Intein-mediated cyclization of randomized peptides in the periplasm of Escherichia coli and their extracellular secretion

Protein-protein interactions enhance the thermal resilience of SpyRing-cyclized enzymes: A molecular dynamic simulation study

Recently a technique based on the interaction between adhesion proteins extracted from Streptococcus pyogenes, known as SpyRing, has been widely used to improve the thermal resilience of enzymes, the assembly of biostructures, cancer cell recognition and other fields. It was believed that the covalent cyclization of protein skeleton caused by SpyRing reduces the conformational entropy of biological structure and improves its rigidity, thus improving the thermal resilience of the target enzyme. However, the effects of SpyTag/ SpyCatcher interaction with this enzyme are poorly understood, and their regulation of enzyme properties remains unclear. Here, for simplicity, we took the single domain enzyme lichenase from Bacillus subtilis 168 as an example, studied the interface interactions in the SpyRing by molecular dynamics simulations, and examined the effects of the changes of electrostatic interaction and van der Waals interaction on the thermal resilience of target enzyme. The simulations showed that the interface between SpyTag/SpyCatcher and the target enzyme is different from that found by geometric matching method and highlighted key mutations at the interface that might have effect on the thermal resilience of the enzyme. Our calculations highlighted interfacial interactions between enzyme and SpyTag/SpyCatcher, which might be useful in rational designs of the SpyRing.

Kinetics study of the natural split Npu DnaE intein in the generation of bispecific IgG antibodies

Rapid and efficient bispecific antibody (BsAb) production for industrial applications is still facing many challenges. We reported a technology platform for generating bispecific IgG antibodies, "Bispecific Antibody by Protein Trans-splicing (BAPTS)." While the "BAPTS" method has shown potential in high-throughput screening of BsAbs, further understanding and optimizing the methodology is desirable. A large number of BsAbs were selected to illustrate the conversion efficiency and kinetics parameters. The temperature of reaction makes no significant influence in conversion efficiency, which can reach more than 70% within 2 h, and CD3 × HER2 BsAb can reach 90%. By fitting trans-splicing reaction to single-component exponential decay curves, the apparent first-order rate constants at a series of temperatures were determined. The rate constant ranges from 0.02 to 0.11 min^-1 at 37 °C, which is a high rate reported for the protein trans-splicing reaction (PTS). The reaction process is activated rapidly with activation energy of 8.9 kcal·mol^-1 (CD3 × HER2) and 5.2 kcal·mol^-1 (CD3 × EGFR). The BsAbs generated by "BAPTS" technology not only had the similar post-translation modifications to the parental antibodies, but also demonstrated excellent in vitro and in vivo bioactivity. The kinetics parameters and activation energy of the reaction illustrate feasible for high-throughput screening and industrial applications using the "BAPTS" approach. KEY POINTS: • The trans-splicing reaction of Npu DnaE intein in "BAPTS" platform is a rapid process with low reaction activation and high rate. • The BsAb generated by "BAPTS" remained effective in tumor cell killing. • The kinetics parameters and activation energy of the reaction illustrate feasible for high-throughput screening and industrial applications using the "BAPTS" approach.

Directed evolution for enzyme development in biocatalysis

As an important sector of the chemical industry, biocatalysis requires the continuous development of enzymes with tailor-made activity, selectivity, stability, or tolerance to unnatural environments. This is now routinely achieved by directed evolution based on iterative cycles of genetic diversification and activity screening. Here, we highlight its recent developments. First, the design of "smarter" libraries by focused mutagenesis may be a crucial start-up for a fast and successful outcome. Then library assembly and expression are also key steps that benefits from modern molecular biology progresses. Finally, various strategies may be considered for library screening depending on the final objective: while low-throughput direct assays have been very successful in generating enzymes for important biocatalytic processes, even in bringing completely new chemistries to the enzyme world, ultrahigh-throughput screening methods are emerging as powerful approaches for engineering the next generation of industrial enzymes.

