Mutant p53 aggregates into prion-like amyloid oligomers and fibrils: implications for cancer

p53 protein aggregation promotes platinum resistance in ovarian cancer

High-grade serous ovarian carcinoma (HGSOC), the most lethal gynecological cancer, often leads to chemoresistant diseases. The p53 protein is a key transcriptional factor regulating cellular homeostasis. A majority of HGSOCs have inactive p53 because of genetic mutations. However, genetic mutation is not the only cause of p53 inactivation. The aggregation of p53 protein has been discovered in different types of cancers and may be responsible for impairing the normal transcriptional activation and pro-apoptotic functions of p53. We demonstrated that in a unique population of HGSOC cancer cells with cancer stem cell properties, p53 protein aggregation is associated with p53 inactivation and platinum resistance. When these cancer stem cells differentiated into their chemosensitive progeny, they lost tumor-initiating capacity and p53 aggregates. In addition to the association of p53 aggregation and chemoresistance in HGSOC cells, we further demonstrated that the overexpression of a p53-positive regulator, p14ARF, inhibited MDM2-mediated p53 degradation and led to the imbalance of p53 turnover that promoted the formation of p53 aggregates. With in vitro and in vivo models, we demonstrated that the inhibition of p14ARF could suppress p53 aggregation and sensitize cancer cells to platinum treatment. Moreover, by two-dimensional gel electrophoresis and mass spectrometry we discovered that the aggregated p53 may function uniquely by interacting with proteins that are critical for cancer cell survival and tumor progression. Our findings help us understand the poor chemoresponse of a subset of HGSOC patients and suggest p53 aggregation as a new marker for chemoresistance. Our findings also suggest that inhibiting p53 aggregation can reactivate p53 pro-apoptotic function. Therefore, p53 aggregation is a potential therapeutic target for reversing chemoresistance. This is paramount for improving ovarian cancer patients' responses to chemotherapy, and thus increasing their survival rate.

Investigating the intrinsic aggregation potential of evolutionarily conserved segments in p53

Protein aggregation and amyloid formation are known to play a role both in diseases and in biological functions. Transcription factor p53 plays a major role in tumor suppression by maintaining genomic stability. Recent studies have suggested that amyloid formation of p53 could lead to its loss of physiological function as a tumor suppressor. Here, we investigated the intrinsic amyloidogenic nature of wild-type p53 using sequence analysis. We used bioinformatics and aggregation prediction algorithms to establish the evolutionarily conserved nature of aggregation-prone sequences in wild-type p53. Further, we analyzed the amyloid forming capacity of conserved and aggregation-prone p53-derived peptides PILTIITL and YFTLQI in vitro using various biophysical techniques, including all atom molecular dynamics simulation. Finally, we probed the seeding ability of the PILTIITL peptide on p53 aggregation in vitro and in cells. Our data demonstrate the intrinsic amyloid forming ability of a sequence stretch of the p53 DNA binding domain (DBD) and its aggregation templating behavior on full-length and p53 core domain. Therefore, p53 aggregation, instigated through an amyloidogenic segment in its DBD, could be a putative driving force for p53 aggregation in vivo.

Arsenic trioxide reactivates proteasome-dependent degradation of mutant p53 protein in cancer cells in part via enhanced expression of Pirh2 E3 ligase

