A PDZ domain recapitulates a unifying mechanism for protein folding

From covalent transition states in chemistry to noncovalent in biology: from β- to Φ-value analysis of protein folding

Solving the mechanism of a chemical reaction requires determining the structures of all the ground states on the pathway and the elusive transition states linking them. 2024 is the centenary of Brønsted's landmark paper that introduced the β-value and structure-activity studies as the only experimental means to infer the structures of transition states. It involves making systematic small changes in the covalent structure of the reactants and analysing changes in activation and equilibrium-free energies. Protein engineering was introduced for an analogous procedure, Φ-value analysis, to analyse the noncovalent interactions in proteins central to biological chemistry. The methodology was developed first by analysing noncovalent interactions in transition states in enzyme catalysis. The mature procedure was then applied to study transition states in the pathway of protein folding - 'part (b) of the protein folding problem'. This review describes the development of Φ-value analysis of transition states and compares and contrasts the interpretation of β- and Φ-values and their limitations. Φ-analysis afforded the first description of transition states in protein folding at the level of individual residues. It revealed the nucleation-condensation folding mechanism of protein domains with the transition state as an expanded, distorted native structure, containing little fully formed secondary structure but many weak tertiary interactions. A spectrum of transition states with various degrees of structural polarisation was then uncovered that spanned from nucleation-condensation to the framework mechanism of fully formed secondary structure. Φ-analysis revealed how movement of the expanded transition state on an energy landscape accommodates the transition from framework to nucleation-condensation mechanisms with a malleability of structure as a unifying feature of folding mechanisms. Such movement follows the rubric of analysis of classical covalent chemical mechanisms that began with Brønsted. Φ-values are used to benchmark computer simulation, and Φ and simulation combine to describe folding pathways at atomic resolution.

A short structural extension dictates the early stages of folding of a PDZ domain

PDZ domains are highly abundant protein-protein interaction modules in human. One of the most extensively characterized PDZ domain, the third PDZ domain from PSD-95 (PDZ3), contains an α-helical C-terminal extension that has a key role in the function of the domain. Here we compared the folding of PDZ3 with a truncated variant (PDZ3Δα3), lacking the additional helix, by means of the so-called Φ-value analysis, an experimental technique that allows inferring the structure of folding transition states. Experiments reveal subtle but detectable differences in the folding of PDZ3Δα3 versus PDZ3, as probed by structural characterization of the folding transition states. These differences appear more remarkable in the early stages of folding, with a detectable shift of the folding nucleus. The presented results allow demonstrating that the native state exerts a weak bias at the early stages of folding, which appear to be characterized by alternative pathways.

Boltzmann Machine Learning and Regularization Methods for Inferring Evolutionary Fields and Couplings From a Multiple Sequence Alignment

The inverse Potts problem to infer a Boltzmann distribution for homologous protein sequences from their single-site and pairwise amino acid frequencies recently attracts a great deal of attention in the studies of protein structure and evolution. We study regularization and learning methods and how to tune regularization parameters to correctly infer interactions in Boltzmann machine learning. Using L₂ regularization for fields, group L₁ for couplings is shown to be very effective for sparse couplings in comparison with L₂ and L₁. Two regularization parameters are tuned to yield equal values for both the sample and ensemble averages of evolutionary energy. Both averages smoothly change and converge, but their learning profiles are very different between learning methods. The Adam method is modified to make stepsize proportional to the gradient for sparse couplings and to use a soft-thresholding function for group L₁. It is shown by first inferring interactions from protein sequences and then from Monte Carlo samples that the fields and couplings can be well recovered, but that recovering the pairwise correlations in the resolution of a total energy is harder for the natural proteins than for the protein-like sequences. Selective temperature for folding/structural constrains in protein evolution is also estimated.

A Glimpse into the Structural Properties of the Intermediate and Transition State in the Folding of Bromodomain 2 Domain 2 by Φ Value Analysis

Bromodomains (BRDs) are small protein interaction modules of about 110 amino acids that selectively recognize acetylated lysine in histones and other proteins. These domains have been identified in a variety of multi-domain proteins involved in transcriptional regulation or chromatin remodeling in eukaryotic cells. BRD inhibition is considered an attractive therapeutic approach in epigenetic disorders, particularly in oncology. Here, we present a Φ value analysis to investigate the folding pathway of the second domain of BRD2 (BRD2(2)). Using an extensive mutational analysis based on 25 site-directed mutants, we provide structural information on both the intermediate and late transition state of BRD2(2). The data reveal that the C-terminal region represents part of the initial folding nucleus, while the N-terminal region of the domain consolidates its structure only later in the folding process. Furthermore, only a small number of native-like interactions have been identified, suggesting the presence of a non-compact, partially folded state with scarce native-like characteristics. Taken together, these results indicate that, in BRD2(2), a hierarchical mechanism of protein folding can be described with non-native interactions that play a significant role in folding.

Unveiling the Folding Mechanism of PDZ Domains

Understanding the mechanism of folding of single domain proteins demands a complete characterization of their equilibrium and kinetic properties. By using a well-studied class of protein domain, the PDZ domain, here we exemplify the typical procedure to address this problem.

