Kinetic folding mechanism of PDZ2 from PTP-BL

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.

Cryptic binding properties of a transient folding intermediate in a PDZ tandem repeat

PDZ domains are the most diffused protein-protein interaction modules of the human proteome and are often present in tandem repeats. An example is PDZD2, a protein characterized by the presence of six PDZ domains that undergoes a proteolytic cleavage producing sPDZD2, comprising a tandem of two PDZ domains, namely PDZ5 and PDZ6. Albeit the physiopathological importance of sPDZD2 is well-established, the interaction with endogenous ligands has been poorly characterized. To understand the determinants of the stability and function of sPDZD2, we investigated its folding pathway. Our data highlights the presence of a complex scenario involving a transiently populated folding intermediate that may be accumulated from the concurrent denaturation of both PDZ5 and PDZ6 domains. Importantly, double jump kinetic experiments allowed us to pinpoint the ability of this transient intermediate to bind the physiological ligand of sPDZD2 with increased affinity compared to the native state. In summary, our results provide an interesting example of a functionally competent misfolded intermediate, which may exert a cryptic function that is not captured from the analysis of the native state only.

Folding and Binding Mechanisms of the SH2 Domain from Crkl

SH2 domains are structural modules specialized in the recognition and binding of target sequences containing a phosphorylated tyrosine residue. They are mostly incorporated in the 3D structure of scaffolding proteins that represent fundamental regulators of several signaling pathways. Among those, Crkl plays key roles in cell physiology by mediating signals from a wide range of stimuli, and its overexpression is associated with several types of cancers. In myeloid cells expressing the oncogene BCR/ABL, one interactor of Crkl-SH2 is the focal adhesion protein Paxillin, and this interaction is crucial in leukemic transformation. In this work, we analyze both the folding pathway of Crkl-SH2 and its binding reaction with a peptide mimicking Paxillin, under different ionic strength and pH conditions, by using means of fluorescence spectroscopy. From a folding perspective, we demonstrate the presence of an intermediate along the reaction. Moreover, we underline the importance of the electrostatic interactions in the early event of recognition, occurring between the phosphorylated tyrosine of the Paxillin peptide and the charge residues of Crkl-SH2. Finally, we highlight a pivotal role of a highly conserved histidine residue in the stabilization of the binding complex. The experimental results are discussed in light of previous works on other SH2 domains.

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.

Determining folding and binding properties of the C-terminal SH2 domain of SHP2

SH2 domains are a class of protein–protein interaction modules with the function to recognize and bind sequences characterized by the presence of a phosphorylated tyrosine. SHP2 is a protein phosphatase involved in the Ras‐ERK1/2 signaling pathway that possess two SH2 domains, namely, N‐SH2 and C‐SH2, that mediate the interaction of SHP2 with various partners and determine the regulation of its catalytic activity. One of the main interactors of the SH2 domains of SHP2 is Gab2, a scaffolding protein with critical role in determining cell differentiation. Despite their key biological role and the importance of a correct native fold to ensure it, the mechanism of binding of SH2 domains with their ligands and the determinants of their stability have been poorly characterized. In this article, we present a comprehensive kinetic study of the folding of the C‐SH2 domain and the binding mechanism with a peptide mimicking a region of Gab2. Our data, obtained at different pH and ionic strength conditions and supported by site‐directed mutagenesis, highlight the role of electrostatic interactions in the early events of recognition. Interestingly, our results suggest a key role of a highly conserved histidine residue among SH2 family in the interaction with negative charges carried by the phosphotyrosine of Gab2. Moreover, the analysis of the equilibrium and kinetic folding data of C‐SH2 describes a complex mechanism implying a change in rate‐limiting step at high denaturant concentrations. Our data are discussed under the light of previous works on N‐SH2 domain of SHP2 and other SH2 domains.

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.

Addressing the role of the α-helical extension in the folding of the third PDZ domain from PSD-95

PDZ domains are one of the most important protein-protein interaction domains in human. While presenting a conserved three dimensional structure, a substantial number of PDZ domains display structural extensions suggested to be involved in their folding and binding mechanisms. The C-terminal α-helix extension (α3) of the third PDZ domain from PSD-95 (PDZ3) has been reported to have a role in function of the domain as well as in the stabilization of the native fold. Here we report an evaluation of the effect of the truncation of this additional helix on the folding and unfolding kinetics of PDZ3. Fluorescent variants of full length and truncated PDZ3 were produced and stopped-flow fluorescence measurements were made under different experimental conditions (pH, ionic strength and temperature) to investigate the folding kinetics of the respective variant. The results show that folding of PDZ3 is robust and that the mechanism is only marginally affected by the truncation, which contributes to a destabilization of the native state, but otherwise do not change the overall observed kinetics. Furthermore, the increase in the unfolding rate constants, but not the folding rate constant upon deletion of α3 suggests that the α-helical extension is largely unstructured in the folding transition state.

