhibit SV40 DNA replication in COS cells.

Simian virus 40 large T antigen induces or activates a protein kinase which phosphorylates the transformation-associated protein p53

The cellular phosphoprotein p53 is presumably involved in simian virus 40 (SV40)-induced transformation. We have monitored changes in the state of phosphorylation of p53 from normal versus SV40-infected or -transformed cells. In normal cells, p 53 was hardly phosphorylated. Upon infection or transformation, a quantitative and qualitative increase in p53 phosphorylation was observed as revealed by two-dimensional phosphopeptide analysis. This increase was dependent on a functional large T antigen. In rat cells, enhanced phosphorylation of p53 resulted in conversion to a second, electrophoretically distinct form. In cells transformed with transformation-defective mutants, phosphorylation of p53 was reduced and conversion to form 2 was inefficient. These data suggest (i) that SV40 large T antigen induces or activates a protein kinase, one substrate of which is p53, (ii) that transformation-defective mutants are impaired in kinase induction, and (iii) that either a certain phosphorylation state of p53 or the SV40-induced kinase is critical for efficient transformation.

Development of a monoclonal antibody against recombinant neuroendocrine 7B2 protein

Mouse monoclonal antibody MON-100 was raised against the neuroendocrine protein 7B2 using bacterially produced hybrid proteins. In Western blot analysis, MON-100 reacted with 3 different 7B2 hybrid proteins and not with the respective carrier proteins. Furthermore, MON-100 was reactive with recombinant 7B2 cleaved from a hybrid protein. In an immunohistochemical study, MON-100 exhibited strong reactivity with the intermediate lobe of the Xenopus pituitary gland, a tissue previously shown to contain 7B2 mRNA. Using MON-100, immunoprecipitation analysis of newly synthesized proteins produced by in vitro incubated Xenopus neurointermediate lobes revealed the biosynthesis of a single protein of Mr 24 kDa, the expected size of the 7B2 protein. It appears, therefore, that the anti-7B2 monoclonal antibody MON-100 can be successfully used for Western blot analysis and immunohistochemical analysis as well as for immunoprecipitation experiments.

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

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

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

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

Intranuclear distribution of SV40 large T-antigen and transformation-related protein p53 in abortively infected cells

The intranuclear localization of SV40 T-antigen (T-Ag) and the cellular protein p53 was studied in SV40 abortively infected baby mouse kidney cells using two complementary methods of ultrastructural immunocytochemistry in combination with preferential staining of nuclear RNP components and electron microscope autoradiography. Both proteins were revealed in association with peri- and interchromatin RNP fibrils containing the newly synthesized hnRNA. In addition, T-Ag and p53 remained bound, at least in part, to the residual internal nuclear matrix following nuclease and salt extractions of infected cells. The localization of T-Ag was different in SV40 lytically infected monkey kidney cells since, in addition to hnRNP fibrils, the viral protein was also associated with cellular chromatin. However, when lytic infection was performed in conditions of blocked viral DNA replication, T-Ag was no longer associated with the cellular chromatin but remained bound to the hnRNP fibrils. We conclude that the transforming and lytic functions of T-Ag can be distinguished by different subnuclear distributions. The significance of the association of T-Ag and p53 with hnRNP fibrils and the internal nuclear matrix is discussed in relation to the role of these structures in the control of cellular mRNA biogenesis.

The transformation-related protein p53 is not bound to the SV40 T antigen in BALB 3T12 cells expressing T antigen

In most murine cells transformed by the SV40 virus, virtually all of the cellular phosphoprotein p53 is in a complex with the SV40 T antigen. Here, we report that, in SV40-infected T-antigen-positive Balb 3T12 mouse cells, most (approximately 80%) of the p53 is not in complex. Complex formation was determined by measuring the amounts of [35S]methionine-labeled p53 which coprecipitated with T antigen when using monoclonal antibody to T antigen. The amount of complex formation was expressed as a percentage of total p53 present, measured by the amount of p53 precipitated with the monoclonal antibody to the p53. The values were confirmed by Western blotting procedure, in which the steady-state levels of the proteins were measured. In these measurements after complete precipitation with antibody to T antigen, the residual p53 in the supernatant was precipitated by antibody to p53, and this amount was denoted as free p53. There was no significant difference seen between the [35S]methionine-labeled tryptic peptides of complexed and the free p53 (or between complexed and free T antigens) as determined by two-dimensional gel electrophoresis and chromatography. Virus rescue experiments and retransformation by the rescued virus showed that there was no mutation in the SV40 DNA coding for the T antigen which could account for the lack of complex formation. Both p53 and T antigen were underphosphorylated in cells which exhibited reduced complex formation. Tumorigenicity in syngeneic mice and anchorage-independent cell growth in culture of various cloned mouse cells with or without T antigen expression was compared. The changes in the biologic properties were explainable solely on the basis of known or expected effects of expression of the T antigen and were independent of complex formation or of absence of complex formation between p53 and T antigen.