Eva S. Cobos

A Thermodynamic and Kinetic Analysis of the Folding Pathway of an SH3 Domain Entropically Stabilised by a Redesigned Hydrophobic Core

The folding thermodynamics and kinetics of the alpha-spectrin SH3 domain with a redesigned hydrophobic core have been studied. The introduction of five replacements, A11V, V23L, M25V, V44I and V58L, resulted in an increase of 16% in the overall volume of the side-chains forming the hydrophobic core but caused no remarkable changes to the positions of the backbone atoms. Judging by the scanning calorimetry data, the increased stability of the folded structure of the new SH3-variant is caused by entropic factors, since the changes in heat capacity and enthalpy upon the unfolding of the wild-type and mutant proteins were identical at 298 K. It appears that the design process resulted in an increase in burying both the hydrophobic and hydrophilic surfaces, which resulted in a compensatory effect upon the changes in heat capacity and enthalpy. Kinetic analysis shows that both the folding and unfolding rate constants are higher for the new variant, suggesting that its transition state becomes more stable compared to the folded and unfolded states. The phi(double dagger-U) values found for a number of side-chains are slightly lower than those of the wild-type protein, indicating that although the transition state ensemble (TSE) did not change overall, it has moved towards a more denatured conformation, in accordance with Hammonds postulate. Thus, the acceleration of the folding-unfolding reactions is caused mainly by an improvement in the specific and/or non-specific hydrophobic interactions within the TSE rather than by changes in the contact order. Experimental evidence showing that the TSE changes globally according to its hydrophobic content suggests that hydrophobicity may modulate the kinetic behaviour and also the folding pathway of a protein.

AS-48: a circular protein with an extremely stable globular structure

The unfolding thermodynamics of the circular enterocin protein AS‐48, produced by Enterococcus faecalis, has been characterized by differential scanning calorimetry. The native structure of the 70‐residue protein is extremely thermally stable. Thus, at pH 2.5 and low ionic strength thermal denaturation occurs under equilibrium at 102°C, while the unfolded state irreversibly aggregates at neutral and alkaline pH. Calorimetric data analysis shows that the specific enthalpy change upon unfolding is unusually small and the heat capacity change is quite normal for a protein of this size, whereas the Gibbs energy change at 25°C is relatively high. At least part of this high stability might be put down to entropic constraints induced by the circular organization of the polypeptide chain.

The denaturation of circular enterocin AS-48 by urea and guanidinium hydrochloride.

The unfolding thermodynamics of the circular enterocin protein AS-48, produced by Enterococcus faecalis, has been studied. The native structure of the 70-amino-acid-long protein turned out to be extremely stable against heat and denaturant-induced unfolding. At pH 2.5 and low ionic strength, it denatures at 102 degrees C, while at 25 degrees C, the structure only unfolds in 6.3 M guanidinium hydrochloride (GuHCl) and does not unfold even in 8 M urea. A comparison of its thermal unfolding in water and in the presence of urea shows a good correspondence between the two deltaGw(298) values, which are about 30 kJ mol(-1) at pH 2.5 and low ionic strength. The stability of the structure is highly dependent upon ionic strength and so GuHCl acts both as a denaturant and a stabilising agent. This seems to be why the deltaGw(298) value calculated from the unfolding data in GuHCl is twice as high as in the absence of this salt. At least part of the high stability of native AS-48 can almost certainly be put down to its circular organization since other structural features are quite normal for a protein of this size.

Single-chain protein mimetics of the N-terminal heptad-repeat region of gp41 with potential as anti–HIV-1 drugs

Significance The envelope subunit gp41 is an attractive target for therapeutic intervention against HIV-1. Interfering with the interaction between the heptad-repeat regions of gp41 is a promising approach to inhibit HIV-1 fusion to the host cell membrane. Here, we present an alternative rational design and protein-engineering approach to produce highly stable single-chain proteins that accurately mimic the trimeric coiled-coil surface of the gp41 N-terminal heptad repeat. This approach has a strong potential for development to HIV-1 drugs, vaccines, or microbicides and could be extendable to the design of proteins interfering with other types of coiled-coil interactions. During HIV-1 fusion to the host cell membrane, the N-terminal heptad repeat (NHR) and the C-terminal heptad repeat (CHR) of the envelope subunit gp41 become transiently exposed and accessible to fusion inhibitors or Abs. In this process, the NHR region adopts a trimeric coiled-coil conformation that can be a target for therapeutic intervention. Here, we present an approach to rationally design single-chain protein constructs that mimic the NHR coiled-coil surface. The proteins were built by connecting with short loops two parallel NHR helices and an antiparallel one with the inverse sequence followed by engineering of stabilizing interactions. The constructs were expressed in Escherichia coli, purified with high yield, and folded as highly stable helical coiled coils. The crystal structure of one of the constructs confirmed the predicted fold and its ability to accurately mimic an exposed gp41 NHR surface. These single-chain proteins bound to synthetic CHR peptides with very high affinity, and furthermore, they showed broad inhibitory activity of HIV-1 fusion on various pseudoviruses and primary isolates.

