Fast Collapse but Slow Formation of Secondary Structure Elements in the Refolding Transition of E.coli Adenylate Kinase

Cooperativity and Folding Kinetics in a Multidomain Protein with Interwoven Chain Topology

Although a large percentage of eukaryotic proteomes consist of proteins with multiple domains, not much is known about their assembly mechanism, especially those with intricate native state architectures. Some have a complex topology in which the structural elements along the sequence are interwoven in such a manner that the domains cannot be separated by cutting at any location along the sequence. Such proteins are multiply connected multidomain proteins (MMPs) with the three-domain (NMP, LID, and CORE) phosphotransferase enzyme adenylate kinase (ADK) being an example. We devised a coarse-grained model to simulate ADK folding initiated by changing either the temperature or guanidinium chloride (GdmCl) concentration. The simulations reproduce the experimentally measured melting temperatures (associated with two equilibrium transitions), FRET efficiency as a function of GdmCl concentration, and the folding times quantitatively. Although the NMP domain orders independently, cooperative interactions between the LID and the CORE domains are required for complete assembly of the enzyme. Kinetic simulations show that, on the collapse time scale, multiple interconnected metastable states are populated, attesting to the folding heterogeneity. The network of kinetically connected states reveals that the CORE domain folds only after the NMP and LID domains, reflecting the interwoven nature of the chain topology.

Heterogeneity in Protein Folding and Unfolding Reactions

Proteins have dynamic structures that undergo chain motions on time scales spanning from picoseconds to seconds. Resolving the resultant conformational heterogeneity is essential for gaining accurate insight into fundamental mechanistic aspects of the protein folding reaction. The use of high-resolution structural probes, sensitive to population distributions, has begun to enable the resolution of site-specific conformational heterogeneity at different stages of the folding reaction. Different states populated during protein folding, including the unfolded state, collapsed intermediate states, and even the native state, are found to possess significant conformational heterogeneity. Heterogeneity in protein folding and unfolding reactions originates from the reduced cooperativity of various kinds of physicochemical interactions between various structural elements of a protein, and between a protein and solvent. Heterogeneity may arise because of functional or evolutionary constraints. Conformational substates within the unfolded state and the collapsed intermediates that exchange at rates slower than the subsequent folding steps give rise to heterogeneity on the protein folding pathways. Multiple folding pathways are likely to represent distinct sequences of structure formation. Insight into the nature of the energy barriers separating different conformational states populated during (un)folding can also be obtained by resolving heterogeneity.

Resolving Site-Specific Heterogeneity of the Unfolded State under Folding Conditions

Understanding the properties of the unfolded state under folding conditions is of fundamental importance for gaining mechanistic insight into folding as well as misfolding reactions. Toward achieving this objective, the folding reaction of a small protein, monellin, has been resolved structurally and temporally, with the use of the multisite time-resolved FRET methodology. The present study establishes that the initial polypeptide chain collapse is not only heterogeneous but also structurally asymmetric and nonuniform. The population-averaged size for the segments spanning parts of the β-sheet decreases much more than that for the α-helix. Multisite measurements enabled specific and nonspecific components of the initial chain collapse to be discerned. The expanded and compact intermediate subensembles have the properties of a nonspecifically collapsed (hence, random-coil-like) and specifically collapsed (hence, globular) polymer, respectively. During subsequent folding, both the subensembles underwent contraction to varying extents at the four monitored segments, which was close to gradual in nature. The expanded intermediate subensemble exhibited an additional very slow contraction, suggestive of the presence of non-native interactions that result in a higher effective viscosity slowing down intrachain motions under folding conditions.

Mapping Distinct Sequences of Structure Formation Differentiating Multiple Folding Pathways of a Small Protein

To determine experimentally how the multiple folding pathways of a protein differ, in the order in which the structural parts are assembled, has been a long-standing challenge. To resolve whether structure formation during folding can progress in multiple ways, the complex folding landscape of monellin has been characterized, structurally and temporally, using the multisite time-resolved FRET methodology. After an initial heterogeneous polypeptide chain collapse, structure formation proceeds on parallel pathways. Kinetic analysis of the population evolution data across various protein segments provides a clear structural distinction between the parallel pathways. The analysis leads to a phenomenological model that describes how and when discrete segments acquire structure independently of each other in different subensembles of protein molecules. When averaged over all molecules, structure formation is seen to progress as α-helix formation, followed by core consolidation, then β-sheet formation, and last end-to-end distance compaction. Parts of the protein that are closer in the primary sequence acquire structure before parts separated by longer sequence.