The p53 gene is mutated in more than 50% of human tumors. Mutant p53 exerts an oncogenic function and is often highly expressed in cancer cells due to evasion of proteasome-dependent degradation. Thus, reactivating proteasome-dependent degradation of mutant p53 protein is an attractive strategy for cancer management. Previously, we found that arsenic trioxide (ATO), a drug for acute promyelocytic leukemia, degrades mutant p53 protein through a proteasome pathway. However, it remains unclear what is the E3 ligase that targets mutant p53 for degradation. In current study, we sought to identify an E3 ligase necessary for ATO-mediated degradation of mutant p53. We found that ATO induces expression of Pirh2 E3 ligase at the transcriptional level. We also found that knockdown of Pirh2 inhibits, whereas ectopic expression of Pirh2 enhances, ATO-induced degradation of mutant p53 protein. Furthermore, we found that Pirh2 E3 ligase physically interacts with and targets mutant p53 for polyubiquitination and subsequently proteasomal degradation. Interestingly, we found that ATO cooperates with HSP90 or HDAC inhibitor to promote mutant p53 degradation and growth suppression in tumor cells. Together, these data suggest that ATO promotes mutant p53 degradation in part via induction of the Pirh2-dependent proteasome pathway.

Prion-like aggregation of mutant p53 in cancer

p53 is a master regulatory protein that participates in cellular processes such as apoptosis, DNA repair, and cell cycle control. p53 functions as a homotetrameric tumor suppressor, and is lost in more than 50% of human cancers. Recent studies have suggested that the formation of mutant p53 aggregates is associated with loss-of-function (LoF), dominant-negative (DN), and gain-of-function (GoF) effects. We propose that these phenomena can be explained by a prion-like behavior of mutant p53. We discuss the shared properties of cancer and neurodegenerative diseases and how the prion-like properties of p53 aggregates offer potential targets for drug development.

Clinical significance of different types of p53 gene alteration in surgically treated prostate cancer

Despite a multitude of p53 immunohistochemistry (IHC) studies, data on the combined effect of nuclear p53 protein accumulation and TP53 genomic inactivation are lacking for prostate cancer. A tissue microarray including 11,152 prostate cancer samples was analyzed by p53 IHC and fluorescence in situ hybridization. Nuclear p53 accumulation was found in 10.1% of patients including 1.4% with high-level and 8.7% with low-level immunostaining. TP53 sequencing revealed that 17 of 22 (77%) cases with high-level p53 immunostaining, but only 3% (1 of 31) low-level p53 cases carried putative dominant-negative mutations. TP53 deletions occurred in 14.8% of cancers. Both deletions and protein accumulation were linked to unfavorable tumor phenotype and prostate specific antigen (PSA) recurrence (p<0.0001 each). The combination of both methods revealed subgroups with remarkable differences in their clinical course. Tumors with either TP53 deletion (14%) or low-level p53 positivity (8.7%) had identical risks of PSA recurrence, which were markedly higher than in cancers without p53 alterations (p<0.0001). Tumors with both p53 deletion and low-level p53 positivity (1.5%) had a worse prognosis than patients with only one of these alterations (p<0.0001). Tumors with strong p53 immunostaining or homozygous inactivation through deletion of one allele and disrupting translocation involving the second allele had the worst outcome, independent from clinical and pathological parameters. These data demonstrate a differential clinical impact of various TP53 alterations in prostate cancer. Strong p53 immunostaining-most likely accompanying dominant negative or oncogenic p53 mutation-has independent prognostic relevance and may thus represent a clinical useful molecular feature of prostate cancer.

δ-Opioid receptor activation reduces α-synuclein overexpression and oligomer formation induced by MPP(+) and/or hypoxia

Hypoxic/ischemic brain injury is a potential cause of Parkinson's disease (PD) with ɑ-synuclein playing a critical role in the pathophysiology. Since δ-opioid receptor (DOR) is neuroprotective against hypoxic/ischemic insults, we sought to determine if DOR regulates ɑ-synuclein under hypoxia and/or MPP(+) stress. We found that in HEK293 cells 1) MPP(+) in normoxia enhanced ɑ-synuclein expression and the formation of ɑ-synuclein oligomers thereby causing cytotoxic injury; 2) hypoxia at 1% O2 for 48h or at 0.5% O2 for 24h also induced ɑ-synuclein overexpression and its oligomer formation with cell injury; 3) however, hypoxia at 1% O2 for 24h, though increasing ɑ-synuclein expression, did not cause ɑ-synuclein oligomer formation and cell injury; 4) UFP-512 mediated DOR activation markedly attenuated the hypoxic cell injury and ɑ-synuclein overexpression, which was largely attenuated by DOR antagonism with naltrindole or siRNA "knock-down" of the DOR; and 5) DOR activation enhanced CREB phosphorylation and prevented the collapse of mitochondrial membrane potential (△ψm). These findings suggest that DOR activation attenuates MPP(+) or severe hypoxia induced ɑ-synuclein expression/aggregation via a CREB pathway.

