In vitro and in silico design of alpha1-antitrypsin mutants with different conformational stabilities

Nascent chains can form co-translational folding intermediates that promote post-translational folding outcomes in a disease-causing protein

During biosynthesis, proteins can begin folding co-translationally to acquire their biologically-active structures. Folding, however, is an imperfect process and in many cases misfolding results in disease. Less is understood of how misfolding begins during biosynthesis. The human protein, alpha-1-antitrypsin (AAT) folds under kinetic control via a folding intermediate; its pathological variants readily form self-associated polymers at the site of synthesis, leading to alpha-1-antitrypsin deficiency. We observe that AAT nascent polypeptides stall during their biosynthesis, resulting in full-length nascent chains that remain bound to ribosome, forming a persistent ribosome-nascent chain complex (RNC) prior to release. We analyse the structure of these RNCs, which reveals compacted, partially-folded co-translational folding intermediates possessing molten-globule characteristics. We find that the highly-polymerogenic mutant, Z AAT, forms a distinct co-translational folding intermediate relative to wild-type. Its very modest structural differences suggests that the ribosome uniquely tempers the impact of deleterious mutations during nascent chain emergence. Following nascent chain release however, these co-translational folding intermediates guide post-translational folding outcomes thus suggesting that Z's misfolding is initiated from co-translational structure. Our findings demonstrate that co-translational folding intermediates drive how some proteins fold under kinetic control, and may thus also serve as tractable therapeutic targets for human disease.

Improvement of the Catalytic Activity of Chitosanase BsCsn46A from Bacillus subtilis by Site-Saturation Mutagenesis of Proline121

BsCsn46A, a GH46 family chitosanase from Bacillus subtilis, has great potential for industrial chitooligosaccharide production due to its high activity and stability. In this study, a special amino acid Pro121 was identified not fit in the helix structure, which was located in the opposite side of the active center in BsCsn46A, by the PoPMuSiC algorithm. Then, saturation mutagenesis was performed to explore the role of the site amino acid 121. Compared with the wild type, the specific activity of P121N, P121C, and P121V was increased by 1.69-, 1.97-, and 2.15-fold, respectively. In particular, the specific activity of P121N was increased without loss of thermostability, indicating that replacing the structural stiffness of proline in the helical structure could significantly improve the chitosanase activity. The K_m values of P121N, P121C, and P121V decreased significantly, indicating that the affinity between the enzyme-substrate complex was enhanced. Through molecular docking, it was found that the increase of hydrogen bonds and van der Waals force between the enzyme-substrate complex and the removal of unfavorable bonds might be the main reason for the change of enzyme properties. In addition, the optimal temperature of the three mutants changed from 60 to 55 °C. These results indicate that the site 121 plays a critical role in the catalytic activity and enzymatic properties of chitosanase. To our knowledge, the results provide novel data on chitosanase activity and identify an excellent candidate of industrial chitosanase.

Computational Approaches to Prioritize Cancer Driver Missense Mutations

Cancer is a complex disease that is driven by genetic alterations. There has been a rapid development of genome-wide techniques during the last decade along with a significant lowering of the cost of gene sequencing, which has generated widely available cancer genomic data. However, the interpretation of genomic data and the prediction of the association of genetic variations with cancer and disease phenotypes still requires significant improvement. Missense mutations, which can render proteins non-functional and provide a selective growth advantage to cancer cells, are frequently detected in cancer. Effects caused by missense mutations can be pinpointed by in silico modeling, which makes it more feasible to find a treatment and reverse the effect. Specific human phenotypes are largely determined by stability, activity, and interactions between proteins and other biomolecules that work together to execute specific cellular functions. Therefore, analysis of missense mutations’ effects on proteins and their complexes would provide important clues for identifying functionally important missense mutations, understanding the molecular mechanisms of cancer progression and facilitating treatment and prevention. Herein, we summarize the major computational approaches and tools that provide not only the classification of missense mutations as cancer drivers or passengers but also the molecular mechanisms induced by driver mutations. This review focuses on the discussion of annotation and prediction methods based on structural and biophysical data, analysis of somatic cancer missense mutations in 3D structures of proteins and their complexes, predictions of the effects of missense mutations on protein stability, protein-protein and protein-nucleic acid interactions, and assessment of conformational changes in protein conformations induced by mutations.

