Dual Specificity PDZ- and 14-3-3-Binding Motifs: A Structural and Interactomics Study

Holdup Multiplex Assay for High-Throughput Measurement of Protein-Ligand Affinity Constants Using a Mass Spectrometry Readout

The accurate description and subsequent modeling of protein interactomes require quantification of their affinities at the proteome-wide scale. Here we develop and validate the Holdup Multiplex, a versatile assay with a mass spectrometry (MS) readout for profiling the affinities of a protein for large pools of peptides. The method can precisely quantify, in one single run, thousands of affinity constants over several orders of magnitude. The throughput, dynamic range, and sensitivity can be pushed to the performance limit of the MS readout. We applied the Holdup Multiplex to quantify in a few sample runs the affinities of the 14-3-3s, phosphoreader proteins highly abundant in humans, for 1000 different phosphopeptides. The seven human 14-3-3 isoforms were found to display similar specificities but staggered affinities, with 14-3-3γ being always the best binder and 14-3-3ε and σ being the weakest. Hundreds of new 14-3-3 binding sites were identified. We also identified dozens of 14-3-3 binding sites, some intervening in key signaling pathways, that were either stabilized or destabilized by the phytotoxin Fusicoccin-A. The results were corroborated by X-ray crystallography. Finally, we demonstrated the transferability of the Holdup Multiplex by quantifying the interactions of a PDZ domain for 5400 PBM peptides at once. The approach is applicable to any category of protein-binding ligands that can be quantifiable by mass spectrometry.

Structural biology of the human papillomavirus

Human papillomavirus (HPV), known for its oncogenic properties, is the primary cause of cervical cancer and significantly contributes to mortality rates. It also plays a considerable role in the globally rising incidences of head and neck cancers. These cancers pose a substantial health burden worldwide. Current limitations in diagnostic and treatment strategies, along with inadequate coverage of preventive vaccines in low- and middle-income countries, hinder the progress toward the World Health Organization (WHO) HPV prevention and control targets set for 2030. In response to these challenges, extensive research in structural virology has explored the properties of HPV proteins, yielding crucial insights into the mechanisms of HPV infection that are important for the development of prevention and therapeutic strategies. This review highlights recent advances in understanding the structures of HPV proteins and discusses achievements and future opportunities for HPV vaccine development.

Uncovering the BIN1-SH3 interactome underpinning centronuclear myopathy

Truncation of the protein-protein interaction SH3 domain of the membrane remodeling Bridging Integrator 1 (BIN1, Amphiphysin 2) protein leads to centronuclear myopathy. Here, we assessed the impact of a set of naturally observed, previously uncharacterized BIN1 SH3 domain variants using conventional in vitro and cell-based assays monitoring the BIN1 interaction with dynamin 2 (DNM2) and identified potentially harmful ones that can be also tentatively connected to neuromuscular disorders. However, SH3 domains are typically promiscuous and it is expected that other, so far unknown partners of BIN1 exist besides DNM2, that also participate in the development of centronuclear myopathy. In order to shed light on these other relevant interaction partners and to get a holistic picture of the pathomechanism behind BIN1 SH3 domain variants, we used affinity interactomics. We identified hundreds of new BIN1 interaction partners proteome-wide, among which many appear to participate in cell division, suggesting a critical role of BIN1 in the regulation of mitosis. Finally, we show that the identified BIN1 mutations indeed cause proteome-wide affinity perturbation, signifying the importance of employing unbiased affinity interactomic approaches.

