Stabilisation of a (betaalpha)8-barrel protein designed from identical half barrels

Directed Evolution of Soluble α-1,2-Fucosyltransferase Using Kanamycin Resistance Protein as a Phenotypic Reporter for Efficient Production of 2'-Fucosyllactose

2'-Fucosyllactose (2'-FL), the most abundant fucosylated oligosaccharide in human milk, has multiple beneficial effects on human health. However, its biosynthesis by metabolically engineered Escherichia coli is often hampered owing to the insolubility and instability of α-1,2-fucosyltransferase (the rate-limiting enzyme). In this study, we aimed to enhance 2'-FL production by increasing the expression of soluble α-1,2-fucosyltransferase from Helicobacter pylori (FucT2). Because structural information regarding FucT2 has not been unveiled, we decided to improve the expression of soluble FucT2 in E. coli via directed evolution using a protein solubility biosensor that links protein solubility to antimicrobial resistance. For such a system to be viable, the activity of kanamycin resistance protein (Kan^R) should be dependent on FucT2 solubility. Kan^R was fused to the C-terminus of mutant libraries of FucT2, which were generated using a combination of error-prone PCR and DNA shuffling. Notably, one round of the directed evolution process, which consisted of mutant library generation and selection based on kanamycin resistance, resulted in a significant increase in the expression level of soluble FucT2. As a result, a batch fermentation with the ΔL M15 pBCGW strain, expressing the FucT2 mutant (F#1-5) isolated from the first round of the directed evolution process, resulted in the production of 0.31 g/l 2'-FL with a yield of 0.22 g 2'-FL/g lactose, showing 1.72- and 1.51-fold increase in the titer and yield, respectively, compared to those of the control strain. The simple and powerful method developed in this study could be applied to enhance the solubility of other unstable enzymes.

A newly introduced salt bridge cluster improves structural and biophysical properties of de novo TIM barrels

Protein stability can be fine‐tuned by modifying different structural features such as hydrogen‐bond networks, salt bridges, hydrophobic cores, or disulfide bridges. Among these, stabilization by salt bridges is a major challenge in protein design and engineering since their stabilizing effects show a high dependence on the structural environment in the protein, and therefore are difficult to predict and model. In this work, we explore the effects on structure and stability of an introduced salt bridge cluster in the context of three different de novo TIM barrels. The salt bridge variants exhibit similar thermostability in comparison with their parental designs but important differences in the conformational stability at 25°C can be observed such as a highly stabilizing effect for two of the proteins but a destabilizing effect to the third. Analysis of the formed geometries of the salt bridge cluster in the crystal structures show either highly ordered salt bridge clusters or only single salt bridges. Rosetta modeling of the salt bridge clusters results in a good prediction of the tendency on stability changes but not the geometries observed in the three‐dimensional structures. The results show that despite the similarities in protein fold, the salt bridge clusters differently influence the structural and stability properties of the de novo TIM barrel variants depending on the structural background where they are introduced.

PDB Code(s): 7OSU, 7OT7, 7OSV, 7OT8 and 7P12;

Evolution, folding, and design of TIM barrels and related proteins

Proteins are chief actors in life that perform a myriad of exquisite functions. This diversity has been enabled through the evolution and diversification of protein folds. Analysis of sequences and structures strongly suggest that numerous protein pieces have been reused as building blocks and propagated to many modern folds. This information can be traced to understand how the protein world has diversified. In this review, we discuss the latest advances in the analysis of protein evolutionary units, and we use as a model system one of the most abundant and versatile topologies, the TIM-barrel fold, to highlight the existing common principles that interconnect protein evolution, structure, folding, function, and design.