Computational tools help improve protein stability but with a solubility tradeoff

Accurately predicting changes in protein stability upon amino acid substitution is a much sought after goal. Destabilizing mutations are often implicated in disease, whereas stabilizing mutations are of great value for industrial and therapeutic biotechnology. Increasing protein stability is an especially challenging task, with random substitution yielding stabilizing mutations in only ∼2% of cases. To overcome this bottleneck, computational tools that aim to predict the effect of mutations have been developed; however, achieving accuracy and consistency remains challenging. Here, we combined 11 freely available tools into a meta-predictor (meieringlab.uwaterloo.ca/stabilitypredict/). Validation against ∼600 experimental mutations indicated that our meta-predictor has improved performance over any of the individual tools. The meta-predictor was then used to recommend 10 mutations in a previously designed protein of moderate thermodynamic stability, ThreeFoil. Experimental characterization showed that four mutations increased protein stability and could be amplified through ThreeFoil's structural symmetry to yield several multiple mutants with >2-kcal/mol stabilization. By avoiding residues within functional ties, we could maintain ThreeFoil's glycan-binding capacity. Despite successfully achieving substantial stabilization, however, almost all mutations decreased protein solubility, the most common cause of protein design failure. Examination of the 600-mutation data set revealed that stabilizing mutations on the protein surface tend to increase hydrophobicity and that the individual tools favor this approach to gain stability. Thus, whereas currently available tools can increase protein stability and combining them into a meta-predictor yields enhanced reliability, improvements to the potentials/force fields underlying these tools are needed to avoid gaining protein stability at the cost of solubility.

An antibody that prevents serpin polymerisation acts by inducing a novel allosteric behaviour

Serpins are important regulators of proteolytic pathways with an antiprotease activity that involves a conformational transition from a metastable to a hyperstable state. Certain mutations permit the transition to occur in the absence of a protease; when associated with an intermolecular interaction, this yields linear polymers of hyperstable serpin molecules, which accumulate at the site of synthesis. This is the basis of many pathologies termed the serpinopathies. We have previously identified a monoclonal antibody (mAb4B12) that, in single-chain form, blocks α1-antitrypsin (α1-AT) polymerisation in cells. Here, we describe the structural basis for this activity. The mAb4B12 epitope was found to encompass residues Glu32, Glu39 and His43 on helix A and Leu306 on helix I. This is not a region typically associated with the serpin mechanism of conformational change, and correspondingly the epitope was present in all tested structural forms of the protein. Antibody binding rendered β-sheet A - on the opposite face of the molecule - more liable to adopt an 'open' state, mediated by changes distal to the breach region and proximal to helix F. The allosteric propagation of induced changes through the molecule was evidenced by an increased rate of peptide incorporation and destabilisation of a preformed serpin-enzyme complex following mAb4B12 binding. These data suggest that prematurely shifting the β-sheet A equilibrium towards the 'open' state out of sequence with other changes suppresses polymer formation. This work identifies a region potentially exploitable for a rational design of ligands that is able to dynamically influence α1-AT polymerisation.

Altered native stability is the dominant basis for susceptibility of α1-antitrypsin mutants to polymerization

Serpins are protease inhibitors whose most stable state is achieved upon transition of a central 5-stranded β-sheet to a 6-stranded form. Mutations, low pH, denaturants and elevated temperatures promote this transition, which can result in a growing polymer chain of inactive molecules. Different types of polymer are possible, but, experimentally only heat has been shown to generate polymers in vitro consistent with ex vivo pathological specimens. Many mutations that alter the rate of heat-induced polymerization have been described, but interpretation is problematic because discrimination is lacking between the effect of global changes in native stability and specific effects on structural mechanism. We show that the temperature midpoint (Tm) of thermal denaturation reflects the transition of α1-antitrypsin to the polymerization intermediate, and determine the relationship with fixed-temperature polymerization half-times (t0.5) in the presence of stabilizing additives [TMAO (trimethylamine N-oxide), sucrose and sodium sulfate], point mutations and disulfide bonds. Combined with a retrospective analysis of 31 mutants characterized in the literature, the results of the present study show that global changes to native state stability are the predominant basis for the effects of mutations and osmolytes on heat-induced polymerization, summarized by the equation: ln(t0.5,mutant/t0.5,wild-type)=0.34×ΔTm. It is deviations from this relationship that hold key information about the polymerization process.

