Phosphorylation of p53 in normal and simian virus 40-transformed NIH 3T3 cells

Understanding p53 functions through p53 antibodies

TP53 is the most frequently mutated gene across all cancer types. Our understanding of its functions has evolved since its discovery four decades ago. Initially thought to be an oncogene, it was later realized to be a critical tumour suppressor. A significant amount of our knowledge about p53 functions have come from the use of antibodies against its various forms. The early anti-p53 antibodies contributed to the recognition of p53 accumulation as a common feature of cancer cells and to our understanding of p53 DNA-binding and transcription activities. They led to the concept that conformational changes can facilitate p53’s activity as a growth inhibitory protein. The ensuing p53 conformational-specific antibodies further underlined p53’s conformational flexibility, collectively forming the basis for current efforts to generate therapeutic molecules capable of altering the conformation of mutant p53. A subsequent barrage of antibodies against post-translational modifications on p53 has clarified p53’s roles further, especially with respect to the mechanistic details and context-dependence of its activity. More recently, the generation of p53 mutation-specific antibodies have highlighted the possibility to go beyond the general framework of our comprehension of mutant p53—and promises to provide insights into the specific properties of individual p53 mutants. This review summarizes our current knowledge of p53 functions derived through the major classes of anti-p53 antibodies, which could be a paradigm for understanding other molecular events in health and disease.

Small changes huge impact: the role of protein posttranslational modifications in cellular homeostasis and disease

Posttranslational modifications (PTMs) modulate protein function in most eukaryotes and have a ubiquitous role in diverse range of cellular functions. Identification, characterization, and mapping of these modifications to specific amino acid residues on proteins are critical towards understanding their functional significance in a biological context. The interpretation of proteome data obtained from the high-throughput methods cannot be deciphered unambiguously without a priori knowledge of protein modifications. An in-depth understanding of protein PTMs is important not only for gaining a perception of a wide array of cellular functions but also towards developing drug therapies for many life-threatening diseases like cancer and neurodegenerative disorders. Many of the protein modifications like ubiquitination play a decisive role in various drug response(s) and eventually in disease prognosis. Thus, many commonly observed PTMs are routinely tracked as disease markers while many others are used as molecular targets for developing target-specific therapies. In this paper, we summarize some of the major, well-studied protein alterations and highlight their importance in various chronic diseases and normal development. In addition, other promising minor modifications such as SUMOylation, observed to impact cellular dynamics as well as disease pathology, are mentioned briefly.

Serine 312 phosphorylation is dispensable for wild-type p53 functions in vivo

Cellular stimulation results in phosphorylation of the tumor suppressor p53 on multiple residues, though the functional relevance is not always clear. It is noteworthy that the serine (S) 315 residue is unique, as it has been suggested to be phosphorylated not only by genotoxic signals, but also during cell-cycle progression and by endoplasmic-reticulum stress. However, in vitro data have been conflicting as phosphorylation at this site was shown to both positively and negatively regulate p53 functions. We have thus generated knock-in mice expressing an unphosphorylable S312 (equivalent to human S315), by substitution with an alanine (A) residue, to clarify the conflicting observations and to evaluate its functional relevance in vivo. Born at Mendelian ratios, the p53(S312A/S312A) mice show no anomalies during development and adulthood. p53 activation, stability, localization and ability to induce apoptosis, cell-cycle arrest and prevent centrosome amplification are not compromised in p53(S312A/S312A) cells. p53(S312A/S312A) mice are unable to rescue mdm2(-/-) lethality, and tumorigenesis--both spontaneous and irradiation/oncogene-induced--is not accentuated. Taken together, the results show that the S312 phosphorylation site is not in itself necessary for efficient p53 function, and advocates the possibility that it is neither relevant in the mouse context nor important for p53 functions in vivo.

Towards inferring time dimensionality in protein-protein interaction networks by integrating structures: the p53 example

Structural data, efficient structural comparison algorithms and appropriate datasets and filters can assist in getting an insight into time dimensionality in interaction networks; in predicting which interactions can and cannot co-exist; and in obtaining concrete predictions consistent with experiment.