Selection and screening strategies in directed evolution to improve protein stability

Protein stability is not only fundamental for experimental, industrial, and therapeutic applications, but is also the baseline for evolving novel protein functions. For decades, stability engineering armed with directed evolution has continued its rapid development and inevitably poses challenges. Generally, in directed evolution, establishing a reliable link between a genotype and any interpretable phenotype is more challenging than diversifying genetic libraries. Consequently, we set forth in a small picture to emphasize the screening or selection techniques in protein stability-directed evolution to secure the link. For a more systematic review, two main branches of these techniques, namely cellular or cell-free display and stability biosensors, are expounded with informative examples.

Search for independent (β/α)4 subdomains in a (β/α)8 barrel β-glucosidase

Proteins that fold as (β/α)₈ barrels are thought to have evolved from half-barrels that underwent duplication and fusion events. The evidence is particularly clear for small barrels, which have almost identical halves. Additionally, computational calculations of the thermodynamic stability of these structures in the presence of denaturants have revealed that (β/α)₈ barrels contain two subunits or domains corresponding to half-barrels. Hence, within (β/α)₈ barrels, half-barrels are self-contained units. Here, we tested this hypothesis using β-glucosidase from the bacterium Thermotoga maritima (bglTm), which has a (β/α)₈ barrel structure. Mutations were introduced to disrupt the noncovalent contacts between its halves and reveal the presence of two domains within bglTm, thus resulting in the creation of mutants T1 (containing W12A and I217A mutations) and T2 (containing W12A, H195A, I217A and F404A mutations). Mutants T1 and T2 were properly folded, as indicated by their fluorescence spectra and enzyme kinetic parameters. T1 and wild-type bglTm were equally stable, as shown by the results of thermal inactivation, differential scanning fluorimetry and guanidine hydrochloride denaturation experiments. However, T2 showed a first-order inactivation at 80°C, a single melting temperature of 82°C and only one transition concentration (c₅₀) in 2.4 M guanidine hydrochloride. Additionally, T1 and T2 exhibited a cooperative denaturation process that followed a two-state model (m-values equal to 1.4 and 1.6 kcal/mol/M, respectively), similar to that of wild-type bglTm (1.2 kcal/mol/M). Hence, T1 and T2 each denatured as a single unit, although they contained different degrees of disruption between their halves. In conclusion, bglTm halves are equivalent in terms of their thermal and chemical stability; thus, their separate contributions to (β/α)₈ barrel unfolding cannot be disentangled.

Creation of active TIM barrel enzymes through genetic fusion of half-barrel domain constructs derived from two distantly related glycosyl hydrolases

Diverse unrelated enzymes that adopt the beta/alpha (or TIM) barrel topology display similar arrangements of beta/alpha units placed in a radial eight-fold symmetry around the barrel's axis. The TIM barrel was originally thought to be a single structural domain; however, it is now thought that TIM barrels arose from duplication and fusion of smaller half-barrels consisting of four beta/alpha units. We describe here the design, expression and purification, as well as characterization of folding, activity and stability, of chimeras of two TIM barrel glycosyl hydrolases, made by fusing different half-barrel domains derived from an endoglucanase from Clostridium cellulolyticum, CelCCA and a beta-glucosidase from Pyrococcus furiosus, CelB. We show that after refolding following purification from inclusion bodies, the two half-barrel fusion chimeras (CelCCACelB and CelBCelCCA) display catalytic activity although they assemble into large soluble oligomeric aggregated species containing chains of mixed beta and alpha structure. CelBCelCCA displays hyperthermophile-like structural stability as well as significant stability to chemical denaturation (C_m of 2.6 m guanidinium hydrochloride), whereas CelCCACelB displays mesophile-like stability (T_m of ~ 71 °C). The endoglucanase activities of both chimeras are an order of magnitude lower than those of CelB or CelCCA, whereas the beta-glucosidase activity of CelBCelCCA is about two orders of magnitude lower than that of CelB. The chimera CelCCACelB shows no beta-glucosidase activity. Our results demonstrate that half-barrel domains from unrelated sources can fold, assemble and function, with scope for improvement.

ENZYME

Pyrococcus furiosus beta-glucosidase (CelB, EC: 3.2.1.21). Clostridium cellulolyticum endoglucanase A (CelCCA, EC: 3.2.1.4).