WW Domains of the Yes-Kinase-Associated-Protein (YAP) Transcriptional Regulator Behave as Independent Units with Different Binding Preferences for PPxY Motif-Containing Ligands

YAP is a WW domain-containing effector of the Hippo tumor suppressor pathway, and the object of heightened interest as a potent oncogene and stemness factor. YAP has two major isoforms that differ in the number of WW domains they harbor. Elucidating the degree of co-operation between these WW domains is important for a full understanding of the molecular function of YAP. We present here a detailed biophysical study of the structural stability and binding properties of the two YAP WW domains aimed at investigating the relationship between both domains in terms of structural stability and partner recognition. We have carried out a calorimetric study of the structural stability of the two YAP WW domains, both isolated and in a tandem configuration, and their interaction with a set of functionally relevant ligands derived from PTCH1 and LATS kinases. We find that the two YAP WW domains behave as independent units with different binding preferences, suggesting that the presence of the second WW domain might contribute to modulate target recognition between the two YAP isoforms. Analysis of structural models and phage-display studies indicate that electrostatic interactions play a critical role in binding specificity. Together, these results are relevant to understand of YAP function and open the door to the design of highly specific ligands of interest to delineate the functional role of each WW domain in YAP signaling.

Thermodynamic characterization of the folding equilibrium of the human Nedd4-WW4 domain: at the frontiers of cooperative folding.

WW domains are the smallest naturally independent beta-sheet protein structures available to date and constitute attractive model systems for investigating the determinants of beta-sheet folding and stability. Nonetheless, their small size and low cooperativity pose a difficult challenge for a quantitative analysis of the folding equilibrium. We describe here a comprehensive thermodynamic characterization of the conformational equilibrium of the fourth WW domain from the human ubiquitin ligase Nedd4 (hNedd4-WW4) using a combination of calorimetric and spectroscopic techniques with several denaturing agents (temperature, pH, and chemical denaturants). Our results reveal that even though the experimental data can be described in terms of a two-state equilibrium, spectral data together with anomalous values for some thermodynamic parameters (a strikingly low temperature of maximum stability, a higher than expected native-state heat capacity, and a small specific enthalpy of unfolding) could be indicative of more complex types of equilibria, such as one-state downhill folding or alternative native conformations. Moreover, double-perturbation experiments reveal some features that, in spite of the apparent linear correlation between the thermodynamic parameters, seem to be indicative of a complex conformational equilibrium in the presence of urea. In summary, the data presented here point toward the existence of a low-energy barrier between the different macrostates of hNedd4-WW4, placing it at the frontier of cooperative folding.

Novel conformational aspects of the third PDZ domain of the neuronal post-synaptic density-95 protein revealed from two 1.4A X-ray structures

The crystal structure of the third PDZ domain of the neuronal post-synaptic density-95 protein (PSD95-PDZ3, residues 302-402) has been solved at 1.4 and 1.35A from two different crystal forms. These structures lack the cloning artefact present in the carboxyl terminal sequence of the former crystallographic structures and they belong to the space groups P4(3) and P1. The new PDZ structures are identical between the two crystal forms and among the four chains of the P1 crystal form. When we compare the new structures with the previous ones, some important conformational differences in the C-terminal alpha-helix and in the loop connecting beta2 and beta3 strands have been found. Additionally, the high resolution of the new structures has allowed us to indentify a succinimide residue at the position corresponding to Asp332 in the beta2-beta3 loop, which may contribute to the alternate conformation of this loop, and at the same time, to the interaction between residues from this loop and the C-terminal alpha-helix. Thus, these features would have implications in the recently proposed allosteric role of this third alpha-helix in the binding of the carboxyl terminal fragments to the PSD95-PDZ3.

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.

A thermodynamic characterization of the interaction of 8-anilino-1-naphthalenesulfonic acid with native globular proteins: the effect of the ligand dimerization in the analysis of the binding isotherms.

8‐Anilino‐1‐naphthalenesulfonic acid (ANS) is a popular fluorescence probe, broadly used for the analysis of proteins, but the nature of its interaction with proteins and the high increase in the fluorescence intensity that takes place upon such process are still unclear. In the last few years, isothermal titration calorimetry has been used to characterize the nature of the interaction of this dye with proteins. The analysis of the binding isotherms of these studies has not considered the dimerization equilibrium of ANS, which is pH dependent, and it can result in serious errors in the data analysis. In the present work we have developed a suitable data analysis by which this process is taken into account. To study the binding of the dye to proteins at different pH values, we have used the Abl‐SH3 domain. Our results suggest that at pH 3 and 5, where the dimerization of the ANS is important, electrostatic interactions are significant for the binding of ANS to the Abl‐SH3 domain. However, at pH 7, ANS behaves mostly as monomer and the interaction with the protein is mainly hydrophobic. The pH dependent behavior of the ANS binding to proteins can be explained in terms of ionization states of both, the protein and the ANS. Copyright

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.