The Conformational Plasticity Vista of PDZ Domains

The PDZ domain (PSD95-Discs large-ZO1) is a widespread modular domain present in the living organisms. A prevalent function in the PDZ family is to serve as scaffolding and adaptor proteins connecting multiple partners in signaling pathways. An explanation of the flexible functionality in this domain family, based just on a static perspective of the structure-activity relationship, might fall short. More dynamic and conformational aspects in the protein fold can be the reasons for such functionality. Folding studies indeed showed an ample and malleable folding landscape for PDZ domains where multiple intermediate states were experimentally detected. Allosteric phenomena that resemble energetic coupling between residues have also been found in PDZ domains. Additionally, several PDZ domains are modulated by post-translational modifications, which introduce conformational switches that affect binding. Altogether, the ability to connect diverse partners might arise from the intrinsic plasticity of the PDZ fold.

A structurally heterogeneous transition state underlies coupled binding and folding of disordered proteins

Many intrinsically disordered proteins (IDPs) attain a well-defined structure in a coupled folding and binding reaction with another protein. Such reactions may involve early to late formation of different native structural regions along the reaction pathway. To obtain insights into the transition state for a coupled binding and folding reaction, we performed restrained molecular dynamics simulations using previously determined experimental binding Φ_b values of the interaction between two IDP domains: the activation domain from the p160 transcriptional co-activator for thyroid hormone and retinoid receptors (ACTR) and the nuclear co-activator binding domain (NCBD) of CREB-binding protein, each forming three well-defined α-helices upon binding. These simulations revealed that both proteins are largely disordered in the transition state for complex formation, except for two helices, one from each domain, that display a native-like structure. The overall transition state structure was extended and largely dynamic with many weakly populated contacts. To test the transition state model, we combined site-directed mutagenesis with kinetic experiments, yielding results consistent with overall diffuse interactions and formation of native intramolecular interactions in the third NCBD helix during the binding reaction. Our findings support the view that the transition state and, by inference, any encounter complex in coupled binding and folding reactions are structurally heterogeneous and largely independent of specific interactions. Furthermore, experimental Φ_b values and Brønsted plots suggested that the transition state is globally robust with respect to most mutations but can display more native-like features for some highly destabilizing mutations, possibly because of Hammond behavior or ground-state effects.

How Many Protein Sequences Fold to a Given Structure? A Coevolutionary Analysis

Quantifying the relationship between protein sequence and structure is key to understanding the protein universe. A fundamental measure of this relationship is the total number of amino acid sequences that can fold to a target protein structure, known as the "sequence capacity," which has been suggested as a proxy for how designable a given protein fold is. Although sequence capacity has been extensively studied using lattice models and theory, numerical estimates for real protein structures are currently lacking. In this work, we have quantitatively estimated the sequence capacity of 10 proteins with a variety of different structures using a statistical model based on residue-residue co-evolution to capture the variation of sequences from the same protein family. Remarkably, we find that even for the smallest protein folds, such as the WW domain, the number of foldable sequences is extremely large, exceeding the Avogadro constant. In agreement with earlier theoretical work, the calculated sequence capacity is positively correlated with the size of the protein, or better, the density of contacts. This allows the absolute sequence capacity of a given protein to be approximately predicted from its structure. On the other hand, the relative sequence capacity, i.e., normalized by the total number of possible sequences, is an extremely tiny number and is strongly anti-correlated with the protein length. Thus, although there may be more foldable sequences for larger proteins, it will be much harder to find them. Lastly, we have correlated the evolutionary age of proteins in the CATH database with their sequence capacity as predicted by our model. The results suggest a trade-off between the opposing requirements of high designability and the likelihood of a novel fold emerging by chance.

A many-body term improves the accuracy of effective potentials based on protein coevolutionary data

The study of correlated mutations in alignments of homologous proteins proved to be successful not only in the prediction of their native conformation but also in the development of a two-body effective potential between pairs of amino acids. In the present work, we extend the effective potential, introducing a many-body term based on the same theoretical framework, making use of a principle of maximum entropy. The extended potential performs better than the two-body one in predicting the energetic effect of 308 mutations in 14 proteins (including membrane proteins). The average value of the parameters of the many-body term correlates with the degree of hydrophobicity of the corresponding residues, suggesting that this term partly reflects the effect of the solvent.

Contribution to the prediction of the fold code: application to immunoglobulin and flavodoxin cases

Background

Folding nucleus of globular proteins formation starts by the mutual interaction of a group of hydrophobic amino acids whose close contacts allow subsequent formation and stability of the 3D structure. These early steps can be predicted by simulation of the folding process through a Monte Carlo (MC) coarse grain model in a discrete space. We previously defined MIRs (Most Interacting Residues), as the set of residues presenting a large number of non-covalent neighbour interactions during such simulation. MIRs are good candidates to define the minimal number of residues giving rise to a given fold instead of another one, although their proportion is rather high, typically [15-20]% of the sequences. Having in mind experiments with two sequences of very high levels of sequence identity (up to 90%) but different folds, we combined the MIR method, which takes sequence as single input, with the “fuzzy oil drop” (FOD) model that requires a 3D structure, in order to estimate the residues coding for the fold. FOD assumes that a globular protein follows an idealised 3D Gaussian distribution of hydrophobicity density, with the maximum in the centre and minima at the surface of the “drop”. If the actual local density of hydrophobicity around a given amino acid is as high as the ideal one, then this amino acid is assigned to the core of the globular protein, and it is assumed to follow the FOD model. Therefore one obtains a distribution of the amino acids of a protein according to their agreement or rejection with the FOD model.