Understanding the role of phosphorylation in the binding mechanism of a PDZ domain

The PDZ domain is one of the most common protein-protein interaction domains in mammalian species. While several studies have demonstrated the importance of phosphorylation in interactions involving PDZ domains, there is a paucity of detailed mechanistic data addressing how the PDZ interaction is affected by phosphorylation. Here, we address this question by equilibrium and kinetic binding experiments using PDZ2 from protein tyrosine phosphatase L1 and its interaction with a peptide from the natural ligand RIL. The results show that phosphorylation of a serine residue in the RIL peptide has dual and opposing effects: it increases both the association and dissociation rate constants, which leads to an overall weakening of binding. Furthermore, we performed binding experiments with a RIL peptide in which the serine was replaced by a glutamate, a commonly used method to mimic phosphorylation in proteins. Strikingly, both the affinity and the ionic strength dependence of the affinity differed markedly for the phosphoserine and glutamate peptides. These results show that, in this particular case, glutamate is a poor mimic of serine phosphorylation.

Common features in the unfolding and misfolding of PDZ domains and beyond: the modulatory effect of domain swapping and extra-elements

PDZ domains are protein-protein interaction modules sharing the same structural arrangement. To discern whether they display common features in their unfolding/misfolding behaviour we have analyzed in this work the unfolding thermodynamics, together with the misfolding kinetics, of the PDZ fold using three archetypical examples: the second and third PDZ domains of the PSD95 protein and the Erbin PDZ domain. Results showed that all domains passed through a common intermediate, which populated upon unfolding, and that this in turn drove the misfolding towards worm-like fibrillar structures. Thus, the unfolding/misfolding behaviour appears to be shared within these domains. We have also analyzed how this landscape can be modified upon the inclusion of extra-elements, as it is in the nNOS PDZ domain, or the organization of swapped species, as happens in the second PDZ domain of the ZO2 protein. Although the intermediates still formed upon thermal unfolding, the misfolding was prevented to varying degrees.

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.

A thermodynamic study of the third PDZ domain of MAGUK neuronal protein PSD-95 reveals a complex three-state folding behavior

The relevance of the C-terminal α helix of the PDZ3 domain of PSD95 in its unfolding process has been explored by achieving the thermodynamic characterization of a construct where the sequence of the nine residues corresponding to such motif has been deleted. Calorimetric traces at neutral pH require the application of a three-state model displaying three different equilibrium processes in which the intermediate state self-associates upon heating, being stable and populated in a wide temperature range. Temperature scans followed by circular dichroism, Fourier transform infrared spectroscopy and dynamic light scattering support the presence of such oligomeric-partially folded species. This study reveals that the deletion of the α3-helix sequence results in a more complex description of the domain unfolding.

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.

Probing the role of backbone hydrogen bonds in protein-peptide interactions by amide-to-ester mutations

One of the most frequent protein-protein interaction modules in mammalian cells is the postsynaptic density 95/discs large/zonula occludens 1 (PDZ) domain, involved in scaffolding and signaling and emerging as an important drug target for several diseases. Like many other protein-protein interactions, those of the PDZ domain family involve formation of intermolecular hydrogen bonds: C-termini or internal linear motifs of proteins bind as β-strands to form an extended antiparallel β-sheet with the PDZ domain. Whereas extensive work has focused on the importance of the amino acid side chains of the protein ligand, the role of the backbone hydrogen bonds in the binding reaction is not known. Using amide-to-ester substitutions to perturb the backbone hydrogen-bonding pattern, we have systematically probed putative backbone hydrogen bonds between four different PDZ domains and peptides corresponding to natural protein ligands. Amide-to-ester mutations of the three C-terminal amides of the peptide ligand severely affected the affinity with the PDZ domain, demonstrating that hydrogen bonds contribute significantly to ligand binding (apparent changes in binding energy, ΔΔG = 1.3 to >3.8 kcal mol(-1)). This decrease in affinity was mainly due to an increase in the dissociation rate constant, but a significant decrease in the association rate constant was found for some amide-to-ester mutations suggesting that native hydrogen bonds have begun to form in the transition state of the binding reaction. This study provides a general framework for studying the role of backbone hydrogen bonds in protein-peptide interactions and for the first time specifically addresses these for PDZ domain-peptide interactions.