Design of symmetric TIM barrel proteins from first principles

Background

Computational protein design is a rapidly maturing field within structural biology, with the goal of designing proteins with custom structures and functions. Such proteins could find widespread medical and industrial applications. Here, we have adapted algorithms from the Rosetta software suite to design much larger proteins, based on ideal geometric and topological criteria. Furthermore, we have developed techniques to incorporate symmetry into designed structures. For our first design attempt, we targeted the (α/β)₈ TIM barrel scaffold. We gained novel insights into TIM barrel folding mechanisms from studying natural TIM barrel structures, and from analyzing previous TIM barrel design attempts.

Methods

Computational protein design and analysis was performed using the Rosetta software suite and custom scripts. Genes encoding all designed proteins were synthesized and cloned on the pET20-b vector. Standard circular dichroism and gel chromatographic experiments were performed to determine protein biophysical characteristics. 1D NMR and 2D HSQC experiments were performed to determine protein structural characteristics.

Results

Extensive protein design simulations coupled with ab initio modeling yielded several all-atom models of ideal, 4-fold symmetric TIM barrels. Four such models were experimentally characterized. The best designed structure (Symmetrin-1) contained a polar, histidine-rich pore, forming an extensive hydrogen bonding network. Symmetrin-1 was easily expressed and readily soluble. It showed circular dichroism spectra characteristic of well-folded alpha/beta proteins. Temperature melting experiments revealed cooperative and reversible unfolding, with a T_m of 44 °C and a Gibbs free energy of unfolding (ΔG°) of 8.0 kJ/mol. Urea denaturing experiments confirmed these observations, revealing a C_m of 1.6 M and a ΔG° of 8.3 kJ/mol. Symmetrin-1 adopted a monomeric conformation, with an apparent molecular weight of 32.12 kDa, and displayed well resolved 1D-NMR spectra. However, the HSQC spectrum revealed somewhat molten characteristics.

Conclusions

Despite the detection of molten characteristics, the creation of a soluble, cooperatively folding protein represents an advancement over previous attempts at TIM barrel design. Strategies to further improve Symmetrin-1 are elaborated. Our techniques may be used to create other large, internally symmetric proteins.

Electronic supplementary material

The online version of this article (doi:10.1186/s12858-015-0047-4) contains supplementary material, which is available to authorized users.

Synthetic biology for the directed evolution of protein biocatalysts: navigating sequence space intelligently

The amino acid sequence of a protein affects both its structure and its function. Thus, the ability to modify the sequence, and hence the structure and activity, of individual proteins in a systematic way, opens up many opportunities, both scientifically and (as we focus on here) for exploitation in biocatalysis. Modern methods of synthetic biology, whereby increasingly large sequences of DNA can be synthesised de novo, allow an unprecedented ability to engineer proteins with novel functions. However, the number of possible proteins is far too large to test individually, so we need means for navigating the 'search space' of possible protein sequences efficiently and reliably in order to find desirable activities and other properties. Enzymologists distinguish binding (Kd) and catalytic (kcat) steps. In a similar way, judicious strategies have blended design (for binding, specificity and active site modelling) with the more empirical methods of classical directed evolution (DE) for improving kcat (where natural evolution rarely seeks the highest values), especially with regard to residues distant from the active site and where the functional linkages underpinning enzyme dynamics are both unknown and hard to predict. Epistasis (where the 'best' amino acid at one site depends on that or those at others) is a notable feature of directed evolution. The aim of this review is to highlight some of the approaches that are being developed to allow us to use directed evolution to improve enzyme properties, often dramatically. We note that directed evolution differs in a number of ways from natural evolution, including in particular the available mechanisms and the likely selection pressures. Thus, we stress the opportunities afforded by techniques that enable one to map sequence to (structure and) activity in silico, as an effective means of modelling and exploring protein landscapes. Because known landscapes may be assessed and reasoned about as a whole, simultaneously, this offers opportunities for protein improvement not readily available to natural evolution on rapid timescales. Intelligent landscape navigation, informed by sequence-activity relationships and coupled to the emerging methods of synthetic biology, offers scope for the development of novel biocatalysts that are both highly active and robust.