Results

We compared and combined MIR and FOD methods to define the minimal nucleus, or keystone, of two populated folds: immunoglobulin-like (Ig) and flavodoxins (Flav). The combination of these two approaches defines some positions both predicted as a MIR and assigned as accordant with the FOD model. It is shown here that for these two folds, the intersection of the predicted sets of residues significantly differs from random selection. It reduces the number of selected residues by each individual method and allows a reasonable agreement with experimentally determined key residues coding for the particular fold. In addition, the intersection of the two methods significantly increases the specificity of the prediction, providing a robust set of residues that constitute the folding nucleus.

Understanding the effect of alternative splicing in the folding and function of the second PDZ from protein tyrosine phosphatase-BL

PDZ domains are the most prominent biological structural domains involved in protein-protein interactions in the human cell. The second PDZ domain of the protein tyrosine phosphatase BL (PDZ2) interacts and binds the C-termini of the tumour suppressor protein APC and of the LIM domain-containing protein RIL. One isoform of PDZ2 (PDZ2as) involves an alternative spliced form that exhibits an insertion of 5 residues in a loop. PDZ2as abrogates binding to its partners, even if the insertion is directly located in its binding pocket. Here, we investigate the folding and function of PDZ2as, in comparison to the previously characterized PDZ2 domain. Data reveal that, whilst the thermodynamic stability of PDZ2as appears as nearly identical to that of PDZ2, the insertion of 5 amino acids induces formation of some weak transient non-native interactions in the folding transition state, as mirrored by a concomitant increase of both the folding and unfolding rate constants. From a functional perspective, we show that the decrease in affinity is caused by a pronounced decrease of the association rate constants (by nearly ten fold), with no effect on the microscopic dissociation rate constants. The results are briefly discussed in the context of previous work on PDZ domains.

Some (dis)assembly required: partial unfolding in the Par-6 allosteric switch

Allostery is commonly described as a functional connection between two distant sites in a protein, where a binding event at one site alters affinity at the other. Here we review the conformational dynamics that encode an allosteric switch in the PDZ domain of Par-6. Par-6 is a scaffold protein that organizes other proteins into a complex required to initiate and maintain cell polarity. NMR measurements revealed that the PDZ domain samples an evolutionarily conserved unfolding intermediate allowing rearrangement of two adjacent loop residues that control ligand binding affinity. Cdc42 binding to Par-6 creates a novel interface between the PDZ domain and the adjoining CRIB motif that stabilizes the high-affinity PDZ conformation. Thermodynamic and kinetic studies suggest that partial PDZ unfolding is an integral part of the Par-6 switching mechanism. The Par-6 CRIB-PDZ module illustrates two important structural aspects of protein evolution: the interface between adjacent domains in the same protein can give rise to allosteric regulation, and thermodynamic stability may be sacrificed to increase the sampling frequency of an unfolding intermediate required for conformational switching.

Understanding the frustration arising from the competition between function, misfolding, and aggregation in a globular protein

Folding and function may impose different requirements on the amino acid sequences of proteins, thus potentially giving rise to conflict. Such a conflict, or frustration, can result in the formation of partially misfolded intermediates that can compromise folding and promote aggregation. We investigate this phenomenon by studying frataxin, a protein whose normal function is to facilitate the formation of iron-sulfur clusters but whose mutations are associated with Friedreich's ataxia. To characterize the folding pathway of this protein we carry out a Φ-value analysis and use the resulting structural information to determine the structure of the folding transition state, which we then validate by a second round of rationally designed mutagenesis. The analysis of the transition-state structure reveals that the regions involved in the folding process are highly aggregation-prone. By contrast, the regions that are functionally important are partially misfolded in the transition state but highly resistant to aggregation. Taken together, these results indicate that in frataxin the competition between folding and function creates the possibility of misfolding, and that to prevent aggregation the amino acid sequence of this protein is optimized to be highly resistant to aggregation in the regions involved in misfolding.

The network of stabilizing contacts in proteins studied by coevolutionary data

The primary structure of proteins, that is their sequence, represents one of the most abundant sets of experimental data concerning biomolecules. The study of correlations in families of co-evolving proteins by means of an inverse Ising-model approach allows to obtain information on their native conformation. Following up on a recent development along this line, we optimize the algorithm to calculate effective energies between the residues, validating the approach both back-calculating interaction energies in a model system, and predicting the free energies associated to mutations in real systems. Making use of these effective energies, we study the network of interactions which stabilizes the native conformation of some well-studied proteins, showing that it displays different properties than the associated contact network.