Flanking domain stability modulates the aggregation kinetics of a polyglutamine disease protein

Spinocerebellar Ataxia Type 3 (SCA3) is one of nine polyglutamine (polyQ) diseases that are all characterized by progressive neuronal dysfunction and the presence of neuronal inclusions containing aggregated polyQ protein, suggesting that protein misfolding is a key part of this disease. Ataxin-3, the causative protein of SCA3, contains a globular, structured N-terminal domain (the Josephin domain) and a flexible polyQ-containing C-terminal tail, the repeat-length of which modulates pathogenicity. It has been suggested that the fibrillogenesis pathway of ataxin-3 begins with a non-polyQ-dependent step mediated by Josephin domain interactions, followed by a polyQ-dependent step. To test the involvement of the Josephin domain in ataxin-3 fibrillogenesis, we have created both pathogenic and nonpathogenic length ataxin-3 variants with a stabilized Josephin domain, and have both stabilized and destabilized the isolated Josephin domain. We show that changing the thermodynamic stability of the Josephin domain modulates ataxin-3 fibrillogenesis. These data support the hypothesis that the first stage of ataxin-3 fibrillogenesis is caused by interactions involving the non-polyQ containing Josephin domain and that the thermodynamic stability of this domain is linked to the aggregation propensity of ataxin-3.

PoPMuSiC 2.1: a web server for the estimation of protein stability changes upon mutation and sequence optimality

Background

The rational design of modified proteins with controlled stability is of extreme importance in a whole range of applications, notably in the biotechnological and environmental areas, where proteins are used for their catalytic or other functional activities. Future breakthroughs in medical research may also be expected from an improved understanding of the effect of naturally occurring disease-causing mutations on the molecular level.

Results

PoPMuSiC-2.1 is a web server that predicts the thermodynamic stability changes caused by single site mutations in proteins, using a linear combination of statistical potentials whose coefficients depend on the solvent accessibility of the mutated residue. PoPMuSiC presents good prediction performances (correlation coefficient of 0.8 between predicted and measured stability changes, in cross validation, after exclusion of 10% outliers). It is moreover very fast, allowing the prediction of the stability changes resulting from all possible mutations in a medium size protein in less than a minute. This unique functionality is user-friendly implemented in PoPMuSiC and is particularly easy to exploit. Another new functionality of our server concerns the estimation of the optimality of each amino acid in the sequence, with respect to the stability of the structure. It may be used to detect structural weaknesses, i.e. clusters of non-optimal residues, which represent particularly interesting sites for introducing targeted mutations. This sequence optimality data is also expected to have significant implications in the prediction and the analysis of particular structural or functional protein regions. To illustrate the interest of this new functionality, we apply it to a dataset of known catalytic sites, and show that a much larger than average concentration of structural weaknesses is detected, quantifying how these sites have been optimized for function rather than stability.

Conclusion

The freely available PoPMuSiC-2.1 web server is highly useful for identifying very rapidly a list of possibly relevant mutations with the desired stability properties, on which subsequent experimental studies can be focused. It can also be used to detect sequence regions corresponding to structural weaknesses, which could be functionally important or structurally delicate regions, with obvious applications in rational protein design.

Identification of amino acid residues responsible for increased thermostability of feruloyl esterase A from Aspergillus niger using the PoPMuSiC algorithm

Feruloyl esterases are key enzymes involved in the complete hydrolysis of hemicellulose. In order to improve the thermostability of feruloyl esterase A (FaeA) from Aspergillus niger CIB 423.1, the PoPMuSiC algorithm was applied to predict the folding free energy change (ΔΔG) of amino acid substitutions. Four amino acid substitutions (S92A, D93G, D174A and S187F) were introduced into the enzyme by site-directed mutagenesis and the enzymes were produced in Pichia pastoris KM71. No obvious changes in thermal stability resulted from substitutions S92A and D174A, but, compared to the wild-type enzyme which has a half-life of inactivation of 8 min, the half-lives of enzymes with a D93G or S187F substitution increased to 9.4 and 60.5 min, respectively. The double mutant D93G/S187F displayed a synergistic effect with a t1/2 value of 77.0 min. It also displayed over 10-fold increase in catalytic turnover frequency. The result will benefit further investigation of the thermostability of feruloyl esterase A.

Fast and accurate predictions of protein stability changes upon mutations using statistical potentials and neural networks: PoPMuSiC-2.0

MOTIVATION

The rational design of proteins with modified properties, through amino acid substitutions, is of crucial importance in a large variety of applications. Given the huge number of possible substitutions, every protein engineering project would benefit strongly from the guidance of in silico methods able to predict rapidly, and with reasonable accuracy, the stability changes resulting from all possible mutations in a protein.

RESULTS

We exploit newly developed statistical potentials, based on a formalism that highlights the coupling between four protein sequence and structure descriptors, and take into account the amino acid volume variation upon mutation. The stability change is expressed as a linear combination of these energy functions, whose proportionality coefficients vary with the solvent accessibility of the mutated residue and are identified with the help of a neural network. A correlation coefficient of R = 0.63 and a root mean square error of sigma(c) = 1.15 kcal/mol between measured and predicted stability changes are obtained upon cross-validation. These scores reach R = 0.79, and sigma(c) = 0.86 kcal/mol after exclusion of 10% outliers. The predictive power of our method is shown to be significantly higher than that of other programs described in the literature.