Design of proteins from smaller fragments-learning from evolution

Nature has generated an impressive set of proteins with diverse folds and functions. It has been able to do so using mechanisms such as duplication and fusion as well as recombination of smaller protein fragments that serve as building blocks. These evolutionary mechanisms provide a template for the rational design of new proteins from fragments of existing proteins. Design by duplication and fusion has been explored for a number of symmetric protein folds, while design by rational recombination has just emerged. First experiments in recombining fragments from the same and different folds are proving successful in building new proteins that harbor easily evolvable properties originating from the parents. Overall, duplication and recombination of smaller fragments shows much potential for future applications in the design of proteins.

Engineering chimaeric proteins from fold fragments: 'hopeful monsters' in protein design

Modern highly complex proteins evolved from much simpler and less specialized subunits. The same concept can be applied in protein engineering to construct new well-folded proteins. Hybrid proteins or chimaeras can be built from contemporary protein fragments through illegitimate recombination. Even parts from different globular folds can be fitted together using rational design methodologies. Furthermore, intrinsic functional properties encoded in the fold fragments allow rapid adaptation of the new proteins and thus provide interesting starting scaffolds for further redesign.

Symmetric protein architecture in protein design: top-down symmetric deconstruction

Top-down symmetric deconstruction (TDSD) is a joint experimental and computational approach to generate a highly stable, functionally benign protein scaffold for intended application in subsequent functional design studies. By focusing on symmetric protein folds, TDSD can leverage the dramatic reduction in sequence space achieved by applying a primary structure symmetric constraint to the design process. Fundamentally, TDSD is an iterative symmetrization process, in which the goal is to maintain or improve properties of thermodynamic stability and folding cooperativity inherent to a starting sequence (the "proxy"). As such, TDSD does not attempt to solve the inverse protein folding problem directly, which is computationally intractable. The present chapter will take the reader through all of the primary steps of TDSD-selecting a proxy, identifying potential mutations, establishing a stability/folding cooperativity screen-relying heavily on a successful TDSD solution for the common β-trefoil fold.

Establishing catalytic activity on an artificial (βα)8-barrel protein designed from identical half-barrels

It has been postulated that the ubiquitous (βα)8-barrel enzyme fold has evolved by duplication and fusion of an ancestral (βα)4-half-barrel. We have previously reconstructed this process in the laboratory by fusing two copies of the C-terminal half-barrel HisF-C of imidazole glycerol phosphate synthase (HisF). The resulting construct HisF-CC was stepwise stabilized to Sym1 and Sym2, which are extremely robust but catalytically inert proteins. Here, we report on the generation of a circular permutant of Sym2 and the establishment of a sugar isomerization reaction on its scaffold. Our results demonstrate that duplication and mutagenesis of (βα)4-half-barrels can readily lead to a stable and catalytically active (βα)8-barrel enzyme.

Emergence of symmetric protein architecture from a simple peptide motif: evolutionary models

Structural symmetry is observed in the majority of fundamental protein folds and gene duplication and fusion evolutionary processes are postulated to be responsible. However, convergent evolution leading to structural symmetry has also been proposed; additionally, there is debate regarding the extent to which exact primary structure symmetry is compatible with efficient protein folding. Issues of symmetry in protein evolution directly impact strategies for de novo protein design as symmetry can substantially simplify the design process. Additionally, when considering gene duplication and fusion in protein evolution, there are two competing models: "emergent architecture" and "conserved architecture". Recent experimental work has shed light on both the evolutionary process leading to symmetric protein folds as well as the ability of symmetric primary structure to efficiently fold. Such studies largely support a "conserved architecture" evolutionary model, suggesting that complex protein architecture was an early evolutionary achievement involving oligomerization of smaller polypeptides.