Periaxin and AHNAK nucleoprotein 2 form intertwined homodimers through domain swapping

Periaxin (PRX) is an abundant protein in the peripheral nervous system, with an important role in myelination. PRX participates in large molecular complexes, most likely through the interactions of its N-terminal PSD-95/Discs-large/ZO-1 (PDZ)-like domain. We present the crystal structures of the PDZ-like domains from PRX and its homologue AHNAK nucleoprotein 2 (AHNAK2). The unique intertwined, domain-swapped dimers provide a structural basis for the homodimerization of both proteins. The core of the homodimer is formed by a 6-stranded antiparallel β sheet, with every other strand from a different chain. The AHNAK2 PDZ domain structure contains a bound class III ligand peptide. The binding pocket is preformed, and the peptide-PDZ interactions have unique aspects, including two salt bridges and weak recognition of the peptide C terminus. Tight homodimerization may be central to the scaffolding functions of PRX and AHNAK2 in molecular complexes linking the extracellular matrix to the cytoskeletal network.

The role of backbone hydrogen bonds in the transition state for protein folding of a PDZ domain

Backbone hydrogen bonds are important for the structure and stability of proteins. However, since conventional site-directed mutagenesis cannot be applied to perturb the backbone, the contribution of these hydrogen bonds in protein folding and stability has been assessed only for a very limited set of small proteins. We have here investigated effects of five amide-to-ester mutations in the backbone of a PDZ domain, a 90-residue globular protein domain, to probe the influence of hydrogen bonds in a β-sheet for folding and stability. The amide-to-ester mutation removes NH-mediated hydrogen bonds and destabilizes hydrogen bonds formed by the carbonyl oxygen. The overall stability of the PDZ domain generally decreased for all amide-to-ester mutants due to an increase in the unfolding rate constant. For this particular region of the PDZ domain, it is therefore clear that native hydrogen bonds are formed after crossing of the rate-limiting barrier for folding. Moreover, three of the five amide-to-ester mutants displayed an increase in the folding rate constant suggesting that the hydrogen bonds are involved in non-native interactions in the transition state for folding.

The folding of a family of three-helix bundle proteins: spectrin R15 has a robust folding nucleus, unlike its homologous neighbours

Three homologous spectrin domains have remarkably different folding characteristics. We have previously shown that the slow-folding R16 and R17 spectrin domains can be altered to resemble the fast folding R15, in terms of speed of folding (and unfolding), landscape roughness and folding mechanism, simply by substituting five residues in the core. Here we show that, by contrast, R15 cannot be engineered to resemble R16 and R17. It is possible to engineer a slow-folding version of R15, but our analysis shows that this protein neither has a rougher energy landscape nor does change its folding mechanism. Quite remarkably, R15 appears to be a rare example of a protein with a folding nucleus that does not change in position or in size when its folding nucleus is disrupted. Thus, while two members of this protein family are remarkably plastic, the third has apparently a restricted folding landscape.

Computational and theoretical methods for protein folding

A computational approach is essential whenever the complexity of the process under study is such that direct theoretical or experimental approaches are not viable. This is the case for protein folding, for which a significant amount of data are being collected. This paper reports on the essential role of in silico methods and the unprecedented interplay of computational and theoretical approaches, which is a defining point of the interdisciplinary investigations of the protein folding process. Besides giving an overview of the available computational methods and tools, we argue that computation plays not merely an ancillary role but has a more constructive function in that computational work may precede theory and experiments. More precisely, computation can provide the primary conceptual clues to inspire subsequent theoretical and experimental work even in a case where no preexisting evidence or theoretical frameworks are available. This is cogently manifested in the application of machine learning methods to come to grips with the folding dynamics. These close relationships suggested complementing the review of computational methods within the appropriate theoretical context to provide a self-contained outlook of the basic concepts that have converged into a unified description of folding and have grown in a synergic relationship with their computational counterpart. Finally, the advantages and limitations of current computational methodologies are discussed to show how the smart analysis of large amounts of data and the development of more effective algorithms can improve our understanding of protein folding.

Allosteric activation of the Par-6 PDZ via a partial unfolding transition

Proteins exist in a delicate balance between the native and unfolded states, where thermodynamic stability may be sacrificed to attain the flexibility required for efficient catalysis, binding, or allosteric control. Partition-defective 6 (Par-6) regulates the Par polarity complex by transmitting a GTPase signal through the Cdc42/Rac interaction binding PSD-95/Dlg/ZO-1 (CRIB-PDZ) module that alters PDZ ligand binding. Allosteric activation of the PDZ is achieved by local rearrangement of the L164 and K165 side chains to stabilize the interdomain CRIB:PDZ interface and reposition a conserved element of the ligand binding pocket. However, microsecond to millisecond dynamics measurements revealed that L164/K165 exchange requires a larger rearrangement than expected. The margin of thermodynamic stability for the PDZ domain is modest (∼3 kcal/mol) and further reduced by transient interactions with the disordered CRIB domain. Measurements of local structural stability revealed that tertiary contacts within the PDZ are disrupted by a partial unfolding transition that enables interconversion of the L/K switch. The unexpected participation of partial PDZ unfolding in the allosteric mechanism of Par-6 suggests that native-state unfolding may be essential for the function of other marginally stable proteins.