Two Orders of Magnitude Variation of Diffusion-Enhanced Förster Resonance Energy Transfer in Polypeptide Chains

A flexible peptide chain displays structural and dynamic properties that correspond to its folding and biological activity. These properties are mirrored in intrachain site-to-site distances and diffusion coefficients of mutual site-to-site motion. Both distance distribution and diffusion determine the extent of Förster resonance energy transfer (FRET) between two sites labeled with a FRET donor and acceptor. The relatively large Förster radii of traditional FRET methods (R₀ > 20 Å) lead to a fairly low contribution of diffusion. We introduced short-distance FRET (sdFRET) where Dbo, an asparagine residue conjugated to 2,3-diazabicyclo[2.2.2]octane, acts as acceptor paired with donors, such as naphtylalanine (NAla), tryptophan, 5-l-fluorotryptophan, or tyrosine. The Förster radii are always close to 10 Å, which makes sdFRET highly sensitive to diffusional motion. We recently found indications that the FRET enhancement caused by diffusion depends symmetrically on the product of the radiative fluorescence lifetime of the donor and the diffusion coefficient. In this study, we varied this product by two orders of magnitude, using both donors of different lifetime, NAla and FTrp, as well as a varying viscogen concentration, to corroborate this statement. We demonstrate the consequences of this relationship in evaluating the impact of viscogenic coadditives on peptide dimensions.

Method-Unifying View of Loop-Formation Kinetics in Peptide and Protein Folding

Protein folding can be described as a probabilistic succession of events in which the peptide chain forms loops closed by specific amino acid residue contacts, herein referred to as loop nodes. To measure loop rates, several photophysical methods have been introduced where a pair of optically active probes is incorporated at selected chain positions and the excited probe undergoes contact quenching (CQ) upon collision with the second probe. The quenching mechanisms involved triplet-triplet energy transfer, photoinduced electron transfer, and collision-induced fluorescence quenching, where the fluorescence of Dbo, an asparagine residue conjugated to 2,3-diazabicyclo[2.2.2]octane, is quenched by tryptophan. The discrepancy between the loop rates afforded from these three CQ techniques has, however, remained unresolved. In analyzing this discrepancy, we now report two short-distance FRET methods where Dbo acts as an energy acceptor in combination with tryptophan and naphtylalanine, two donors with largely different fluorescence lifetimes of 1.3 and 33 ns, respectively. Despite the different quenching mechanisms, the rates from FRET and CQ methods were, surprisingly, of comparable magnitude. This combination of FRET and CQ data led to a unifying physical model and to the conclusion that the rate of loop formation in folding reactions varies not only with the kind and number of residues that constitute the chain but also in particular with the size and properties of the residues that constitute the loop node.

Effect of ligand binding on a protein with a complex folding landscape

Ligand binding to a protein can stabilize it significantly against unfolding. The variation of the folding free energy, ΔΔG⁰, due to ligand binding can be derived from a simple reaction scheme involving exclusive binding to the native state. One obtains the following expression: , where K_d is the ligand dissociation constant and L is its concentration, R is the universal gas constant and T is the temperature. This expression has been shown to correctly describe experimental results on multiple proteins. In the current work we studied the effect of ligand binding on the stability of the multi-domain protein adenylate kinase from E. coli (AKE). Unfolding experiments were conducted using single-molecule FRET spectroscopy, which allowed us to directly obtain the fraction of unfolded protein in a model-free way from FRET efficiency histograms. Surprisingly, it was found that the effect of two inhibitors (Ap₅A and AMPPNP) and a substrate (AMP) on the stability of AKE was much smaller than expected based on K_d values obtained independently using microscale thermophoresis. To shed light on this issue, we measured the K_d for Ap₅A over a range of chemical denaturant concentrations where the protein is still folded. It was found that K_d increases dramatically over this range, likely due to the population of folding intermediates, whose binding to the ligand is much weaker than that of the native state. We propose that binding to folding intermediates may dominate the effect of ligands on the stability of multi-domain proteins, and could therefore have a strong impact on protein homeostasis in vivo.

Site-specific time-resolved FRET reveals local variations in the unfolding mechanism in an apparently two-state protein unfolding transition

Using multi-site time-resolved FRET, it is shown that equilibrium unfolding of monellin is not only heterogeneous, but that the degree of non-cooperativity differs between the sole α-helix and different parts of the β-sheet.

Microsecond Rearrangements of Hydrophobic Clusters in an Initially Collapsed Globule Prime Structure Formation during the Folding of a Small Protein

Determining how polypeptide chain collapse initiates structure formation during protein folding is a long standing goal. It has been challenging to characterize experimentally the dynamics of the polypeptide chain, which lead to the formation of a compact kinetic molten globule (MG) in about a millisecond. In this study, the sub-millisecond events that occur early during the folding of monellin from the guanidine hydrochloride-unfolded state have been characterized using multiple fluorescence and fluorescence resonance energy transfer probes. The kinetic MG is shown to form in a noncooperative manner from the unfolded (U) state as a result of at least three different processes happening during the first millisecond of folding. Initial chain compaction completes within the first 37μs, and further compaction occurs only after structure formation commences at a few milliseconds of folding. The transient nonnative and native-like hydrophobic clusters with side chains of certain residues buried form during the initial chain collapse and the nonnative clusters quickly disassemble. Subsequently, partial chain desolvation occurs, leading to the formation of a kinetic MG. The initial chain compaction and subsequent chain rearrangement appear to be barrierless processes. The two structural rearrangements within the collapsed globule appear to prime the protein for the actual folding transition.

