Protein folding and stability using denaturants

Exploring the sequence and structural determinants of the energy landscape from thermodynamically stable and kinetically trapped subtilisins: ISP1 and SbtE

A protein's energy landscape, all the accessible conformations, their populations, and their dynamics of interconversion, is encoded in its primary sequence. While we have a good understanding of how a protein's primary sequence encodes its native state, we have a much weaker understanding of how sequence encodes the kinetic barriers such as unfolding and refolding. Here we have looked at two subtiliase homologs from the Bacillus subtilis, Intracellular Subtilisin Protease 1 (ISP1) and Subtilisin E (SbtE) that are expected to have very different dynamics. As an intracellular protein, ISP1 has a small pro-domain thought to act simply as a zymogen, whereas the extracellular SbtE has a large pro-domain required for folding. We examined the global and local energetics of the mature proteases and how each pro-domain impacts their landscapes. We find that ISP1's pro-domain has limited impact on the energy landscape while the mature SbtE is thermodynamically unstable and kinetically trapped. The impact of the pro-domain has opposite effects on the flexibility of the core of the protein. ISP1's core becomes more flexible while SbtE's core becomes more rigid. ISP1 contains a conserved amino-acid insertion not present in extracellular subtilisin proteases, which points to a potential source for these differences. These homologs are an extreme example of how changes in the primary sequence can dramatically alter a proteins energy landscape, both stability and dynamics, and highlight the need for large scale, high throughput studies on the relationship between primary sequence and conformational dynamics.

Cumulative asparagine to aspartate deamidation fails to perturb γD-crystallin structure and stability

Deamidation frequently is invoked as an important driver of crystallin aggregation and cataract formation. Here, we characterized the structural and biophysical consequences of cumulative Asn to Asp changes in γD-crystallin. Using NMR spectroscopy, we demonstrate that N- or C-terminal domain-confined or fully Asn to Asp changed γD-crystallin exhibits essentially the same 1H-15N HSQC spectrum as the wild-type protein, implying that the overall structure is retained. Only a very small thermodynamic destabilization for the overall Asn to Asp γD-crystallin variants was noted by chaotropic unfolding, and assessment of the colloidal stability, by measuring diffusion interaction parameters, yielded no substantive differences in association propensities. Furthermore, using molecular dynamics simulations, no significant changes in dynamics for proteins with Asn to Asp or iso-Asp changes were detected. Our combined results demonstrate that substitution of all Asn by Asp residues, reflecting an extreme case of deamidation, did not affect the structure and biophysical properties of γD-crystallin. This suggests that these changes alone cannot be the major determinant in driving cataract formation.

The importance of input sequence set to consensus-derived proteins and their relationship to reconstructed ancestral proteins

A protein sequence encodes its energy landscape-all the accessible conformations, energetics, and dynamics. The evolutionary relationship between sequence and landscape can be probed phylogenetically by compiling a multiple sequence alignment of homologous sequences and generating common ancestors via Ancestral Sequence Reconstruction or a consensus protein containing the most common amino acid at each position. Both ancestral and consensus proteins are often more stable than their extant homologs-questioning the differences between them and suggesting that both approaches serve as general methods to engineer thermostability. We used the Ribonuclease H family to compare these approaches and evaluate how the evolutionary relationship of the input sequences affects the properties of the resulting consensus protein. While the consensus protein derived from our full Ribonuclease H sequence alignment is structured and active, it neither shows properties of a well-folded protein nor has enhanced stability. In contrast, the consensus protein derived from a phylogenetically-restricted set of sequences is significantly more stable and cooperatively folded, suggesting that cooperativity may be encoded by different mechanisms in separate clades and lost when too many diverse clades are combined to generate a consensus protein. To explore this, we compared pairwise covariance scores using a Potts formalism as well as higher-order sequence correlations using singular value decomposition (SVD). We find the SVD coordinates of a stable consensus sequence are close to coordinates of the analogous ancestor sequence and its descendants, whereas the unstable consensus sequences are outliers in SVD space.

Selection pressures on evolution of ribonuclease H explored with rigorous free-energy-based design

Understanding natural protein evolution and designing novel proteins are motivating interest in development of high-throughput methods to explore large sequence spaces. In this work, we demonstrate the application of multisite λ dynamics (MSλD), a rigorous free energy simulation method, and chemical denaturation experiments to quantify evolutionary selection pressure from sequence-stability relationships and to address questions of design. This study examines a mesophilic phylogenetic clade of ribonuclease H (RNase H), furthering its extensive characterization in earlier studies, focusing on E. coli RNase H (ecRNH) and a more stable consensus sequence (AncCcons) differing at 15 positions. The stabilities of 32,768 chimeras between these two sequences were computed using the MSλD framework. The most stable and least stable chimeras were predicted and tested along with several other sequences, revealing a designed chimera with approximately the same stability increase as AncCcons, but requiring only half the mutations. Comparing the computed stabilities with experiment for 12 sequences reveals a Pearson correlation of 0.86 and root mean squared error of 1.18 kcal/mol, an unprecedented level of accuracy well beyond less rigorous computational design methods. We then quantified selection pressure using a simple evolutionary model in which sequences are selected according to the Boltzmann factor of their stability. Selection temperatures from 110 to 168 K are estimated in three ways by comparing experimental and computational results to evolutionary models. These estimates indicate selection pressure is high, which has implications for evolutionary dynamics and for the accuracy required for design, and suggests accurate high-throughput computational methods like MSλD may enable more effective protein design.

The importance of input sequence set to consensus-derived proteins and their relationship to reconstructed ancestral proteins

A protein sequence encodes its energy landscape - all the accessible conformations, energetics, and dynamics. The evolutionary relationship between sequence and landscape can be probed phylogenetically by compiling a multiple sequence alignment of homologous sequences and generating common ancestors via Ancestral Sequence Reconstruction or a consensus protein containing the most common amino acid at each position. Both ancestral and consensus proteins are often more stable than their extant homologs - questioning the differences and suggesting that both approaches serve as general methods to engineer thermostability. We used the Ribonuclease H family to compare these approaches and evaluate how the evolutionary relationship of the input sequences affects the properties of the resulting consensus protein. While the overall consensus protein is structured and active, it neither shows properties of a well-folded protein nor has enhanced stability. In contrast, the consensus protein derived from a phylogenetically-restricted region is significantly more stable and cooperatively folded, suggesting that cooperativity may be encoded by different mechanisms in separate clades and lost when too many diverse clades are combined to generate a consensus protein. To explore this, we compared pairwise covariance scores using a Potts formalism as well as higher-order couplings using singular value decomposition (SVD). We find the SVD coordinates of a stable consensus sequence are close to coordinates of the analogous ancestor sequence and its descendants, whereas the unstable consensus sequences are outliers in SVD space.

