Kinetic stabilisation of a modular protein by domain interactions

Retracing the evolution of a modern periplasmic binding protein

Investigating the evolution of structural features in modern multidomain proteins helps to understand their immense diversity and functional versatility. The class of periplasmic binding proteins (PBPs) offers an opportunity to interrogate one of the main processes driving diversification: the duplication and fusion of protein sequences to generate new architectures. The symmetry of their two-lobed topology, their mechanism of binding, and the organization of their operon structure led to the hypothesis that PBPs arose through a duplication and fusion event of a single common ancestor. To investigate this claim, we set out to reverse the evolutionary process and recreate the structural equivalent of a single-lobed progenitor using ribose-binding protein (RBP) as our model. We found that this modern PBP can be deconstructed into its lobes, producing two proteins that represent possible progenitor halves. The isolated halves of RBP are well folded and monomeric proteins, albeit with a lower thermostability, and do not retain the original binding function. However, the two entities readily form a heterodimer in vitro and in-cell. The x-ray structure of the heterodimer closely resembles the parental protein. Moreover, the binding function is fully regained upon formation of the heterodimer with a ligand affinity similar to that observed in the modern RBP. This highlights how a duplication event could have given rise to a stable and functional PBP-like fold and provides insights into how more complex functional structures can evolve from simpler molecular components.

Thermodynamic and kinetic analysis of the LAO binding protein and its isolated domains reveal non-additivity in stability, folding and function

Substrate-binding proteins (SBPs) are used by organisms from the three domains of life for transport and signalling. SBPs are composed of two domains that collectively trap ligands with high affinity and selectivity. To explore the role of the domains and the integrity of the hinge region between them in the function and conformation of SBPs, here, we describe the ligand binding, conformational stability and folding kinetics of the Lysine Arginine Ornithine (LAO) binding protein from Salmonella thiphimurium and constructs corresponding to its two independent domains. LAO is a class II SBP formed by a continuous and a discontinuous domain. Contrary to the expected behaviour based on their connectivity, the discontinuous domain shows a stable native-like structure that binds l-arginine with moderate affinity, whereas the continuous domain is barely stable and shows no detectable ligand binding. Regarding folding kinetics, studies of the entire protein revealed the presence of at least two intermediates. While the unfolding and refolding of the continuous domain exhibited only a single intermediate and simpler and faster kinetics than LAO, the folding mechanism of the discontinuous domain was complex and involved multiple intermediates. These findings suggest that in the complete protein the continuous domain nucleates folding and that its presence funnels the folding of the discontinuous domain avoiding nonproductive interactions. The strong dependence of the function, stability and folding pathway of the lobes on their covalent association is most likely the result of the coevolution of both domains as a single unit.

Kinetic Stability of Long-Lived Human Lens γ-Crystallins and Their Isolated Double Greek Key Domains

The γ-crystallins of the eye lens nucleus are among the longest-lived proteins in the human body. Synthesized in utero, they must remain folded and soluble throughout adulthood to maintain lens transparency and avoid cataracts. γD- and γS-crystallin are two major monomeric crystallins of the human lens. γD-crystallin is concentrated in the oldest lens fiber cells, the lens nucleus, whereas γS-crystallin is concentrated in the younger cells of the lens cortex. The kinetic stability parameters of these two-domain proteins and their isolated domains were determined and compared. Kinetic unfolding experiments monitored by fluorescence spectroscopy in varying concentrations of guanidinium chloride were used to extrapolate unfolding rate constants and half-lives of the crystallins in the absence of the denaturant. Consistent with their long lifespans in the lens, extrapolated half-lives for the initial unfolding step were on the timescale of years. Both proteins' isolated N-terminal domains were less kinetically stable than their respective C-terminal domains at denaturant concentrations predicted to disrupt the domain interface, but at low denaturant concentrations, the relative kinetic stabilities were reversed. Cataract-associated aggregation has been shown to proceed from partially unfolded intermediates in these proteins; their extreme kinetic stability likely evolved to protect the lens from the initiation of aggregation reactions. Our findings indicate that the domain interface is the source of significant kinetic stability. The gene duplication and fusion event that produced the modern two-domain architecture of vertebrate lens crystallins may be the origin of their high kinetic as well as thermodynamic stability.