The Folding process of Human Profilin-1, a novel protein associated with familial amyotrophic lateral sclerosis

Human profilin-1 is a novel protein associated with a recently discovered form of familial amyotrophic lateral sclerosis. This urges the characterization of possible conformational states, different from the fully folded state, potentially able to initiate self-assembly. Under native conditions, profilin-1 is monomeric and possesses a well-defined secondary and tertiary structure. When incubated at low pH or with high urea concentrations, profilin-1 remains monomeric but populates unfolded states exhibiting larger hydrodynamic radius and disordered structure, as assessed by dynamic light scattering, far-UV circular dichroism and intrinsic fluorescence. Refolding from the urea-unfolded state was studied at equilibrium and in real-time using a stopped-flow apparatus. The results obtained with intrinsic fluorescence and circular dichroism indicate a single phase without significant changes of the corresponding signals before the major refolding transition. However, such a transition is preceded by a burst phase with an observed increase of ANS fluorescence, which indicates the conversion into a transiently populated collapsed state possessing solvent-exposed hydrophobic clusters. Kinetic analysis reveals that such state has a conformational stability comparable to that of the fully unfolded state. To our knowledge, profilin-1 is the first example of an amyloid-related protein where folding occurs in the absence of thermodynamically stable partially folded states.

In the multi-domain protein adenylate kinase, domain insertion facilitates cooperative folding while accommodating function at domain interfaces

Having multiple domains in proteins can lead to partial folding and increased aggregation. Folding cooperativity, the all or nothing folding of a protein, can reduce this aggregation propensity. In agreement with bulk experiments, a coarse-grained structure-based model of the three-domain protein, E. coli Adenylate kinase (AKE), folds cooperatively. Domain interfaces have previously been implicated in the cooperative folding of multi-domain proteins. To understand their role in AKE folding, we computationally create mutants with deleted inter-domain interfaces and simulate their folding. We find that inter-domain interfaces play a minor role in the folding cooperativity of AKE. On further analysis, we find that unlike other multi-domain proteins whose folding has been studied, the domains of AKE are not singly-linked. Two of its domains have two linkers to the third one, i.e., they are inserted into the third one. We use circular permutation to modify AKE chain-connectivity and convert inserted-domains into singly-linked domains. We find that domain insertion in AKE achieves the following: (1) It facilitates folding cooperativity even when domains have different stabilities. Insertion constrains the N- and C-termini of inserted domains and stabilizes their folded states. Therefore, domains that perform conformational transitions can be smaller with fewer stabilizing interactions. (2) Inter-domain interactions are not needed to promote folding cooperativity and can be tuned for function. In AKE, these interactions help promote conformational dynamics limited catalysis. Finally, using structural bioinformatics, we suggest that domain insertion may also facilitate the cooperative folding of other multi-domain proteins.

Most individual protein domains fold in an all or nothing fashion. This cooperative folding is important because it reduces the existence of partially folded proteins which can stick to each other and create disease causing aggregates. However, numerous proteins have multiple domains, independent units of folding, stability and/or function. Several such proteins also fold cooperatively. It is thought that strong interactions between individual domains allow the folding to propagate from a nucleating domain to neighbouring ones and this enables cooperative folding in multi-domain proteins. Here, we computationally study the folding of the three-domain protein AKE and find instead that the topology of the protein, wherein the two less stable domains are inserted into the more stable one, promotes folding cooperativity. When the more stable domain is folded, the ends of the inserted domains are constrained and this allows them to fold easily. In such a protein topology, strong inter-domain interactions are not needed to promote folding cooperativity. Interface amino acids which would have been involved in ensuring that the domains fit together correctly can now be tuned for binding or catalysis or conformational transitions. Thus, inserted domains may be present in multi-domain proteins to promote both function and folding.

Kinetics of fast changing intramolecular distance distributions obtained by combined analysis of FRET efficiency kinetics and time-resolved FRET equilibrium measurements

Detailed studies of the mechanisms of macromolecular conformational transitions such as protein folding are enhanced by analysis of changes of distributions for intramolecular distances during the transitions. Time-resolved Förster resonance energy transfer (FRET) measurements yield such data, but the more readily available kinetics of mean FRET efficiency changes cannot be analyzed in terms of changes in distances because of the sixth-power dependence on the mean distance. To enhance the information obtained from mean FRET efficiency kinetics, we combined the analyses of FRET efficiency kinetics and equilibrium trFRET experiments. The joint analysis enabled determination of transient distance distributions along the folding reaction both in cases where a two-state transition is valid and in some cases consisting of a three-state scenario. The procedure and its limits were tested by simulations. Experimental data obtained from stopped-flow measurements of the refolding of Escherichia coli adenylate kinase were analyzed. The distance distributions between three double-labeled mutants, in the collapsed transient state, were determined and compared to those obtained experimentally using the double-kinetics technique. The proposed method effectively provides information on distance distributions of kinetically accessed intermediates of fast conformational transitions induced by common relaxation methods.