Pathological implications of nucleic acid interactions with proteins associated with neurodegenerative diseases

Protein misfolding disorders (PMDs) refer to a group of diseases related to the misfolding of particular proteins that aggregate and deposit in the cells and tissues of humans and other mammals. The mechanisms that trigger protein misfolding and aggregation are still not fully understood. Increasing experimental evidence indicates that abnormal interactions between PMD-related proteins and nucleic acids (NAs) can induce conformational changes. Here, we discuss these protein-NA interactions and address the role of deoxyribonucleic (DNA) and ribonucleic (RNA) acid molecules in the conformational conversion of different proteins that aggregate in PMDs, such as Alzheimer's, Parkinson's, and prion diseases. Studies on the affinity, stability, and specificity of proteins involved in neurodegenerative diseases and NAs are specifically addressed. A landscape of reciprocal effects resulting from the binding of prion proteins, amyloid-β peptides, tau proteins, huntingtin, and α-synuclein are presented here to clarify the possible role of NAs, not only as encoders of genetic information but also in triggering PMDs.

Self-propagation of pathogenic protein aggregates in neurodegenerative diseases

For several decades scientists have speculated that the key to understanding age-related neurodegenerative disorders may be found in the unusual biology of the prion diseases. Recently, owing largely to the advent of new disease models, this hypothesis has gained experimental momentum. In a remarkable variety of diseases, specific proteins have been found to misfold and aggregate into seeds that structurally corrupt like proteins, causing them to aggregate and form pathogenic assemblies ranging from small oligomers to large masses of amyloid. Proteinaceous seeds can therefore serve as self-propagating agents for the instigation and progression of disease. Alzheimer's disease and other cerebral proteopathies seem to arise from the de novo misfolding and sustained corruption of endogenous proteins, whereas prion diseases can also be infectious in origin. However, the outcome in all cases is the functional compromise of the nervous system, because the aggregated proteins gain a toxic function and/or lose their normal function. As a unifying pathogenic principle, the prion paradigm suggests broadly relevant therapeutic directions for a large class of currently intractable diseases.

Chaperone-mediated autophagy degrades mutant p53

Missense mutations in the gene TP53, which encodes p53, one of the most important tumor suppressors, are common in human cancers. Accumulated mutant p53 proteins are known to actively contribute to tumor development and metastasis. Thus, promoting the removal of mutant p53 proteins in cancer cells may have therapeutic significance. Here we investigated the mechanisms that govern the turnover of mutant p53 in nonproliferating tumor cells using a combination of pharmacological and genetic approaches. We show that suppression of macroautophagy by multiple means promotes the degradation of mutant p53 through chaperone-mediated autophagy in a lysosome-dependent fashion. In addition, depletion of mutant p53 expression due to macroautophagy inhibition sensitizes the death of dormant cancer cells under nonproliferating conditions. Taken together, our results delineate a novel strategy for killing tumor cells that depend on mutant p53 expression by the activation of chaperone-mediated autophagy and potential pharmacological means to reduce the levels of accumulated mutant p53 without the restriction of mutant p53 conformation in quiescent tumor cells.

