Uncovering quantitative protein interaction networks for mouse PDZ domains using protein microarrays

One of the principal challenges in systems biology is to uncover the networks of protein-protein interactions that underlie most biological processes. To date, experimental efforts directed at this problem have largely produced only qualitative networks that are replete with false positives and false negatives. Here, we describe a domain-centered approach--compatible with genome-wide investigations--that enables us to measure the equilibrium dissociation constant (K(D)) of recombinant PDZ domains for fluorescently labeled peptides that represent physiologically relevant binding partners. Using a pilot set of 22 PDZ domains, 4 PDZ domain clusters, and 20 peptides, we define a gold standard dataset by determining the K(D) for all 520 PDZ-peptide combinations using fluorescence polarization. We then show that microarrays of PDZ domains identify interactions of moderate to high affinity (K(D) < or = 10 microM) in a high-throughput format with a false positive rate of 14% and a false negative rate of 14%. By combining the throughput of protein microarrays with the fidelity of fluorescence polarization, our domain/peptide-based strategy yields a quantitative network that faithfully recapitulates 85% of previously reported interactions and uncovers new biophysical interactions, many of which occur between proteins that are co-expressed. From a broader perspective, the selectivity data produced by this effort reveal a strong concordance between protein sequence and protein function, supporting a model in which interaction networks evolve through small steps that do not involve dramatic rewiring of the network.     

Analysis of leukocyte membrane protein interactions using protein microarrays

Background  

Protein microarrays represent an emerging class of proteomic tools to investigate multiple protein-protein interactions in parallel. A sufficient proportion of immobilized proteins must maintain an active conformation and an orientation that allows for the sensitive and specific detection of antibody and ligand binding. In order to establish protein array technology for the characterization of the weak interactions between leukocyte membrane proteins, we selected the human leukocyte membrane protein CD200 (OX2) and its cell surface receptor (hCD200R) as a model system. As antibody-antigen reactions are generally of higher affinity than receptor-ligand binding, we first analyzed the reactivity of monoclonal antibodies (mAb) to normal and mutant forms of immobilized CD200R.     

A systematic family-wide investigation reveals that ~30% of mammalian PDZ domains engage in PDZ-PDZ interactions

PDZ domains are independently folded modules that typically mediate protein-protein interactions by binding to the C termini of their target proteins. However, in a few instances, PDZ domains have been reported to dimerize with other PDZ domains. To investigate this noncanonical-binding mode further, we used protein microarrays comprising virtually every mouse PDZ domain to systematically query all possible PDZ-PDZ pairs. We then used fluorescence polarization to retest and quantify interactions and coaffinity purification to test biophysically validated interactions in the context of their full-length proteins. Overall, we discovered 37 PDZ-PDZ interactions involving 46 PDZ domains (~30% of all PDZ domains tested), revealing that dimerization is a more frequently used binding mode than was previously appreciated. This suggests that many PDZ domains evolved to form multiprotein complexes by simultaneously interacting with more than one ligand.     

Engineering temperature-sensitive SH3 domains.

BACKGROUND: The ability to control specific protein-protein interactions conditionally in vivo would be extremely helpful for analyzing protein-protein interaction networks. SH3 (Src homology 3) modular protein binding domains are found in many signaling proteins and they play a crucial role in signal transduction by binding to proline-rich sequences. RESULTS: Random in vitro mutagenesis coupled with yeast two-hybrid screening was used to identify mutations in the second SH3 domain of Nck that render interaction with its ligand temperature sensitive. Four of the mutants were functionally temperature sensitive in mammalian cells, where temperature sensitivity was correlated with a pronounced instability of the mutant domains at the nonpermissive temperature. Two of the mutations affect conserved residues in the hydrophobic core (Val133 and Val160), suggesting a general strategy for engineering temperature-sensitive SH3-containing proteins. Indeed mutagenesis of the corresponding positions in another SH3 domain, that of Crk-1, rendered the full-length Crk-1 protein temperature sensitive for function and stability in mammalian cells. CONCLUSIONS: Construction of temperature-sensitive SH3 domains is a novel approach to regulating the function of SH3 domains in vivo. Such mutants will be valuable in dissecting SH3-mediated signaling pathways. Furthermore, the methodology described here to isolate temperature-sensitive domains should be widely applicable to any domain involved in protein-protein interactions.     