Folding properties of cytosine monophosphate kinase from E. coli indicate stabilization through an additional insert in the NMP binding domain

The globular 25 kDa protein cytosine monophosphate kinase (CMPK, EC ID: 2.7.4.14) from E. coli belongs to the family of nucleoside monophosphate (NMP) kinases (NMPK). Many proteins of this family share medium to high sequence and high structure similarity including the frequently found α/β topology. A unique feature of CMPK in the family of NMPKs is the positioning of a single cis-proline residue in the CORE-domain (cis-Pro124) in conjunction with a large insert in the NMP binding domain. This insert is not found in other well studied NMPKs such as AMPK or UMP/CMPK. We have analyzed the folding pathway of CMPK using time resolved tryptophan and FRET fluorescence as well as CD. Our results indicate that unfolding at high urea concentrations is governed by a single process, whereas refolding in low urea concentrations follows at least a three step process which we interpret as follows: Pro124 in the CORE-domain is in cis in the native state (N(c)) and equilibrates with its trans-isomer in the unfolded state (U(c) - U(t)). Under refolding conditions, at least the U(t) species and possibly also the U(c) species undergo a fast initial collapse to form intermediates with significant amount of secondary structure, from which the trans-Pro124 fraction folds to the native state with a 100-fold lower rate constant than the cis-Pro124 species. CMPK thus differs from homologous NMP kinases like UMP/CMP kinase or AMP kinase, where folding intermediates show much lower content of secondary structure. Importantly also unfolding is up to 100-fold faster compared to CMPK. We therefore propose that the stabilizing effect of the long NMP-domain insert in conjunction with a subtle twist in the positioning of a single cis-Pro residue allows for substantial stabilization compared to other NMP kinases with α/β topology.

The loop hypothesis: contribution of early formed specific non-local interactions to the determination of protein folding pathways

The extremely fast and efficient folding transition (in seconds) of globular proteins led to the search for some unifying principles embedded in the physics of the folding polypeptides. Most of the proposed mechanisms highlight the role of local interactions that stabilize secondary structure elements or a folding nucleus as the starting point of the folding pathways, i.e., a "bottom-up" mechanism. Non-local interactions were assumed either to stabilize the nucleus or lead to the later steps of coalescence of the secondary structure elements. An alternative mechanism was proposed, an "up-down" mechanism in which it was assumed that folding starts with the formation of very few non-local interactions which form closed long loops at the initiation of folding. The possible biological advantage of this mechanism, the "loop hypothesis", is that the hydrophobic collapse is associated with ordered compactization which reduces the chance for degradation and misfolding. In the present review the experiments, simulations and theoretical consideration that either directly or indirectly support this mechanism are summarized. It is argued that experiments monitoring the time-dependent development of the formation of specifically targeted early-formed sub-domain structural elements, either long loops or secondary structure elements, are necessary. This can be achieved by the time-resolved FRET-based "double kinetics" method in combination with mutational studies. Yet, attempts to improve the time resolution of the folding initiation should be extended down to the sub-microsecond time regime in order to design experiments that would resolve the classes of proteins which first fold by local or non-local interactions.

An instrument for fast acquisition of fluorescence decay curves at picosecond resolution designed for "double kinetics" experiments: application to fluorescence resonance excitation energy transfer study of protein folding

The information obtained by studying fluorescence decay of labeled biopolymers is a major resource for understanding the dynamics of their conformations and interactions. The lifetime of the excited states of probes attached to macromolecules is in the nanosecond time regime, and hence, a series of snapshot decay curves of such probes might - in principle - yield details of fast changes of ensembles of labeled molecules down to sub-microsecond time resolution. Hence, a major current challenge is the development of instruments for the low noise detection of fluorescence decay curves within the shortest possible time intervals. Here, we report the development of an instrument, picosecond double kinetics apparatus, that enables recording of multiple fluorescence decay curves with picosecond excitation pulses over wide spectral range during microsecond data collection for each curve. The design is based on recording and averaging multiphoton pulses of fluorescence decay using a fast 13 GHz oscilloscope during microsecond time intervals at selected time points over the course of a chemical reaction or conformational transition. We tested this instrument in a double kinetics experiment using reference probes (N-acetyl-tryptophanamide). Very low stochastic noise level was attained, and reliable multi-parameter analysis such as derivation of distance distributions from time resolved FRET (fluorescence resonance excitation energy transfer) measurements was achieved. The advantage of the pulse recording and averaging approach used here relative to double kinetics methods based on the established time correlated single photon counting method, is that in the pulse recording approach, averaging of substantially fewer kinetic experiments is sufficient for obtaining the data. This results in a major reduction in the consumption of labeled samples, which in many cases, enables the performance of important experiments that were not previously feasible.