Same but not alike: Structure, flexibility and energetics of domains in multi-domain proteins are influenced by the presence of other domains

The majority of the proteins encoded in the genomes of eukaryotes contain more than one domain. Reasons for high prevalence of multi-domain proteins in various organisms have been attributed to higher stability and functional and folding advantages over single-domain proteins. Despite these advantages, many proteins are composed of only one domain while their homologous domains are part of multi-domain proteins. In the study presented here, differences in the properties of protein domains in single-domain and multi-domain systems and their influence on functions are discussed. We studied 20 pairs of identical protein domains, which were crystallized in two forms (a) tethered to other proteins domains and (b) tethered to fewer protein domains than (a) or not tethered to any protein domain. Results suggest that tethering of domains in multi-domain proteins influences the structural, dynamic and energetic properties of the constituent protein domains. 50% of the protein domain pairs show significant structural deviations while 90% of the protein domain pairs show differences in dynamics and 12% of the residues show differences in the energetics. To gain further insights on the influence of tethering on the function of the domains, 4 pairs of homologous protein domains, where one of them is a full-length single-domain protein and the other protein domain is a part of a multi-domain protein, were studied. Analyses showed that identical and structurally equivalent functional residues show differential dynamics in homologous protein domains; though comparable dynamics between in-silico generated chimera protein and multi-domain proteins were observed. From these observations, the differences observed in the functions of homologous proteins could be attributed to the presence of tethered domain. Overall, we conclude that tethered domains in multi-domain proteins not only provide stability or folding advantages but also influence pathways resulting in differences in function or regulatory properties.

High prevalence of multi-domain proteins in proteomes has been attributed to higher stability and functional and folding advantages of the multi-domain proteins. Influence of tethering of domains on the overall properties of proteins has been well studied but its influence on the properties of the constituent domains is largely unaddressed. Here, we investigate the influence of tethering of domains in multi-domain proteins on the structural, dynamics and energetics properties of the constituent domains and its implications on the functions of proteins. To this end, comparative analyses were carried out for identical protein domains crystallized in tethered and untethered forms. Also, comparative analyses of single-domain proteins and their homologous multi-domain proteins were performed. The analyses suggest that tethering influences the structural, dynamic and energetic properties of constituent protein domains. Our observations hint at regulation of protein domains by tethered domains in multi-domain systems, which may manifest at the differential function observed between single-domain and homologous multi-domain proteins.

Protein Assembly and Building Blocks: Beyond the Limits of the LEGO Brick Metaphor

Proteins, like other biomolecules, have a modular and hierarchical structure. Various building blocks are used to construct proteins of high structural complexity and diverse functionality. In multidomain proteins, for example, domains are fused to each other in different combinations to achieve different functions. Although the LEGO brick metaphor is justified as a means of simplifying the complexity of three-dimensional protein structures, several fundamental properties (such as allostery or the induced-fit mechanism) make deviation from it necessary to respect the plasticity, softness, and cross-talk that are essential to protein function. In this work, we illustrate recently reported protein behavior in multidomain proteins that deviates from the LEGO brick analogy. While earlier studies showed that a protein domain is often unaffected by being fused to another domain or becomes more stable following the formation of a new interface between the tethered domains, destabilization due to tethering has been reported for several systems. We illustrate that tethering may sometimes result in a multidomain protein behaving as "less than the sum of its parts". We survey these cases for which structure additivity does not guarantee thermodynamic additivity. Protein destabilization due to fusion to other domains may be linked in some cases to biological function and should be taken into account when designing large assemblies.

Single-molecule Force Spectroscopy Reveals the Calcium Dependence of the Alternative Conformations in the Native State of a βγ-Crystallin Protein

Although multidomain proteins predominate the proteome of all organisms and are expected to display complex folding behaviors and significantly greater structural dynamics as compared with single-domain proteins, their conformational heterogeneity and its impact on their interaction with ligands are poorly understood due to a lack of experimental techniques. The multidomain calcium-binding βγ-crystallin proteins are particularly important because their deterioration and misfolding/aggregation are associated with melanoma tumors and cataracts. Here we investigate the mechanical stability and conformational dynamics of a model calcium-binding βγ-crystallin protein, Protein S, and elaborate on its interactions with calcium. We ask whether domain interactions and calcium binding affect Protein S folding and potential structural heterogeneity. Our results from single-molecule force spectroscopy show that the N-terminal (but not the C-terminal) domain is in equilibrium with an alternative conformation in the absence of Ca(2+), which is mechanically stable in contrast to other proteins that were observed to sample a molten globule under similar conditions. Mutagenesis experiments and computer simulations reveal that the alternative conformation of the N-terminal domain is caused by structural instability produced by the high charge density of a calcium binding site. We find that this alternative conformation in the N-terminal domain is diminished in the presence of calcium and can also be partially eliminated with a hitherto unrecognized compensatory mechanism that uses the interaction of the C-terminal domain to neutralize the electronegative site. We find that up to 1% of all identified multidomain calcium-binding proteins contain a similarly highly charged site and therefore may exploit a similar compensatory mechanism to prevent structural instability in the absence of ligand.

