Insights into the structure and protein-protein interactions of the pro-apoptotic protein ASPP2

Gill physiological and transcriptomic response of the threatened freshwater mussel Solenaia oleivora to salinity shift

Solenaia oleivora, a freshwater shellfish endemic to China, is becoming one of the most threatened freshwater mussels owing to water pollution, habitat fragmentation, and overfishing. Hence, exploring its response to different environmental factors is important for its conservation. In this work, we investigated the physiological and transcriptomic response of S. oleivora to increased salinity. We found that increased salinity caused the death of S. oleivora. High salinity caused shrinking and deformation of gill filaments, reduced gill cilia, and induced cell apoptosis in gills. The activities of superoxide dismutase (SOD), catalase (CAT), acid phosphatase (ACP), alkaline phosphatase (AKP), as well as glutathione (GSH) content were increased at the beginning of salinity stress (3-12 h), while SOD and ACP activities decreased at 48 h. Transcriptome data revealed that high salinity stress (48 h) induced 766 differentially expressed genes (DEGs). Among these DEGs, the majority of the stress response and ion transport-related genes were up-regulated, while most of the immune-related genes were down-regulated. In conclusion, these findings suggest that the antioxidant and immune functions of S. oleivora can be inhibited by high salinity, which may be one of the main reasons for its low survival rate under conditions of increasing salinity.

Phosphorylation and dephosphorylation of Ser852 and Ser889 control the clustering, localization and function of PAR3

Cell polarity is essential for various asymmetric cellular events, and the partitioning defective (PAR) protein PAR3 (encoded by PARD3 in mammals) plays a unique role as a cellular landmark to establish polarity. In epithelial cells, PAR3 localizes at the subapical border, such as the tight junction in vertebrates, and functions as an apical determinant. Although we know a great deal about the regulators of PAR3 localization, how PAR3 is concentrated and localized to a specific membrane domain remains an important question to be clarified. In this study, we demonstrate that ASPP2 (also known as TP53BP2), which controls PAR3 localization, links PAR3 and protein phosphatase 1 (PP1). The ASPP2-PP1 complex dephosphorylates a novel phosphorylation site, Ser852, of PAR3. Furthermore, Ser852- or Ser889-unphosphorylatable PAR3 mutants form protein clusters, and ectopically localize to the lateral membrane. Concomitance of clustering and ectopic localization suggests that PAR3 localization is a consequence of local clustering. We also demonstrate that unphosphorylatable forms of PAR3 exhibited a low molecular turnover and failed to coordinate rapid reconstruction of the tight junction, supporting that both the phosphorylated and dephosphorylated states are essential for the functional integrity of PAR3.

Alternative splicing of the tumor suppressor ASPP2 results in a stress-inducible, oncogenic isoform prevalent in acute leukemia

Background

Apoptosis-stimulating Protein of TP53-2 (ASPP2) is a tumor suppressor enhancing TP53-mediated apoptosis via binding to the TP53 core domain. TP53 mutations found in cancers disrupt ASPP2 binding, arguing for an important role of ASPP2 in TP53-mediated tumor suppression. We now identify an oncogenic splicing variant, ASPP2κ, with high prevalence in acute leukemia.

Methods

An mRNA screen to detect ASPP2 splicing variants was performed and ASPP2κ was validated using isoform-specific PCR approaches. Translation into a genuine protein isoform was evaluated after establishing epitope-specific antibodies. For functional studies cell models with forced expression of ASPP2κ or isoform-specific ASPP2κ-interference were created to evaluate proliferative, apoptotic and oncogenic characteristics of ASPP2κ.

Findings

Exon skipping generates a premature stop codon, leading to a truncated C-terminus, omitting the TP53-binding sites. ASPP2κ translates into a dominant-negative protein variant impairing TP53-dependent induction of apoptosis. ASPP2κ is expressed in CD34+ leukemic progenitor cells and functional studies argue for a role in early oncogenesis, resulting in perturbed proliferation and impaired induction of apoptosis, mitotic failure and chromosomal instability (CIN) – similar to TP53 mutations.

Importantly, as expression of ASPP2κ is stress-inducible it defines a novel class of dynamic oncogenes not represented by genomic mutations.

Interpretation

Our data demonstrates that ASPP2κ plays a distinctive role as an antiapoptotic regulator of the TP53 checkpoint, rendering cells to a more aggressive phenotype as evidenced by proliferation and apoptosis rates – and ASPP2κ expression results in acquisition of genomic mutations, a first initiating step in leukemogenesis. We provide proof-of-concept to establish ASPP2κ as a clinically relevant biomarker and a target for molecule-defined therapy.