Inspection of protein–protein interaction maps illustrates that a hub protein can interact with a very large number of proteins, reaching tens and even hundreds. Since a single protein cannot interact with such a large number of partners at the same time, this presents a challenge: can we figure out which interactions can occur simultaneously and which are mutually excluded? Addressing this question adds a fourth dimension into interaction maps: that of time. Including the time dimension in structural networks is an immense asset; time dimensionality transforms network node-and-edge maps into cellular processes, assisting in the comprehension of cellular pathways and their regulation. While the time dimensionality can be further enhanced by linking protein complexes to time series of mRNA expression data, current robust, network experimental data are lacking. Here we outline how, using structural data, efficient structural comparison algorithms and appropriate datasets and filters can assist in getting an insight into time dimensionality in interaction networks; in predicting which interactions can and cannot co-exist; and in obtaining concrete predictions consistent with experiment. As an example, we present p53-linked processes.

Frequent phosphorylation at serine 392 in overexpressed p53 protein due to missense mutation in carcinoma of the urinary tract

Post-transcriptional modification of p53 by phosphorylation has been proposed to be an important mechanism of p53 stabilization and functional regulation. Phosphorylation of p53 Ser392, in particular, activates specific DNA binding functions by stabilizing p53 tetramer formation. This study evaluated the relationship between p53 Ser392 phosphorylation and various types of p53 missense mutation detected in urothelial transitional cell carcinomas (TCCs), with stratification of the mutations according to the functional domains elucidated by the crystal structure of the p53 protein. Of 41 TCCs with missense mutations, 26 (63.4%) exhibited immunopositivity with Ser392 phospho-specific p53 antibody. In comparison to structural mutations, the missense mutations at exon 7 (p=0.0307) or located in regions that affect direct DNA binding ability (p = 0.0273) were significantly associated with Ser392 immunopositivity. No statistically significant relationship was found between Ser392 immunoreactivity and other different types of p53 mutation. The prevalence of cases exhibiting Ser392-positive immunostaining was higher for high-grade (p <0.0001) and advanced-stage TCCs (p =0.0119) than for TCCs with wild-type p53. No significant relationship was found between Ser392 immunoreactivity and apoptotic index in urothelial TCCs. These in vivo findings indicate that Ser392 phosphorylation frequently occurs in mutant form p53 in TCCs. Because mutant form p53 can act dominant-negatively by heterooligomerization with wild-type p53, these findings also suggest that Ser392 phosphorylation might activate tetramer formation to promote the dominant-negative effects of mutant form p53, and thereby contribute to proliferation of aggressive TCCs.

Protein kinase CK2-dependent regulation of p53 function: evidence that the phosphorylation status of the serine 386 (CK2) site of p53 is constitutive and stable

The p53 tumour suppressor protein is regulated by several mechanisms including multisite phosphorylation. One of the protein kinases which has an established role in regulating p53 function is the protein kinase CK2. The regulation by CK2 occurs both through interaction of p53 with CK2 itself (the regulatory beta subunit) and phosphorylation at the penultimate residue of p53, serine 386 (murine p53). Strikingly, this phosphorylation event controls several independent functions of p53 including site-specific DNA binding, strand renaturation, transcriptional repression and the anti-proliferative function of p53. However, CK2 is a constitutively-active enzyme and therefore the mechanism by which the phosphorylation of p53 at serine 386 is itself regulated, or indeed the question as to whether phosphorylation of this site is regulated at all, remains unresolved. In this paper we provide evidence that serine 386 is highly resistant to dephosphorylation in cultured cells, even though this site can be dephosphorylated in vitro by recombinant protein phosphatase 1. These data suggest that, once phosphorylated at the CK2 site, a p53 molecule remains in this modified form throughout its lifespan. To address the issue of whether the level of serine 386 phosphorylation may be regulated through controlling the subcellular compartmentalisation of p53 and CK2, we examined the subcellular localisation of p53 and CK2alpha in C57MG cells and Rat-1 fibroblasts by immunofluorescence staining. Both proteins were present in the cytoplasm and enriched in the nucleus, with minor variations in the intensity of subcellular location over the course of the cell cycle. Similarly, activation of p53 by UV irradiation or DNA damage-inducing drugs had no effect on either the localisation or levels of CK2alpha, even although significant nuclear p53 accumulation was observed. A striking observation arising from these studies was the intense staining of CK2alpha with the centrosomes, suggesting a potentially important role for this kinase in microtubule formation and/or chromosomal segregation.