Tolerance of protein folding to a circular permutation in a PDZ domain

Circular permutation is a common molecular mechanism for evolution of proteins. However, such re-arrangement of secondary structure connectivity may interfere with the folding mechanism causing accumulation of folding intermediates, which in turn can lead to misfolding. We solved the crystal structure and investigated the folding pathway of a circularly permuted variant of a PDZ domain, SAP97 PDZ2. Our data illustrate how well circular permutation may work as a mechanism for molecular evolution. The circular permutant retains the overall structure and function of the native protein domain. Further, unlike most examples in the literature, this circular permutant displays a folding mechanism that is virtually identical to that of the wild type. This observation contrasts with previous data on the circularly permuted PDZ2 domain from PTP-BL, for which the folding pathway was remarkably affected by the same mutation in sequence connectivity. The different effects of this circular permutation in two homologous proteins show the strong influence of sequence as compared to topology. Circular permutation, when peripheral to the major folding nucleus, may have little effect on folding pathways and could explain why, despite the dramatic change in primary structure, it is frequently tolerated by different protein folds.

Equilibrium unfolding of the PDZ domain of β2-syntrophin

β2-syntrophin, a dystrophin-associated protein, plays a pivotal role in insulin secretion by pancreatic β-cells. It contains a PDZ domain (β2S-PDZ) that, in complex with protein-tyrosine phosphatase ICA512, anchors the dense insulin granules to actin filaments. The phosphorylation state of β2-syntrophin allosterically regulates the affinity of β2S-PDZ for ICA512, and the disruption of the complex triggers the mobilization of the insulin granule stores. Here, we investigate the thermal unfolding of β2S-PDZ at different pH and urea concentrations. Our results indicate that, unlike other PDZ domains, β2S-PDZ is marginally stable. Thermal denaturation experiments show broad transitions and cold denaturation, and a two-state model fit reveals a significant unfolded fraction under physiological conditions. Furthermore, T(m) and T(max) denaturant-dependent shifts and noncoincidence of melting curves monitored at different wavelengths suggest that two-state and three-state models fail to explain the equilibrium data properly and are in better agreement with a downhill scenario. Its higher stability at pH >9 and the results of molecular dynamics simulations indicate that this behavior of β2S-PDZ might be related to its charge distribution. All together, our results suggest a link between the conformational plasticity of the native ensemble of this PDZ domain and the regulation of insulin secretion.

An expanded view of the protein folding landscape of PDZ domains

Most protein domains fold in an apparently co-operative and two-state manner with only the native and denatured states significantly populated at any experimental condition. However, the protein folding energy landscape is often rugged and different transition states may be rate limiting for the folding reaction under different conditions, as seen for the PDZ protein domain family. We have here analyzed the folding kinetics of two PDZ domains and found that a previously undetected third transition state is rate limiting under conditions that stabilize the native state relative to the denatured state. In light of these results, we have re-analyzed previous folding data on PDZ domains and present a unified folding mechanism with three distinct transition states separated by two high-energy intermediates. Our data show that sequence composition tunes the relative stabilities of folding transition states within the PDZ family, while the overall mechanism is determined by topology. This model captures the kinetic folding mechanism of all PDZ domains studied to date.

Overview of protein folding mechanisms: experimental and theoretical approaches to probing energy landscapes

We present an overview of the current experimental and theoretical approaches to studying protein folding mechanisms, set against current models of the folding energy landscape. We describe how stability and folding kinetics can be determined experimentally and how this data can be interpreted in terms of the characteristic features of various models from the simplest two-state pathway to a multi-state mechanism. We summarize the pros and cons of a range of spectroscopic methods for measuring folding rates and present a theoretical framework, coupled with protein engineering approaches, for elucidating folding mechanisms and structural features of folding transition states. A series of case studies are used to show how experimental kinetic data can be interpreted in the context of non-native interactions, populated intermediates, parallel folding pathways, and sequential transition states. We also show how computational methods now allow transient species of high energy, such as folding transition states, to be modeled on the basis of experimental Φ-value analysis derived from the effects of point mutations on folding kinetics.

Morphogenesis of a protein: folding pathways and the energy landscape1

Current knowledge on the reaction whereby a protein acquires its native three-dimensional structure was obtained by and large through characterization of the folding mechanism of simple systems. Given the multiplicity of amino acid sequences and unique folds, it is not so easy, however, to draw general rules by comparing folding pathways of different proteins. In fact, quantitative comparison may be jeopardized not only because of the vast repertoire of sequences but also in view of a multiplicity of structures of the native and denatured states. We have tackled the problem of the relationships between the sequence information and the folding pathway of a protein, using a combination of kinetics, protein engineering and computational methods, applied to relatively simple systems. Our strategy has been to investigate the folding mechanism determinants using two complementary approaches, i.e. (i) the study of members of the same family characterized by a common fold, but substantial differences in amino acid sequence, or (ii) heteromorphic pairs characterized by largely identical sequences but with different folds. We discuss some recent data on protein-folding mechanisms by presenting experiments on different members of the PDZ domain family and their circularly permuted variants. Characterization of the energetics and structures of intermediates and TSs (transition states), obtained by Φ-value analysis and restrained MD (molecular dynamics) simulations, provides a glimpse of the malleability of the dynamic states and of the role of the topology of the native states and of the denatured states in dictating folding and misfolding pathways.