Kinesin Regulation in the Proximal Axon is Essential for Dendrite-selective Transport

Neurons are polarized and typically extend multiple dendrites and one axon. To maintain polarity, vesicles carrying dendritic proteins are arrested upon entering the axon. To determine whether kinesin regulation is required for terminating anterograde axonal transport, we overexpressed the dendrite-selective kinesin KIF13A. This caused mistargeting of dendrite-selective vesicles to the axon and a loss of dendritic polarity. Polarity was not disrupted if the kinase MARK2/Par1b was coexpressed. MARK2/Par1b is concentrated in the proximal axon, where it maintains dendritic polarity-likely by phosphorylating S1371 of KIF13A, which lies in a canonical 14-3-3 binding motif. We probed for interactions of KIF13A with 14-3-3 isoforms and found that 14-3-3β and 14-3-3ζ bound KIF13A. Disruption of MARK2 or 14-3-3 activity by small molecule inhibitors caused a loss of dendritic polarity. These data show that kinesin regulation is integral for dendrite-selective transport. We propose a new model in which KIF13A that moves dendrite-selective vesicles in the proximal axon is phosphorylated by MARK2. Phosphorylated KIF13A is then recognized by 14-3-3, which causes dissociation of KIF13A from the vesicle and termination of transport. These findings define a new paradigm for the regulation of vesicle transport by localized kinesin tail phosphorylation, to restrict dendrite-selective vesicles from entering the axon.

Genetic Encoding of Phosphorylated Amino Acids into Proteins

Reversible phosphorylation is a fundamental mechanism for controlling protein function. Despite the critical roles phosphorylated proteins play in physiology and disease, our ability to study individual phospho-proteoforms has been hindered by a lack of versatile methods to efficiently generate homogeneous proteins with site-specific phosphoamino acids or with functional mimics that are resistant to phosphatases. Genetic code expansion (GCE) is emerging as a transformative approach to tackle this challenge, allowing direct incorporation of phosphoamino acids into proteins during translation in response to amber stop codons. This genetic programming of phospho-protein synthesis eliminates the reliance on kinase-based or chemical semisynthesis approaches, making it broadly applicable to diverse phospho-proteoforms. In this comprehensive review, we provide a brief introduction to GCE and trace the development of existing GCE technologies for installing phosphoserine, phosphothreonine, phosphotyrosine, and their mimics, discussing both their advantages as well as their limitations. While some of the technologies are still early in their development, others are already robust enough to greatly expand the range of biologically relevant questions that can be addressed. We highlight new discoveries enabled by these GCE approaches, provide practical considerations for the application of technologies by non-GCE experts, and also identify avenues ripe for further development.

PDZome-wide and structural characterization of the PDZ-binding motif of VANGL2

VANGL2 is a core component of the non-canonical Wnt/Planar Cell Polarity signaling pathway that uses its highly conserved carboxy-terminal type 1 PDZ-binding motif (PBM) to bind a variety of PDZ proteins. In this study, we characterize and quantitatively assess the largest VANGL2 PDZome-binding profile documented so far, using orthogonal methods. The results of our holdup approach support VANGL2 interactions with a large panel of both long-recognized and unprecedented PDZ domains. Truncation and point mutation analyses of the VANGL2 PBM establish that, beyond the strict requirement of the P-0 / V521 and P-2 / T519 amino acids, upstream residues, including E518, Q516 and R514 at, respectively, P-3, P-5 and P-7 further contribute to the robustness of VANGL2 interactions with two distinct PDZ domains, SNX27 and SCRIBBLE-PDZ3. In agreement with these data, incremental amino-terminal deletions of the VANGL2 PBM causes its overall affinity to progressively decline. Moreover, the holdup data establish that the PDZome binding repertoire of VANGL2 starts to diverge significantly with the truncation of E518. A structural analysis of the SYNJ2BP-PDZ/VANGL2 interaction with truncated PBMs identifies a major conformational change in the binding direction of the PBM peptide after the P-2 position. Finally, we report that the PDZome binding profile of VANGL2 is dramatically rearranged upon phosphorylation of S517, T519 and S520. Our crystallographic approach illustrates how SYNJ2BP accommodates a S520-phosphorylated PBM peptide through the ideal positioning of two basic residues, K48 and R86. Altogether our data provides a comprehensive view of the VANGL2 PDZ network and how this network specifically responds to the post-translation modification of distinct PBM residues. These findings should prove useful in guiding future functional and molecular studies of the key PCP component VANGL2.