The βγ-crystallins: native state stability and pathways to aggregation

The βγ-crystallins are among the most stable and long-lived proteins in the human body. With increasing age, however, they transform to high molecular weight light-scattering aggregates, resulting in cataracts. This occurs despite the presence in the lens of high concentrations of the a-crystallin chaperones. Aggregation of crystallins can be induced in vitro by a variety of stresses, including acidic pH, ultraviolet light, oxidative damage, heating or freezing, and specific amino acid substitutions. Accumulating evidence points to the existence of specific biochemical pathways of protein: protein interaction and polymerization. We review the methods used for studying crystallin stability and aggregation and discuss the sometimes counterintuitive relationships between factors that favor native state stability and those that favor non-native aggregation. We discuss the behavior of βγ-crystallins in mixtures and their chaperone ability; the consequences of missense mutations and covalent damage to the side-chains; and the evolutionary strategies that have shaped these proteins. Efforts are ongoing to reveal the nature of cataractous crystallin aggregates and understand the mechanisms of aggregation in the context of key models of protein polymerization: amyloid, native-state, and domain-swapped. Such mechanistic understanding is likely to be of value for the development of therapeutic interventions and draw attention to unanswered questions about the relationship between a protein's native state stability and its transformation to an aggregated state.

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.

Cooperative assembly and misfolding of CFTR domains in vivo

The cystic fibrosis transmembrane conductance regulator (CFTR) architecture consists of two membrane spanning domains (MSD1 and -2), two nucleotide binding domains (NBD1 and -2), and a regulatory (R) domain. Several point mutations lead to the channel misprocessing, with limited structural perturbation of the mutant domain. To gain more insight into the basis of CFTR folding defect, the contribution of domain-wise and cooperative domain folding was assessed by determining 1) the minimal domain combination that is recognized as native and can efficiently escape the endoplasmic reticulum (ER) retention and 2) the impact of mutation on the conformational coupling among domains. One-, two-, three-, and most of the four-domain assemblies were retained at the ER. Solubilization mutations, however, rescued the NBD1 processing defect conceivably by thermodynamic stabilization. The smallest folding unit that traversed the secretory pathway was composed of MSD1-NBD1-R-MSD2 as a linear or split polypeptide. Cystic fibrosis-causing missense mutations in the MSD1, NBD1, MSD2, and NBD2 caused conformational defect in multiple domains. We propose that cooperative posttranslational folding is required for domain stabilization and provides a plausible explanation for the global misfolding caused by point mutations dispersed along the full-length CFTR.

Cooperativity, connectivity, and folding pathways of multidomain proteins

Multidomain proteins are ubiquitous in both prokaryotic and eukaryotic proteomes. Study on protein folding, however, has concentrated more on the isolated single domains of proteins, and there have been relatively few systematic studies on the effects of domain-domain interactions on folding. We here discuss this issue by examining human gammaD-crystallin, spore coat protein S, and a tandem array of the R16 and R17 domains of spectrin as example proteins by using a structure-based model of folding. The calculated results consistently explain the experimental data on folding pathways and effects of mutational perturbations, supporting the view that the connectivity of two domains and the distribution of domain-domain interactions in the native conformation are factors to determine kinetic and equilibrium properties of cooperative folding.

The folding pathway of a single domain in a multidomain protein is not affected by its neighbouring domain

Domains are the structural, functional, and evolutionary components of proteins. Most folding studies to date have concentrated on the folding of single domains, but more than 70% of human proteins contain more than one domain, and interdomain interactions can affect both the stability and the folding kinetics. Whether the folding pathway is altered by interdomain interactions is not yet known. Here we investigated the effect of a folded neighbouring domain on the folding pathway of spectrin R16 (the 16th α-helical repeat from chicken brain α-spectrin) by using the two-domain construct R1516. The R16 folds faster and unfolds more slowly in the presence of its folded neighbour R15 (the 15th α-helical repeat from chicken brain α-spectrin). An extensive Φ-value analysis of the R16 domain in R1516 was completed to compare the transition state of the R16 domain alone with that of the R16 domain in a multidomain construct. The results indicate that the folding pathways are the same. This result validates the current approach of breaking up larger proteins into domains for the study of protein folding pathways.