Inside-Out Design of Zinc-Binding Proteins with Non-Native Backbones

The de novo design of functional proteins requires specification of tertiary structure and incorporation of molecular binding sites. Here, we develop an inside-out design strategy in the molecular modeling program Rosetta that begins with amino acid side chains from one or two α-helices making well-defined contacts with a ligand. A full-sized protein is then built around the ligand by adding additional helices that promote the formation of a protein core and allow additional contacts with the ligand. The protocol was tested by designing 12 zinc-binding proteins, each with 4-5 helices. Four of the designs were folded and bound to zinc with equilibrium dissociation constants varying between 95 nM and 1.1 μM. The design with the tightest affinity for zinc, N12, adopts a unique conformation in the folded state as assessed with nuclear magnetic resonance (NMR) and the design model closely matches (backbone root-mean-square deviation (RMSD) < 1 Å) an AlphaFold model of the sequence. Retrospective analysis with AlphaFold suggests that the sequences of many of the failed designs did not encode the desired tertiary packing.

Surface residues and non-additive interactions stabilize a consensus homeodomain protein

Despite the widely reported success of consensus design in producing highly stabilized proteins, little is known about the physical mechanisms underlying this stabilization. Here we explore the potential sources of stabilization by performing a systematic analysis of the 29 substitutions that we previously found to collectively stabilize a consensus homeodomain compared to an extant homeodomain. By separately introducing groups of consensus substitutions that alter or preserve charge state, occur at varying degrees of residue burial, and occur at positions of varying degrees of conservation, we determine the extent to which these three features contribute to the consensus stability enhancement. Surprisingly, we find that the largest total contribution to stability comes from consensus substitutions on the protein surface and that the largest per-substitution contributions come from substitutions that maintain charge state. This finding suggests that although consensus proteins are often enriched in charged residues, consensus stabilization does not result primarily from interactions involving charged residues. Although consensus substitutions at strongly conserved positions also contribute disproportionately to stabilization, significant stabilization is also contributed from substitutions at weakly conserved positions. Furthermore, we find that identical consensus substitutions show larger stabilizing effects when introduced into the consensus background than when introduced into an extant homeodomain, indicating that synergistic, stabilizing interactions among the consensus residues contribute to consensus stability enhancement of the homeodomain. By measuring DNA binding affinity for the same set of variants, we find that although consensus design of the homeodomain increases both affinity and folding stability, it does so using a largely non-overlapping set of substitutions.

Refolding of Hemoglobin Under Macromolecular Confinement: Impersonating In Vivo Volume Exclusion

Biomacromolecules evolve and function inside the cell under crowded conditions. The effect of macromolecular crowding and confinement on nature and interactions of biomacromolecules cannot be ruled out. This study demonstrates the effect of volume exclusion due to macromolecular crowding on refolding rate of Gn-HCl induced unfolded hemoglobin. The in vivo like crowding milieu was created using dextran 70. Unfolding of Hb was followed by the absorbance at 280 nm and intrinsic fluorescence intensity along with a bathochromic shift that shows the destabilization of Hb in the presence of the denaturing agent. This was supported by a decrease in soret absorbance, increased hydrodynamic radii and loss in secondary structure, evidenced from dynamic light scattering and circular dichroism experiments respectively. Refolding process of Hb was followed by an increase in soret absorbance, decrease in intrinsic fluorescence intensity with a hypsochromic shift, decreased hydrodynamic radii and gain in secondary structural content. The results revealed that the effect of confinement and volume exclusion is insignificant on the process of Hb refolding.

A pH-dependent protein stability switch coupled to the perturbed pKa of a single ionizable residue

The contribution of electrostatic interactions in protein stability has not been fully understood. Burial of an ionizable amino acid inside the hydrophobic protein core can affect its ionization equilibrium and shift its pKa differentially in the native (N) and unfolded (U) states of a protein and this coupling between the folding/unfolding cycle and the ionization equilibria of the ionizable residue can substantially influence the protein stability. Here, we studied the coupling of the folding/unfolding cycle with the ionization of a buried ionizable residue in a multi-domain protein, Human Serum Albumin (HSA) using fluorescence spectroscopy. A pH-dependent change in the stability of HSA was observed in the near native pH range (pH 6.0-9.0). The protonation-deprotonation equilibrium of a single thiol residue that is buried in the protein structure was identified to give rise to the pH-dependent protein stability. We quantified the pKa of the thiol residue in the N and the U states. The mean pKa of the thiol in the N state was upshifted by 0.5 units to 8.7 due to the burial of the thiol in the protein structure. Surprisingly, the mean pKa of the thiol in the U state was observed to be downshifted by 1.3 units to 6.9. These results indicate that some charged residues are spatially proximal to the thiol group in the U state. Our results suggest that, in addition to the N state, electrostatic interactions in the U state are important determinants of protein stability.

Linking thermodynamics and measurements of protein stability

We review the background, theory and general equations for the analysis of equilibrium protein unfolding experiments, focusing on denaturant and heat-induced unfolding. The primary focus is on the thermodynamics of reversible folding/unfolding transitions and the experimental methods that are available for extracting thermodynamic parameters. We highlight the importance of modelling both how the folding equilibrium depends on a perturbing variable such as temperature or denaturant concentration, and the importance of modelling the baselines in the experimental observables.

Molecular diagnostic of toxigenic Corynebacterium diphtheriae strain by DNA sensor potentially suitable for electrochemical point-of-care diagnostic

The presented study is focused on the development of electrochemical genosensor for detection of tox gene fragment of toxigenic Corynebacterium diphtheriae strain. Together with our previous studies it fulfils the whole procedure for fast and accurate diagnostic of diphtheria at its early stage of infection with the use of electrochemical methods. The developed DNA sensor potentially can be used in more sophisticated portable device. After the electrochemical stem-loop probe structure optimization the conditions for real asymmetric PCR (aPCR) product detection were selected. As was shown it was crucial to optimize the magnesium and organic solvent concentrations in detection buffer. Under optimal conditions it was possible to selectively detect as low as 20.8 nM of complementary stand in 5 min or 0.5 nM in 30 min with sensitivity of 12.81 and 0.24 1⋅μM^-1 respectively. The unspecific biosensor response was elucidated with the use of new electrode blocking agent, diethyldithiocarbamate. Its application in electrochemical genosensors lead to significant higher current values and the biosensor response even in conditions with magnesium ion depletion. The developed biosensor selectivity was examined using samples containing genetic material originated from a number of non-target bacterial species which potentially can be present in the human upper respiratory tract.