Pathogenic mutations of human phosphorylation sites affect protein-protein interactions

Despite their lack of a defined 3D structure, intrinsically disordered regions (IDRs) of proteins play important biological roles. Many IDRs contain short linear motifs (SLiMs) that mediate protein-protein interactions (PPIs), which can be regulated by post-translational modifications like phosphorylation. 20% of pathogenic missense mutations are found in IDRs, and understanding how such mutations affect PPIs is essential for unraveling disease mechanisms. Here, we employ peptide-based interaction proteomics to investigate 36 disease-associated mutations affecting phosphorylation sites. Our results unveil significant differences in interactomes between phosphorylated and non-phosphorylated peptides, often due to disrupted phosphorylation-dependent SLiMs. We focused on a mutation of a serine phosphorylation site in the transcription factor GATAD1, which causes dilated cardiomyopathy. We find that this phosphorylation site mediates interaction with 14-3-3 family proteins. Follow-up experiments reveal the structural basis of this interaction and suggest that 14-3-3 binding affects GATAD1 nucleocytoplasmic transport by masking a nuclear localisation signal. Our results demonstrate that pathogenic mutations of human phosphorylation sites can significantly impact protein-protein interactions, offering insights into potential molecular mechanisms underlying pathogenesis.

Detection and Analysis of Short Linear Motif-Based Protein-Protein Interactions with SLiMAn2 Web Server

Interactomics is bringing a deluge of data regarding protein-protein interactions (PPIs) which are involved in various molecular processes in all types of cells. However, this information does not easily translate into direct and precise molecular interfaces. This limits our understanding of each interaction network and prevents their efficient modulation. A lot of the detected interactions involve recognition of short linear motifs (SLiMs) by a folded domain while others rely on domain-domain interactions. Functional SLiMs hide among a lot of spurious ones, making deeper analysis of interactomes tedious. Hence, actual contacts and direct interactions are difficult to identify.Consequently, there is a need for user-friendly bioinformatic tools, enabling rapid molecular and structural analysis of SLiM-based PPIs in a protein network. In this chapter, we describe the use of the new webserver SLiMAn to help digging into SLiM-based PPIs in an interactive fashion.

Large-scale phosphomimetic screening identifies phospho-modulated motif-based protein interactions

Phosphorylation is a ubiquitous post-translation modification that regulates protein function by promoting, inhibiting or modulating protein-protein interactions. Hundreds of thousands of phosphosites have been identified but the vast majority have not been functionally characterised and it remains a challenge to decipher phosphorylation events modulating interactions. We generated a phosphomimetic proteomic peptide-phage display library to screen for phosphosites that modulate short linear motif-based interactions. The peptidome covers ~13,500 phospho-serine/threonine sites found in the intrinsically disordered regions of the human proteome. Each phosphosite is represented as wild-type and phosphomimetic variant. We screened 71 protein domains to identify 248 phosphosites that modulate motif-mediated interactions. Affinity measurements confirmed the phospho-modulation of 14 out of 18 tested interactions. We performed a detailed follow-up on a phospho-dependent interaction between clathrin and the mitotic spindle protein hepatoma-upregulated protein (HURP), demonstrating the essentiality of the phospho-dependency to the mitotic function of HURP. Structural characterisation of the clathrin-HURP complex elucidated the molecular basis for the phospho-dependency. Our work showcases the power of phosphomimetic ProP-PD to discover novel phospho-modulated interactions required for cellular function.

The next wave of interactomics: Mapping the SLiM-based interactions of the intrinsically disordered proteome

Short linear motifs (SLiMs) are a unique and ubiquitous class of protein interaction modules that perform key regulatory functions and drive dynamic complex formation. For decades, interactions mediated by SLiMs have accumulated through detailed low-throughput experiments. Recent methodological advances have opened this previously underexplored area of the human interactome to high-throughput protein-protein interaction discovery. In this article, we discuss that SLiM-based interactions represent a significant blind spot in the current interactomics data, introduce the key methods that are illuminating the elusive SLiM-mediated interactome of the human cell on a large scale, and discuss the implications for the field.