A selective irreversible inhibitor targeting a PDZ protein interaction domain

Irreversible inhibitors of proteases have proven themselves useful tools for determining which proteases are active under given conditions in tissues or cells and for studying the functional role that a protease plays in physiological processes. The application of such techniques to the study of the activity and function of protein-protein interactions has been hindered by the lack of guiding principles for the mechanistic design of irreversible inhibitors targeting the "active site" of a protein interaction. We report herein the first example of a mechanism-based irreversible inhibitor of a protein interaction that has been specifically targeted to one member of the PDZ family of protein interaction domains: the second PDZ domain of the membrane-associated guanylate kinase MAGI3. This inhibitor was designed using rationally directed computational evaluation to take advantage of a conserved histidine in the PDZ domain by introducing an ionizable group that will be held in close proximity to that nucleophile during binding. The novel compound exhibits all of the characteristics of an irreversible inhibitor of the interaction of the tumor suppressor PTEN with MAGI3 in in vitro models. In cells, the inhibitor can be shown to release PTEN from sequestration by MAGI3 and consequently upregulate the PKB signaling pathway.     

Analysis of protein interaction networks using mass spectrometry compatible techniques

The ability to map protein-protein interactions has grown tremendously over the last few years, making it possible to envision the mapping of whole or targeted protein interaction networks and to elucidate their temporal dynamics. The use of mass spectrometry for the study of protein complexes has proven to be an invaluable tool due to its ability to unambiguously identify proteins from a variety of biological samples. Furthermore, when affinity purification is combined with mass spectrometry analysis, the identification of multimeric protein complexes is greatly facilitated. Here, we review recent developments for the analysis of protein interaction networks by mass spectrometry and discuss the integration of different bioinformatic tools for predicting, validating, and managing interaction datasets.     

Use of photolithography to encode cell adhesive domains into protein microarrays

Protein microarrays are rapidly emerging as valuable tools in creating combinatorial cell culture systems where inducers of cellular differentiation can be identified in a rapid and multiplexed fashion. In the present study, protein microarraying was combined with photoresist lithography to enable printing of extracellular matrix (ECM) protein arrays while precisely controlling "on-the-spot" cell-cell interactions. In this surface engineering approach, the micropatterned photoresist layer formed on a glass substrate served as a temporary stencil during the microarray printing, defining the micrometer-scale dimensions and the geometry of the cell-adhesion domains within the printed protein spots. After removal of the photoresist, the glass substrates contained micrometer-scale cell-adhesive regions that were encoded within 300 or 500 microm diameter protein domains. Fluorescence microscopy and atomic force microscopy (AFM) were employed to characterize protein micropatterns. When incubated with micropatterned surfaces, hepatic (HepG2) cells attached on 300 or 500 mum diameter protein spots; however, the extent of cell-cell contacts within each spot varied in accordance with dimensions of the photoresist stencil, from single cells attaching on 30 microm diameter features to multicell clusters residing on 100 or 200 microm diameter regions. Importantly, the photoresist removal process was shown to have no detrimental effects on the ability of several ECM proteins (collagens I, II, and IV and laminin) to support functional hepatic cultures. The micropatterning approach described here allows for a small cell population seeded onto a single cell culture substrate to be exposed to multiple scenarios of cell-cell and cell-surface interactions in parallel. This technology will be particularly useful for high-throughput screening of biological stimuli required for tissue specification of stem cells or for maintenance of differentiated phenotype in scarce primary cells.     