Molten globule of hemoglobin proceeds into aggregates and advanced glycated end products

Conformational alterations of bovine hemoglobin (Hb) upon sequential addition of glyoxal over a range of 0–90% v/v were investigated. At 20% v/v glyoxal, molten globule (MG) state of Hb was observed by altered tryptophan fluorescence, high ANS binding, existence of intact heme, native-like secondary structure as depicted by far-UV circular dichroism (CD) and ATR-FTIR spectra as well as loss in tertiary structure as confirmed by near-UV CD spectra. In addition, size exclusion chromatography analysis depicted that MG state at 20% v/v glyoxal corresponded to expanded pre-dissociated dimers. Aggregates of Hb were detected at 70% v/v glyoxal. These aggregates of Hb had altered tryptophan environment, low ANS binding, exposed heme, increased β-sheet secondary structure, loss in tertiary structure, enhanced thioflavin T (ThT) fluorescence and red shifted Congo Red (CR) absorbance. On incubating Hb with 30% v/v glyoxal for 0–20 days, advanced glycation end products (AGEs) were detected on day 20. These AGEs were characterised by enhanced tryptophan fluorescence at 450 nm, exposure of heme, increase in intermolecular β-sheets, enhanced ThT fluorescence and red shift in CR absorbance. Comet assay revealed aggregates and AGEs to be genotoxic in nature. Scanning electron microscopy confirmed the amorphous structure of aggregates and branched fibrils of AGEs. The transformation of α-helix to β-sheet usually alters the normal protein to amyloidogenic resulting in a variety of protein conformational disorders such as diabetes, prion and Huntington's.

Expanding the prion concept to cancer biology: dominant-negative effect of aggregates of mutant p53 tumour suppressor

p53 is a key protein that participates in cell-cycle control, and its malfunction can lead to cancer. This tumour suppressor protein has three main domains; the N-terminal transactivation domain, the CTD (C-terminal domain) and the core domain (p53C) that constitutes the sequence-specific DBD (DNA-binding region). Most p53 mutations related to cancer development are found in the DBD. Aggregation of p53 into amyloid oligomers and fibrils has been shown. Moreover, amyloid aggregates of both the mutant and WT (wild-type) forms of p53 were detected in tumour tissues. We propose that if p53 aggregation occurred, it would be a crucial aspect of cancer development, as p53 would lose its WT functions in an aggregated state. Mutant p53 can also exert a dominant-negative regulatory effect on WT p53. Herein, we discuss the dominant-negative effect in light of p53 aggregation and the fact that amyloid-like mutant p53 can convert WT p53 into more aggregated species, leading into gain of function in addition to the loss of tumour suppressor function. In summary, the results obtained in the last decade indicate that cancer may have characteristics in common with amyloidogenic and prion diseases.

p53 Aggregates penetrate cells and induce the co-aggregation of intracellular p53

Prion diseases are unique pathologies in which the infectious particles are prions, a protein aggregate. The prion protein has many particular features, such as spontaneous aggregation, conformation transmission to other native PrP proteins and transmission from an individual to another. Protein aggregation is now frequently associated to many human diseases, for example Alzheimer’s disease, Parkinson’s disease or type 2 diabetes. A few proteins associated to these conformational diseases are part of a new category of proteins, called prionoids: proteins that share some, but not all, of the characteristics associated with prions. The p53 protein, a transcription factor that plays a major role in cancer, has recently been suggested to be a possible prionoid. The protein has been shown to accumulate in multiple cancer cell types, and its aggregation has also been reproduced in vitro by many independent groups. These observations suggest a role for p53 aggregates in cancer development. This study aims to test the «prion-like» features of p53. Our results show in vitro aggregation of the full length and N-terminally truncated protein (p53C), and penetration of these aggregates into cells. According to our findings, the aggregates enter cells using macropinocytosis, a non-specific pathway of entry. Lastly, we also show that once internalized by the cell, p53C aggregates can co-aggregate with endogenous p53 protein. Together, these findings suggest prion-like characteristics for p53 protein, based on the fact that p53 can spontaneously aggregate, these aggregates can penetrate cells and co-aggregate with cellular p53.