Additive energetic contributions of multiple peptide positions determine the relative promiscuity of viral and human sequences for PDZ domain targets

Protein-protein interactions that involve recognition of short peptides are critical in cellular processes. Protein-peptide interaction surface areas are relatively small and shallow, and there are often overlapping specificities in families of peptide-binding domains. Therefore, dissecting selectivity determinants can be challenging. PDZ domains are a family of peptide-binding domains located in several intracellular signaling and trafficking pathways. These domains are also directly targeted by pathogens, and a hallmark of many oncogenic viral proteins is a PDZ-binding motif. However, amidst sequences that target PDZ domains, there is a wide spectrum in relative promiscuity. For example, the viral HPV16 E6 oncoprotein recognizes over double the number of PDZ domain-containing proteins as the cystic fibrosis transmembrane conductance regulator (CFTR) in the cell, despite similar PDZ targeting-sequences and identical motif residues. Here, we determine binding affinities for PDZ domains known to bind either HPV16 E6 alone or both CFTR and HPV16 E6, using peptides matching WT and hybrid sequences. We also use energy minimization to model PDZ-peptide complexes and use sequence analyses to investigate this difference. We find that while the majority of single mutations had marginal effects on overall affinity, the additive effect on the free energy of binding accurately describes the selectivity observed. Taken together, our results describe how complex and differing PDZ interactomes can be programmed in the cell.

Additive energetic contributions of multiple peptide positions determine the relative promiscuity of viral and human sequences for PDZ domain targets

Protein-protein interactions that include recognition of short sequences of amino acids, or peptides, are critical in cellular processes. Protein-peptide interaction surface areas are relatively small and shallow, and there are often overlapping specificities in families of peptide-binding domains. Therefore, dissecting selectivity determinants can be challenging. PDZ domains are an example of a peptide-binding domain located in several intracellular signaling and trafficking pathways, which form interactions critical for the regulation of receptor endocytic trafficking, tight junction formation, organization of supramolecular complexes in neurons, and other biological systems. These domains are also directly targeted by pathogens, and a hallmark of many oncogenic viral proteins is a PDZ-binding motif. However, amidst sequences that target PDZ domains, there is a wide spectrum in relative promiscuity. For example, the viral HPV16 E6 oncoprotein recognizes over double the number of PDZ domain-containing proteins as the cystic fibrosis transmembrane conductance regulator (CFTR) in the cell, despite similar PDZ targeting-sequences and identical motif residues. Here, we determine binding affinities for PDZ domains known to bind either HPV16 E6 alone or both CFTR and HPV16 E6, using peptides matching WT and hybrid sequences. We also use energy minimization to model PDZ-peptide complexes and use sequence analyses to investigate this difference. We find that while the majority of single mutations had a marginal effect on overall affinity, the additive effect on the free energy of binding accurately describes the selectivity observed. Taken together, our results describe how complex and differing PDZ interactomes can be programmed in the cell.

Native holdup (nHU) to measure binding affinities from cell extracts

Characterizing macromolecular interactions is essential for understanding cellular processes, yet most methods currently used to detect protein interactions from cells are qualitative. Here, we introduce the native holdup (nHU) approach to estimate equilibrium binding constants of protein interactions directly from cell extracts. Compared to other pull-down-based assays, nHU requires less sample preparation and can be coupled to any analytical methods as readouts, such as Western blotting or mass spectrometry. We use nHU to explore interactions of SNX27, a cargo adaptor of the retromer complex and find good agreement between in vitro affinities and those measured directly from cell extracts using nHU. We discuss the strengths and limitations of nHU and provide simple protocols that can be implemented in most laboratories.