High-affinity chelator thiols for switchable and oriented immobilization of histidine-tagged proteins: a generic platform for protein chip technologies

Protein micro-/nanoarrays are becoming increasingly important in systematic approaches for the exploration of protein-protein interactions and dynamic protein networks, so there is a high demand for specific, generic, stable, uniform, and locally addressable protein immobilization on solid supports. Here we present multivalent metal-chelating thiols that are suitable for stable binding of histidine-tagged proteins on biocompatible self-assembled monolayers (SAMs). The architectures and physicochemical properties of these SAMs have been probed by various surface-sensitive techniques such as contact angle goniometry, ellipsometry, and infrared reflection-absorption spectroscopy. The specific molecular organization of proteins and protein complexes was demonstrated by surface plasmon resonance, confocal laser scanning, and atomic force microscopy. In contrast to the mono-NTA/His6 tag interaction, which has major drawbacks because of its low affinity and fast dissociation, drastically improved stability of protein binding by these multivalent chelator surfaces was observed. The immobilized histidine-tagged proteins are uniformly oriented and retain their function. At the same time, proteins can be removed from the chip surface under mild conditions (switchability). This new platform for switchable and oriented immobilization should assist proteome-wide wide analyses of protein-protein interactions as well as structural and single-molecule studies.     

Engineering modular protein interaction switches by sequence overlap

Many cellular signaling pathways contain proteins whose interactions change in response to upstream inputs, allowing for conditional activation or repression of the interaction based on the presence of the input molecule. The ability to engineer similar regulation into protein interaction elements would provide us with powerful tools for controlling cell signaling. Here we describe an approach for engineering diverse synthetic protein interaction switches. Specifically, by overlapping the sequences of pairs of protein interaction domains and peptides, we have been able to generate mutually exclusive regulation over their interactions. Thus, the hybrid protein (which is composed of the two overlapped interaction modules) can bind to either of the two respective ligands for those modules, but not to both simultaneously. We show that these synthetic switch proteins can be used to regulate specific protein-protein interactions in vivo. These switches allow us to disrupt an interaction with the addition or activation of a protein input that has no natural connection to the interaction in question. Therefore, they give us the ability to make novel connections between normally unrelated signaling pathways and to rewire the input/output relationships of cellular behaviors. Our experiments also suggest a possible mechanism by which complex regulatory proteins might have evolved from simpler components.     

Molecular aptamers for real-time protein-protein interaction study

Protein-protein interactions play critical roles in cellular functions, but current techniques for real-time study of these interactions are limited. We report the real-time monitoring of protein-protein interactions without labeling either of the two interacting proteins; this procedure poses minimum effects on the binding properties of the proteins. Our strategy uses a protein/aptamer complex to probe the interactions in a competitive assay where the binding of an aptamer to its target protein is altered by a second protein that interacts with the target protein. Two signal transduction strategies, fluorescence resonance energy transfer (FRET) and fluorescence anisotropy, have been designed to study the interactions of human alpha-thrombin with different proteins by using two aptamers specific for two binding sites on alpha-thrombin. Our method has been shown to be simple and effective, does not require labeling of proteins, makes use of easily obtainable aptamers, provides detailed protein-protein interaction information and has excellent sensitivity for protein detection and protein-protein interaction studies. The FRET and the fluorescent anisotropy approaches complement each other in providing insight into the kinetics, mechanisms, binding sites and binding dynamics of the interacting proteins.     

Thermodynamic basis for promiscuity and selectivity in protein-protein interactions: PDZ domains, a case study