Inteins in Science: Evolution to Application

Inteins are mobile genetic elements that apply standard enzymatic strategies to excise themselves post-translationally from the precursor protein via protein splicing. Since their discovery in the 1990s, recent advances in intein technology allow for them to be implemented as a modern biotechnological contrivance. Radical improvement in the structure and catalytic framework of cis- and trans-splicing inteins devised the development of engineered inteins that contribute to various efficient downstream techniques. Previous literature indicates that implementation of intein-mediated splicing has been extended to in vivo systems. Besides, the homing endonuclease domain also acts as a versatile biotechnological tool involving genetic manipulation and control of monogenic diseases. This review orients the understanding of inteins by sequentially studying the distribution and evolution pattern of intein, thereby highlighting a role in genetic mobility. Further, we include an in-depth summary of specific applications branching from protein purification using self-cleaving tags to protein modification, post-translational processing and labelling, followed by the development of intein-based biosensors. These engineered inteins offer a disruptive approach towards research avenues like biomaterial construction, metabolic engineering and synthetic biology. Therefore, this linear perspective allows for a more comprehensive understanding of intein function and its diverse applications.

Site-Selective Peptide Macrocyclization

Cyclized peptides have seen a rise in popularity in the pharmaceutical industry as drug molecules. As such, new macrocyclization methodologies have become abundant in the last several decades. However, efficient methods of cyclization without the formation of side products remain a great challenge. Herein, we review cyclization approaches that focus on site-selective chemistry. Site selectivity in macrocyclization decreases the generation of side products, leading to a greater yield of the desired peptide macrocycles. We will also take an in-depth look at the new exclusively intramolecular N-terminal site-selective CyClick strategy for the synthesis of cyclic peptides. The CyClick method uses imine formation between an aldehyde and the N terminus. The imine is then trapped by a nucleophilic attack from the second amidic nitrogen in an irreversible site-selective fashion.

Biotechnological Applications of Protein Splicing

Protein splicing domains, also called inteins, have become a powerful biotechnological tool for applications involving molecular biology and protein engineering. Early applications of inteins focused on self-cleaving affinity tags, generation of recombinant polypeptide α-thioesters for the production of semisynthetic proteins and backbone cyclized polypeptides. The discovery of naturallyoccurring split-inteins has allowed the development of novel approaches for the selective modification of proteins both in vitro and in vivo. This review gives a general introduction to protein splicing with a focus on their role in expanding the applications of intein-based technologies in protein engineering and chemical biology.

QuickLib, a method for building fully synthetic plasmid libraries by seamless cloning of degenerate oligonucleotides

Incorporation of synthetic degenerate oligonucleotides into plasmids for building highly diverse genetic libraries requires efficient and quantitative DNA manipulation. We present a fast and seamless method for generating libraries of PCR-synthesized plasmids designed with a degenerate sequence and short overlapping ends. Our method called QuickLib should find many applications in synthetic biology; as an example, we easily prepared genetic libraries of Escherichia coli expressing billions of different backbone cyclic peptides.

Intein-mediated backbone cyclization of VP1 protein enhanced protection of CVB3-induced viral myocarditis

CVB3 is a common human pathogen to be highly lethal to newborns and causes viral myocarditis and pancreatitis in adults. However, there is no vaccine available for clinical use. CVB3 capsid protein VP1 is an immunodominant structural protein, containing several B- and T-cell epitopes. However, immunization of mice with VP1 protein is ineffective. Cyclization of peptide is commonly used to improve their in vivo stability and biological activity. Here, we designed and synthesizd cyclic VP1 protein by using engineered split Rma DnaB intein and the cyclization efficiency was 100% in E. coli. As a result, the cyclic VP1 was significantly more stable against irreversible aggregation upon heating and against carboxypeptidase in vitro and the degradation rate was more slowly in vivo. Compared with linear VP1, immunization mice with circular VP1 significantly increased CVB3-specific serum IgG level and augmented CVB3-specific cellular immune responses, consequently afforded better protection against CVB3-induced viral myocarditis. The cyclic VP1 may be a novel candidate protein vaccine for preventing CVB3 infection and similar approaches could be employed to a variety of protein vaccines to enhance their protection effect.

Recombinant expression of backbone-cyclized polypeptides

Here we review the different biochemical approaches available for the expression of backbone-cyclized polypeptides, including peptides and proteins. These methods allow for the production of circular polypeptides either in vitro or in vivo using standard recombinant DNA expression techniques. Polypeptide circularization provides a valuable tool to study the effects of topology on protein stability and folding kinetics. Furthermore, having biosynthetic access to backbone-cyclized polypeptides makes the production of genetically encoded libraries of cyclic polypeptides possible. The production of such libraries, which was previously restricted to the domain of synthetic chemistry, now offers biologists access to highly diverse and stable molecular libraries that can be screened using high-throughput methods for the rapid selection of novel cyclic polypeptide sequences with new biological activities.