Expression of p53 and ALZ-50 immunoreactivity in rat cortex: effect of prenatal exposure to ethanol

Neuronal death is an active process that results in the upregulation of antigens recognized by ALZ-50 and p53. Since prenatal exposure to ethanol can induce the postnatal death of cortical neurons, we examined the effects of ethanol on the in vivo expression of both the ALZ-50-positive antigen and p53. Pregnant rats were fed one of three diets, a liquid diet containing ethanol (Et), an isocaloric and isonutritive diet (Ct), or chow and water (Ch). Segments of frontoparietal cortex from fetuses and pups were examined for ethanol-induced changes (a) in the expression of ALZ-50 and p53 immunoreactivity using a quantitative immunoblotting assay and (b) in the distribution of ALZ-50- and p53-positive cells using immunohistochemistry. In control rats, ALZ-50 identified a 56-kDa peptide that was transiently expressed postnatally and peak expression occurred on postnatal day (P) 6 to P12. In Et-treated rats, peak expression was attained earlier (on P3) and was about three times of that achieved in the controls. The anti-p53 antibody identified three proteins (28, 56, and 58 kDa). Peak expression in control rats occurred during the second postnatal week and only the 58-kDa protein was expressed in appreciable amounts in adult cortex. Each p53-positive protein was affected by ethanol exposure. The 28- and 56-kDa p53-positive proteins were affected by ethanol much in the same way as was the ALZ-50-positive antigen. That is, the timing and amount of peak expression were earlier and lower, respectively, in the Et-treated rats. The postnatal expression of the 58-kDa protein was halved following prenatal exposure to ethanol. Both ALZ-50 and anti-p53 immunoprecipitated proteins are p53- and ALZ-50-positive, respectively. Thus, ethanol alters the expression of the ALZ-50- and p53-positive proteins and presumably the timing of neuronal death in the developing cortex. The parallel effects of prenatal ethanol exposure on the 56-kDa ALZ-50-positive antigen and the 28- and 56-kDa p53-positive proteins and the coprecipitation of the proteins are consistent with the notion that ALZ-50 recognizes a form of p53.

Multisite phosphorylation and the integration of stress signals at p53

The p53 tumour suppressor protein is a potent transcription factor that plays a major role in the defence against tumour development. p53 exists in a latent form that can be activated by a range of stresses including DNA damage, hypoxia, cytokines, metabolic changes, viral infection, and activated oncogenes. Activation of p53 can lead to cellular growth arrest prior to entry into either S phase or mitosis or can trigger cell death through apoptosis. The modification of p53 by multisite phosphorylation provides a potential link between stress signalling and the regulation of p53 activity, and there is now striking evidence that agents that activate p53 can lead to selective changes in its phosphorylation status. Topologically, the phosphorylation sites in p53 fall into two discrete functional domains. Four phosphorylation events take place within the N-terminal 83 amino acids containing the transactivation domain and a region involved in transcription-independent growth suppression. At least three of these modifications occur in response to agents that cause cellular stress such as DNA damage. At the C-terminus, there are three phosphorylation events, each of which can independently regulate the specific DNA-binding function of p53, suggesting convergent control by different signalling pathways. The multiplicity of these covalent modifications and their responsiveness to a wide range of signals suggest that p53 activity is tightly and coordinately controlled in response to stresses and changes in the cellular environment.

Neonatal transection of the infraorbital nerve increases the expression of proteins related to neuronal death in the principal sensory nucleus of the trigeminal nerve