Quantitative fragmentomics allow affinity mapping of interactomes

Human protein networks have been widely explored but most binding affinities remain unknown, hindering quantitative interactome-function studies. Yet interactomes rely on minimal interacting fragments displaying quantifiable affinities. Here, we measure the affinities of 65,000 interactions involving PDZ domains and their target PDZ-binding motifs (PBM) within a human interactome region particularly relevant for viral infection and cancer. We calculate interactomic distances, identify hot spots for viral interference, generate binding profiles and specificity logos, and explain selected cases by crystallographic studies. Mass spectrometry experiments on cell extracts and literature surveys show that quantitative fragmentomics effectively complements protein interactomics by providing affinities and completeness of coverage, putting a full human interactome affinity survey within reach. Finally, we show that interactome hijacking by the viral PBM of human papillomavirus E6 oncoprotein substantially impacts the host cell proteome beyond immediate E6 binders, illustrating the complex system-wide relationship between interactome and function.

Protein networks have been widely explored but most binding affinities remain unknown, limiting the quantitative interpretation of interactomes. Here the authors measure affinities of 65,000 interactions involving human PDZ domains and target sequence motifs relevant for viral infection and cancer.

Structural basis for the recognition by 14-3-3 proteins of a conditional binding site within the oligomerization domain of human nucleophosmin

Nucleophosmin 1 (NPM1) is a multifunctional protein regulating ribosome biogenesis, centrosome duplication and chromatin remodeling. Being a major nucleolar protein, NPM1 can migrate to the nucleus and the cytoplasm, which is controlled by changes of NPM1 oligomerization and interaction with other cell factors. NPM1 forms a stable pentamer with its N-terminal structured domain, where two nuclear export signals and several phosphorylation sites reside. This domain undergoes dissociation and disordering upon Ser48 phosphorylation in the subunit interface. Recent studies indicated that Ser48 is important for NPM1 interaction with other proteins including 14-3-3, the well-known phosphoserine/phosphothreonine binders, but the structural basis for 14-3-3/NPM1 interaction remained unaddressed. By fusing human 14-3-3ζ with an NPM1 segment surrounding Ser48, which was phosphorylated inside Escherichia coli cells by co-expressed protein kinase A, here we obtained the desired protein/phosphopeptide complex and determined its crystal structure. While biochemical data indicated that the interaction is driven by Ser48 phosphorylation, the crystallographic 14-3-3/phosphopeptide interface reveals an NPM1 conformation distinctly different from that in the NPM1 pentamer. Given the canonical phosphopeptide-binding mode observed in our crystal structure, Ser48 emerges as a conditional binding site whose recognition by 14-3-3 proteins is enabled by NPM1 phosphorylation, disassembly and disordering under physiological circumstances.

Identification of HCN1 as a 14-3-3 client

Hyperpolarization activated cyclic nucleotide-gated channel 1 (HCN1) is expressed throughout the nervous system and is critical for regulating neuronal excitability, with mutations being associated with multiple forms of epilepsy. Adaptive modulation of HCN1 has been observed, as has pathogenic dysregulation. While the mechanisms underlying this modulation remain incompletely understood, regulation of HCN1 has been shown to include phosphorylation. A candidate phosphorylation-dependent regulator of HCN1 channels is 14-3-3. We used bioinformatics to identify three potential 14-3-3 binding sites in HCN1. We confirmed that 14-3-3 could pull down HCN1 from multiple tissue sources and used HEK293 cells to detail the interaction. Two sites in the intrinsically disordered C-terminus of HCN1 were necessary and sufficient for a phosphorylation-dependent interaction with 14-3-3. The same region of HCN1 containing the 14-3-3 binding peptides is required for phosphorylation-independent protein degradation. We propose a model in which phosphorylation of mouse S810 and S867 (human S789 and S846) recruits 14-3-3 to inhibit a yet unidentified factor signaling for protein degradation, thus increasing the half-life of HCN1.