Sequence-specific long range networks in PSD-95/discs large/ZO-1 (PDZ) domains tune their binding selectivity

Protein-protein interactions mediated by modular protein domains are critical for cell scaffolding, differentiation, signaling, and ultimately, evolution. Given the vast number of ligands competing for binding to a limited number of domain families, it is often puzzling how specificity can be achieved. Selectivity may be modulated by intradomain allostery, whereby a remote residue is energetically connected to the functional binding site via side chain or backbone interactions. Whereas several energetic pathways, which could mediate intradomain allostery, have been predicted in modular protein domains, there is a paucity of experimental data to validate their existence and roles. Here, we have identified such functional energetic networks in one of the most common protein-protein interaction modules, the PDZ domain. We used double mutant cycles involving site-directed mutagenesis of both the PDZ domain and the peptide ligand, in conjunction with kinetics to capture the fine energetic details of the networks involved in peptide recognition. We performed the analysis on two homologous PDZ-ligand complexes and found that the energetically coupled residues differ for these two complexes. This result demonstrates that amino acid sequence rather than topology dictates the allosteric pathways. Furthermore, our data support a mechanism whereby the whole domain and not only the binding pocket is optimized for a specific ligand. Such cross-talk between binding sites and remote residues may be used to fine tune target selectivity.

Crystallographic and nuclear magnetic resonance evaluation of the impact of peptide binding to the second PDZ domain of protein tyrosine phosphatase 1E

PDZ (PSD95/Discs large/ZO-1) domains are ubiquitous protein interaction motifs found in scaffolding proteins involved in signal transduction. Despite the fact that many PDZ domains show a limited tendency to undergo structural change, the PDZ family has been associated with long-range communication and allostery. One of the PDZ domains studied most in terms of structure and biophysical properties is the second PDZ ("PDZ2") domain from protein tyrosine phosphatase 1E (PTP1E, also known as PTPL1). Previously, we showed through NMR relaxation studies that binding of the RA-GEF2 C-terminal peptide substrate results in long-range propagation of side-chain dynamic changes in human PDZ2 [Fuentes, E. J., et al. (2004) J. Mol. Biol. 335, 1105-1115]. Here, we present the first X-ray crystal structures of PDZ2 in the absence and presence of RA-GEF2 ligand, determined to resolutions of 1.65 and 1.3 Å, respectively. These structures deviate somewhat from previously determined NMR structures and indicate that very minor structural changes in PDZ2 accompany peptide binding. NMR residual dipolar couplings confirm the crystal structures to be accurate models of the time-averaged atomic coordinates of PDZ2. The impact on side-chain dynamics was further tested with a C-terminal peptide from APC, which showed results nearly identical to those of RA-GEF2. Thus, allosteric transmission in PDZ2 induced by peptide binding is conveyed purely and robustly by dynamics. (15)N relaxation dispersion measurements did not detect appreciable populations of a kinetic structural intermediate. Collectively, for ligand binding to PDZ2, these data support a lock-and-key binding model from a structural perspective and an allosteric model from a dynamical perspective, which together suggest a complex energy landscape for functional transitions within the ensemble.

The plastic energy landscape of protein folding: a triangular folding mechanism with an equilibrium intermediate for a small protein domain

Protein domains usually fold without or with only transiently populated intermediates, possibly to avoid misfolding, which could result in amyloidogenic disease. Whether observed intermediates are productive and obligatory species on the folding reaction pathway or dispensable by-products is a matter of debate. Here, we solved the crystal structure of a small protein domain, SAP97 PDZ2 I342W C378A, and determined its folding pathway. The presence of a folding intermediate was demonstrated both by single and double-mixing kinetic experiments using urea-induced (un)folding as well as ligand-induced folding. This protein domain was found to fold via a triangular scheme, where the folding intermediate could be either on- or off-pathway, depending on the experimental conditions. Furthermore, we found that the intermediate was present at equilibrium, which is rarely seen in folding reactions of small protein domains. The folding mechanism observed here illustrates the roughness and plasticity of the protein folding energy landscape, where several routes may be employed to reach the native state. The results also reconcile the folding mechanisms of topological variants within the PDZ domain family.

What lessons can be learned from studying the folding of homologous proteins?

The studies of the folding of structurally related proteins have proved to be a very important tool for investigating protein folding. Here we review some of the insights that have been gained from such studies. Our highlighted studies show just how such an investigation should be designed and emphasise the importance of the synergy between experiment and theory. We also stress the importance of choosing the right system carefully, exploiting the excellent structural and sequence databases at our disposal.