Like other protein-protein interaction domains, PDZ domains are involved in many key cellular processes. These processes often require that specific multiprotein complexes be assembled, a task that PDZ domains accomplish by binding to specific peptide motifs in target proteins. However, a growing number of experimental studies show that PDZ domains (like other protein-protein interaction domains) can engage in a variety of interactions and bind distinct peptide motifs. Such promiscuity in ligand recognition raises intriguing questions about the molecular and thermodynamic mechanisms that can sustain it. To identify possible sources of promiscuity and selectivity underlying PDZ domain interactions, we performed molecular dynamics simulations of 20 to 25 ns on a set of 12 different PDZ domain complexes (for the proteins PSD-95, Syntenin, Erbin, GRIP, NHERF, Inad, Dishevelled, and Shank). The electrostatic, nonpolar, and configurational entropy binding contributions were evaluated using the MM/PBSA method combined with a quasi-harmonic analysis. The results revealed that PDZ domain interactions are characterized by overwhelmingly favorable nonpolar contributions and almost negligible electrostatic components, a mix that may readily sustain promiscuity. In addition, despite the structural similarity in fold and in recognition modes, the entropic and other dynamical aspects of binding were remarkably variable not only across PDZ domains but also for the same PDZ domain bound to distinct ligands. This variability suggests that entropic and dynamical components can play a role in determining selectivity either of PDZ domain interactions with peptide ligands or of PDZ domain complexes with downstream effectors.     

Synthesis of a phosphoserine mimetic prodrug with potent 14-3-3 protein inhibitory activity

Many protein-protein interactions in cells are mediated by functional domains that recognize and bind to motifs containing phosphorylated serine and threonine residues. To create small molecules that inhibit such interactions, we developed methodology for the synthesis of a prodrug that generates a phosphoserine peptidomimetic in cells. For this study, we synthesized a small molecule inhibitor of 14-3-3 proteins that incorporates a nonhydrolyzable difluoromethylenephosphoserine prodrug moiety. The prodrug is cytotoxic at low micromolar concentrations when applied to cancer cells and induces caspase activation resulting in apoptosis. The prodrug reverses the 14-3-3-mediated inhibition of FOXO3a resulting from its phosphorylation by Akt1 in a concentration-dependent manner that correlates well with its ability to inhibit cell growth. This methodology can be applied to target a variety of proteins containing phosphoserine and other phosphoamino acid binding domains.     

Computational analyses of high-throughput protein-protein interaction data

Protein-protein interactions play important roles in nearly all events that take place in a cell. High-throughput experimental techniques enable the study of protein-protein interactions at the proteome scale through systematic identification of physical interactions among all proteins in an organism. High-throughput protein-protein interaction data, with ever-increasing volume, are becoming the foundation for new biological discoveries. A great challenge to bioinformatics is to manage, analyze, and model these data. In this review, we describe several databases that store, query, and visualize protein-protein interaction data. Comparison between experimental techniques shows that each high-throughput technique such as yeast two-hybrid assay or protein complex identification through mass spectrometry has its limitations in detecting certain types of interactions and they are complementary to each other. In silico methods using protein/DNA sequences, domain and structure information to predict protein-protein interaction can expand the scope of experimental data and increase the confidence of certain protein-protein interaction pairs. Protein-protein interaction data correlate with other types of data, including protein function, subcellular location, and gene expression profile. Highly connected proteins are more likely to be essential based on the analyses of the global architecture of large-scale interaction network in yeast. Use of protein-protein interaction networks, preferably in conjunction with other types of data, allows assignment of cellular functions to novel proteins and derivation of new biological pathways. As demonstrated in our study on the yeast signal transduction pathway for amino acid transport, integration of high-throughput data with traditional biology resources can transform the protein-protein interaction data from noisy information into knowledge of cellular mechanisms.     

Affinity-guided protein conjugation: the trilogy of covalent protein labeling,assembly and inhibition