Interaction of p53 with the CCT complex promotes protein folding and wild-type p53 activity

p53 is a transcription factor that mediates tumor suppressor responses. Correct folding of the p53 protein is essential for these activities, and point mutations that induce conformational instability of p53 are frequently found in cancers. These mutant p53s not only lose wild-type activity but can also acquire the ability to promote invasion and metastasis. We show that folding of wild-type p53 is promoted by an interaction with the chaperonin CCT. Depletion of this chaperone in cells results in the accumulation of misfolded p53, leading to a reduction in p53-dependent gene expression. Intriguingly, p53 proteins mutated to prevent the interaction with CCT show conformational instability and acquire an ability to promote invasion and random motility that is similar to the activity of tumor-derived p53 mutants. Our data therefore suggest that both growth suppression and cell invasion may be differentially regulated functions of wild-type p53.

Folding of a cyclin box: linking multitarget binding to marginal stability, oligomerization, and aggregation of the retinoblastoma tumor suppressor AB pocket domain

The retinoblastoma tumor suppressor (Rb) controls the proliferation, differentiation, and survival of cells in most eukaryotes with a role in the fate of stem cells. Its inactivation by mutation or oncogenic viruses is required for cellular transformation and eventually carcinogenesis. The high conservation of the Rb cyclin fold prompted us to investigate the link between conformational stability and ligand binding properties of the RbAB pocket domain. RbAB unfolding presents a three-state transition involving cooperative secondary and tertiary structure changes and a partially folded intermediate that can oligomerize. The first transition corresponds to unfolding of the metastable B subdomain containing the binding site for the LXCXE motif present in cellular and viral targets, and the second transition corresponds to the stable A subdomain. The low thermodynamic stability of RbAB translates into a propensity to rapidly oligomerize and aggregate at 37 °C (T50 = 28 min) that is suppressed by human papillomavirus E7 and E2F peptide ligands, suggesting that Rb is likely stabilized in vivo through binding to target proteins. We propose that marginal stability and associated oligomerization may be conserved for function as a "hub" protein, allowing the formation of multiprotein complexes, which could constitute a robust mechanism to retain its cell cycle regulatory role throughout evolution. Decreased stability and oligomerization are shared with the p53 tumor suppressor, suggesting a link between folding and function in these two essential cell regulators that are inactivated in most cancers and operate within multitarget signaling pathways.

p53 mutations in cancer

In the past fifteen years, it has become apparent that tumour-associated p53 mutations can provoke activities that are different to those resulting from simply loss of wild-type tumour-suppressing p53 function. Many of these mutant p53 proteins acquire oncogenic properties that enable them to promote invasion, metastasis, proliferation and cell survival. Here we highlight some of the emerging molecular mechanisms through which mutant p53 proteins can exert these oncogenic functions.

Live-cell imaging of p53 interactions using a novel Venus-based bimolecular fluorescence complementation system

p53 plays an important role in regulating a wide variety of cellular processes, such as cell cycle arrest and/or apoptosis. Dysfunction of p53 is frequently associated with several pathologies, such as cancer and neurodegenerative diseases. In recent years substantial progress has been made in developing novel p53-activating molecules. Importantly, modulation of p53 interaction with its main inhibitor, Mdm2, has been highlighted as a promising therapeutic target. In this regard, bimolecular fluorescence complementation (BiFC) analysis, by providing direct visualization of protein interactions in living cells, offers a straightforward method to identify potential modulators of protein interactions. In this study, we developed a simple and robust Venus-based BiFC system to screen for modulators of p53-p53 and p53-Mdm2 interactions in live mammalian cells. We used nutlin-3, a well-known disruptor of p53-Mdm2 interaction, to validate the specificity of the assay. The reduction of BiFC signal mediated by nutlin-3 was correlated with an increase in Puma transactivation, PARP cleavage, and cell death. Finally, this novel BiFC approach was exploited to identify potential modulators of p53-Mdm2 complex formation among a commercially available chemical library of 33 protein phosphatase inhibitors. Our results constitute "proof-of-concept" that this model has strong potential as an alternative to traditional target-based drug discovery strategies. Identification of new modulators of p53-p53 and p53-Mdm2 interactions will be useful to achieve synergistic drug efficacy with currently used anti-tumor therapies.