Neonatal lesion of the primary afferents in the infraorbital nerve causes the death of one-third of the neurons in the second-order target, the principal sensory nucleus of the trigeminal nerve (PSN). We examined the expression of two candidate 'death' proteins, p53 and the antigen recognized by the antibody ALZ-50, in the normal and deafferented PSN. In addition, the effect of neonatal transection of the infraorbital nerve (a major component of the trigeminal nerve) on protein expression was examined. The expression of c-fos in the developing PSN was also studied as an index of metabolic activity. Protein expression was measured using quantitative analyses of immunoblots and immunohistochemical preparations. The expression of p53- and ALZ-50-immunoreactivity in the normal PSN peaked during the first postnatal week. Transection of the infraorbital nerve directly affected the expression of p53 and the ALZ-50-positive antigen. The immunoblots showed that whereas p53 amounts were unaffected by the lesion, ALZ-50 expression was significantly upregulated in the ipsilateral PSN 2 h and 2 days postlesion. The density of p53- and ALZ-50-immunoreactive neurons was significantly higher in the ventral ipsilateral PSN (i.e., the target of the transected infraorbital nerve) than in the contralateral PSN. c-fos expression selectively and transiently rose in the ventral ipsilateral PSN within 2 h of the lesion. Thus, both p53 and the ALZ-50-positive antigen are involved in neuronal death. In light of data suggesting that ALZ-50 recognizes a phosphorylated form of p53, we conclude that neuronal death in the developing nervous system involves the post-translational modification of an existing protein, p53. The increase in ALZ-50 expression apparently occurs during a catabolic phase of neuronal death, as indicated by the increase in c-fos expression.

Mutation of phosphoserine 389 affects p53 function in vivo

To study the importance of phosphorylation for p53 transactivation function, we generated mutations at each of its known phosphorylated serine amino acids. Mutations of murine p53 serine residues individually to either alanine or glutamic acid at positions 7, 9, 12, 18, 37, 312, and 389 resulted in equivalent levels of transcriptional activation in standard transient transfection experiments. However, when p53 transcriptional activity was measured in cells that attain G1 arrest upon contact inhibition, wild-type p53 was inactive, and only alteration at serine 389 to glutamic acid resulted in a functional p53 protein. This Ser --> Glu mutant also has an increased ability to bind DNA. Elimination of the phosphorylation site by substitution of an alanine amino acid resulted in loss of transcriptional activity. We also demonstrated that specific phosphorylation of p53 at serine 389 is induced by cyclin E overexpression in high-density cells. Our data establish for the first time that phosphorylation of p53 at serine 389 is important in activating its function in vivo.

p53 Phosphorylation: biochemical and functional consequences

The p53 protein plays a vital role in suppressing the development of cancer. Posttranslational modification through phosphorylation has been postulated to be an important regulatory mechanism of p53 function. Data describing the role of phosphorylation in terms of its effects on several biochemical properties and cellular functions of p53 are examined in the context of how p53 might be "phospho-regulated."

Analysis of genomic instability in Li-Fraumeni fibroblasts with germline p53 mutations

Germline p53 mutations are frequently observed in the normal DNA of cancer-prone patients with Li-Fraumeni syndrome (LFS). Fibroblasts from LFS patients develop chromosomal aberrations, loss of cell cycle control, and spontaneous immortalization. We transfected four different mutant p53 genes into human skin fibroblasts from normal donors with two copies of wild-type p53 (p53(wt/wt)). Each mutant p53 expression-plasmid induced genomic instability equivalent to that seen in LFS cells. To test the role of wild-type and mutant p53 alleles in DNA replication and fidelity in LFS cells, we analysed the replication of the SV40-based shuttle vector pZ189 in four types of cells. We used p53(wt/mut) and p53(mut/-) LFS fibroblasts, and p53(-/-) non-LFS cells. Replication of pZ189 in vivo was significantly reduced by the presence of a p53(wt) allele. To show that this was not just due to inhibition of the function of T-antigen in SV40-based replication, we constructed a shuttle vector, pZ402, that contains a mutation in SV40 T-antigen which blocks its ability to interact with p53. Replication of pZ402 in LFS cells was also reduced by the presence of p53(wt), indicating that p53 can inhibit replication by interacting with proteins within the cellular replication machinery. Replicative errors in this shuttle vector are detected as mutations in a marker gene, supF. In addition to supF mutations, we observed deletion of a portion of the SV40 T-antigen gene in 100% of replicated plasmid pZ189 mutants (supF-) from the p53(wt/mut) fibroblasts and in 88% of the supF mutants from the p53(mut/-) (amino acid 175 arg to his) LFS cells. In one cell strain of immortal LFS cells, P53(mut/-) , containing a p53 frameshift mutation at amino acid 184, pZ189 replication yielded very few of these deleted shuttle vector plasmids (15%). These large deletions were not detected in plasmids replicated in p53(-/-) non-LFS cells, Saos-2 cells. Replicated plasmids with a normal supF gene were never found to have this large deletion regardless of the cell from which they were derived. Because the supF gene is not in the same region of the shuttle vector as the T-antigen gene it appears that second, independent gene deletions are frequent when replicative errors in supF occur in cells with a mutant p53. We conclude, therefore, that p53(wt/mut) LFS cells contain an activity that promotes mutations. Such an activity, which is likely to be due to the p53(mut), could result in the high rate of chromosomal instability and allelic loss of the wild-type p53 observed as these cells spontaneously immortalize.