Exploring the Evolutionary History of Kinetic Stability in the α-Lytic Protease Family

In addition to encoding the tertiary fold and stability, the primary sequence of a protein encodes the folding trajectory and kinetic barriers that determine the speed of folding. How these kinetic barriers are encoded is not well understood. Here, we use evolutionary sequence variation in the α-lytic protease (αLP) protein family to probe the relationship between sequence and energy landscape. αLP has an unusual energy landscape: the native state of αLP is not the most thermodynamically favored conformation and, instead, remains folded due to a large kinetic barrier preventing unfolding. To fold, αLP utilizes an N-terminal pro region similar in size to the protease itself that functions as a folding catalyst. Once folded, the pro region is removed, and the native state does not unfold on a biologically relevant time scale. Without the pro region, αLP folds on the order of millennia. A phylogenetic search uncovers αLP homologs with a wide range of pro region sizes, including some with no pro region at all. In the resulting phylogenetic tree, these homologs cluster by pro region size. By studying homologs naturally lacking a pro region, we demonstrate they can be thermodynamically stable, fold much faster than αLP, yet retain the same fold as αLP. Key amino acids thought to contribute to αLP's extreme kinetic stability are lost in these homologs, supporting their role in kinetic stability. This study highlights how the entire energy landscape plays an important role in determining the evolutionary pressures on the protein sequence.

Dissecting the statistical properties of the linear extrapolation method of determining protein stability

The linear extrapolation method to determine protein stability from denaturant-induced unfolding experiments is based on the observation that the free energy of unfolding is often a linear function of the denaturant concentration. The value in the absence of denaturant is then estimated by extrapolation from this linear relationship. Parameters and their confidence intervals are typically estimated by nonlinear least-squares regression. We have compared different methods for calculating confidence intervals and found that a simple method based on linear theory gives accurate results. We have also compared three different parameterizations of the linear extrapolation method and show that the most commonly used form is problematic since the stability and m-value are correlated in the nonlinear least-squares analysis. Parameter correlation can in some cases causes problems in the estimation of confidence intervals and regions and should be avoided when possible. Two alternative parameterizations show much less correlation between parameters.

Biophysical characterization of SARAH domain-mediated multimerization of Hippo pathway complexes in Drosophila

Hippo pathway signaling limits cell growth and proliferation and maintains the stem-cell niche. These cellular events result from the coordinated activity of a core kinase cassette that is regulated, in part, by interactions involving Hippo, Salvador, and dRassF. These interactions are mediated by a conserved coiled-coil domain, termed SARAH, in each of these proteins. SARAH domain-mediated homodimerization of Hippo kinase leads to autophosphorylation and activation. Paradoxically, SARAH domain-mediated heterodimerization between Hippo and Salvador enhances Hippo kinase activity in cells, whereas complex formation with dRassF inhibits it. To better understand the mechanism by which each complex distinctly modulates Hippo kinase and pathway activity, here we biophysically characterized the entire suite of SARAH domain-mediated complexes. We purified the three SARAH domains from Drosophila melanogaster and performed an unbiased pulldown assay to identify all possible interactions, revealing that isolated SARAH domains are sufficient to recapitulate the cellular assemblies and that Hippo is a universal binding partner. Additionally, we found that the Salvador SARAH domain homodimerizes and demonstrate that this interaction is conserved in Salvador's mammalian homolog. Using native MS, we show that each of these complexes is dimeric in solution. We also measured the stability of each SARAH domain complex, finding that despite similarities at both the sequence and structural levels, SARAH domain complexes differ in stability. The identity, stoichiometry, and stability of these interactions characterized here comprehensively reveal the nature of SARAH domain-mediated complex formation and provide mechanistic insights into how SARAH domain-mediated interactions influence Hippo pathway activity.

A Second Backbone: The Contribution of a Buried Asparagine Ladder to the Global and Local Stability of a Leucine-Rich Repeat Protein

Parallel β-sheet-containing repeat proteins often display a structural motif in which conserved asparagines form a continuous ladder buried within the hydrophobic core. In such "asparagine ladders", the asparagine side-chain amides form a repetitive pattern of hydrogen bonds with neighboring main-chain NH and CO groups. Although asparagine ladders have been thought to be important for stability, there is little experimental evidence to support such speculation. Here we test the contribution of a minimal asparagine ladder from the leucine-rich repeat protein pp32 to stability and investigate lattice rigidity and hydrogen bond character using solution nuclear magnetic resonance (NMR) spectroscopy. Point substitutions of the two ladder asparagines of pp32 are strongly destabilizing and decrease the cooperativity of unfolding. The chemical shifts of the ladder side-chain HZ protons are shifted significantly downfield in the NMR spectrum and have low temperature coefficients, indicative of strong hydrogen bonding. In contrast, the HE protons are shifted upfield and have temperature coefficients close to zero, suggesting an asymmetry in hydrogen bond strength along the ladder. Ladder NH2 groups have weak 1H-15N cross-peak intensities; 1H-15N nuclear Overhauser effect and 15N CPMG experiments show this to be the result of high rigidity. Hydrogen exchange measurements demonstrate that the ladder NH2 groups exchange very slowly, with rates approaching the global exchange limit. Overall, these results show that the asparagine side chains are held in a very rigid, nondynamic structure, making a significant contribution to the overall stability. In this regard, buried asparagine ladders can be considered "second backbones" within the cores of their elongated β-sheet host proteins.

Using Single-Molecule Chemo-Mechanical Unfolding to Simultaneously Probe Multiple Structural Parameters in Protein Folding

While single-molecule force spectroscopy has greatly advanced the study of protein folding, there are limitations to what can be learned from studying the effect of force alone. We developed a novel technique, chemo-mechanical unfolding, that combines multiple perturbants—force and chemical denaturant—to more fully characterize the folding process by simultaneously probing multiple structural parameters—the change in end-to-end distance, and solvent accessible surface area. Here, we describe the theoretical background, experimental design, and data analysis for chemo-mechanical unfolding experiments probing protein folding thermodynamics and kinetics. This technique has been applied to characterize parallel protein folding pathways, the protein denatured state, protein folding on the ribosome, and protein folding intermediates.