A highly stable protein chimera built from fragments of different folds

Proteins increased in complexity during the course of evolution. Domains as well as subdomain-sized fragments were recruited and adapted to form new proteins and novel folds. This concept can be used in engineering to construct new proteins. We previously reported the combination of fragments from two ancient protein folds, a flavodoxin-like and a (βα)₈-barrel protein. Here we report two further attempts at engineering a chimeric protein from fragments of these folds. While one of the constructs showed a high tendency to aggregate, the other turned out to be a highly stable, well-structured protein. In terms of stability against heat and chemical denaturation this chimera, named NarLHisF, is superior to the earlier presented CheYHisF. This is the second instance of a chimera build from two different protein folds, which demonstrates how easily recombination can lead to the development and diversification of new proteins--a mechanism that most likely occurred frequently in the course of evolution. Based on the results of the failed and the successful chimera, we discuss important considerations for a general design strategy for fold chimeras.

Designing proteins from simple motifs: opportunities in Top-Down Symmetric Deconstruction

The purpose of this review is to describe the development of 'top-down' approaches to protein design. It will be argued that a diverse number of studies over the past decade, involving many investigators, and focused upon elucidating the role of symmetry in protein evolution and design, are converging into a novel top-down approach to protein design. Top-down design methodologies have successfully produced comparatively simple polypeptide 'building blocks' (typically comprising 40-60 amino acids) useful in generating complex protein architecture, and have produced compelling data in support of macro-evolutionary pathways of protein structure. Furthermore, a distillation of the experimental approaches utilized in such studies suggests the potential for method formalism, one that may accelerate future success in this field.

Potential of fragment recombination for rational design of proteins

It is hypothesized that protein domains evolved from smaller intrinsically stable subunits via combinatorial assembly. Illegitimate recombination of fragments that encode protein subunits could have quickly led to diversification of protein folds and their functionality. This evolutionary concept presents an attractive strategy to protein engineering, e.g., to create new scaffolds for enzyme design. We previously combined structurally similar parts from two ancient protein folds, the (βα)(8)-barrel and the flavodoxin-like fold. The resulting "hopeful monster" differed significantly from the intended (βα)(8)-barrel fold by an extra β-strand in the core. In this study, we ask what modifications are necessary to form the intended structure and what potential this approach has for the rational design of functional proteins. Guided by computational design, we optimized the interface between the fragments with five targeted mutations yielding a stable, monomeric protein whose predicted structure was verified experimentally. We further tested binding of a phosphorylated compound and detected that some affinity was already present due to an intact phosphate-binding site provided by one fragment. The affinity could be improved quickly to the level of natural proteins by introducing two additional mutations. The study illustrates the potential of recombining protein fragments with unique properties to design new and functional proteins, offering both a possible pathway of protein evolution and a protocol to rapidly engineer proteins for new applications.

Exploring symmetry as an avenue to the computational design of large protein domains

It has been demonstrated previously that symmetric, homodimeric proteins are energetically favored, which explains their abundance in nature. It has been proposed that such symmetric homodimers underwent gene duplication and fusion to evolve into protein topologies that have a symmetric arrangement of secondary structure elements--"symmetric superfolds". Here, the ROSETTA protein design software was used to computationally engineer a perfectly symmetric variant of imidazole glycerol phosphate synthase and its corresponding symmetric homodimer. The new protein, termed FLR, adopts the symmetric (βα)(8) TIM-barrel superfold. The protein is soluble and monomeric and exhibits two-fold symmetry not only in the arrangement of secondary structure elements but also in sequence and at atomic detail, as verified by crystallography. When cut in half, FLR dimerizes readily to form the symmetric homodimer. The successful computational design of FLR demonstrates progress in our understanding of the underlying principles of protein stability and presents an attractive strategy for the in silico construction of larger protein domains from smaller pieces.