p53 and translational control

The tumor suppressor p53 plays a role in mediating a G1 arrest (for example, in response to DNA damage), in the cellular commitment to apoptosis and in suppression of transformation. The mechanism of action of p53 in each of these biological outcomes is likely to be overlapping. Current data indicate that p53 functions as a sequence specific transcriptional activator. p53 can also repress transcription from certain promoters. One way in which p53 mediates a G1 arrest after DNA damage appears to be clear. Cells exposed to ionizing radiation show elevated levels of p53 protein. The increase in p53 levels is thought to be responsible for the increase in the cyclin-dependent kinase (cdk) inhibitor p21 mediated through the p53 binding sites in the p21 promoter. With regard to the ability of p53 to suppress transformation, there is data suggesting that p53 functions other than, or in addition to, its transcriptional activation function may be necessary. Similar data exist for p53-dependent apoptosis. Recently a role for p53 at another level of gene regulation, namely, translational regulation has been proposed. p53 associates with various components of the translation machinery and has been implicated in the translational regulation of both the p53 and CDK4 mRNAs. Here we will summarize the evidence suggesting a role for p53 in translation and how this regulation might be achieved.

Radiation induced G1-block and p53 status in six human cell lines

Considerable attention has recently been focused on the fact that the tumor suppressor protein p53 is involved in the cellular response to radiation. In its wild-type form the protein appears to control a cell cycle checkpoint, preventing entry into S-phase following DNA damage. A number of authors observed a radiation induced G1-block in cells expressing wild-type p53, but not in p53 mutant cells. We obtained similar results with four human tumour cell lines as well as two strains of human fibroblasts, whose p53 status was ascertained at the protein as well as DNA levels. In addition to cell cycle delays in exponentially growing cell cultures, we have studied the possible role of the p53 in the transition from quiescence to active proliferation. Cells were irradiated after 6 days of serum-starvation and labelled with BrdU at different times after addition of fresh medium. Entry into S-phase was found to be delayed by several hours in the p53 wild-type cells, but no such effect was observed in the p53 mutants. Where a delay occurred, it was roughly proportional to the X-ray dose. Although it remains to be clarified, whether the cells were delayed only in G1 or also in G0, it is interesting to note that entry into S-phase can be delayed by irradiation in a quiescent state immediately before serum-stimulation, provided the cells are wild-type with respect to p53. Certain differences in the cell cycle response of transformed and untransformed cells were noted.

Phosphorylation studies on rat p53 using the baculovirus expression system. Manipulation of the phosphorylation state with okadaic acid and influence on DNA binding

To elucidate the role of phosphorylation of p53 we used the baculovirus expression system to obtain high yields of protein eventually in distinct phosphorylation states. Initially, we obtained only marginal phosphorylation, despite high levels of expression. Two-dimensional phosphopeptide maps exhibited the same pattern as known from rat cells although some sites were underrepresented. Coexpression of simian virus 40 (SV40) large T antigen or cyclin-dependent kinases, cdc2 or cdk2, had only marginal effects on the phosphorylation state of p53. However, when we employed the phosphatase inhibitor okadaic acid, overall phosphorylation of p53 was drastically enhanced in a dose-dependent manner and resembled that of p53 from SV40-transformed rat cells. This hyperphosphorylation resulted in enhanced binding of a consensus oligonucleotide as revealed by electrophoretic mobility shift assays. To assess the role of individual phosphorylation sites, we generated a set of mutants at putative or identified sites. All mutants retained the ability to bind wild-type conformation-specific antibody Pab1620, to complex with SV40 large T antigen, and to bind to the consensus oligonucleotide. Moreover, most mutants exhibited enhanced DNA binding upon okadaic acid treatment, except for a mutant at the cdk site which failed to do so. These data show that: (a) insect cells contain all the protein kinases necessary for phosphorylation of a mammalian protein, p53; (b) in insect cells the ratio of kinase/phosphatase activities differs from that in mammalian cells so that underphosphorylation of recombinant proteins in this system may result from high phosphatase activities rather than saturation of kinases with recombinant substrate; (c) the system can be manipulated to obtain subpopulations of recombinant protein in a desired phosphorylation state, and (d) phosphorylation may regulate the DNA-binding activity of p53.