How, when and why proteins collapse: the relation to folding

Unfolded proteins under strongly denaturing conditions are highly expanded. However, when the conditions are more close to native, an unfolded protein may collapse to a compact globular structure distinct from the folded state. This transition is akin to the coil-globule transition of homopolymers. Single-molecule FRET experiments have been particularly conducive in revealing the collapsed state under conditions of coexistence with the folded state. The collapse can be even more readily observed in natively unfolded proteins. Time-resolved studies, using FRET and small-angle scattering, have shown that the collapse transition is a very fast event, probably occurring on the submicrosecond time scale. The forces driving collapse are likely to involve both hydrophobic and backbone interactions. The loss of configurational entropy during collapse makes the unfolded state less stable compared to the folded state, thus facilitating folding.

Single-molecule fluorescence spectroscopy maps the folding landscape of a large protein

Proteins attain their function only after folding into a highly organized three-dimensional structure. Much remains to be learned about the mechanisms of folding of large multidomain proteins, which may populate metastable intermediate states on their energy landscapes. Here we introduce a novel method, based on high-throughput single-molecule fluorescence experiments, which is specifically geared towards tracing the dynamics of folding in the presence of a plethora of intermediates. We employ this method to characterize the folding reaction of a three-domain protein, adenylate kinase. Using thousands of single-molecule trajectories and hidden Markov modelling, we identify six metastable states on adenylate kinase's folding landscape. Remarkably, the connectivity of the intermediates depends on denaturant concentration; at low concentration, multiple intersecting folding pathways co-exist. We anticipate that the methodology introduced here will find broad applicability in the study of folding of large proteins, and will provide a more realistic scenario of their conformational dynamics.

Time-resolved FRET detection of subtle temperature-induced conformational biases in ensembles of α-synuclein molecules

The α-synuclein (αS) molecule, a polypeptide of 140 residues, is an intrinsically disordered protein that is involved in the onset of Parkinson's disease. We applied time-resolved excitation energy transfer measurements in search of specific deviations from the disordered state in segments of the αS backbone that might be involved in the initiation of aggregation. Since at higher temperatures, the αS molecule undergoes accelerated aggregation, we studied the temperature dependence of the distributions of intramolecular segmental end-to-end distances and their fast fluctuations in eight labeled chain segments of the αS molecule. Over the temperature range of 5-40 °C, no temperature-induced unfolding or folding was detected at the N-terminal domain (residues 1-66) of the αS molecule. The intramolecular diffusion coefficient of the segments' ends relative to each other increased monotonously with temperature. A common very high upper limiting value of ∼25 A²/ns was reached at 40 °C, another indication of a fully disordered state. Three exceptions were two segments with reduced values of the diffusion coefficients (the shortest segment where the excluded volume effect is dominant and the segment labeled in the NAC domain) and a nonlinear cooperative transition in the N-terminal segment. These specific subtle deviations from the common pattern of temperature dependence reflect specific structural constraints that could be critical in controlling the stability of the soluble monomer, or for its aggregation. Such very weak effects might be dominant in determination of the fate of ensembles of disordered polypeptides either to folding or to misfolding.

Evidence for initial non-specific polypeptide chain collapse during the refolding of the SH3 domain of PI3 kinase

Refolding of the SH3 domain of PI3 kinase from the guanidine hydrochloride (GdnHCl)-unfolded state has been probed with millisecond (stopped flow) and sub-millisecond (continuous flow) measurements of the change in fluorescence, circular dichroism, ANS fluorescence and three-site fluorescence resonance energy transfer (FRET) efficiency. Fluorescence measurements are unable to detect structural changes preceding the rate-limiting step of folding, whereas measurements of changes in ANS fluorescence and FRET efficiency indicate that polypeptide chain collapse precedes the major structural transition. The initial chain collapse reaction is complete within 150 μs. The collapsed form at this time possesses hydrophobic clusters to which ANS binds. Each of the three measured intra-molecular distances has contracted to an extent predicted by the dependence of the FRET signal in completely unfolded protein on denaturant concentration, indicating that contraction is non-specific. The extent of contraction of each intra-molecular distance in the collapsed product of sub-millisecond folding increases continuously with a decrease in [GdnHCl]. The gradual contraction is continuous with the gradual contraction seen in completely unfolded protein, and its dependence on [GdnHCl] is not indicative of an all-or-none collapse reaction. The dependence of the extent of contraction on [GdnHCl] was similar for the three distances, indicating that chain collapse occurs in a synchronous manner across different segments of the polypeptide chain. The sub-millisecond measurements of folding in GdnHCl were unable to determine whether hydrophobic cluster formation, probed by ANS fluorescence measurement, precedes chain contraction probed by FRET. To determine whether hydrogen bonding plays a role in initial chain collapse, folding was initiated by dilution of the urea-unfolded state. The extent of contraction of at least one intra-molecular distance in the collapsed product of sub-millisecond folding in urea is similar to that seen in GdnHCl, and the initial contraction in urea too appears to be gradual.