Improving the resistance of a eukaryotic β-barrel protein to thermal and chemical perturbations

β-Barrel membrane proteins have regular structures with extensive hydrogen-bond networks between their transmembrane (TM) β-strands, which stabilize their protein fold. Nevertheless, weakly stable TM regions, which are important for the protein function and interaction with other proteins, exist. Here, we report on the apparent stability of human Tom40A, a member of the "mitochondrial porin family" and main constituent of the mitochondrial protein-conducting channel TOM (translocase of the outer membrane). Using a physical interaction model, TmSIP, for β-barrel membrane proteins, we have identified three unfavorable β-strands in the TM domain of the protein. Substitution of key residues inside these strands with hydrophobic amino acids results in a decreased sensitivity of the protein to chemical and/or thermal denaturation. The apparent melting temperature observed when denatured at a rate of 1 °C per minute is shifted from 73 to 84 °C. Moreover, the sensitivity of the protein to denaturant agents is significantly lowered. Further, we find a reduced tendency for the mutated protein to form dimers. We propose that the identified weakly stable β-strands 1, 2 and 9 of human Tom40A play an important role in quaternary protein-protein interactions within the mammalian TOM machinery. Our results show that the use of empirical energy functions to model the apparent stability of β-barrel membrane proteins may be a useful tool in the field of nanopore bioengineering.

A fast and precise approach for computational saturation mutagenesis and its experimental validation by using an artificial (βα)8-barrel protein

We present a computational saturation mutagenesis protocol (CoSM) that predicts the impact on stability of all possible amino acid substitutions for a given site at an internal protein interface. CoSM is an efficient algorithm that uses a combination of rotamer libraries, side-chain flips, energy minimization, and molecular dynamics equilibration. Because CoSM considers full side-chain and backbone flexibility in the local environment of the mutated position, amino acids larger than the wild-type residue are also modeled in a proper manner. To assess the performance of CoSM, the effect of point mutations on the stability of an artificial (βα)(8)-barrel protein that has been designed from identical (βα)(4)-half barrels, was studied. In this protein, position 234(N) is a previously identified stability hot-spot that is located at the interface of the two half barrels. By using CoSM, changes in protein stability were predicted for all possible single point mutations replacing wild-type Val234(N). In parallel, the stabilities of 14 representative mutants covering all amino acid classes were experimentally determined. A linear correlation of computationally and experimentally determined energy values yielded an R(2) value of 0.90, which is statistically significant. This degree of coherence is stronger than the ones we obtained for established computational methods of mutational analysis.

Stabilization of a metabolic enzyme by library selection in Thermus thermophilus

The anthranilate phosphoribosyl transferase from the hyperthermophilic archaeon Sulfolobus solfataricus (sAnPRT, encoded by strpD), which catalyzes the third step in tryptophan biosynthesis, is a thermostable homodimer with low enzymatic activity at room temperature. We have combined two mutations leading to the monomerization and two mutations leading to the activation of sAnPRT. The resulting "activated monomer" sAnPRT-I36E-M47D+D83G-F149S, which is much more labile than wild-type sAnPRT, was stabilized by a combination of random mutagenesis and metabolic library selection using the extremely thermophilic bacterium Thermus thermophilus as host. This approach led to the identification of five mutations that individually increased the thermal stability of sAnPRT-I36E-M47D+D83G-F149S by 1 to 8 °C, and by 13 °C when combined. The beneficial exchanges were located in different parts of the protein structure, but none of them led to the "re-dimerization" of the enzyme. We observed a negative correlation between thermal stability and catalytic activity of the mutants; this suggests that conformational flexibility is required for catalysis by sAnPRT.