Specific and dynamic biological interactions pave the blueprint of signal networks in cell. For example, a great variety of specific protein-ligand interactions define how intracellular signals flow. Taking advantage of the specificity of these interactions, we postulate an “affinity-guided covalent conjugation” strategy to lock binding ligands through covalent reactions between the ligand and the receptor protein. The presence of a nucleophile close to the ligand binding site of a protein is sine qua none of this reaction. Specific noncovalent interaction of a ligand derivative (which contains an electrophile at a designed position) to the ligand binding site of the protein brings the electrophile to the close proximity of the nucleophile. Subsequently, a conjugation reaction spontaneously takes place between the nucleophile and the electrophile, and leads to an intermolecular covalent linkage. This strategy was first showcased in coiled coil peptides which include a cysteine mutation at a selected position. The short peptide sequence was used for covalent labeling of cell surface receptors. The same strategy was then used to guide the design of a set of protein Lego bricks for covalent assembly of protein complexes of unnatural geometry. We finally made “reactive peptides” for natural adaptor proteins that play significant roles in signal transduction. The peptides were designed to react with a single domain of the multidomain adaptor protein, delivered into the cytosol of neurons, and re-directed the intracellular signal of neuronal migration. The trilogy of protein labeling, assembly, and inhibition of intracellular signals, all through a specific covalent bond, fully demonstrated the generality and versatility of “affinity-guided covalent conjugation” in various applications.     

Conformational changes in salivary proline-rich protein 1 upon adsorption to calcium phosphate crystals

Conformational analyses of PRP1, a proline-rich acidic salivary protein and major component of the acquired enamel pellicle, have been carried out in solution and upon binding to two enamel prototypes, hydroxyapatite (HA) and carbonated hydroxyapatite (CHA), using Fourier transform infrared spectroscopy (FTIR) in attenuated total reflection (ATR) mode. We have shown for the first time that, in solution, large portions of PRP1 adopt the hydrated polyproline type II (PPII) helical structure in addition to the random coil structure, with the maximum absorbance of the amide I band around 1620 cm(-1). Upon binding to HA or CHA, the protein undergoes significant conformational changes, loosing a considerable portion of hydrated PPII and random coil domains with a shift in the maximum absorbance to 1666 cm(-1), indicating that a large fraction of the protein is composed of beta turns. A small fraction of PPII in a calcium-bound or anhydrous form (approximately 1642 cm(-1)) was also observed in the HA- and CHA-bound proteins, which could play a role in protein-mineral interactions. The conformational changes in PRP1 adsorbed on CHA and HA were similar in nature; however, these changes were greater in the protein bound to HA. Interestingly, these results are in agreement with protein adsorption data that show that less protein is adsorbed onto CHA than onto HA. Our results demonstrate that binding to apatitic mineral surfaces leads to major conformational changes in PRP1, which might reflect the expulsion of water and the formation of protein-mineral and/or protein-protein interactions in the adsorbed layer.     

Mini-review: computational structure-based design of inhibitors that target protein surfaces

Finding drugs that inhibit protein-protein interactions is usually difficult. While computer-aided design is used widely to facilitate the drug discovery process for protein targets with well-defined binding pockets, its application to the design of inhibitors targeting a protein surface is very limited. In this mini-review we address two aspects of this issue: firstly, we overview the current state of design methodology for inhibitors specifically targeting protein surfaces, and secondly, we briefly outline recent advances in computational methods for structure-based drug design. These methods are closely related to protein docking and protein recognition, the difference being that in ligand design, ligands are built on a fragment-by-fragment basis. A novel scheme of computational combinatorial ligand design developed for the design of inhibitors that interfere with protein-protein interaction is described in detail. Current applications and limitations of this methodology, as well as its future prospects, are discussed.     

Use of selective Trp side chain labeling to characterize protein-protein and protein-ligand interactions by NMR spectroscopy

Recent studies on amino acid occurrence in protein binding sites suggest that only a reduced number of residues are responsible for most interaction energy in protein-protein and protein-ligand interactions. Above all, tryptophan (Trp) seems to be the most frequent residue in protein's hot spots. Here we report a novel, efficient, and cost-effective method to selectively incorporate specific isotope labels into the side chains of Trp residues in recombinant proteins. We show that the method proposed allows selective NMR observation of Trp side chains that enables studies of ligand binding, protein-protein interactions, hydrogen binding, protein folding, and side chain dynamics. Examples with the protein BIR3 will be given.