AVAILABILITY

http://babylone.ulb.ac.be/popmusic

Expression, purification and characterization of recombinant Z alpha(1)-antitrypsin--the most common cause of alpha(1)-antitrypsin deficiency

Alpha(1)-antitrypsin (alpha(1)AT), the most abundant proteinase inhibitor circulating in the blood, protects extracellular matrix proteins of the lung against proteolytic destruction by neutrophil elastase. alpha(1)AT deficiency predisposes patients to emphysema, juvenile cirrhosis and hepatocellular carcinoma. Over 90% of clinical cases of severe alpha(1)AT deficiency are caused by the Z variant (E342K) of alpha(1)AT. The presence of the Z mutation results in misfolding and polymerization of alpha(1)AT. Due to its inherent propensity to polymerize there are no reported cases of recombinant Z alpha(1)AT production. This has created a major impediment to studying the effect of the Z mutation on alpha(1)AT. Here we report our attempts to produce recombinant Z alpha(1)AT using both Escherichia coli and Pichia pastoris as host systems. Using a range of expression vectors in E. coli we were unable to produce soluble active Z alpha(1)AT. Cytosolic expression of the Z alpha(1)AT gene in P. pastoris was successful. Monomeric and active recombinant Z alpha(1)AT was purified from the yeast cytosol using affinity chromatography and anion exchange chromatography. Biochemical analyses demonstrated that the recombinant Z alpha(1)AT has identical properties to its native counterpart purified from plasma of patients homozygous for the Z allele. A recombinant source of pathological Z alpha(1)AT will increase the chances of elucidating the mechanism of its polymerization and thus the development of therapeutic strategies.

Enhancing the stability and solubility of TEV protease using in silico design

The ability to rationally increase the stability and solubility of recombinant proteins has long been a goal of biotechnology and has significant implications for biomedical research. Poorly soluble enzymes, for example, result in the need for larger reaction volumes, longer incubation times, and more restricted reaction conditions, all of which increase the cost and have a negative impact on the feasibility of the process. Rational design is achieved here by means of the PoPMuSiC program, which performs in silico predictions of stability changes upon single-site mutations. We have used this program to increase the stability of the tobacco etch virus (TEV) protein. TEV is a 27-kDa nuclear inclusion protease with stringent specificity that is commonly used for the removal of solubility tags during protein purification protocols. However, while recombinant TEV can be produced in large quantities, a limitation is its relatively poor solubility (generally approximately 1 mg/mL), which means that large volumes and often long incubation times are required for efficient cleavage. Following PoPMuSiC analysis of TEV, five variants predicted to be more stable than the wild type were selected for experimental analysis of their stability, solubility, and activity. Of these, two were found to enhance the solubility of TEV without compromising its functional activity. In addition, a fully active double mutant was found to remain soluble at concentrations in excess of 40 mg/mL. This modified TEV appears thus as an interesting candidate to be used in recombinant protein technology.

Aeropin from the extremophile Pyrobaculum aerophilum bypasses the serpin misfolding trap

Serpins are metastable proteinase inhibitors. Serpin metastability drives both a large conformational change that is utilized during proteinase inhibition and confers an inherent structural flexibility that renders serpins susceptible to aggregation under certain conditions. These include point mutations (the basis of a number of important human genetic diseases), small changes in pH, and an increase in temperature. Many studies of serpins from mesophilic organisms have highlighted an inverse relationship: mutations that confer a marked increase in serpin stability compromise inhibitory activity. Here we present the first biophysical characterization of a metastable serpin from a hyperthermophilic organism. Aeropin, from the archaeon Pyrobaculum aerophilum, is both highly stable and an efficient proteinase inhibitor. We also demonstrate that because of high kinetic barriers, aeropin does not readily form the partially unfolded precursor to serpin aggregation. We conclude that stability and activity are not mutually exclusive properties in the context of the serpin fold, and propose that the increased stability of aeropin is caused by an unfolding pathway that minimizes the formation of an aggregation-prone intermediate ensemble, thereby enabling aeropin to bypass the misfolding fate observed with other serpins.