Slow formation of aggregation-resistant beta-sheet folding intermediates

Protein folding has been studied extensively for decades, yet our ability to predict how proteins reach their native state from a mechanistic perspective is still rudimentary at best, limiting our understanding of folding-related processes in vivo and our ability to manipulate proteins in vitro. Here, we investigate the in vitro refolding mechanism of a large beta-helix protein, pertactin, which has an extended, elongated shape. At 55 kDa, this single domain, all-beta-sheet protein allows detailed analysis of the formation of beta-sheet structure in larger proteins. Using a combination of fluorescence and far-UV circular dichroism spectroscopy, we show that the pertactin beta-helix refolds remarkably slowly, with multiexponential kinetics. Surprisingly, despite the slow refolding rates, large size, and beta-sheet-rich topology, pertactin refolding is reversible and not complicated by off-pathway aggregation. The slow pertactin refolding rate is not limited by proline isomerization, and 30% of secondary structure formation occurs within the rate-limiting step. Furthermore, site-specific labeling experiments indicate that the beta-helix refolds in a multistep but concerted process involving the entire protein, rather than via initial formation of the stable core substructure observed in equilibrium titrations. Hence pertactin provides a valuable system for studying the refolding properties of larger, beta-sheet-rich proteins, and raises intriguing questions regarding the prevention of aggregation during the prolonged population of partially folded, beta-sheet-rich refolding intermediates. Proteins 2010. (c) 2009 Wiley-Liss, Inc.

Noncooperative folding of subdomains in adenylate kinase

Conformational change is regulating the biological activity of a large number of proteins and enzymes. Efforts in structural biology have provided molecular descriptions of the interactions that stabilize the stable ground states on the reaction trajectories during conformational change. Less is known about equilibrium thermodynamic stabilities of the polypeptide segments that participate in structural changes and whether the stabilities are relevant for the reaction pathway. Adenylate kinase (Adk) is composed of three subdomains: CORE, ATPlid, and AMPbd. ATPlid and AMPbd are flexible nucleotide binding subdomains where large-scale conformational changes are directly coupled to catalytic activity. In this report, the equilibrium thermodynamic stabilities of Adk from both mesophilic and hyperthermophilic bacteria were investigated using solution state NMR spectroscopy together with protein engineering experiments. Equilibrium hydrogen to deuterium exchange experiments indicate that the flexible subdomains are of significantly lower thermodynamic stability compared to the CORE subdomain. Using site-directed mutagenesis, parts of ATPlid and AMPbd could be selectively unfolded as a result of perturbation of hydrophobic clusters located in these respective subdomains. Analysis of the perturbed Adk variants using NMR spin relaxation and C(alpha) chemical shifts shows that the CORE subdomain can fold independently of ATPlid and AMPbd; consequently, folding of the two flexible subdomains occurs independently of each other. Based on the experimental results it is apparent that the flexible subdomains fold into their native structure in a noncooperative manner with respect to the CORE subdomain. These results are discussed in light of the catalytically relevant conformational change of ATPlid and AMPbd.

Collapse transition in proteins

The coil-globule transition, a tenet of the physics of polymers, has been identified in recent years as an important unresolved aspect of the initial stages of the folding of proteins. We describe the basics of the collapse transition, starting with homopolymers and continuing with proteins. Studies of denatured-state collapse under equilibrium are then presented. An emphasis is placed on single-molecule fluorescence experiments, which are particularly useful for measuring properties of the denatured state even under conditions of coexistence with the folded state. Attempts to understand the dynamics of collapse, both theoretically and experimentally, are then described. Only an upper limit for the rate of collapse has been obtained so far. Improvements in experimental and theoretical methodology are likely to continue to push our understanding of the importance of the denatured-state thermodynamics and dynamics for protein folding in the coming years.

Early closure of a long loop in the refolding of adenylate kinase: a possible key role of non-local interactions in the initial folding steps