Fund

Unrestricted grant support from the Wilhelm Sander Foundation for Cancer Research, the IZKF Program of the Medical Faculty Tübingen, the Brigitte Schlieben-Lange Program and the Margarete von Wrangell Program of the State Ministry Baden-Wuerttemberg for Science, Research and Arts and the Athene Program of the excellence initiative of the Eberhard-Karls University, Tübingen.

The roles of intrinsic disorder-based liquid-liquid phase transitions in the "Dr. Jekyll-Mr. Hyde" behavior of proteins involved in amyotrophic lateral sclerosis and frontotemporal lobar degeneration

Pathological developments leading to amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are associated with misbehavior of several key proteins, such as SOD1 (superoxide dismutase 1), TARDBP/TDP-43, FUS, C9orf72, and dipeptide repeat proteins generated as a result of the translation of the intronic hexanucleotide expansions in the C9orf72 gene, PFN1 (profilin 1), GLE1 (GLE1, RNA export mediator), PURA (purine rich element binding protein A), FLCN (folliculin), RBM45 (RNA binding motif protein 45), SS18L1/CREST, HNRNPA1 (heterogeneous nuclear ribonucleoprotein A1), HNRNPA2B1 (heterogeneous nuclear ribonucleoprotein A2/B1), ATXN2 (ataxin 2), MAPT (microtubule associated protein tau), and TIA1 (TIA1 cytotoxic granule associated RNA binding protein). Although these proteins are structurally and functionally different and have rather different pathological functions, they all possess some levels of intrinsic disorder and are either directly engaged in or are at least related to the physiological liquid-liquid phase transitions (LLPTs) leading to the formation of various proteinaceous membrane-less organelles (PMLOs), both normal and pathological. This review describes the normal and pathological functions of these ALS- and FTLD-related proteins, describes their major structural properties, glances at their intrinsic disorder status, and analyzes the involvement of these proteins in the formation of normal and pathological PMLOs, with the ultimate goal of better understanding the roles of LLPTs and intrinsic disorder in the "Dr. Jekyll-Mr. Hyde" behavior of those proteins.

Wrecked regulation of intrinsically disordered proteins in diseases: pathogenicity of deregulated regulators

Biologically active proteins without stable tertiary structure are common in all known proteomes. Functions of these intrinsically disordered proteins (IDPs) are typically related to regulation, signaling, and control. Cellular levels of these important regulators are tightly regulated by a variety mechanisms ranging from firmly controlled expression to precisely targeted degradation. Functions of IDPs are controlled by binding to specific partners, alternative splicing, and posttranslational modifications among other means. In the norm, right amounts of precisely activated IDPs have to be present in right time at right places. Wrecked regulation brings havoc to the ordered world of disordered proteins, leading to protein misfolding, misidentification, and missignaling that give rise to numerous human diseases, such as cancer, cardiovascular disease, neurodegenerative diseases, and diabetes. Among factors inducing pathogenic transformations of IDPs are various cellular mechanisms, such as chromosomal translocations, damaged splicing, altered expression, frustrated posttranslational modifications, aberrant proteolytic degradation, and defective trafficking. This review presents some of the aspects of deregulated regulation of IDPs leading to human diseases.

Structure of the Helicobacter pylori CagA oncoprotein bound to the human tumor suppressor ASPP2

The Cytotoxin associated gene A (CagA) protein of Helicobacter pylori is associated with increased virulence and risk of cancer. Recent proteomic studies have demonstrated an association of CagA with the human tumor suppressor Apoptosis-stimulating Protein of p53-2 (ASPP2). We present here a genetic, biochemical, and structural analysis of CagA with ASPP2. Domain delineation of the 120-kDa CagA protein revealed a stable N-terminal subdomain that was used in a yeast two-hybrid screen that identified the proline-rich domain of ASPP2 as a host cellular target. Biochemical experiments confirm this interaction. The cocrystal structure to 2.0-Å resolution of this N-terminal subdomain of CagA with a 7-kDa proline-rich sequence of ASPP2 reveals that this domain of CagA forms a highly specialized three-helix bundle, with large insertions in the loops connecting the helices. These insertions come together to form a deep binding cleft for a highly conserved 20-aa peptide of ASPP2. ASPP2 forms an extended helix in this groove of CagA, burying more than 1,000 Å(2) of surface area. This interaction is disrupted in vitro and in vivo by structure-based, loss-of-contact point mutations of key residues in either CagA or ASPP2. Disruption of CagA and ASPP2 binding alters the function of ASPP2 and leads to the decreased survival of H. pylori-infected cells.