In silico analysis of the thermodynamic stability changes of psychrophilic and mesophilic alpha-amylases upon exhaustive single-site mutations

Identifying sequence modifications that distinguish psychrophilic from mesophilic proteins is important for designing enzymes with different thermodynamic stabilities and to understand the underlying mechanisms. The PoPMuSiC algorithm is used to introduce, in silico, all the single-site mutations in four mesophilic and one psychrophilic chloride-dependent alpha-amylases and to evaluate the changes in thermodynamic stability. The analysis of the distribution of the sequence positions that could be stabilized upon mutation shows a clear difference between the three domains of psychrophilic and mesophilic alpha-amylases. Most of the mutations stabilizing the psychrophilic enzyme are found in domains B and C, contrary to the mesophilic proteins where they are preferentially situated in the catalytic domain A. Moreover, the calculations show that the environment of some residues responsible for the activity of the psychrophilic protein has evolved to reinforce favorable interactions with these residues. In the second part, these results are exploited to propose rationally designed mutations that are predicted to confer to the psychrophilic enzyme mesophilic-like thermodynamic properties. Interestingly, most of the mutations found in domain C strengthen the interactions with domain A, in agreement with suggestions made on the basis of structural analyses. Although this study focuses on single-site mutations, the thermodynamic effects of the recommended mutations should be additive if the mutated residues are not close in space.

Alpha-1-antitrypsin associated panniculitis: the MS variant

Over 90 mutant alleles of the alpha-1-antitrypsin (AAT) gene are recognized and classified by mobility on an acid starch gel. The four major categories include: F=fast, M=medium, S=slow, Z=very slow. Among 41 reported cases of AAT panniculitis, most have the ZZ phenotype with AAT levels below normal. We report two cases of AAT panniculitis with MS phenotype and normal AAT levels. In addition, we review the pathophysiology, epidemiology, and extracutaneous manifestations of AAT disease and propose a diagnostic algorithm for ulcerative panniculitis. A 42-year-old man presented with a solitary plaque on the left thigh exacerbated by trauma or excessive activity. The lesion frequently suppurated with a yellowish oily material. Twenty years before, he had fractured his left femur which was repaired with a metal plate. X-rays, histology with special stains for organisms, and cultures were negative. AAT phenotype was MS and AAT value was normal. A 43-year-old woman presented with multiple plaques on the proximal extremities which suppurated with exercise or trauma. AAT phenotype was MS and AAT level was normal. Histologic exam for both patients showed a dense neutrophilic infiltrate with septal and lobular panniculitis and areas of necrobiosis in the lower reticular dermis.

Different Conformational Changes within the F-Helix Occur during Serpin Folding, Polymerization, and Proteinase Inhibition†

The intrinsic metastability of the serpin native state is the thermodynamic driving force for both proteinase inhibition and the formation of inactive polymers. A number of mechanisms has been proposed to explain how both these conformational changes are achieved. However, one aspect that has received little attention is the movement of the F-helix, which physically impedes both these events. We have applied a protein engineering approach to investigate the conformational changes of this helix during proteinase inhibition, serpin folding, and polymerization. We systematically mutated two highly conserved hydrophobic residues on the F-helix, V161 and I157, and in addition, removed a hydrogen bond between D149 and the first turn of the helix. Our data demonstrate that while all three interactions are important for the stability and folding of the molecule, their contribution during inhibition and polymerization differ. The presence of I157 is crucial to all conformational changes as its loss results in inactivation of the serpin and rapid polymerization. The replacement of D149 does not affect activity but significantly increases the polymerization rate. The interactions formed by V161 play an important role only in maintaining the native conformation. Taken together, these data suggest that the F-helix undergoes a reversible conformational change in both its N- and C-termini during proteinase inhibition only the C-terminus undergoes changes during polymerization, but there is a global change required for folding.

Sequence-structure signals of 3D domain swapping in proteins

Three-dimensional domain swapping occurs when two or more identical proteins exchange identical parts of their structure to generate an oligomeric unit. It affects proteins with diverse sequences and structures, and is expected to play important roles in evolution, functional regulation and even conformational diseases. Here, we search for traces of domain swapping in the protein sequence, by means of algorithms that predict the structure and stability of proteins using database-derived potentials. Regions whose sequences are not optimal with regard to the stability of the native structure, or showing marked intrinsic preferences for non-native conformations in absence of tertiary interactions are detected in most domain-swapping proteins. These regions are often located in areas crucial in the swapping process and are likely to influence it on a kinetic or thermodynamic level. In addition, cation-pi interactions are frequently observed to zip up the edges of the interface between intertwined chains or to involve hinge loop residues, thereby modulating stability. We end by proposing a set of mutations altering the swapping propensities, whose experimental characterization would contribute to refine our in silico derived hypotheses.