Recent advances in structural studies of 14-3-3 protein complexes

Being phosphopeptide-binding hubs, 14-3-3 proteins coordinate multiple cellular processes in eukaryotes, including the regulation of apoptosis, cell cycle, ion channels trafficking, transcription, signal transduction, and hormone biosynthesis. Forming constitutive α-helical dimers, 14-3-3 proteins predominantly recognize specifically phosphorylated Ser/Thr sites within their partners; this generally stabilizes phosphotarget conformation and affects its activity, intracellular distribution, dephosphorylation, degradation and interactions with other proteins. Not surprisingly, 14-3-3 complexes are involved in the development of a range of diseases and are considered promising drug targets. The wide interactome of 14-3-3 proteins encompasses hundreds of different phosphoproteins, for many of which the interaction is well-documented in vitro and in vivo but lack the structural data that would help better understand underlying regulatory mechanisms and develop new drugs. Despite obtaining structural information on 14-3-3 complexes is still lagging behind the research of 14-3-3 interactions on a proteome-wide scale, recent works provided some advances, including methodological improvements and accumulation of new interesting structural data, that are discussed in this review.

A scalable strategy to solve structures of PDZ domains and their complexes

The human PDZome represents one of the largest globular domain families in the human proteome, with 266 instances. These globular domains typically interact with C-terminal peptide motifs found in thousands of human proteins. Despite previous efforts, not all PDZ domains have experimentally solved structures and most of their complexes remain to be solved. Here, a simple and cost-effective strategy is proposed for the crystallization of PDZ domains and their complexes. A human annexin A2 fusion tag was used as a crystallization chaperone and the structures of nine PDZ domains were solved, including five domains that had not yet been solved. Finally, these novel experimental structures were compared with AlphaFold predictions and it is speculated how predictions and experimental methods could cooperate in order to investigate the structural landscapes of entire domain families and interactomes.

Orchestrating serine/threonine phosphorylation and elucidating downstream effects by short linear motifs

Cellular function is based on protein-protein interactions. A large proportion of these interactions involves the binding of short linear motifs (SLiMs) by folded globular domains. These interactions are regulated by post-translational modifications, such as phosphorylation, that create and break motif binding sites or tune the affinity of the interactions. In addition, motif-based interactions are involved in targeting serine/threonine kinases and phosphatases to their substrate and contribute to the specificity of the enzymatic actions regulating which sites are phosphorylated. Here, we review how SLiM-based interactions assist in determining the specificity of serine/threonine kinases and phosphatases, and how phosphorylation, in turn, affects motif-based interactions. We provide examples of SLiM-based interactions that are turned on/off, or are tuned by serine/threonine phosphorylation and exemplify how this affects SLiM-based protein complex formation.

PDZ-Containing Proteins Targeted by the ACE2 Receptor

Angiotensin-converting enzyme 2 (ACE2) is a main receptor for SARS-CoV-2 entry to the host cell. Indeed, the first step in viral entry is the binding of the viral trimeric spike (S) protein to ACE2. Abundantly present in human epithelial cells of many organs, ACE2 is also expressed in the human brain. ACE2 is a type I membrane protein with an extracellular N-terminal peptidase domain and a C-terminal collectrin-like domain that ends with a single transmembrane helix and an intracellular 44-residue segment. This C-terminal segment contains a PDZ-binding motif (PBM) targeting protein-interacting domains called PSD-95/Dlg/ZO-1 (PDZ). Here, we identified the human PDZ specificity profile of the ACE2 PBM using the high-throughput holdup assay and measuring the binding intensities of the PBM of ACE2 against the full human PDZome. We discovered 14 human PDZ binders of ACE2 showing significant binding with dissociation constants' values ranging from 3 to 81 μM. NHERF, SHANK, and SNX27 proteins found in this study are involved in protein trafficking. The PDZ/PBM interactions with ACE2 could play a role in ACE2 internalization and recycling that could be of benefit for the virus entry. Interestingly, most of the ACE2 partners we identified are expressed in neuronal cells, such as SHANK and MAST families, and modifications of the interactions between ACE2 and these neuronal proteins may be involved in the neurological symptoms of COVID-19.