Distinguishing specific and nonspecific interdomain interactions in multidomain proteins

Multidomain proteins account for over two-thirds of the eukaryotic genome. Although there have been extensive studies into the biophysical properties of isolated domains, few have investigated how the domains interact. Spectrin is a well-characterized multidomain protein with domains linked in tandem array by contiguous helices. Several of these domains have been shown to be stabilized by their neighbors. Until now, this stabilization has been attributed to specific interactions between the natural neighbors, however we have recently observed that nonnatural neighboring domains can also induce a significant amount of stabilization. Here we investigate this nonnative stabilizing effect. We created spectrin-titin domain pairs of both spectrin R16 and R17 with a single titin I27 domain at either the N- or the C-terminus and found that spectrin domains are significantly stabilized, through slowed unfolding, by nonnative interactions at the C-terminus only. Of particular importance, we show that specific interactions between natural folded neighbors at either terminus confer even greater stability by additionally increasing the folding rate constants. We demonstrate that it is possible to distinguish between natural stabilizing interactions and nonspecific stabilizing effects through examination of the kinetics of well chosen mutant proteins. This work adds to the complexity of studying multidomain proteins.

The folding and evolution of multidomain proteins

Analyses of genomes show that more than 70% of eukaryotic proteins are composed of multiple domains. However, most studies of protein folding focus on individual domains and do not consider how interactions between domains might affect folding. Here, we address this by analysing the three-dimensional structures of multidomain proteins that have been characterized experimentally and observe that where the interface is small and loosely packed, or unstructured, the folding of the domains is independent. Furthermore, recent studies indicate that multidomain proteins have evolved mechanisms to minimize the problems of interdomain misfolding.

Apparent cooperativity in the folding of multidomain proteins depends on the relative rates of folding of the constituent domains

Approximately 75% of eukaryotic proteins contain more than one so-called independently folding domain. However, there have been relatively few systematic studies to investigate the effect of interdomain interactions on protein stability and fewer still on folding kinetics. We present the folding of pairs of three-helix bundle spectrin domains as a paradigm to indicate how complex such an analysis can be. Equilibrium studies show an increase in denaturant concentration required to unfold the domains with only a single unfolding transition; however, in some cases, this is not accompanied by the increase in m value, which would be expected if the protein is a truly cooperative, all-or-none system. We analyze the complex kinetics of spectrin domain pairs, both wild-type and carefully selected mutants. By comparing these pairs, we are able to demonstrate that equilibrium data alone are insufficient to describe the folding of multidomain proteins and to quantify the effects that one domain can have on its neighbor.

Effect of linker segments on the stability of epithelial cadherin domain 2

Epithelial cadherin is a transmembrane protein that is essential in calcium-dependent cell-cell recognition and adhesion. It contains five independently folded globular domains in its extracellular region. Each domain has a seven-strand beta-sheet immunoglobulin fold. Short seven-residue peptide segments connect the globular domains and provide oxygens to chelate calcium ions at the interface between the domains (Nagar et al., Nature 1995;380:360-364). Recently, stability studies of ECAD2 (Prasad et al., Biochemistry 2004;43:8055-8066) were undertaken with the motivation that Domain 2 is a representative domain for this family of proteins. The definition of a domain boundary is somewhat arbitrary; hence, it was important to examine the effect of the adjoining linker regions that connect Domain 2 to the adjacent domains. Present studies employ temperature-denaturation and proteolytic susceptibility to provide insight into the impact of these linkers on Domain 2. The significant findings of our present study are threefold. First, the linker segments destabilize the core domain in the absence of calcium. Second, the destabilization due to addition of the linker segments can be partially reversed by the addition of calcium. Third, sodium chloride stabilizes all constructs. This result implies that electrostatic repulsion is a contributor to destabilization of the core domain by addition of the linkers. Thus, the context of Domain 2 within the whole molecule affects its thermodynamic characteristics.