The structure of a truncated phosphoribosylanthranilate isomerase suggests a unified model for evolution of the (βα)8 barrel fold

The (βα)(8) barrel is one of the most common protein folds, and enzymes with this architecture display a remarkable range of catalytic activities. Many of these functions are associated with ancient metabolic pathways, and phylogenetic reconstructions suggest that the (βα)(8) barrel was one of the very first protein folds to emerge. Consequently, there is considerable interest in understanding the evolutionary processes that gave rise to this fold. In particular, much attention has been focused on the plausibility of (βα)(8) barrel evolution from homodimers of half barrels. However, we previously isolated a three-quarter-barrel-sized fragment of a (βα)(8) barrel, termed truncated phosphoribosylanthranilate isomerase (trPRAI), that is soluble and almost as thermostable as full-length N-(5'-phosphoribosyl)anthranilate isomerase (PRAI). Here, we report the NMR-derived structure of trPRAI. The subdomain is monomeric, is well ordered and adopts a native-like structure in solution. Side chains from strands β(1) (Glu3 and Lys5), β(2) (Tyr25) and β(6) (Lys122) of trPRAI repack to shield the hydrophobic core from the solvent. This result demonstrates that three-quarter barrels were viable intermediates in the evolution of the (βα)(8) barrel fold. We propose a unified model for (βα)(8) barrel evolution that combines our data, previously published work and plausible scenarios for the emergence of (initially error-prone) genetic systems. In this model, the earliest proto-cells contained diverse pools of part-barrel subdomains. Combinatorial assembly of these subdomains gave rise to many distinct lineages of (βα)(8) barrel proteins, that is, our model excludes the possibility that there was a single (βα)(8) barrel from which all present examples are descended.

Mimicking the evolution of a thermally stable monomeric four-helix bundle by fusion of four identical single-helix peptides

Internal symmetry is a common feature of the tertiary structures of proteins and protein domains. Probably, because the genes of homo-oligomeric proteins duplicated and fused, their evolutionary descendants are proteins with internal symmetry. To identify any advantages that cause monomeric proteins with internal symmetry to be selected evolutionarily, we characterized some of the physical properties of a recombinant protein with a sequence consisting of two tandemly fused copies of the Escherichia coli Lac repressor C-terminal alpha-helix. This polypeptide exists in solution mainly as dimer that likely maintains a four-helix bundle motif. Thermal unfolding experiments demonstrate that the protein is considerably more stable at elevated temperatures than is a homotetramer consisting of four non-covalently associated copies of a 21-residue polypeptide similar in sequence to that of the Lac repressor C-terminal alpha-helix. A tandem duplication of our helix-loop-helix polypeptide yields an even more thermally stable protein. Our results exemplify the concept that fusion of non-covalently assembled polypeptide chains leads to enhanced protein stability. Herein, we discuss how our work relates to the evolutionary selective-advantages realized when symmetrical homo-oligomers evolve into monomers. Moreover, our thermally stable single-chain four-helix bundle protein may provide a robust scaffold for development of new biomaterials.

Establishing wild-type levels of catalytic activity on natural and artificial (beta alpha)8-barrel protein scaffolds

The generation of high levels of new catalytic activities on natural and artificial protein scaffolds is a major goal of enzyme engineering. Here, we used random mutagenesis and selection in vivo to establish a sugar isomerisation reaction on both a natural (beta alpha)(8)-barrel enzyme and a catalytically inert chimeric (beta alpha)(8)-barrel scaffold, which was generated by the recombination of 2 (beta alpha)(4)-half barrels. The best evolved variants show turnover numbers and substrate affinities that are similar to those of wild-type enzymes catalyzing the same reaction. The determination of the crystal structure of the most proficient variant allowed us to model the substrate sugar in the novel active site and to elucidate the mechanistic basis of the newly established activity. The results demonstrate that natural and inert artificial protein scaffolds can be converted into highly proficient enzymes in the laboratory, and provide insights into the mechanisms of enzyme evolution.