The biology of papillomavirus PDZ associations: what do they offer papillomaviruses?

The high-risk α-type papillomaviruses have a C-terminal PDZ-binding motif (PBM) on one of the two major oncoproteins E6 or E7; the vast majority on E6. The PBM is essential for the high-risk HPV life cycle, for episomal maintenance of the virus genome, and for maintaining the mitotic stability of the infected cell. The question is why only these viruses have PBMs - are there specific constraints imposed by the mucosal epithelium in which these viruses replicate? However the low-risk α-HPVs, such as HPV-6 and HPV-11 replicate extremely efficiently without a PBM, while viruses of the alpha8 group, such as HPV-40, replicate well with a very primitive PBM. So what does PDZ-binding capacity contribute to the fitness of the virus?

Host PDZ-containing proteins targeted by SARS-CoV-2

Small linear motifs targeting protein interacting domains called PSD‐95/Dlg/ZO‐1 (PDZ) have been identified at the C terminus of the severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) proteins E, 3a, and N. Using a high‐throughput approach of affinity‐profiling against the full human PDZome, we identified sixteen human PDZ binders of SARS‐CoV‐2 proteins E, 3A, and N showing significant interactions with dissociation constants values ranging from 3 to 82 μm. Six of them (TJP1, PTPN13, HTRA1, PARD3, MLLT4, LNX2) are also recognized by SARS‐CoV while three (NHERF1, MAST2, RADIL) are specific to SARS‐CoV‐2 E protein. Most of these SARS‐CoV‐2 protein partners are involved in cellular junctions/polarity and could be also linked to evasion mechanisms of the immune responses during viral infection. Among the binders of the SARS‐CoV‐2 proteins E, 3a, or N, seven significantly affect viral replication under knock down gene expression in infected cells. This PDZ profiling identifying human proteins potentially targeted by SARS‐CoV‐2 can help to understand the multifactorial severity of COVID19 and to conceive effective anti‐coronaviral agents for therapeutic purposes.

Hierarchized phosphotarget binding by the seven human 14-3-3 isoforms

The seven 14-3-3 isoforms are highly abundant human proteins encoded by similar yet distinct genes. 14-3-3 proteins recognize phosphorylated motifs within numerous human and viral proteins. Here, we analyze by X-ray crystallography, fluorescence polarization, mutagenesis and fusicoccin-mediated modulation the structural basis and druggability of 14-3-3 binding to four E6 oncoproteins of tumorigenic human papillomaviruses. 14-3-3 isoforms bind variant and mutated phospho-motifs of E6 and unrelated protein RSK1 with different affinities, albeit following an ordered affinity ranking with conserved relative KD ratios. Remarkably, 14-3-3 isoforms obey the same hierarchy when binding to most of their established targets, as supported by literature and a recent human complexome map. This knowledge allows predicting proportions of 14-3-3 isoforms engaged with phosphoproteins in various tissues. Notwithstanding their individual functions, cellular concentrations of 14-3-3 may be collectively adjusted to buffer the strongest phosphorylation outbursts, explaining their expression variations in different tissues and tumors.

Phosphorylation-induced changes in the PDZ domain of Dishevelled 3

The PDZ domain of Dishevelled 3 protein belongs to a highly abundant protein recognition motif which typically binds short C-terminal peptides. The affinity of the PDZ towards the peptides could be fine-tuned by a variety of post-translation modifications including phosphorylation. However, how phosphorylations affect the PDZ structure and its interactions with ligands remains elusive. Combining molecular dynamics simulations, NMR titration, and biological experiments, we explored the role of previously reported phosphorylation sites and their mimetics in the Dishevelled PDZ domain. Our observations suggest three major roles for phosphorylations: (1) acting as an on/off PDZ binding switch, (2) allosterically affecting the binding groove, and (3) influencing the secondary binding site. Our simulations indicated that mimetics had similar but weaker effects, and the effects of distinct sites were non-additive. This study provides insight into the Dishevelled regulation by PDZ phosphorylation. Furthermore, the observed effects could be used to elucidate the regulation mechanisms in other PDZ domains.