Slow Motion Protein Dance Visualized Using Red-Edge Excitation Shift of a Buried Fluorophore

It has been extremely challenging to detect protein structures with a dynamic core, such as dry molten globules, that remain in equilibrium with the tightly packed native (N) state and that are important for a myriad of entropy-driven protein functions. Here, we detect the higher entropy conformations of a human serum protein, using red-edge excitation shift experiments. We covalently introduced a fluorophore inside the protein core and observed that in a subset of native population, the side chains of the polar and buried residues have different spatial arrangements than the mean population and that they solvate the fluorophore on a timescale much slower than the nanosecond timescale of fluorescence. Our results provide direct evidence for the dense fluidity of protein core and show that alternate side-chain packing arrangements exist in the core that might be important for multiple binding functions of this protein.

The burst-phase folding intermediate of ribonuclease H changes conformation over evolutionary history

The amino acid sequence encodes the energy landscape of a protein. Therefore, we expect evolutionary mutations to change features of the protein energy landscape, including the conformations adopted by a polypeptide as it folds to its native state. Ribonucleases H (RNase H) from Escherichia coli and Thermus thermophilus both fold via a partially folded intermediate in which the core region of the protein (helices A-D and strands 4-5) is structured. Strand 1, however, uniquely contributes to the T. thermophilus RNase H folding intermediate (Icore+1 ), but not the E. coli RNase H intermediate (Icore ) (Rosen & Marqusee, PLoS One 2015). We explore the origin of this difference by characterizing the folding intermediate of seven ancestral RNases H spanning the evolutionary history of these two homologs. Using fragment models with or without strand 1 and FRET probes to characterize the folding intermediate of each ancestor, we find a distinct evolutionary trend across the family-the involvement of strand 1 in the folding intermediate is an ancestral feature that is maintained in the thermophilic lineage and is gradually lost in the mesophilic lineage. Evolutionary sequence changes indeed modulate the conformations present on the folding landscape and altered the folding trajectory of RNase H.

Salvador has an extended SARAH domain that mediates binding to Hippo kinase

The Hippo pathway controls cell proliferation and differentiation through the precisely tuned activity of a core kinase cassette. The activity of Hippo kinase is modulated by interactions between its C-terminal coiled-coil, termed the SARAH domain, and the SARAH domains of either dRassF or Salvador. Here, we wanted to understand the molecular basis of SARAH domain-mediated interactions and their influence on Hippo kinase activity. We focused on Salvador, a positive effector of Hippo activity and the least well-characterized SARAH domain-containing protein. We determined the crystal structure of a complex between Salvador and Hippo SARAH domains from Drosophila This structure provided insight into the organization of the Salvador SARAH domain including a folded N-terminal extension that expands the binding interface with Hippo SARAH domain. We also found that this extension improves the solubility of the Salvador SARAH domain, enhances binding to Hippo, and is unique to Salvador. We therefore suggest expanding the definition of the Salvador SARAH domain to include this extended region. The heterodimeric assembly observed in the crystal was confirmed by cross-linked MS and provided a structural basis for the mutually exclusive interactions of Hippo with either dRassF or Salvador. Of note, Salvador influenced the kinase activity of Mst2, the mammalian Hippo homolog. In co-transfected HEK293T cells, human Salvador increased the levels of Mst2 autophosphorylation and Mst2-mediated phosphorylation of select substrates, whereas Salvador SARAH domain inhibited Mst2 autophosphorylation in vitro These results suggest Salvador enhances the effects of Hippo kinase activity at multiple points in the Hippo pathway.

Conformational sampling of the intrinsically disordered dsRBD-1 domain from Arabidopsis thaliana DCL1

Partial folding and stability of DCL1-dsRBD1.

Exploring the Denatured State Ensemble by Single-Molecule Chemo-Mechanical Unfolding: The Effect of Force, Temperature, and Urea

While it is widely appreciated that the denatured state of a protein is a heterogeneous conformational ensemble, there is still debate over how this ensemble changes with environmental conditions. Here, we use single-molecule chemo-mechanical unfolding, which combines force and urea using the optical tweezers, together with traditional protein unfolding studies to explore how perturbants commonly used to unfold proteins (urea, force, and temperature) affect the denatured-state ensemble. We compare the urea m-values, which report on the change in solvent accessible surface area for unfolding, to probe the denatured state as a function of force, temperature, and urea. We find that while the urea- and force-induced denatured states expose similar amounts of surface area, the denatured state at high temperature and low urea concentration is more compact. To disentangle these two effects, we use destabilizing mutations that shift the T_m and C_m. We find that the compaction of the denatured state is related to changing temperature as the different variants of acyl-coenzyme A binding protein have similar m-values when they are at the same temperature but different urea concentration. These results have important implications for protein folding and stability under different environmental conditions.

Creating a Homeodomain with High Stability and DNA Binding Affinity by Sequence Averaging

There is considerable interest in generating proteins with both high stability and high activity for biomedical and industrial purposes. One approach that has been used successfully to increase the stability of linear repeat proteins is consensus design. It is unclear the extent over which the consensus design approach can be used to produce folded and hyperstable proteins, and importantly, whether such stabilized proteins would retain function. Here we extend the consensus strategy to design a globular protein. We show that a consensus-designed homeodomain (HD) sequence adopts a cooperatively folded homeodomain structure. The unfolding free energy of the consensus-HD is 5 kcal·mol^-1 higher than that of the naturally occurring engrailed-HD from Drosophila melanogaster. Remarkably, the consensus-HD binds the engrailed-HD cognate DNA in a similar mode as the engrailed-HD with approximately 100-fold higher affinity. ¹⁵N relaxation studies show a decrease in ps-ns backbone dynamics in the free state of consensus-HD, suggesting that increased affinity is not a result of increased plasticity. In addition to demonstrating the potential for consensus design of globular proteins with increased stability, these results demonstrate that greatly stabilized proteins can bind cognate substrates with increased affinities, showing that high stability is compatible with function.