The selective BH4-domain biology of Bcl-2-family members: IP3Rs and beyond

Anti-apoptotic Bcl-2-family members not only neutralize pro-apoptotic proteins but also directly regulate intracellular Ca(2+) signaling from the endoplasmic reticulum (ER), critically controlling cellular health, survival, and death initiation. Furthermore, distinct Bcl-2-family members may selectively regulate inositol 1,4,5-trisphosphate receptor (IP3R): Bcl-2 likely acts as an endogenous inhibitor of the IP3R, preventing pro-apoptotic Ca(2+) transients, while Bcl-XL likely acts as an endogenous IP3R-sensitizing protein promoting pro-survival Ca(2+) oscillations. Furthermore, distinct functional domains in Bcl-2 and Bcl-XL may underlie the divergence in IP3R regulation. The Bcl-2 homology (BH) 4 domain, which targets the central modulatory domain of the IP3R, is likely to be Bcl-2's determining factor. In contrast, the hydrophobic cleft targets the C-terminal Ca(2+)-channel tail and might be more crucial for Bcl-XL's function. Furthermore, one amino acid critically different in the sequence of Bcl-2's and Bcl-XL's BH4 domains underpins their selective effect on Ca(2+) signaling and distinct biological properties of Bcl-2 versus Bcl-XL. This difference is evolutionary conserved across five classes of vertebrates and may represent a fundamental divergence in their biological function. Moreover, these insights open novel avenues to selectively suppress malignant Bcl-2 function in cancer cells by targeting its BH4 domain, while maintaining essential Bcl-XL functions in normal cells. Thus, IP3R-derived molecules that mimic the BH4 domain's binding site on the IP3R may function synergistically with BH3-mimetic molecules selectivity suppressing Bcl-2's proto-oncogenic activity. Finally, a more general role for the BH4 domain on IP3Rs, rather than solely anti-apoptotic, may not be excluded as part of a complex network of molecular interactions.

Regulation of ASPP2 interaction with p53 core domain by an intramolecular autoinhibitory mechanism

ASPP2 is a key protein in regulating apoptosis both in p53-dependent and-independent pathways. The C-terminal part of ASPP2 contains four ankyrin repeats and an SH3 domain (Ank-SH3) that mediate the interactions of ASPP2 with apoptosis related proteins such as p53, Bcl-2 and the p65 subunit of NFκB. p53 core domain (p53CD) binds the n-src loop and the RT loop of ASPP2 SH3. ASPP2 contains a disordered proline rich domain (ASPP2 Pro) that forms an intramolecular autoinhibitory interaction with the Ank-SH3 domains. Here we show how this intramolecular interaction affects the intermolecular interactions of ASPP2 with p53, Bcl-2 and NFkB. We used biophysical methods to obtain better understanding of the relationship between ASPP2 and its partners for getting a comprehensive view on ASPP2 pathways. Fluorescence anisotropy competition experiments revealed that both ASPP2 Pro and p53CD competed for binding the n-src loop of the ASPP2 SH3, indicating regulation of p53CD binding to this loop by ASPP2 Pro. Peptides derived from the ASPP2-binding interface of Bcl-2 did not compete with p53CD or NFkB peptides for binding the ASPP2 n-src loop. However, p53CD displaced the NFκB peptide (residues 303–332) from its complex with ASPP2 Ank-SH3, indicating that NFκB 303–332 and p53CD bind a partly overlapping site in ASPP2 SH3, mostly in the RT loop. These results are in agreement with previous docking studies, which showed that ASPP2 Ank-SH3 binds Bcl-2 and NFκB mostly via distinct sites from p53. However they show some overlap between the binding sites of p53CD and NFkB in ASPP2 Ank-SH3. Our results provide experimental evidence that the intramolecular interaction in ASPP2 regulates its binding to p53CD and that ASPP2 Ank-SH3 binds Bcl-2 and NFκB via distinct sites.

N terminus of ASPP2 binds to Ras and enhances Ras/Raf/MEK/ERK activation to promote oncogene-induced senescence

The ASPP2 (also known as 53BP2L) tumor suppressor is a proapoptotic member of a family of p53 binding proteins that functions in part by enhancing p53-dependent apoptosis via its C-terminal p53-binding domain. Mounting evidence also suggests that ASPP2 harbors important nonapoptotic p53-independent functions. Structural studies identify a small G protein Ras-association domain in the ASPP2 N terminus. Because Ras-induced senescence is a barrier to tumor formation in normal cells, we investigated whether ASPP2 could bind Ras and stimulate the protein kinase Raf/MEK/ERK signaling cascade. We now show that ASPP2 binds to Ras-GTP at the plasma membrane and stimulates Ras-induced signaling and pERK1/2 levels via promoting Ras-GTP loading, B-Raf/C-Raf dimerization, and C-Raf phosphorylation. These functions require the ASPP2 N terminus because BBP (also known as 53BP2S), an alternatively spliced ASPP2 isoform lacking the N terminus, was defective in binding Ras-GTP and stimulating Raf/MEK/ERK signaling. Decreased ASPP2 levels attenuated H-RasV12-induced senescence in normal human fibroblasts and neonatal human epidermal keratinocytes. Together, our results reveal a mechanism for ASPP2 tumor suppressor function via direct interaction with Ras-GTP to stimulate Ras-induced senescence in nontransformed human cells.