A Computational Protocol to Analyze PDZ/PBM Affinity Data Obtained by High-Throughput Holdup Assay

The holdup assay is an automated high-throughput comparative chromatographic retention approach that allows to measure quantitative binding intensities (BI) for a large number of domain-motif pairs and deduce equilibrium binding affinity constants. We routinely apply this approach to obtain quantitative binding specificity profiles of particular PDZ-binding motifs (PBMs) toward the full library of known human PDZ domains (the PDZome). The quality of the electropherograms extracted from the capillary electrophoresis instrument at the final step of the holdup assay may vary, influencing the accuracy and reproducibility of the measurement. By using bioinformatic tools, we can solve these issues to extract more reliable BIs by means of a better superimposition of the electropherograms. The protocol presented in this chapter describes the main principles and strategies of our curated method to process holdup data and new ways to plot and compare the BIs for the PBM-PDZ interactions. For this particular protocol, all the necessary computing commands are freely available in open Python packages.

Interactomic affinity profiling by holdup assay: Acetylation and distal residues impact the PDZome-binding specificity of PTEN phosphatase

Protein domains often recognize short linear protein motifs composed of a core conserved consensus sequence surrounded by less critical, modulatory positions. PTEN, a lipid phosphatase involved in phosphatidylinositol 3-kinase (PI3K) pathway, contains such a short motif located at the extreme C-terminus capable to recognize PDZ domains. It has been shown that the acetylation of this motif could modulate the interaction with several PDZ domains. Here we used an accurate experimental approach combining high-throughput holdup chromatographic assay and competitive fluorescence polarization technique to measure quantitative binding affinity profiles of the PDZ domain-binding motif (PBM) of PTEN. We substantially extended the previous knowledge towards the 266 known human PDZ domains, generating the full PDZome-binding profile of the PTEN PBM. We confirmed that inclusion of N-terminal flanking residues, acetylation or mutation of a lysine at a modulatory position significantly altered the PDZome-binding profile. A numerical specificity index is also introduced as an attempt to quantify the specificity of a given PBM over the complete PDZome. Our results highlight the impact of modulatory residues and post-translational modifications on PBM interactomes and their specificity.

Reading the phosphorylation code: binding of the 14-3-3 protein to multivalent client phosphoproteins

Many major protein-protein interaction networks are maintained by 'hub' proteins with multiple binding partners, where interactions are often facilitated by intrinsically disordered protein regions that undergo post-translational modifications, such as phosphorylation. Phosphorylation can directly affect protein function and control recognition by proteins that 'read' the phosphorylation code, re-wiring the interactome. The eukaryotic 14-3-3 proteins recognizing multiple phosphoproteins nicely exemplify these concepts. Although recent studies established the biochemical and structural basis for the interaction of the 14-3-3 dimers with several phosphorylated clients, understanding their assembly with partners phosphorylated at multiple sites represents a challenge. Suboptimal sequence context around the phosphorylated residue may reduce binding affinity, resulting in quantitative differences for distinct phosphorylation sites, making hierarchy and priority in their binding rather uncertain. Recently, Stevers et al. [Biochemical Journal (2017) 474: 1273-1287] undertook a remarkable attempt to untangle the mechanism of 14-3-3 dimer binding to leucine-rich repeat kinase 2 (LRRK2) that contains multiple candidate 14-3-3-binding sites and is mutated in Parkinson's disease. By using the protein-peptide binding approach, the authors systematically analyzed affinities for a set of LRRK2 phosphopeptides, alone or in combination, to a 14-3-3 protein and determined crystal structures for 14-3-3 complexes with selected phosphopeptides. This study addresses a long-standing question in the 14-3-3 biology, unearthing a range of important details that are relevant for understanding binding mechanisms of other polyvalent proteins.