p53 is phosphorylated in vitro and in vivo by an ultraviolet radiation-induced protein kinase characteristic of the c-Jun kinase, JNK1

The p53 tumor suppressor protein is thought to play a major role in the defense of the cell against agents that damage DNA. In this report, we describe the identification and characterization of a protein kinase that phosphorylates mouse p53 at a single site, serine 34, a major site of phosphorylation in the cell. The protein kinase is activated strikingly following treatment of cells with ultraviolet radiation, has a native molecular weight of approximately 45,000, and can be resolved from mitogen-activated protein (MAP) kinase by chromatography on Superose 6 and DEAE-cellulose. The p53 kinase activity co-purifies with UV-activated c-Jun kinase activity on heparin-Sepharose and on a c-Jun (but not a v-Jun-) affinity column. Treatment of the partially purified kinase with CL100, a protein phosphatase that specifically dephosphorylates MAP kinase homologues, inhibits its activity. Taken together, the data suggest that this p53 kinase is likely to be activated by phosphorylation and may be a member of the stress-activated protein kinase subfamily of MAP kinases. UV irradiation of SV3T3 cells leads to increased phosphorylation of p53 at serine 34, indicating that phosphorylation of p53 by this kinase is likely to be physiological. Phosphorylation of p53 by this protein kinase may be a key event in a signal transduction mechanism that coordinately controls key nuclear proteins in response to oxidative stress or DNA damaging agents.

Tumor suppressor p53 mutations and breast cancer: a critical analysis

Alterations in the tumor suppressor gene p53 are the most commonly identified changes in cancer, including neoplasia of the breast. The activity of p53 is regulated post-translationally. Phosphorylation state, subcellular localization, and interaction with any of a number of cellular proteins are likely to influence the function of p53. The exact effect of p53-mediated growth suppression seems to be cell-type specific but appears to be directly related to the ability of p53 to act as a specific transcriptional activator. The role that transcriptional repression plays in the function of WT p53 is less clear. It is also possible that p53 has a more direct activity in DNA replication and repair. Most documented p53 mutations result in single amino acid substitutions which may confer one or more of a spectrum of transforming abilities on the protein. Mutation may lead to nuclear accumulation of p53 protein; however, inactivation of p53 by nuclear exclusion and interaction with the mdm2 protein also appear to be important in tumorigenesis. Used in conjunction with other established factors, accumulation of cellular p53 may be a useful prognostic indicator in breast cancer. A syngeneic mouse model system yielded evidence that p53 mutations are important in the early, preneoplastic stages of mammary tumorigenesis. This murine system may provide the ability to investigate the functions of p53 in the early stages of breast cancer which are technically difficult to examine in the human system.

Phosphopeptide mapping and phosphoamino acid analysis by electrophoresis and chromatography on thin-layer cellulose plates

Identification of protein phosphorylation sites is essential in order to evaluate the contribution of individual sites to the regulation of a particular protein by phosphorylation. Here we review a method we have developed for the identification of phosphorylation sites based on digestion of 32P-labeled proteins with site-specific proteases and separation of the digestion products in two dimensions on thin-layer cellulose plates using electrophoresis in the first dimension followed by chromatography. This method is very sensitive, requiring only a few hundred 32P-disintegrations per minute to obtain reproducible phosphopeptide maps. We also report methods for the analysis of the phosphoamino acid content of both intact phosphoproteins and individual phosphopeptides recovered from two-dimensional separations, in which the material is subjected to partial acid hydrolysis, and the hydrolysis products are separated on thin-layer cellulose plates by electrophoresis in one or two dimensions. Finally, we describe methods for analyzing the structure of isolated phosphopeptides by secondary digestion with site-specific proteases, by manual Edman degradation, and by immunoprecipitation, and indicate how this information can be used in conjunction with the two-dimensional mobility of the peptide to deduce the identity of a phosphopeptide from the known sequence of a protein.