Experimental snapshots of a protein-DNA binding landscape

Protein recognition of DNA sites is a primary event for gene function. Its ultimate mechanistic understanding requires an integrated structural, dynamic, kinetic, and thermodynamic dissection that is currently limited considering the hundreds of structures of protein-DNA complexes available. We describe a protein-DNA-binding pathway in which an initial, diffuse, transition state ensemble with some nonnative contacts is followed by formation of extensive nonnative interactions that drive the system into a kinetic trap. Finally, nonnative contacts are slowly rearranged into native-like interactions with the DNA backbone. Dissimilar protein-DNA interfaces that populate along the DNA-binding route are explained by a temporary degeneracy of protein-DNA interactions, centered on "dual-role" residues. The nonnative species slow down the reaction allowing for extended functionality.

Deciphering the folding transition state structure and denatured state properties of nucleophosmin C-terminal domain

Nucleophosmin (NPM1), one of the most abundant nucleolar proteins, is a frequent target of oncogenic mutations in acute myeloid leukaemia (AML). Mutation-induced changes at the C-terminal domain of NPM1 (Cter-NPM1) compromise its stability and cause the aberrant translocation of NPM1 to the cytosol. Hence, this protein represents a suitable candidate to investigate the relations between folding and disease. Since Cter-NPM1 folds via a compact denatured state, stabilization of the folded state of the mutated variants demands detailed structural information on both the native and denatured states. Here, we present the characterization of the complete folding pathway of Cter-NPM1 and provide molecular details for both the transition and the denatured states. The structure of the transition state was assessed by Phi-value analysis, whereas residual structure in the denatured state was mapped by evaluating the effect of mutations as modulated by conditions promoting denatured state compaction. Data reveal that folding of Cter-NPM1 proceeds via an extended nucleus and that the denatured state retains significant malleable structure at the interface between the second and third helices. Our observations constitute the essential prerequisite for structure-based drug-design studies, aimed at identifying molecules that may rescue pathological NPM1 mutants by stabilizing the native-like state.

Distinguishing between smooth and rough free energy barriers in protein folding

Analysis of curved chevron plots is a powerful tool in investigating protein folding pathways, as the curvatures can be used to gain information about both early and late folding events. When and if accumulation of low-energy intermediates can be ruled out, two different models have classically been applied to describe curved chevron plots, namely , (i) Hammond effects along smooth barrier profiles and (ii) changes in the rate-limiting step between two discrete transition states. The two models lead to very similar numerical solutions, which are generally indistinguishable. This is not surprising, since the smooth barrier assumption approximates barrier profiles with a more complex topology involving multiple local maxima that are too close, or too broad, to yield clear-cut kinks in the chevron data. In this work, we have reconstructed the transition state shifts as a function of protein stability over a wide stability range for three small globular proteins, to screen for fingerprints more sensitive for different barrier profiles. We show that such an analysis represents a valuable test for the discrimination between the two different scenarios.

Engineering a switchable toxin: the potential use of PDZ domains in the expression, targeting and activation of modified saporin variants

A critical problem in studying ribosome-inactivating proteins (RIPs) lies in the very limited possibility to produce them in heterologous systems. In fact, their inherent toxicity for the producing organism nearly always prevents their recombinant expression. In this study, we designed, expressed and characterized an engineered form of the RIP saporin (SapVSAV), bearing a C-terminal extra sequence that is recognized and bound by the second PDZ domain from murine PTP-BL protein (PDZ2). The co-expression of SapVSAV and PDZ2 in Escherichia coli BL21 cells greatly enhances the production of the toxin in a soluble form. The increase of production was surprisingly not due to protection from bacterial intoxication, but may arise from a stabilization effect of PDZ2 on the toxin molecule during biosynthesis. We found that once purified, SapVSAV is stable but is not toxic to free ribosomes, while it is fully active against human cancer cells. This strategy of co-expression of a toxin moiety and a soluble PDZ domain may represent a new system to increase the production of recombinant toxic proteins and could allow the selection of new extra sequences to target PDZ domains inside specific mammalian cellular domains.

Structural determinants of protein folding

The last several decades have seen an explosion of knowledge in the field of structural biology. With critical advances in spectroscopic techniques in examining structures of biomacromolecules, in maturation of molecular biology techniques, as well as vast improvements in computation prowess, protein structures are now being elucidated at an unprecedented rate. In spite of all the recent advances, the protein folding puzzle remains as one of the fundamental biochemical challenges. A facet to this empiric problem is the structural determinants of protein folding. What are the driving forces that pivot a polypeptide chain to a specific conformation amongst the vast conformation space? In this review, we shall discuss some of the structural determinants to protein folding that have been identified in the recent decades.

Agitation and high ionic strength induce amyloidogenesis of a folded PDZ domain in native conditions

Amyloid fibril formation is a distinctive hallmark of a number of degenerative diseases. In this process, protein monomers self-assemble to form insoluble structures that are generally referred to as amyloid fibrils. We have induced in vitro amyloid fibril formation of a PDZ domain by combining mechanical agitation and high ionic strength under conditions otherwise close to physiological (pH 7.0, 37 degrees C, no added denaturants). The resulting aggregates enhance the fluorescence of the thioflavin T dye via a sigmoidal kinetic profile. Both infrared spectroscopy and circular dichroism spectroscopy detect the formation of a largely intermolecular beta-sheet structure. Atomic force microscopy shows straight, rod-like fibrils that are similar in appearance and height to mature amyloid-like fibrils. Under these conditions, before aggregation, the protein domain adopts an essentially native-like structure and an even higher conformational stability (DeltaG(U-F)(H2O)). These results show a new method for converting initially folded proteins into amyloid-like aggregates. The methodological approach used here does not require denaturing conditions; rather, it couples agitation with a high ionic strength. Such an approach offers new opportunities to investigate protein aggregation under conditions in which a globular protein is initially folded, and to elucidate the physical forces that promote amyloid fibril formation.