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.

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.

GB1 is not a two-state folder: identification and characterization of an on-pathway intermediate

The folding pathway of the small α/β protein GB1 has been extensively studied during the past two decades using both theoretical and experimental approaches. These studies provided a consensus view that the protein folds in a two-state manner. Here, we reassessed the folding of GB1, both by experiments and simulations, and detected the presence of an on-pathway intermediate. This intermediate has eluded earlier experimental characterization and is distinct from the collapsed state previously identified using ultrarapid mixing. Failure to identify the presence of an intermediate affects some of the conclusions that have been drawn for GB1, a popular model for protein folding studies.

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.

An oligomeric equilibrium intermediate as the precursory nucleus of globular and fibrillar supramacromolecular assemblies in a PDZ domain

The equilibrium unfolding at neutral pH of the third PDZ domain of PSD95, as followed by DSC, is characterized by the presence of an equilibrium intermediate with clear signs of oligomerization. DLS and SEC measurements indicate that at 60-70 degrees C small oligomers populate, showing a typical beta-sheet far-UV CD spectrum. These intermediate species lead to the formation of rodlike particulates of approximately 12 nm, which remain in solution after 2 weeks incubation and grow until they adopt annular/spherical shapes of approximately 50 nm and protofibrils, which are subsequently fully transformed into fibrils. The fibrils can also disaggregate after the addition of 1:1 buffer dilution followed by cooling to room temperature, thus returning to the initial monomeric state. Growth kinetics, as shown by ThT and ANS fluorescence, show that the organization of the different supramacromolecular structures comes from a common nucleation unit, the small oligomers, which organize themselves before reaching the incubation temperature of 60 degrees C. Our experiments point toward the existence of a well-defined reversible, stepwise, and downhill organization of the processes involved in the association-dissociation of the intermediate. We estimate the enthalpy change accompanying the association-dissociation equilibria to be 130 kJ x mol(-1). Furthermore, the coalescence under essentially reversible conditions of different kinds of supramacromolecular assemblies renders this protein system highly interesting for biophysical studies aimed at our further understanding of amyloid pathological conditions.

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.

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.

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.

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.

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.

Combining experiment and simulation in protein folding: closing the gap for small model systems

All-atom molecular dynamics (MD) simulations on increasingly powerful computers have been combined with experiments to characterize protein folding in detail over wider time ranges. The folding of small ultrafast folding proteins is being simulated on micros timescales, leading to improved structural predictions and folding rates. To what extent is 'closing the gap' between simulation and experiment for such systems providing insights into general mechanisms of protein folding?

Diffusion-limited unbinding of small peptides from PDZ domains

PDZ domains are typical examples of binding motifs mediating the formation of protein-protein assemblies in many different cells. A quantitative characterization of the mechanisms intertwining structure, chemistry, and dynamics with the PDZ function represent a challenge in molecular biology. Here, we investigated the influence of native state topology on the thermodynamics and dissociation kinetics for a PDZ/peptide complex via molecular dynamics simulations based on a coarse-grained description of PDZ domains. Our native-centric approach neglects chemical details but incorporates the basic structural information to reproduce the protein functional dynamics as it couples to binding. We found that at physiological temperatures the unbinding of a peptide from the PDZ domain becomes increasingly diffusive rather than thermally activated, as a consequence of the significant reduction of the free energy barrier with temperature. In turn, this results in a significant slowing down of the process of 2 orders of magnitude with respect to the conventional Arrhenius extrapolation from low-temperature calculations. Finally, a detailed analysis of a typical unbinding event based on the rupture times of single peptide-PDZ contacts allows us to shed further light on the dissociation mechanism and to elaborate a coherent picture of the relation between function and dynamics in PDZ domains.

Identification and characterization of protein folding intermediates

In order to understand the mechanism by which a polypeptide chain folds into its functionally active native state it is necessary to characterize in detail all the species accumulated along the pathway. The elusive nature of protein folding intermediates poses their identification and characterization as an extremely difficult task in the protein folding field. In the case of small single domain proteins, the direct measurement of the thermodynamics and structural parameters of protein folding intermediates has provided new insights on the nature of the forces involved in the stabilization of nascent protein structures. Here we summarize some of the experimental approaches aimed at the detection and characterization of folding intermediates along with a discussion of some general structural features emerging from these studies.