Calcium and domain interactions contribute to the thermostability of domains of the multimodular cellobiohydrolase, CbhA, a subunit of the Clostridium thermocellum cellulosome

Each of three internal domains of multi-modular cellobiohydrolase CbhA from Clostridium thermocellum, X1(1), X1(2) (previously designated as fibronectin type 3-like modules, Fn3(1) and Fn3(2)) and family 3 carbohydrate-binding module (CBM3) binds 1 mol of Ca(2+). Structures and thermal stabilities of X1(1), X1(2), CBM3, X1(1)X1(2), and X1(1)X1(2)-CBM3 containing Ca(2+) (holo-proteins) and without Ca(2+) (apo-proteins) have been studied using CD spectroscopy. All domains are beta-proteins with irregular far-UV CD spectra due to the aromatic side chain contributions. The positive signal at 294 nm in the near-UV CD spectrum of X1(1) lacking a tryptophan residue might be attributed to the presence of aromatic clusters. Thermal denaturation of all proteins is reversible and results in the total loss of tertiary structure and preservation of significant amount of ordered secondary structure. Removal of Ca(2+) destabilizes polypeptides in a different way and to a different extent. It decreases the melting temperature ( T (m)) (by 20 degrees C) and co-operativity of thermal transition of X1(1), increases the number of transitions and lowers the co-operativity of unfolding of CBM3, and slightly decreases T (m)s (2.4-4.2 degrees C) of X1(2), X1(1)X1(2), and X1(1)X1(2)-CBM3. Transitions of X1(1)X1(2) and X1(1)X1(2)-CBM3 follow a two-state model regardless of the presence of Ca(2+). X1(1) is strongly stabilized in the apo-X1(1)X1(2) and apo-X1(1)X1(2)-CBM3 as they display T (m)s similar to those of individual and combined holo-modules. Observed CD spectra of X1(1)X1(2) and X1(1)X1(2)-CBM3 differ from those calculated as the simple weighted sum of individual modules. These differences are more prominent in spectra of apo-proteins. The results indicate the presence of inter-domain interactions in CbhA. Holo-modules, i.e. containing Ca(2+), behave essentially independently, but in the absence of Ca(2+) domain interactions are more important for the conformation of the polypeptides.

Alternate routes to conformational specificity in a Greek key beta barrel protein

The N-terminal domain of protein S, a Greek key calcium-binding protein from Myxococcus xanthus, forms an atypical molten globule in the calcium-free state. The structure of this state is characterized by significant conformational fluctuations, which are localized to a subdomain that is not contiguous along the polypeptide chain. The conformational instability of this subdomain appears to arise from repulsive electrostatic interactions of four acidic side chains that are clustered together but are removed from the calcium-binding sites. This domain can be induced to form a native-like state through two different routes, calcium binding or reduction of pH. Acid-induced folding stabilizes the locally unfolded subdomain by selectively removing repulsive interactions without significantly affecting global stability. In contrast, calcium binding appears to increase local stability indirectly by causing global stabilization.

Do domain interactions of glycosyl hydrolases from Clostridium thermocellum contribute to protein thermostability?

Cellulolytic and hemicellulolytic enzymes usually have a domain composition. The mutual influence of a cellulose-binding domain and a catalytic domain was investigated with cellobiohydrolase CelK and xylanase XynZ from Clostridium thermocellum. CelK is composed of an N-terminal family IV cellulose-binding domain (CBDIV(CelK)), a family 9 glycosyl hydrolase domain (Gh9(CelK)) and a dockerin domain (DD). CelK without the DD, (CBDIV-Gh9)(CelK) and CBDIV(CelK) bound cellulose. The thermostability of (CBDIV-Gh9)(CelK) was significantly higher than that of CBDIV(CelK) and Gh9(CelK). The temperature optima of (CBDIV-Gh9)(CelK) and Gh9(CelK) were 65 and 45 degrees C, respectively. XynZ consists of an N-terminal feruloyl esterase domain (FAE(XynZ)), a linker (L), a family VI CBD (CBDVI(XynZ)), a DD and a xylanase domain. FAE(XynZ) and (FAE-L-CBDVI)(XynZ), used in the present study did not bind cellulose, but both were highly thermostable. Replacement of CBDVI(XynZ) with CBDIV(CelK) resulted in chimeras with feruloyl esterase activity and the ability to bind cellulose. CBDIV(CelK)-FAE(XynZ) bound cellulose with parameters similar to that of (CBDIV-Gh9)(CelK). (FAE-L)(XynZ)-CBDIV(CelK) and FAE(XynZ)-CBDIV(CelK) had lower relative affinities and binding capacities than those of (CBDIV-Gh9)(CelK). The three chimeras were much less thermostable than FAE(XynZ) and (FAE-L-CBDVI)(XynZ). The results indicate that domains of glycosyl hydrolases are not randomly combined and that domain interactions affect properties of these domain-structured enzymes.