Aromatic residues engineered into the beta-turn nucleation site of ubiquitin lead to a complex folding landscape, non-native side-chain interactions, and kinetic traps

The fast folding of small proteins is likely to be the product of evolutionary pressures that balance the search for native-like contacts in the transition state with the minimum number of stable non-native interactions that could lead to partially folded states prone to aggregation and amyloid formation. We have investigated the effects of non-native interactions on the folding landscape of yeast ubiquitin by introducing aromatic substitutions into the beta-turn region of the N-terminal beta-hairpin, using both the native G-bulged type I turn sequence (TXTGK) as well as an engineered 2:2 XNGK type I' turn sequence. The N-terminal beta-hairpin is a recognized folding nucleation site in ubiquitin. The folding kinetics for wt-Ub (TLTGK) and the type I' turn mutant (TNGK) reveal only a weakly populated intermediate, however, substitution with X = Phe or Trp in either context results in a high propensity to form a stable compact intermediate where the initial U-->I collapse is visible as a distinct kinetic phase. The introduction of Trp into either of the two host turn sequences results in either complex multiphase kinetics with the possibility of parallel folding pathways, or formation of a highly compact I-state stabilized by non-native interactions that must unfold before refolding. Sequence substitutions with aromatic residues within a localized beta-turn capable of forming non-native hydrophobic contacts in both the native state and partially folded states has the undesirable consequence that folding is frustrated by the formation of stable compact intermediates that evolutionary pressures at the sequence level may have largely eliminated.

Comparison of successive transition states for folding reveals alternative early folding pathways of two homologous proteins

The energy landscape theory provides a general framework for describing protein folding reactions. Because a large number of studies, however, have focused on two-state proteins with single well-defined folding pathways and without detectable intermediates, the extent to which free energy landscapes are shaped up by the native topology at the early stages of the folding process has not been fully characterized experimentally. To this end, we have investigated the folding mechanisms of two homologous three-state proteins, PTP-BL PDZ2 and PSD-95 PDZ3, and compared the early and late transition states on their folding pathways. Through a combination of Phi value analysis and molecular dynamics simulations we obtained atomic-level structures of the transition states of these homologous three-state proteins and found that the late transition states are much more structurally similar than the early ones. Our findings thus reveal that, while the native state topology defines essentially in a unique way the late stages of folding, it leaves significant freedom to the early events, a result that reflects the funneling of the free energy landscape toward the native state.

Helix mutations stabilize a late productive intermediate on the folding pathway of ubiquitin

We have investigated the relative placement of rate-limiting energy barriers and the role of productive or obstructive intermediates on the folding pathway of yeast wild-type ubiquitin ( wt-Ub) containing the F45W mutation. To manipulate the folding barriers, we have designed a family of mutants in which stabilizing substitutions have been introduced incrementally on the solvent-exposed surface of the main alpha-helix (residues 23-34), which has a low intrinsic helical propensity in the native sequence. Although the U --> I and I --> N transitions are not clearly delineated in the kinetics of wt-Ub, we show that an intermediate becomes highly populated and more clearly resolved as the predicted stability of the helix increases. The observed acceleration in the rate of folding correlates with helix stability and is consistent with the I-state representing a productive rather than misfolded state. A Leffler analysis of the effects on kinetics of changes in stability within the family of helix mutants results in a biphasic correlation in both the refolding and unfolding rates that suggest a shift from a nucleation-condensation mechanism (weakly stabilized helix) toward a diffusion-collision model (highly stabilized helix). Through the introduction of helix-stabilizing mutations, we are able to engineer a well-resolved I-state on the folding pathway of ubiquitin which is likely to be structurally distinct from that which is only weakly populated on the folding pathway of wild-type ubiquitin.

Folding and misfolding in a naturally occurring circularly permuted PDZ domain

One of the most extreme and fascinating examples of naturally occurring mutagenesis is represented by circular permutation. Circular permutations involve the linking of two chain ends and cleavage at another site. Here we report the first description of the folding mechanism of a naturally occurring circularly permuted protein, a PDZ domain from the green alga Scenedesmus obliquus. Data reveal that the folding of the permuted protein is characterized by the presence of a low energy off-pathway kinetic trap. This finding contrasts with what was previously observed for canonical PDZ domains that, although displaying a similar primary structure when structurally re-aligned, fold via an on-pathway productive intermediate. Although circular permutation of PDZ domains may be necessary for a correct orientation of their functional sites in multi-domain protein scaffolds, such structural rearrangement may compromise their folding pathway. This study provides a straightforward example of the divergent demands of folding and function.