Most globular protein chains, when transferred from high to low denaturant concentrations, collapse instantly before they refold to their native state. The initial compaction of the protein molecule is assumed to have a key effect on the folding pathway, but it is not known whether the earliest structures formed during or instantly after collapse are defined by local or by non-local interactions--that is, by secondary structural elements or by loop closure of long segments of the protein chain. Stable closure of one or several long loops can reduce the chain entropy at a very early stage and can prevent the protein from following non-productive pathways whose number grows exponentially with the length of the protein chain. In Escherichia coli adenylate kinase (AK), about seven long loops define the topology of the native structure. We selected four loop-forming sections of the chain and probed the time course of loop formation during refolding of AK. We labeled the termini of the loop segments with tryptophan and cysteine-5-amidosalicylic acid. This donor-acceptor pair of probes used with fluorescence resonance excitation energy transfer spectroscopy (FRET) is suitable for detecting very short distances and thus is able to distinguish between random and specific compactions. Refolding of AK was initiated by stopped-flow mixing, followed simultaneously by donor and acceptor fluorescence, and analyzed in terms of energy transfer efficiency and distance. In the collapsed state of AK, observed after the 5-ms dead time of the instrument, one of the selected segments shows a native-like separation of its termini; it forms a loop already in the collapsed state. A second segment that includes the first but is longer by 15 residues shows an almost native-like separation of its termini. In contrast, a segment that is shorter but part of the second segment shows a distance separation of its termini as high as a segment that spans almost the whole protein chain. We conclude that a specific network of non-local interactions, the closure of one or several loops, can play an important role in determining the protein folding pathway at its early phases.

The influence of proline isomerization and off-pathway intermediates on the folding mechanism of eukaryotic UMP/CMP Kinase

The globular 22-kDa protein UMP/CMP from Dictyostelium discoideum (UmpK) belongs to the family of nucleoside monophosphate (NMP) kinases. These enzymes not only show high sequence and structure similarities but also share the alpha/beta-fold, a very common protein topology. We investigated the protein folding mechanism of UmpK as a representative for this ubiquitous enzyme class. Equilibrium stability towards urea and the unfolding and refolding kinetics were studied by means of fluorescence and far-UV CD spectroscopy. Although the unfolding can be described by a two-state process, folding kinetics are rather complex with four refolding phases that can be resolved and an additional burst phase. Moreover, two of these phases exhibit a pronounced rollover in the refolding limb that cannot be explained by aggregation. Whilst secondary structure formation is not observed in the burst phase reaction, folding to the native structure is strongly influenced by the slowest phase, since 30% of the alpha-helical CD signal is restored therein. This process can be assigned to proline isomerization and is strongly accelerated by the Escherichia coli peptidyl-prolyl isomerase trigger factor. The analysis of our single-mixing and double-mixing experiments suggests the occurrence of an off-pathway intermediate and an unproductive collapsed structure, which appear to be rate limiting for the folding of UmpK.

Using fluorescence correlation spectroscopy to study conformational changes in denatured proteins

Fluorescence correlation spectroscopy (FCS) is a sensitive analytical tool that allows dynamics and hydrodynamics of biomolecules to be studied under a broad range of experimental conditions. One application of FCS of current interest is the determination of the size of protein molecules in the various states they sample along their folding reaction coordinate, which can be accessed through the measurement of diffusion coefficients. It has been pointed out that the analysis of FCS curves is prone to artifacts that may lead to erroneous size determination. To set the stage for FCS studies of unfolded proteins, we first show that the diffusion coefficients of small molecules as well as proteins can be determined accurately even in the presence of high concentrations of co-solutes that change the solution refractive index significantly. Indeed, it is found that the Stokes-Einstein relation between the measured diffusion coefficient and solution viscosity holds even in highly concentrated glycerol or guanidinium hydrochloride (GuHCl) solutions. These measurements form the basis for an investigation of the structure of the denatured state of two proteins, the small protein L and the larger, three-domain protein adenylate kinase (AK). FCS is found useful for probing expansion in the denatured state beyond the unfolding transition. It is shown that the denatured state of protein L expands as the denaturant concentration increases, in a process akin to the transition from a globule to a coil in polymers. This process continues at least up to 5 M GuHCl. On the other hand, the denatured state of AK does not seem to expand much beyond 2 M GuHCl, a result that is in qualitative accord with single-molecule fluorescence histograms. Because both the unfolding transition and the coil-globule transition of AK occur at a much lower denaturant concentration than those of protein L, a possible correlation between the two phenomena is suggested.

Mapping protein collapse with single-molecule fluorescence and kinetic synchrotron radiation circular dichroism spectroscopy

We have used the combination of single-molecule Förster resonance energy transfer and kinetic synchrotron radiation circular dichroism experiments to probe the conformational ensemble of the collapsed unfolded state of the small cold shock protein CspTm under near-native conditions. This regime is physiologically most relevant but difficult to access experimentally, because the equilibrium signal in ensemble experiments is dominated by folded molecules. Here, we avoid this problem in two ways. One is the use of single-molecule Förster resonance energy transfer, which allows the separation of folded and unfolded subpopulations at equilibrium and provides information on long-range intramolecular distance distributions. From experiments with donor and acceptor chromophores placed at different positions within the chain, we find that the distance distributions in unfolded CspTm agree surprisingly well with a Gaussian chain not only at high concentrations of denaturant, where the polypeptide chain is expanded, but also at low denaturant concentrations, where the chain is collapsed. The second, complementary approach is synchrotron radiation circular dichroism spectroscopy of collapsed unfolded molecules transiently populated with a microfluidic device that enables rapid mixing. The results indicate a beta-structure content of the collapsed unfolded state of approximately 20% compared with the folded protein. This suggests that collapse can induce secondary structure in an unfolded state without interfering with long-range distance distributions characteristic of a random coil, which were previously found only for highly expanded unfolded proteins.