Staphylokinase Displays Surprisingly Low Mechanical Stability

Single-molecule force spectroscopy (SMFS) and molecular dynamics (MD) simulations have revealed that shear topology is an important structural feature for mechanically stable proteins. Proteins containing a β-grasp fold display the typical shear topology and are generally of significant mechanical stability. In an effort to experimentally identify mechanically strong proteins using single-molecule atomic force microscopy, we found that staphylokinase (SAK), which has a typical β-grasp fold and was predicted to be mechanically stable in coarse-grained MD simulations, displays surprisingly low mechanical stability. At a pulling speed of 400 nm/s, SAK unfolds at ∼60 pN, making it the mechanically weakest protein among the β-grasp fold proteins that have been characterized experimentally. In contrast, its structural homologous protein streptokinase β domain displays significant mechanical stability under the same experimental condition. Our results showed that the large malleability of native-state SAK is largely responsible for its low mechanical stability. The molecular origin of this large malleability of SAK remains unknown. Our results reveal a hidden complexity in protein mechanics and call for a detailed investigation into the molecular determinants of the protein mechanical malleability.

Evolutionary trend toward kinetic stability in the folding trajectory of RNases H

Proper folding of proteins is critical to producing the biological machinery essential for cellular function. The rates and energetics of a protein's folding process, which is described by its energy landscape, are encoded in the amino acid sequence. Over the course of evolution, this landscape must be maintained such that the protein folds and remains folded over a biologically relevant time scale. How exactly a protein's energy landscape is maintained or altered throughout evolution is unclear. To study how a protein's energy landscape changed over time, we characterized the folding trajectories of ancestral proteins of the ribonuclease H (RNase H) family using ancestral sequence reconstruction to access the evolutionary history between RNases H from mesophilic and thermophilic bacteria. We found that despite large sequence divergence, the overall folding pathway is conserved over billions of years of evolution. There are robust trends in the rates of protein folding and unfolding; both modern RNases H evolved to be more kinetically stable than their most recent common ancestor. Finally, our study demonstrates how a partially folded intermediate provides a readily adaptable folding landscape by allowing the independent tuning of kinetics and thermodynamics.

Negative Charge Neutralization in the Loops and Turns of Outer Membrane Phospholipase A Impacts Folding Hysteresis at Neutral pH

Hysteresis in equilibrium protein folding titrations is an experimental barrier that must be overcome to extract meaningful thermodynamic quantities. Traditional approaches to solving this problem involve testing a spectrum of solution conditions to find ones that achieve path independence. Through this procedure, a specific pH of 3.8 was required to achieve path independence for the water-to-bilayer equilibrium folding of outer membrane protein OmpLA. We hypothesized that the neutralization of negatively charged side chains (Asp and Glu) at pH 3.8 could be the physical basis for path-independent folding at this pH. To test this idea, we engineered variants of OmpLA with Asp → Asn and Glu → Gln mutations to neutralize the negative charges within various regions of the protein and tested for reversible folding at neutral pH. Although not fully resolved, our results show that these mutations in the periplasmic turns and extracellular loops are responsible for 60% of the hysteresis in wild-type folding. Overall, our study suggests that negative charges impact the folding hysteresis in outer membrane proteins and their neutralization may aid in protein engineering applications.

Protein folding and unfolding under force

The recent revolution in optics and instrumentation has enabled the study of protein folding using extremely low mechanical forces as the denaturant. This exciting development has led to the observation of the protein folding process at single molecule resolution and its response to mechanical force. Here, we describe the principles and experimental details of force spectroscopy on proteins, with a focus on the optical tweezers instrument. Several recent results will be discussed to highlight the importance of this technique in addressing a variety of questions in the protein folding field.

A Naturally Occurring Repeat Protein with High Internal Sequence Identity Defines a New Class of TPR-like Proteins

Linear repeat proteins often have high structural similarity and low (∼25%) pairwise sequence identities (PSI) among modules. We identified a unique P. anserina (Pa) sequence with tetratricopeptide repeat (TPR) homology, which contains longer (42 residue) repeats (42PRs) with an average PSI >91%. We determined the crystal structure of five tandem Pa 42PRs to 1.6 Å, and examined the stability and solution properties of constructs containing three to six Pa 42PRs. Compared with 34-residue TPRs (34PRs), Pa 42PRs have a one-turn extension of each helix, and bury more surface area. Unfolding transitions shift to higher denaturant concentration and become sharper as repeats are added. Fitted Ising models show Pa 42PRs to be more cooperative than consensus 34PRs, with increased magnitudes of intrinsic and interfacial free energies. These results demonstrate the tolerance of the TPR motif to length variation, and provide a basis to understand the effects of helix length on intrinsic/interfacial stability.

Sequential unfolding of the hemolysin two-partner secretion domain from Proteus mirabilis

Protein secretion is a major contributor to Gram-negative bacterial virulence. Type Vb or two-partner secretion (TPS) pathways utilize a membrane bound β-barrel B component (TpsB) to translocate large and predominantly virulent exoproteins (TpsA) through a nucleotide independent mechanism. We focused our studies on a truncated TpsA member termed hemolysin A (HpmA265), a structurally and functionally characterized TPS domain from Proteus mirabilis. Contrary to the expectation that the TPS domain of HpmA265 would denature in a single cooperative transition, we found that the unfolding follows a sequential model with three distinct transitions linking four states. The solvent inaccessible core of HpmA265 can be divided into two different regions. The C-proximal region contains nonpolar residues and forms a prototypical hydrophobic core as found in globular proteins. The N-proximal region of the solvent inaccessible core, however, contains polar residues. To understand the contributions of the hydrophobic and polar interiors to overall TPS domain stability, we conducted unfolding studies on HpmA265 and site-specific mutants of HpmA265. By correlating the effect of individual site-specific mutations with the sequential unfolding results we were able to divide the HpmA265 TPS domain into polar core, nonpolar core, and C-terminal subdomains. Moreover, the unfolding studies provide quantitative evidence that the folding free energy for the polar core subdomain is more favorable than for the nonpolar core and C-terminal subdomains. This study implicates the hydrogen bonds shared among these conserved internal residues as a primary means for stabilizing the N-proximal polar core subdomain.