Dual role of p53 amyloid formation in cancer; loss of function and gain of toxicity

The tumor suppressor p53 plays an important role in genome integrity. It is frequently mutated in all types of human cancers, making p53 a key factor in cancer progression. Two phenotypic consequences of these alterations are dominant; a loss of function and a gain of function of p53, which, in several cases, accumulates in intracellular aggregates. Although the nature of such aggregates is still unclear, recent evidence indicates that p53 can undergo conformational transitions leading to amyloid formation. Amyloid diseases, such as, Alzheimer's disease, are characterized by the accumulation of insoluble aggregates displaying the fibrillar conformation. We decided to investigate the propensity of wild type p53 to aggregate and its consequent assembly into different amyloid species, such as oligomers and fibrils; and to determine if these changes in conformation lead to a loss of function of p53. Furthermore, we analyzed cases of Basal Cell Carcinoma (BCC), for the presence of p53 amyloids. Here, we show that p53 forms amyloid oligomers and fibrils, which coincide with p53 inability of binding to DNA consensus sequences. Both p53 amyloid oligomers and fibrils were detected in BCC cancer samples. Additionally, we demonstrate that p53 oligomers are the most cytotoxic to human cell cultures. Our study reveals p53 amyloid formation and demonstrates its dual role in the pathogenesis of cancer by producing a loss of protein function and a gain of toxic function, extensively described in several amyloidogenic diseases. Our results suggest that under certain circumstances, cancer could be considered a protein-conformation disease.

Transient transfection of a wild-type p53 gene triggers resveratrol-induced apoptosis in cancer cells

Resveratrol is a promising chemopreventive agent that mediates many cellular targets involved in cancer signaling pathways. p53 has been suggested to play a role in the anticancer properties of resveratrol. We investigated resveratrol-induced cytotoxicity in H1299 cells, which are non-small lung cancer cells that have a partial deletion of the gene that encodes the p53 protein. The results for H1299 cells were compared with those for three cell lines that constitutively express wild-type p53: breast cancer MCF-7, adenocarcinomic alveolar basal epithelia A549 and non-small lung cancer H460. Cell viability assays revealed that resveratrol reduced the viability of all four of these cell lines in a dose- and time-dependent manner. MCF-7, A549 and H460 cells were more sensitive to resveratrol than were H1299 cells when exposed to the drug for 24 h at concentrations above 100 µM. Resveratrol also increased the p53 protein levels in MCF-7 cells without altering the p53 mRNA levels, suggesting a post-translational modulation of the protein. The resveratrol-induced cytotoxicity in these cells was partially mediated by p53 and involved the activation of caspases 9 and 7 and the cleavage of PARP. In H1299 cells, resveratrol-induced cytotoxicity was less pronounced and (in contrast to MCF-7 cells) cell death was not accompanied by caspase activation. These findings are consistent with the observation that MCF-7 cells were positively labeled by TUNEL following exposure to 100 µM resveratrol whereas H1299 cells under similar conditions were not labeled by TUNEL. The transient transfection of a wild-type p53-GFP gene caused H1299 cells to become more responsive to the pro-apoptotic properties of resveratrol, similarly to findings in the p53-positive MCF-7 cells. Our results suggest a possible therapeutic strategy based on the use of resveratrol for the treatment of tumors that are typically unresponsive to conventional therapies because of the loss of normal p53 function.