Criteria for downhill protein folding: Calorimetry, chevron plot, kinetic relaxation, and single-molecule radius of gyration in chain models with subdued degrees of cooperativity

Recent investigations of possible downhill folding of small proteins such as BBL have focused on the thermodynamics of non-two-state, "barrierless" folding/denaturation transitions. Downhill folding is noncooperative and thermodynamically "one-state," a phenomenon underpinned by a unimodal conformational distribution over chain properties such as enthalpy, hydrophobic exposure, and conformational dimension. In contrast, corresponding distributions for cooperative two-state folding are bimodal with well-separated population peaks. Using simplified atomic modeling of a three-helix bundle-in a scheme that accounts for hydrophobic interactions and hydrogen bonding-and coarse-grained C(alpha) models of four real proteins with various degrees of cooperativity, we evaluate the effectiveness of several observables at defining the underlying distribution. Bimodal distributions generally lead to sharper transitions, with a higher heat capacity peak at the transition midpoint, compared with unimodal distributions. However, the observation of a sigmoidal transition is not a reliable criterion for two-state behavior, and the heat capacity baselines, used to determine the van't Hoff and calorimetric enthalpies of the transition, can introduce ambiguity. Interestingly we find that, if the distribution of the single-molecule radius of gyration were available, it would permit discrimination between unimodal and bimodal underlying distributions. We investigate kinetic implications of thermodynamic noncooperativity using Langevin dynamics. Despite substantial chevron rollovers, the relaxation of the models considered is essentially single-exponential over an extended range of native stabilities. Consistent with experiments, significant deviations from single-exponential behavior occur only under strongly folding conditions.

Coil-globule transition in the denatured state of a small protein

Upon transfer from strongly denaturing to native conditions, proteins undergo a collapse that either precedes folding or occurs simultaneously with it. This collapse is similar to the well known coil-globule transition of polymers. Here we employ single-molecule fluorescence methods to fully characterize the equilibrium coil-globule transition in the denatured state of the IgG-binding domain of protein L. By using FRET measurements on freely diffusing individual molecules, we determine the radius of gyration of the protein, which shows a gradual expansion as the concentration of the denaturant, guanidinium hydrochloride, is increased all the way up to 7 M. This expansion is observed also in fluorescence correlation spectroscopy measurements of the hydrodynamic radius of the protein. We analyze the radius of gyration measurements using the theory of the coil-globule transition of Sanchez [Sanchez, I. C. (1979) Macromolecules 12, 980-988], which balances the excluded volume entropy of the chain with the average interresidue interaction energy. In particular, we calculate the solvation energy of the denatured protein, a property that is not readily accessible in other experiments. The dependence of this energy on denaturant concentration is nonlinear, contrasting with the common linear extrapolation method used to describe denaturation energy. Interestingly, a fit to the binding model of chemical denaturation suggests a single denaturant binding site per protein residue. The size of the denatured protein under native conditions can be extrapolated from the data as well, showing that the fully collapsed state of protein is only approximately 10% larger than the folded state.

Specificity of the initial collapse in the folding of the cold shock protein

The two-state folding reaction of the cold shock protein from Bacillus caldolyticus (Bc-Csp) is preceded by a rapid chain collapse. A fast shortening of intra-protein distances was revealed by Förster resonance energy transfer (FRET) measurements with protein variants that carried individual pairs of donor and acceptor chromophores at various positions along the polypeptide chain. Here we investigated the specificity of this rapid compaction. Energy transfer experiments that probed the stretching of strand beta2 and the close approach between the strands beta1 and beta2 revealed that the beta1-beta2 hairpin is barely formed in the collapsed form, although it is native-like in the folding transition state of Bc-Csp. The time course of the collapse could not be resolved by pressure or temperature jump experiments, indicating that the collapsed and extended forms are not separated by an energy barrier. The co-solute (NH4)2SO4 stabilizes both native Bc-Csp and the collapsed form, which suggests that the large hydrated SO4(2-) ions are excluded from the surface of the collapsed form in a similar fashion as they are excluded from folded Bc-Csp. Ethylene glycol increases the stability of proteins because it is excluded preferentially from the backbone, which is accessible in the unfolded state. The collapsed form of Bc-Csp resembles the unfolded form in its interaction with ethylene glycol, suggesting that in the collapsed form the backbone is still accessible to water and small molecules. Our results thus rule out that the collapsed form is a folding intermediate with native-like chain topology. It is better described as a mixture of compact conformations that belong to the unfolded state ensemble. However, some of its structural elements are reminiscent of the native protein.