Protein design through systematic catalytic loop exchange in the (beta/alpha)8 fold

Protein engineering by directed evolution has proven effective in achieving various functional modifications, but the well-established protocols for the introduction of variability, typically limited to random point mutations, seriously restrict the scope of the approach. In an attempt to overcome this limitation, we sought to explore variant libraries with richer diversity at regions recognized as functionally important through an exchange of natural components, thus combining design with combinatorial diversity. With this approach, we expected to maintain interactions important for protein stability while directing the introduction of variability to areas important for catalysis. Our strategy consisted in loop exchange over a (beta/alpha)(8) fold. Phosphoribosylanthranilate isomerase was chosen as scaffold, and we investigated its tolerance to loop exchange by fusing variant libraries to the chloramphenicol acetyl transferase coding gene as an in vivo folding reporter. We replaced loops 2, 4, and 6 of phosphoribosylanthranilate isomerase with loops of varied types and sizes from enzymes sharing the same fold. To allow for a better structural fit, saturation mutagenesis was adopted at two amino acid positions preceding the exchanged loop. Our results showed that 30% to 90% of the generated mutants in the different libraries were folded. Some variants were selected for further characterization after removal of chloramphenicol acetyl transferase gene, and their stability was studied by circular dichroism and fluorescence spectroscopy. The sequences of 545 clones show that the introduction of variability at "hinges" connecting the loops with the scaffold exhibited a noticeable effect on the appearance of folded proteins. Also, we observed that each position accepted foreign loops of different sizes and sequences. We believe our work provides the basis of a general method of exchanging variably sized loops within the (beta/alpha)(8) fold, affording a novel starting point for the screening of novel activities as well as modest diversions from an original activity.

Random dissection to select for protein split sites and its application in protein fragment complementation

To identify protein split sites quickly, a selection procedure by using chloramphenicol acetyl transferase (CAT) as reporter was introduced to search for folded protein fragments from libraries generated by random digestion and reassembly of the target gene, which yielded an abundant amount of DNA fragments with controllable lengths. Experimental results of tryptophan synthase alpha subunit (TSalpha) and TEM-1 beta-lactamase agreed well with what the literature has reported. The solubility of these fragments correlated roughly with the minimum inhibitory concentrations of the CAT fusions. The application of this dissection protocol to protein fragment complementation assay (PCA) was evaluated using aminoglycoside-3'-phosphotransferase I (APH(3')-I) as a model protein. Three nearly bisectional sites and a number of possible split points were identified, and guided by this result, four novel pairs of fragments were tested for complementation. Three out of four pairs partially restored the APH activity with the help of leucine zippers, and a truncated but active APH(3')-I (Delta1-25) was also found. Finally, the weakly active APH(3')-I-(1-253)NZ/CZ (254-271) containing a short 18 residue tag was further improved by error-prone PCR, and a best mutant was obtained showing a fourfold improvement after just one round of evolution. These results demonstrate that protein random dissection based on the CAT selection can provide an efficient search for protein breakage points and guide the design of fragments for protein complementation assay. Furthermore, more active fragment pairs can be achieved with the classical directed evolution approach.

A beta alpha-barrel built by the combination of fragments from different folds

Combinatorial assembly of protein domains plays an important role in the evolution of proteins. There is also evidence that protein domains have come together from stable subdomains. This concept of modular assembly could be used to construct new well folded proteins from stable protein fragments. Here, we report the construction of a chimeric protein from parts of a (betaalpha)(8)-barrel enzyme from histidine biosynthesis pathway (HisF) and a protein of the (betaalpha)(5)-flavodoxin-like fold (CheY) from Thermotoga maritima that share a high structural similarity. We expected this construct to fold into a full (betaalpha)(8)-barrel. Our results show that the chimeric protein is a stable monomer that unfolds with high cooperativity. Its three-dimensional structure, which was solved to 3.1 A resolution by x-ray crystallography, confirms a barrel-like fold in which the overall structures of the parent proteins are highly conserved. The structure further reveals a ninth strand in the barrel, which is formed by residues from the HisF C terminus and an attached tag. This strand invades between beta-strand 1 and 2 of the CheY part closing a gap in the structure that might be due to a suboptimal fit between the fragments. Thus, by a combination of parts from two different folds and a small arbitrary fragment, we created a well folded and stable protein.