Enhanced thermal stability of lichenase from Bacillus subtilis 168 by SpyTag/SpyCatcher-mediated spontaneous cyclization

BACKGROUND

SpyTag is a peptide that can form an irreversible covalent linkage to its 12 kDa partner SpyCatcher via a spontaneous isopeptide bond. Herein, we fused SpyTag at the N-terminal of lichenase and SpyCatcher at C-terminal so that the termini of lichenase were locked together by the covalent interaction between the partners. In addition, an elastin-like polypeptides tag was subsequently attached to the C-terminus of SpyCatcher, thereby facilitating the non-chromatographic purification of cyclized lichenase.

RESULTS

The study showed that the optimum temperature of the cyclized lichenase was about 5 °C higher in comparison to its linear counterpart. Moreover, nearly 80 % of the cyclized lichenase activities were retained after 100 °C exposure, whereas the linear form lost almost all of its activities. Therefore, the cyclized variant displayed a significantly higher thermal stability as temperature elevated and was resistant to hyperthermal denaturation. Besides, the Km value of the cyclized lichenase (7.58 ± 0.92 mg/mL) was approximately 1.7-fold lower than that of the linear one (12.96 ± 1.93 mg/mL), indicating a higher affinity with substrates.

CONCLUSIONS

This new SpyTag/SpyCatcher cyclization strategy is deemed as a generalized reference for enhancing enzyme stability and can be effectively customized to the cyclization of various enzymes, hence a tremendous potential for successful application in the biocatalytic conversion of biomass to produce fuels and chemicals.

Chemical and biological production of cyclotides

Cyclotides are fascinating naturally occurring micro-proteins (≈30 residues long) present in several plant families, and display various biological properties such as protease inhibitory, anti-microbial, insecticidal, cytotoxic, anti-HIV and hormone-like activities. Cyclotides share a unique head-to-tail circular knotted topology of three disulfide bridges, with one disulfide penetrating through a macrocycle formed by the two other disulfides and interconnecting peptide backbones, forming what is called a cystine knot topology. This cyclic cystine knot (CCK) framework gives the cyclotides exceptional rigidity, resistance to thermal and chemical denaturation, and enzymatic stability against degradation. Interestingly, cyclotides have been shown to be orally bioavailable, and other cyclotides have been shown to cross the cell membranes. Moreover, recent reports have also shown that engineered cyclotides can be efficiently used to target extracellular and intracellular protein-protein interactions, therefore making cyclotides ideal tools for drug development to selectively target protein-protein interactions. In this work we will review all the available methods for production of these interesting proteins using chemical or biological methods.

Bioinspired strategy for the ribosomal synthesis of thioether-bridged macrocyclic peptides in bacteria

Inspired by the biosynthetic logic of lanthipeptide natural products, a new methodology was developed to direct the ribosomal synthesis of macrocyclic peptides constrained by an intramolecular thioether bond. As a first step, a robust and versatile strategy was implemented to enable the cyclization of ribosomally derived peptide sequences via a chemoselective reaction between a genetically encoded cysteine and a cysteine-reactive unnatural amino acid (O-(2-bromoethyl)-tyrosine). Combination of this approach with intein-catalyzed protein splicing furnished an efficient route to achieve the spontaneous, post-translational formation of structurally diverse macrocyclic peptides in bacterial cells. The present peptide cyclization strategy was also found to be amenable to integration with split intein-mediated circular ligation, resulting in the intracellular synthesis of conformationally constrained peptides featuring a bicyclic architecture.

Intein applications: from protein purification and labeling to metabolic control methods

The discovery of inteins in the early 1990s opened the door to a wide variety of new technologies. Early engineered inteins from various sources allowed the development of self-cleaving affinity tags and new methods for joining protein segments through expressed protein ligation. Some applications were developed around native and engineered split inteins, which allow protein segments expressed separately to be spliced together in vitro. More recently, these early applications have been expanded and optimized through the discovery of highly efficient trans-splicing and trans-cleaving inteins. These new inteins have enabled a wide variety of applications in metabolic engineering, protein labeling, biomaterials construction, protein cyclization, and protein purification.

pH-dependent activation of Streptomyces hygroscopicus transglutaminase mediated by intein