Stability of p53 homologs

Most proteins have not evolved for maximal thermal stability. Some are only marginally stable, as for example, the DNA-binding domains of p53 and its homologs, whose kinetic and thermodynamic stabilities are strongly correlated. Here, we applied high-throughput methods using a real-time PCR thermocycler to study the stability of several full-length orthologs and paralogs of the p53 family of transcription factors, which have diverse functions, ranging from tumour suppression to control of developmental processes. From isothermal denaturation fluorimetry and differential scanning fluorimetry, we found that full-length proteins showed the same correlation between kinetic and thermodynamic stability as their isolated DNA-binding domains. The stabilities of the full-length p53 orthologs were marginal and correlated with the temperature of their organism, paralleling the stability of the isolated DNA-binding domains. Additionally, the paralogs p63 and p73 were significantly more stable and long-lived than p53. The short half-life of p53 orthologs and the greater persistence of the paralogs may be biologically relevant.

S100A6 amyloid fibril formation is calcium-modulated and enhances superoxide dismutase-1 (SOD1) aggregation

S100A6 is a small EF-hand calcium- and zinc-binding protein involved in the regulation of cell proliferation and cytoskeletal dynamics. It is overexpressed in neurodegenerative disorders and a proposed marker for Amyotrophic Lateral Sclerosis (ALS). Following recent reports of amyloid formation by S100 proteins, we investigated the aggregation properties of S100A6. Computational analysis using aggregation predictors Waltz and Zyggregator revealed increased propensity within S100A6 helices H(I) and H(IV). Subsequent analysis of Thioflavin-T binding kinetics under acidic conditions elicited a very fast process with no lag phase and extensive formation of aggregates and stacked fibrils as observed by electron microscopy. Ca(2+) exerted an inhibitory effect on the aggregation kinetics, which could be reverted upon chelation. An FT-IR investigation of the early conformational changes occurring under these conditions showed that Ca(2+) promotes anti-parallel β-sheet conformations that repress fibrillation. At pH 7, Ca(2+) rendered the fibril formation kinetics slower: time-resolved imaging showed that fibril formation is highly suppressed, with aggregates forming instead. In the absence of metals an extensive network of fibrils is formed. S100A6 oligomers, but not fibrils, were found to be cytotoxic, decreasing cell viability by up to 40%. This effect was not observed when the aggregates were formed in the presence of Ca(2+). Interestingly, native S1006 seeds SOD1 aggregation, shortening its nucleation process. This suggests a cross-talk between these two proteins involved in ALS. Overall, these results put forward novel roles for S100 proteins, whose metal-modulated aggregation propensity may be a key aspect in their physiology and function.

Kinetic mechanism of p53 oncogenic mutant aggregation and its inhibition

Aggregation of destabilized mutants of the tumor suppressor p53 is a major route for its loss of activity. In order to assay drugs that inhibit aggregation of p53, we established the basic kinetics of aggregation of its core domain, using the mutant Y220C that has a mutation-induced, druggable cavity. Aggregation monitored by light scattering followed lag kinetics. Electron microscopy revealed the formation of small aggregates that subsequently grew to larger amorphous aggregates. The kinetics of aggregation produced surprising results: progress curves followed either by the binding of Thioflavin T or the fluorescence of the protein at 340 nm fitted well to simple two-step sequential first-order lag kinetics with rate constants k(1) and k(2) that were independent of protein concentration, and not to classical nucleation-growth. We suggest a mechanism of first-order formation of an aggregation competent state as being rate determining followed by rapid polymerization with the higher order kinetics. By measuring the inhibition kinetics of k(1) and k(2), we resolved that the process with the higher rate constant followed that of the lower. Further, there was only partial inhibition of k(1) and k(2), which showed two parallel pathways of aggregation, one via a state that requires unfolding of the protein and the other of partial unfolding with the ligand still bound. Inhibition kinetics of ligands provides a useful tool for probing an aggregation mechanism.