Disordered allostery: lessons from glucocorticoid receptor

Allostery is a biological regulation mechanism of significant importance in cell signaling, metabolism, and disease. Although the ensemble basis of allostery has been known for years, only recently has emphasis shifted from interpreting allosteric mechanism in terms of discrete structural pathways to ones that focus on the statistical nature of the signal propagation process, providing a vehicle to unify allostery in structured, dynamic, and disordered systems. In particular, intrinsically disordered (ID) proteins (IDPs), which lack a unique, stable structure, have been directly demonstrated to exhibit allostery in numerous systems, a reality that challenges traditional structure-based models that focus on allosteric pathways. In this chapter, we will discuss the historical context of allostery and focus on studies from human glucocorticoid receptor (GR), a member of the steroid hormone receptor (SHR) family. The numerous translational isoforms of the disordered N-terminal domain of GR consist of coupled thermodynamic domains that contribute to the delicate balance of states in the ensemble and hence in vivo activity. The data are quantitatively interpreted using the ensemble allosteric model (EAM) that considers only the intrinsic and measurable energetics of allosteric systems. It is demonstrated that the EAM provides mechanistic insight into the distribution of states in solution and provides an interpretation for how certain translational isoforms of GR display enhanced and repressed transcriptional activities. The ensemble nature of allostery illuminated from these studies lends credence to the EAM and provides ground rules for allostery in all systems.

Highly polarized C-terminal transition state of the leucine-rich repeat domain of PP32 is governed by local stability

The leucine-rich repeat domain of PP32 is composed of five β-strand-containing repeats anchored by terminal caps. These repeats differ in sequence but are similar in structure, providing a means to connect topology, sequence, and folding pathway selection. Through kinetic studies of PP32, we find folding to be rate-limited by the formation of an on-pathway intermediate. Destabilizing core substitutions reveal a transition state ensemble that is highly polarized toward the C-terminal repeat and cap. To determine if this nucleus for folding corresponds to the most stable region of PP32, we monitored amide hydrogen exchange by NMR spectroscopy. Indeed, we find the highest protection to be biased toward the C terminus. Sequence manipulations that destabilize the C terminus spread out the transition state toward the middle of the protein. Consistent with results for helical ankyrin repeat proteins, these results suggest that local stabilities determine folding pathways.

Probing conformational stability and dynamics of erythroid and nonerythroid spectrin: effects of urea and guanidine hydrochloride

We have studied the conformational stability of the two homologous membrane skeletal proteins, the erythroid and non-erythroid spectrins, in their dimeric and tetrameric forms respectively during unfolding in the presence of urea and guanidine hydrochloride (GuHCl). Fluorescence and circular dichroism (CD) spectroscopy have been used to study the changes of intrinsic tryptophan fluorescence, anisotropy, far UV-CD and extrinsic fluorescence of bound 1-anilinonapthalene-8-sulfonic acid (ANS). Chemical unfolding of both proteins were reversible and could be described as a two state transition. The folded erythroid spectrin and non-erythroid spectrin were directly converted to unfolded monomer without formation of any intermediate. Fluorescence quenching, anisotropy, ANS binding and dynamic light scattering data suggest that in presence of low concentrations of the denaturants (up-to 1M) hydrogen bonding network and van der Waals interaction play a role inducing changes in quaternary as well as tertiary structures without complete dissociation of the subunits. This is the first report of two large worm like, multi-domain proteins obeying twofold rule which is commonly found in small globular proteins. The free energy of stabilization (ΔGuH20) for the dimeric spectrin has been 20 kcal/mol lesser than the tetrameric from.

Capping motifs stabilize the leucine-rich repeat protein PP32 and rigidify adjacent repeats

Capping motifs are found to flank most β-strand-containing repeat proteins. To better understand the roles of these capping motifs in organizing structure and stability, we carried out folding and solution NMR studies on the leucine-rich repeat (LRR) domain of PP32, which is composed of five tandem LRR, capped by α-helical and β-hairpin motifs on the N- and C-termini. We were able to purify PP32 constructs lacking either cap and containing destabilizing substitutions. Removing the C-cap results in complete unfolding of PP32. Removing the N-cap has a much less severe effect, decreasing stability but retaining much of its secondary structure. In contrast, the dynamics and tertiary structure of the first two repeats are significantly perturbed, based on (1)H-(15)N relaxation studies, chemical shift perturbations, and residual dipolar couplings. However, more distal repeats (3 to C-cap) retain their native tertiary structure. In this regard, the N-cap drives the folding of adjacent repeats from what appears to be a molten-globule-like state. This interpretation is supported by extensive analysis using core packing substitutions in the full-length and N-cap-truncated PP32. This work highlights the importance of caps to the stability and structural integrity of β-strand-containing LRR proteins, and emphasizes the different contributions of the N- and C-terminal caps.

Protein domain definition should allow for conditional disorder

Proteins are often classified in a binary fashion as either structured or disordered. However this approach has several deficits. Firstly, protein folding is always conditional on the physiochemical environment. A protein which is structured in some circumstances will be disordered in others. Secondly, it hides a fundamental asymmetry in behavior. While all structured proteins can be unfolded through a change in environment, not all disordered proteins have the capacity for folding. Failure to accommodate these complexities confuses the definition of both protein structural domains and intrinsically disordered regions. We illustrate these points with an experimental study of a family of small binding domains, drawn from the RNA polymerase of mumps virus and its closest relatives. Assessed at face value the domains fall on a structural continuum, with folded, partially folded, and near unstructured members. Yet the disorder present in the family is conditional, and these closely related polypeptides can access the same folded state under appropriate conditions. Any heuristic definition of the protein domain emphasizing conformational stability divides this domain family in two, in a way that makes no biological sense. Structural domains would be better defined by their ability to adopt a specific tertiary structure: a structure that may or may not be realized, dependent on the circumstances. This explicitly allows for the conditional nature of protein folding, and more clearly demarcates structural domains from intrinsically disordered regions that may function without folding.

Direct observation of a force-induced switch in the anisotropic mechanical unfolding pathway of a protein

Many biological processes generate force, and proteins have evolved to resist and respond to tension along different force axes. Single-molecule force spectroscopy allows for molecular insight into the behavior of proteins under force and the mechanism of protein folding in general. Here, we have used src SH3 to investigate the effect of different pulling axes under the low-force regime afforded by an optical trap. We find that this small cooperatively folded protein shows an anisotropic response to force; the protein is more mechanically resistant to force applied along a longitudinal axis compared to force applied perpendicular to the terminal β strand. In the longitudinal axis, we observe an unusual biphasic behavior revealing a force-induced switch in the unfolding mechanism suggesting the existence of two parallel unfolding pathways. A site-specific variant can selectively affect one of these pathways. Thus, even this simple two-state protein demonstrates a complex mechanical unfolding trajectory, accessing multiple unfolding pathways under the low-force regime of the optical trap; the specific unfolding pathway depends on the perturbation axis and the applied force.