p53 and the Li-Fraumeni syndrome

The Li-Fraumeni familial cancer syndrome was initially described in 1969 in a retrospective epidemiologic review of more than 600 pediatric sarcoma patients. The clinical definition of the syndrome has been refined in the last two decades by prospective analyses of several families. Despite these exhaustive studies, the gene or genes responsible for the unusual constellation of tumors in these families remained elusive until 1990, when it was demonstrated that germline abnormalities of the p53 tumor suppressor gene could account for the occurrence of cancer in many classic Li-Fraumeni families. Identification of the molecular events that yield this phenotype has led many researchers to pursue several lines of investigation to improve our understanding of the significance of such alterations. We discuss the clinical, epidemiologic, genetic, and biologic aspects of the association between p53 and the Li-Fraumeni family cancer syndrome.

Regulation of the specific DNA binding function of p53

The DNA binding activity of p53 is required for its tumor suppressor function; we show here that this activity is cryptic but can be activated by cellular factors acting on a C-terminal regulatory domain of p53. A gel mobility shift assay demonstrated that recombinant wild-type human p53 binds DNA sequence specifically only weakly, but a monoclonal antibody binding near the C terminus activated the cryptic DNA binding activity stoichiometrically. p53 DNA binding could be activated by a C-terminal deletion of p53, mild proteolysis of full-length p53, E. coli dnaK (which disrupts protein-protein complexes), or casein kinase II (and coincident phosphorylation of a C-terminal site on p53). Activation of p53 DNA binding may be critical in regulation of its ability to arrest cell growth and thus its tumor suppressor function.

Domains required for in vitro association between the cellular p53 and the adenovirus 2 E1B 55K proteins

The 55K protein encoded by the adenovirus 2 E1B gene is required for complete cellular transformation and binds the cellular protein p53. Using an in vitro immunoprecipitation assay, we mapped the domains in both 55K and p53 required for the interaction of the two proteins. The domain in p53 mapped to the amino terminal 123 residues. There are several domains in the 495 residue 55K polypeptide which contribute to stable association with p53, with the most essential region mapping between residues 224 and 354. Mutations which prevented 55K-p53 binding were not more defective for transformation than other mutations which did not affect binding.

Transformation by simian virus 40 does not involve the mutational activation of p53 to an oncogenic form

We investigated whether the p53 protein of SV40-transformed mouse cells reacted with the conformation-dependent monoclonal antibody pAb246. This antibody can usually distinguish between a p53 with anti-proliferative activity like the wild-type protein (pAb246+) and a mutated form of p53 with oncogenic activity (pAb246-). Of the 13 cell lines that were screened, 12 contained the pAb246+ form of p53 and one had the pAb246- form. We showed that SV40 did not induce an activating mutation in the p53 of this latter cell line, because the cells from which it was derived were also pAb246-. Cascade immunoprecipitation experiments demonstrated that in three SV40-transformed cell lines that were examined, all of the p53 was of the pAb246+ form making it unlikely that small amounts of pAb246- p53 were responsible for the transformation properties of these cells. We therefore concluded that SV40-mediated transformation of murine cells is not dependent on the activation of their p53 to an oncogenic form, and that, in all probability, transformation is allowed to occur in part because the anti-proliferative activity of p53 is blocked by SV40 T antigen.

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.

Mapping of a phosphorylation site in the 176R (19 kDa) early region 1B protein of human adenovirus type 5

The 176-residue (176R) early region 1B (E1B) protein of human adenovirus type 5 (Ad5) was shown to be phosphorylated at serine in lytically infected KB cells at a level estimated to be about one phosphate group per 28 176R molecules. Through the analysis of peptides generated by cleavage with cyanogen bromide and Staphylococcus aureus V-8 protease the phosphorylation site was mapped to Ser-164. Using site-directed mutagenesis, a mutant was produced in which the codon for Ser-164 was changed to that of asparagine while leaving the coding sequence for the overlapping 496R protein unchanged. This virus, which replicated well on human KB cells, produced normal levels of 176R, but in an unphosphorylated form. The mutant transformed baby rat kidney cells in cooperation with E1A at an efficiency about one-half that obtained with wt E1B. These data therefore gave little indication that phosphorylation is essential for the function of 176R.