Gamma S-crystallin of bovine and human eye lens: solution structure, stability and folding of the intact two-domain protein and its separate domains

Human and bovine gammaS-crystallin (HgammaS and BgammaS) and their isolated N- and C-terminal domains were cloned and expressed as recombinant proteins in E. coli. HgammaS and BgammaS are found to be authentic according to their spectral and hydrodynamic properties. Both full-length proteins and isolated domains are monomeric and exhibit high thermal and pH stabilities. The thermodynamic characterization made use of chemically and thermally-induced equilibrium unfolding transitions at varying pH. In spite of its exemplary two-domain structure, gammaS-crystallin does not show bimodal unfolding characteristics. In the case of BgammaS, at pH 7.0, the C-terminal domain is less stable than the N-terminal one, whereas for HgammaS the opposite holds true. Differential scanning calorimetry confirms the results of chemically-induced equilibrium unfolding transitions. Over the whole pH range between 2.0 and 11.5, HgammaS-crystallin and its isolated domains (HgammaS-N and HgammaS-C) follow the two-state model. The two-state unfolding of the intact two-domain protein points to the close similarity of the stabilities of the constituent domains. Obviously, interactions between the domains do not contribute significantly to the overall stability of gammaS-crystallin. In contrast, the structurally closely related gammaB-crystallin owes much of its extreme stability to domain interactions.

Calorimetric analysis of the Ca(2+)-binding betagamma-crystallin homolog protein S from Myxococcus xanthus: intrinsic stability and mutual stabilization of domains

The betagamma-crystallin superfamily consists of a class of homologous two-domain proteins with Greek-key fold. Protein S, a Ca(2+)-binding spore-coat protein from the soil bacterium Myxococcus xanthus exhibits a high degree of sequential and structural homology with gammaB-crystallin from the vertebrate eye lens. In contrast to gammaB-crystallin, which undergoes irreversible aggregation upon thermal unfolding, protein S folds reversibly and may therefore serve as a model in the investigation of the thermodynamic stability of the eye-lens crystallins. The thermal denaturation of recombinant protein S (PS) and its isolated domains was studied by differential scanning calorimetry in the absence and in the presence of Ca(2+) at varying pH. Ca(2+)-binding leads to a stabilization of PS and its domains and increases the cooperativity of their equilibrium unfolding transitions. The isolated N-terminal and C-terminal domains (NPS and CPS) obey the two-state model, independent of the pH and Ca(2+)-binding; in the case of PS, under all conditions, an equilibrium intermediate is populated. The first transition of PS may be assigned to the denaturation of the C-terminal domain and the loss of domain interactions, whereas the second one coincides with the denaturation of the isolated N-terminal domain. At pH 7.0, in the presence of Ca(2+), where PS exhibits maximal stability, the domain interactions at 20 degrees C contribute 20 kJ/mol to the overall stability of the intact protein.

Kinetic and thermodynamic stabilization of the betagamma-crystallin homolog spherulin 3a from Physarum polycephalum by calcium binding

Globular proteins may be stabilized, either intrinsically, at the various levels of the structural hierarchy, or extrinsically, by ligand binding. In the case of the dormant all-beta protein spherulin 3a (S3a) from the slime mold Physarum polycephalum, binding of calcium ions causes extreme kinetic and thermodynamic stabilization. S3a is the only known single-domain member of the two Greek key superfamily of betagamma-crystallins sharing the extreme long-term stability of its homologs in vertebrate eye lens. Spectral analysis allows two Ca2+-binding sites with KD=9 microM and 200 microM to be distinguished. Unfolding in the absence and in the presence of Ca2+gives evidence for extreme kinetic stabilization of the protein: In the absence of Ca2+, the half-time of unfolding in 2. 5 M guanidinium chloride (GdmCl) equals 8.3 minutes, whereas in the presence of Ca2+, even in 7.5 M GdmCl, it exceeds nine hours. To reach the equilibrium of unfolding in the absence and in the presence of Ca2+takes one day and eight weeks, respectively. The corresponding Gibbs free energies (based on the two-state model) are 77 and 135 kJ/mol. Saturation of S3a with Ca2+leads to an upward shift of the temperature-induced equilibrium transition by ca 20 deg. C. The in situ Ca2+concentration in the spherules is sufficient for the complete complexation of S3a in vivo.