Static and Dynamic Aspects of Supramolecular Interactions of Coumarin 153 and Fluorescein with Bovine Serum Albumin

The static and dynamic aspects of supramolecular interactions between coumarin 153 (C153) and fluorescein (FL) with bovine serum albumin (BSA) has been studied by spectroscopic techniques. Both dyes were found to form 1 : 1 complexes with BSA, with binding constants 2.9 ± 0.3 × 105 M–1 and 2.1 ± 0.2 × 105 M–1 for C153 and FL respectively. The binding site of C153 has been determined by steady-state fluorescence resonance energy transfer, site marker competitive experiments, and a molecular docking study. Our studies indicate that C153 binds to domain IIIA of BSA whereas FL binds non-specifically. Denaturation characteristics of the C153 and FL binding region of BSA were found to be very different to global denaturation. Furthermore, kinetics of binding has been studied by the stopped-flow method. The observed rate constants were found to be 8.8 s–1 and 5.9 s–1 for C153 and FL respectively.

Mapping the Deltex-binding surface on the notch ankyrin domain using analytical ultracentrifugation

The Notch signal transduction pathway controls cell fate determination during metazoan development. The Notch gene encodes a transmembrane receptor that is cleaved upon activation, liberating the Notch intracellular domain, which enters the nucleus and assembles transcriptional activation complexes that drive expression of Notch-responsive genes. The most conserved region of the Notch intracellular domain is an ankyrin domain (Nank), which binds directly to the cytosolic effector protein Deltex (Dx), controlling intracellular Notch activity. However, the structural and energetic basis for this interaction remains unknown. Here, we analyze the thermodynamics and hydrodynamics of the Nank:Dx heteroassociation, as well as a weaker Nank self-association, using sedimentation velocity analytical ultracentrifugation. By comparing g(s*) and c(s) distributions, and by direct fitting of sedimentation boundaries with thermodynamic association models, we were able to characterize the Nank:Dx heterodimer, measure its affinity, and map the interaction on the surface on Nank. N- and C-terminal deletions of whole ankyrin units implicate repeats 3 and 4 as key for mediating heteroassociation. An alanine scan across the interaction loops of Nank identifies a conserved hot spot in repeats 3 and 4, centered at R127, as critical for Dx binding. In addition, we were able to detect weak but reproducible Nank homodimerization (K(d) in the millimolar range). This association is disrupted by substitution of a conserved arginine (R107) with alanine, a residue previously implicated in a functionally relevant mode of interaction within dimeric transcription complexes. The distinct binding surfaces on Nank for homotypic versus Dx interaction appear to be compatible with teterameric Notch(2):Dx(2) assembly.

Overcoming hysteresis to attain reversible equilibrium folding for outer membrane phospholipase A in phospholipid bilayers

The free energy of unfolding of a membrane protein from lipids into water (ΔG(o)(w,l)) describes its equilibrium thermodynamic stability. Knowing this parameter gives insight into a membrane protein's sequence-structure-energy relationships. However, there are few measures of membrane protein stability because of the technical difficulties associated with unfolded and partially folded states. Here, we describe the experimental process that allowed us to measure the ΔG(o)(w,l) of the outer membrane phospholipase A into large unilamellar vesicles (LUVs) of 1,2-dilauroyl-sn-glycero-3-phosphocholine. To arrive at this reversible folding condition, we screened a large number of experimental variables: temperature, incubation time, salt concentration, pH, lipid composition and liposome morphology. The principal challenge we encountered under most conditions was hysteresis between folding and unfolding titrations. A second factor that compromised reversible folding was the observation that a fraction of the protein population tended to aggregate. We found that hysteresis could be completely eliminated on a feasible timescale by conducting experiments at acidic pH, by the slow dilution of the protein in the initial titration setup and by utilizing a low concentration of a detergent as a temporary "holdase" to solubilize the protein upon its initial dilution into folding conditions. We confirmed that the detergent did not disrupt the LUVs using fluorescence emission of lipid-sensitive dyes and light scattering. The results of our parameter search should be generally useful for efforts to measure ΔG(o)(w,l) for other membrane proteins.

Deletion of internal structured repeats increases the stability of a leucine-rich repeat protein, YopM

Mapping the stability distributions of proteins in their native folded states provides a critical link between structure, thermodynamics, and function. Linear repeat proteins have proven more amenable to this kind of mapping than globular proteins. C-terminal deletion studies of YopM, a large, linear leucine-rich repeat (LRR) protein, show that stability is distributed quite heterogeneously, yet a high level of cooperativity is maintained [1]. Key components of this distribution are three interfaces that strongly stabilize adjacent sequences, thereby maintaining structural integrity and promoting cooperativity. To better understand the distribution of interaction energy around these critical interfaces, we studied internal (rather than terminal) deletions of three LRRs in this region, including one of these stabilizing interfaces. Contrary to our expectation that deletion of structured repeats should be destabilizing, we find that internal deletion of folded repeats can actually stabilize the native state, suggesting that these repeats are destabilizing, although paradoxically, they are folded in the native state. We identified two residues within this destabilizing segment that deviate from the consensus sequence at a position that normally forms a stacked leucine ladder in the hydrophobic core. Replacement of these nonconsensus residues with leucine is stabilizing. This stability enhancement can be reproduced in the context of nonnative interfaces, but it requires an extended hydrophobic core. Our results demonstrate that different LRRs vary widely in their contribution to stability, and that this variation is context-dependent. These two factors are likely to determine the types of rearrangements that lead to folded, functional proteins, and in turn, are likely to restrict the pathways available for the evolution of linear repeat proteins.

Side-chain hydrophobicity scale derived from transmembrane protein folding into lipid bilayers

The transfer free energies of the twenty natural amino acid side chains from water to phospholipid bilayers make a major contribution to the assembly and function of membrane proteins. Measurements of those transfer free energies will facilitate the identification of membrane protein sequences and aid in the understanding of how proteins interact with membranes during key biological events. We report the first water-to-bilayer transfer free energy scale (i.e., a "hydrophobicity scale") for the twenty natural amino acid side chains measured in the context of a native transmembrane protein and a phospholipid bilayer. Our measurements reveal parity for apolar side-chain contributions between soluble and membrane proteins and further demonstrate that an arginine side-chain placed near the middle of a lipid bilayer is accommodated with much less energetic cost than predicted by molecular dynamics simulations.