The ASPP interaction network: electrostatic differentiation between pro- and anti-apoptotic proteins

The ASPP proteins are apoptosis regulators: ASPP1 and ASPP2 promote, while iASPP inhibits, apoptosis. The mechanism by which these different outcomes are achieved is still unknown. The C-terminal ankyrin repeats and SH3 domain (ANK-SH3) mediate the interactions of the ASPP proteins with major apoptosis regulators such as p53, Bcl-2, and NFκB. The structure of the complex between ASPP2(ANK-SH3) and the core domain of p53 (p53CD) was previously determined. We have recently characterized the individual interactions of ASPP2(ANK-SH3) with Bcl-2 and NFκB, as well as a regulatory intramolecular interaction with the proline rich domain of ASPP2. Here we compared the ASPP interactions at two levels: ASPP2(ANK-SH3) with different proteins, and different ASPP family members with each protein partner. We found that the binding sites of ASPP2 to p53CD, Bcl-2, and NFκB are different, yet lie on the same face of ASPP2(ANK-SH3) . The intramolecular binding site to the proline rich domain overlaps the three intermolecular binding sites. To reveal the basis of functional diversity in the ASPP family, we compared their protein-binding domains. A subset of surface-exposed residues differentiates ASPP1 and ASPP2 from iASPP: ASPP1/2 are more negatively charged in specific residues that contact positively charged residues of p53CD, Bcl-2, and NFκB. We also found a gain of positive charge at the non-protein binding face of ASPP1/2, suggesting a role in electrostatic direction towards the negatively charged protein binding face. The electrostatic differences in binding interfaces between the ASPP proteins may be one of the causes for their different function.

From peptides to proteins: lessons from my years at the Centre for Protein Engineering

The MRC Centre for Protein Engineering (CPE) hosted and trained many scientists over the years. It is a unique research environment that shaped the career of many scientists in all aspects. These include research directions and methodologies, but even more important--issues such as how to approach scientific problems and how to manage a research team. Alan Fersht was the director of the CPE when I joined it as a postdoc in the year 2000. In the current article for the PEDS special CPE issue, I will review how my scientific research and my approach to science developed from the days I arrived to the CPE as a young peptide chemist and throughout the years I spent at the CPE, and how it shaped my current research interests and attitude. I will focus on two major fields: (i) Using peptides to study and modulate the structure and interactions of proteins; (ii) Using quantitative biophysical methods to study proteins and their interactions at the molecular level.

Molecular basis of the interaction between the antiapoptotic Bcl-2 family proteins and the proapoptotic protein ASPP2

We have characterized the molecular basis of the interaction between ASPP2 and Bcl-2, which are key proteins in the apoptotic pathway. The C-terminal ankyrin repeats and SH3 domain of ASPP2 (ASPP2(Ank-SH3)) mediate its interactions with the antiapoptotic protein Bcl-2. We used biophysical and computational methods to identify the interaction sites of Bcl-2 and its homologues with ASPP2. Using peptide array screening, we found that ASPP2(Ank-SH3) binds two homologous sites in all three Bcl proteins tested: (i) the conserved BH4 motif, and (ii) a binding site for proapoptotic regulators. Quantitative binding studies revealed that binding of ASPP2(Ank-SH3) to the Bcl-2 family members is selective at two levels: (i) interaction with Bcl-2-derived peptides is the tightest compared to peptides from the other family members, and (ii) within Bcl-2, binding of ASPP2(Ank-SH3) to the BH4 domain is tightest. Sequence alignment of the ASPP2-binding peptides combined with binding studies of mutated peptides revealed that two nonconserved positions where only Bcl-2 contains positively charged residues account for its tighter binding. The experimental binding results served as a basis for docking analysis, by which we modeled the complexes of ASPP2(Ank-SH3) with the full-length Bcl proteins. Using peptide arrays and quantitative binding studies, we found that Bcl-2 binds three loops in ASPP2(Ank-SH3) with similar affinity, in agreement with our predicted model. Based on our results, we propose a mechanism in which ASPP2 induces apoptosis by inhibiting functional sites of the antiapoptotic Bcl-2 proteins.