Microbial transglutaminase (MTG) from Streptomyces is naturally secreted as a zymogen (pro-MTG), which is then activated by the removal of its N-terminal proregion by additional proteases. Inteins are protein-intervening sequences that catalyze protein splicing without cofactors. In this study, a pH-dependent Synechocystis sp. strain PCC6803 DnaB mini-intein (SDB) was introduced into pro-MTG to simplify its activation process by controlling pH. The recombinant protein (pro-SDB-MTG) was obtained, and the activation process was determined to take 24 h at pH 7 in vitro. To investigate the effect of the first residue in MTG on the activity and the cleavage time, two variants, pro-SDB-MTG(D1S) and pro-SDB-MTG(ΔD1), were expressed, and the activation time was found to be 6 h and 30 h, respectively. The enzymatic property and secondary structure of the recombinant MTG and two variants were similar to those of the wild type, indicating that the insertion of mini-intein did not affect the function of MTG. This insignificant effect was further illustrated by molecular dynamics simulations. This study revealed a controllable and effective strategy to regulate the activation process of pro-MTG mediated by a mini-intein, and it may have great potential for industrial MTG production.

Recombinant production of peptide C-terminal α-amides using an engineered intein

Peptides are of increasing interest as therapeutics in a wide range of diseases, including metabolic diseases such as diabetes and obesity. In the latter, peptide hormones such as peptide YY (PYY) and pancreatic peptide (PP) are important templates for drug design. Characteristic for these peptides is that they contain a C-terminal that is α-amidated, and this amidation is crucial for biological function. A challenge is to generate such peptides by recombinant means and particularly in a production scale. Here, we have examined an intein-mediated approach to generate a PYY derivative in a larger scale. Initially, we experienced challenges with hydrolysis of the intein fusion protein, which was reduced by a T3C mutation in the intein. Subsequently, we further engineered the intein to decrease the absolute size and improve the relative yield of the PYY derivative, which was achieved by substituting 54 residues of the 198 amino acid intein with an eight amino acid linker. The optimized intein construct was used to produce the PYY derivative under high cell density cultivation conditions, generating the peptide thioester precursor in good yields and subsequent amidation provided the target peptide.

Design, synthesis, and diversification of ribosomally derived peptide macrocycles

Ring topologies are widespread structural motifs among biologically active peptides found in nature. The recurrence of this motif is linked to the inherent advantages resulting from backbone cyclization, which include increased resistance against proteolytic degradation, improved cell permeability, and tighter and more specific interaction with the respective biomolecular target. Inspired by these natural product topologies, a number of groups have recently focused on developing methodologies that hinge upon the chemical elaboration of ribosomally derived polypeptides toward the synthesis and diversification of macrocyclic peptide structures. In this review, we highlight recent advances in this emerging new area and discuss the opportunities created by these methods toward the discovery of new functional entities.

Biological synthesis of circular polypeptides

Here, we review the use of different biochemical approaches for biological synthesis of circular or backbone-cyclized proteins and peptides. These methods allow the production of circular polypeptides either in vitro or in vivo using standard recombinant DNA expression techniques. Protein circularization can significantly impact protein engineering and research in protein folding. Basic polymer theory predicts that circularization should lead to a net thermodynamic stabilization of a folded protein by reducing the entropy associated with the unfolded state. Protein cyclization also provides a valuable tool for exploring the effects of topology on protein folding kinetics. Furthermore, the biological production of cyclic polypeptides makes possible the production of cyclic polypeptide libraries. The generation of such libraries, which was previously restricted to the domain of synthetic chemists, now offers biologists access to highly diverse and stable molecular libraries for probing protein structure and function.

Various mechanisms in cyclopeptide production from precursors synthesized independently of non-ribosomal peptide synthetases

An increasing number of cyclopeptides have been discovered as products of ribosomal synthetic pathway. The biosynthetic study of these cyclopeptides has revealed interesting new mechanisms for cyclization. This review highlighted the recent discoveries in cyclization mechanisms for cyclopeptides synthesized independently of non-ribosomal peptide synthetases, including endopeptidase-catalyzed cyclization, intein-mediated cyclization, and peptide synthetase-catalyzed cyclization. This information may help to design hybrid ribosomal and non-ribosomal biosynthetic systems to produce novel cyclopeptides with various bioactivities.

Protein trans-splicing as a protein ligation tool to study protein structure and function

Protein trans-splicing (PTS) exerted by split inteins is a protein ligation reaction which enables overcoming the barriers of conventional heterologous protein production. We provide an overview of the current state-of-the-art in split intein engineering, as well as the achievements of PTS technology in the realm of protein structure-function analyses, including incorporation of natural and artificial protein modifications, controllable protein reconstitution, segmental isotope labeling and protein cyclization. We further discuss factors crucial for the successful implementation of PTS in these protein engineering approaches, and speculate on necessary future endeavours to make PTS a universally applicable protein ligation tool.