Folding kinetics of the cooperatively folded subdomain of the IκBα ankyrin repeat domain

The ankyrin repeat (AR) domain of IκBα consists of a cooperative folding unit of roughly four ARs (AR1-AR4) and of two weakly folded repeats (AR5 and AR6). The kinetic folding mechanism of the cooperative subdomain, IκBα(67-206), was analyzed using rapid mixing techniques. Despite its apparent architectural simplicity, IκBα(67-206) displays complex folding kinetics, with two sequential on-pathway high-energy intermediates. The effect of mutations to or away from the consensus sequences of ARs on folding behavior was analyzed, particularly the GXTPLHLA motif, which have not been examined in detail previously. Mutations toward the consensus generally resulted in an increase in folding stability, whereas mutations away from the consensus resulted in decreased overall stability. We determined the free energy change upon mutation for three sequential transition state ensembles along the folding route for 16 mutants. We show that folding initiates with the formation of the interface of the outer helices of AR3 and AR4, and then proceeds to consolidate structure in these repeats. Subsequently, AR1 and AR2 fold in a concerted way in a single kinetic step. We show that this mechanism is robust to the presence of AR5 and AR6 as they do not strongly affect the folding kinetics. Overall, the protein appears to fold on a rather smooth energy landscape, where the folding mechanism conforms a one-dimensional approximation. However, we note that the AR does not necessarily act as a single folding element.

Using tryptophan fluorescence to measure the stability of membrane proteins folded in liposomes

Accurate measurements of the thermodynamic stability of folded membrane proteins require methods for monitoring their conformation that are free of experimental artifacts. For tryptophan fluorescence emission experiments with membrane proteins folded into liposomes, there are two significant sources of artifacts: the first is light scattering by the liposomes; the second is the nonlinear relationship of some tryptophan spectral parameters with changes in protein conformation. Both of these sources of error can interfere with the method of determining the reversible equilibrium thermodynamic stability of proteins using titrations of chemical denaturants. Here, we present methods to manage light scattering by liposomes for tryptophan emission experiments and to properly monitor tryptophan spectra as a function of protein conformation. Our methods are tailored to the titrations of membrane proteins using common chemical denaturants. One of our recommendations is to collect and analyze the right-angle light scattering peak that occurs around the excitation wavelength in a fluorescence experiment. Another recommendation is to use only those tryptophan spectral parameters that are linearly proportional to the protein conformational population. We show that other commonly used spectral parameters lead to errors in protein stability measurements.

Consequences of IkappaB alpha hydroxylation by the factor inhibiting HIF (FIH)

The factor inhibiting HIF-1 (FIH-1) hydroxylates many ankyrin repeat-containing proteins including IκBα. It is widely speculated that hydroxylation of IκBα has functional consequences, but the effects of hydroxylation have not been demonstrated. We prepared hydroxylated IκBα and compared it to the unhydroxylated protein. Urea denaturation and amide H/D exchange experiments showed no change in the "foldedness" upon hydroxylation. Surface plasmon resonance measurements of binding to NFκB showed no difference in the NFκB binding kinetics or thermodynamics. Ubiquitin-independent proteasomal degradation experiments showed no difference in the half-life of the protein. Thus, it appears that hydroxylation of IκBα by FIH-1 is inconsequential, at least for the functions we could assay in vitro.

Thermodynamic analysis of the CSL x Notch interaction: distribution of binding energy of the Notch RAM region to the CSL beta-trefoil domain and the mode of competition with the viral transactivator EBNA2

The Notch signaling pathway is a cell-cell communication network giving rise to cell differentiation during metazoan development. Activation of the pathway releases the intracellular portion of the Notch receptor to translocate to the nucleus, where it is able to interact with the effector transcription factor CSL, converting CSL from a transcriptional repressor to an activator. This conversion is dependent upon the high affinity binding of the RAM region of the Notch receptor to the beta-trefoil domain (BTD) of CSL. Here we probe the energetics of binding to BTD of each conserved residue of RAM through the use of isothermal titration calorimetry and single residue substitution. We find that although the highly conserved PhiW PhiP motif is the largest determinant of binding, energetically significant interactions are contributed by N-terminal residues, including a conserved Arg/Lys-rich region. Additionally, we present a thermodynamic analysis of the interaction between the Epstein-Barr virus protein EBNA2 with BTD and explore the extent to which the EBNA2- and RAM-binding sites on BTD are nonoverlapping, as proposed by Fuchs et al. (Fuchs, K. P., Bommer, G., Dumont, E., Christoph, B., Vidal, M., Kremmer, E., and Kempkes, B. (2001) Eur. J. Biochem. 268, 4639-4646). Combining these results with displacement isothermal titration calorimetry, we propose a mechanism by which the PhiW PhiP motif of RAM and EBNA2 compete with one another for binding at the hydrophobic pocket of BTD using overlapping but specific interactions that are unique to each BTD ligand.

C-terminal deletion of leucine-rich repeats from YopM reveals a heterogeneous distribution of stability in a cooperatively folded protein

Terminal deletions of units from alpha-helical repeat proteins have provided insight into the physical origins of their cooperativity. To test if the same principles governing cooperativity apply to beta-sheet-containing repeat proteins, we have created a series of C-terminal deletion constructs from a large leucine-rich repeat (LRR) protein, YopM. We have examined the structure and stability of the resulting deletion constructs by a combination of solution spectroscopy, equilibrium denaturation studies, and limited proteolysis. Surprisingly, a high degree of nonuniformity was found in the stability distribution of YopM. Unlike previously studied repeat proteins, we identified several key LRR that on deletion disrupt nearby structure, at distances as far away as up to three repeats, in YopM. This partial unfolding model is supported by limited proteolysis studies and by point substitution in repeats predicted to be disordered as a result of deletion of adjacent repeats. We show that key internal- and terminal-caps must be present to maintain the structural integrity in adjacent regions (roughly four LRRs long) of decreased stability. The finding that full-length YopM maintains a high level of cooperativity in equilibrium unfolding underscores the importance of interfacial interactions in stabilizing locally unstable regions of structure.

Equilibrium and kinetic approaches for studying oligomeric protein folding

This chapter describes the approaches and considerations necessary for extension of current protein folding methods to the equilibrium and kinetic reactions of oligomeric proteins, using dimers as the primary example. Spectroscopic and transport methods to monitor folding and unfolding transitions are summarized. The data collection and analyses to determine protein stability and kinetic folding mechanisms are discussed in the context of the additional dimension of complexity that arises in higher order folding processes, compared to first order monomeric proteins. As a case study to illustrate the data analysis process, equilibrium, and kinetic data are presented for SmtB, a homodimeric DNA-binding protein from Synechococcus PCC7942.