An On-pathway Intermediate in the Folding of a PDZ Domain

The folding pathways of some proteins include the population of partially structured species en route to the native state. Identification and characterization of these folding intermediates are particularly difficult as they are often only transiently populated and play different mechanistic roles, being either on-pathway productive species or off-pathway kinetic traps. To define the role of folding intermediates, a quantitative analysis of the folding and unfolding rate constants over a wide range of denaturant concentration is often required. Such a task is further complicated by the reversible nature of the folding reaction, which implies the observed kinetics to be governed by a complex combination of different microscopic rate constants. Here, we tackled this problem by measuring directly the folding rate constant under highly denaturing conditions, namely by inducing the folding of a PDZ domain through a quasi-irreversible binding reaction with a specific peptide. In analogy with previous works based on hydrogen exchange experiments, we present evidence that the folding pathway of the PDZ domain involves the formation of an obligatory on-pathway intermediate. The results presented exemplify a novel type of kinetic test to detect an on-pathway folding intermediate.

A conserved folding mechanism for PDZ domains

An important question in protein folding is whether the folding mechanism is sequence dependent and conserved for homologous proteins. In this work we compared the kinetic folding mechanism of five postsynaptic density protein-95, disc-large tumor suppressor protein, zonula occludens-1 (PDZ) domains, sharing similar topology but having different primary structures. Investigation of the different proteins under various experimental conditions revealed that the folding kinetics of each member of the PDZ family can be described by a model with two transition states separated by an intermediate. Moreover, the positions of the two transition states along the reaction coordinate (as given by their beta(T)-values) are fairly constant for the five PDZ domains.

A PDZ domain recapitulates a unifying mechanism for protein folding

A unifying view has been recently proposed according to which the classical diffusion-collision and nucleation-condensation models may represent two extreme manifestations of an underlying common mechanism for the folding of small globular proteins. We report here the characterization of the folding process of the PDZ domain, a protein that recapitulates the three canonical steps involved in this unifying mechanism, namely: (i) the early formation of a weak nucleus that determines the native-like topology of a large portion of the structure, (ii) a global collapse of the entire polypeptide chain, and (iii) the consolidation of the remaining partially structured regions to achieve the native state conformation. These steps, which are clearly detectable in the PDZ domain investigated here, may be difficult to distinguish experimentally in other proteins, which would thus appear to follow one of the two limiting mechanisms. The analysis of the (un)folding kinetics for other three-state proteins (when available) appears consistent with the predictions ensuing from this unifying mechanism, thus providing a powerful validation of its general nature.

Demonstration of Long-Range Interactions in a PDZ Domain by NMR, Kinetics, and Protein Engineering

Understanding the basis of communication within protein domains is a major challenge in structural biology. We present structural and dynamical evidence for allosteric effects in a PDZ domain, PDZ2 from the tyrosine phosphatase PTP-BL, upon binding to a target peptide. The NMR structures of its free and peptide-bound states differ in the orientation of helix alpha2 with respect to the remainder of the molecule, concomitant with a readjustment of the hydrophobic core. Using an ultrafast mixing instrument, we detected a deviation from simple bimolecular kinetics for the association with peptide that is consistent with a rate-limiting conformational change in the protein (k(obs) approximately 7 x 10(3) s(-1)) and an induced-fit model. Furthermore, the binding kinetics of 15 mutants revealed that binding is regulated by long-range interactions, which can be correlated with the structural rearrangements resulting from peptide binding. The homologous protein PSD-95 PDZ3 did not display a similar ligand-induced conformational change.

The kinetics of PDZ domain-ligand interactions and implications for the binding mechanism

PDZ domains are protein adapter modules present in a few hundred human proteins. They play important roles in scaffolding and signal transduction. PDZ domains usually bind to the C termini of their target proteins. To assess the binding mechanism of this interaction we have performed the first in-solution kinetic study for PDZ domains and peptides corresponding to target ligands. Both PDZ3 from postsynaptic density protein 95 and PDZ2 from protein tyrosine phosphatase L1 bind their respective target peptides through an apparent A + B --> A.B mechanism without rate-limiting conformational changes. But a mutant with a fluorescent probe (Trp) outside of the binding pocket suggests that slight changes in the structure take place upon binding in protein tyrosine phosphatase-L1 PDZ2. For PDZ3 from postsynaptic density protein 95 the pH dependence of the binding reaction is consistent with a one-step mechanism with one titratable group. The salt dependence of the interaction shows that the formation of electrostatic interactions is rate-limiting for the association reaction but not for dissociation of the complex.