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

In situ intein-mediated multiprotein assembly via engineered cross-species consortia

Inteins can connect flanking external proteins into a new protein fragment and excise themselves. Here, we report the in situ splicing of proteins by engineered microbial consortia. This study pioneers a programmable microbial consortia platform enabling in situ protein splicing through split intein-mediated assembly. Engineered Escherichia coli with the ePop autolysis system release intein-fused protein fragments via synchronized lysis, while Pichia pastoris secretes complementary domains, enabling extracellular reconstitution directly in culture. With the application of integrating quorum-sensing controls and eukaryotic secretion pathways, this approach bypasses in vitro purification, supporting scalable one-pot synthesis of multiple functional proteins. The platform's versatility in logic computation and antibiotic resistance engineering highlights its potential for adaptive biomanufacturing and context-aware biomaterial design.

A cysteine-less and ultra-fast split intein rationally engineered from being aggregation-prone to highly efficient in protein trans-splicing

Split inteins catalyze protein trans-splicing by ligating their extein sequences while undergoing self-excision, enabling diverse protein modification applications. However, many purified split intein precursors exhibit partial or no splicing activity for unknown reasons. The Aes123 PolB1 intein, a representative of the rare cysteine-less split inteins, is of particular interest due to its resistance to oxidative conditions and orthogonality to thiol chemistries. In this work, we identify β-sheet-dominated aggregation of its N-terminal intein fragment as the origin of its low (~30%) splicing efficiency. Using computational, biochemical, and biophysical analyses, we characterize the fully active monomeric fraction and pinpoint aggregation-prone regions. Supported by a crystal structure, we design stably monomeric mutants with nearly complete splicing activity. The optimized CLm intein (Cysteine-Less and monomeric) retains the wild-type's ultra-fast reaction rate and serves as an efficient, thiol-independent protein modification tool. We find that other benchmark split inteins show similar precursor aggregation, suggesting that this general phenomenon arises from the intrinsic challenge to maintain the precursor in a partially disordered state while promoting stable folding upon fragment association.

Controllable intein splicing and N-terminal cleavage at mesophilic temperatures

Inteins (intervening proteins) interrupt host proteins and are removed through a protein splicing reaction that ligates adjacent N- and C-exteins. The ability of inteins to specifically rearrange peptide bonds has proven exceptionally useful in protein engineering, thus, methods to control intein activity are of considerable interest. One particularly useful application of inteins is for the removal of an affinity tag following purification of a target protein through N-terminal cleavage (NTC). Typically, extended incubation at high temperature (greater than 50°C) or with an external nucleophile (e.g., dithiothreitol) is required to drive NTC, conditions that compromise the folding of many target proteins. Here, we characterize a variant of the Thermococcus kodakarensis RadA intein that can perform NTC at moderate temperatures in the absence of an external nucleophile. While we find that while NTC is largely inhibited during expression in Escherichia coli at 15°C, rapid and efficient NTC can be activated 37°C. Our results provide an alternative intein-based system - one that does not require either an external nucleophile or prolonged incubation at high temperature to stimulate NTC - that controls intein activity within a temperature range amenable to most mesophilic experimental organisms.

Site-directed display of zearalenone lactonase on spilt-intein functionalized nanocarrier for green and efficient detoxification of zearalenone

In this study, we prepared a functional organic-inorganic hybrid nanoflower (InHNF) via split intein moiety in a biomineralization process without using organic solvents. InHNF could specifically bind the target enzymes from crude cell lysates within seconds and site-directedly display them on the surface by forming a peptide bond with enzyme's terminal amino acid residue. This unique feature enabled InHNF to increase the specific activity of zearalenone detoxifying enzyme ZHD518 by 40 ∼ 60% at all tested temperatures and prevented enzyme denaturation even under extreme pH conditions (pH 3-11). Furthermore, it exhibited excellent operational stability, with a residual activity of over 70% after eight reaction cycles. Strikingly, InHNF-ZHD518 achieved above 50% ZEN degradation despite the near inactivation of free ZHD518 in beer sample. Overall, InHNF nanocarriers can achieve environmentally friendly, purification-free, and site-directed immobilization of food enzymes and enhance their catalytic properties, making them suitable for a wide range of industrial applications.

Intein Based Fusion Proteins: Great Tags for the Soluble Production and Convenient Purification of Recombinant Proteins

Background

The main problem in the recombinant protein expression in E. coli strains, especially for high-yield production, is the accumulation in un-folded and inactive inclusion bodies. A suitable solution is the direction into the soluble cytoplasmic products by solubilizing tags. The use of inteins with self-cleaving ability, in addition to increase the chance of soluble protein expression, facilitates their purification process.

Evidence Acquisition

In this review article, papers related to the use of intein tags for soluble expression or protein purification were collected regardless the time limit. Available databases including Pubmed, google scholar, ScienceDirect, Web of Science, Scopus, and Embase was searched. The best condition for soluble expression or purification was focused in all articles.

Results

There are various intein tags commercially available in expression vectors that results in gaining our goal in facilitating the recombinant protein solubilization as well as its simple purification. It is enough to induce the self-cleavage property of the intein, which varies according to the type of intein used. In this way, the target protein is easily separated from the purification tag without the need to add protease enzymes such as enterokinase or treatment with various chemicals. The most common affinity tag in intein-based systems is Chitin Binding Domain attached to the chitin resin.

Conclusions

In this review article, we introduced proteins or peptides which produced in fusion to intein tags and discussed about their expression condition and purification process in order to enhance the chance of soluble expression and intein cleavage in a single stage, respectively.

The three-dimensional structure of the Vint domain from Tetrahymena thermophila suggests a ligand-regulated cleavage mechanism by the HINT fold

Vint proteins have been identified in unicellular metazoans as a novel hedgehog-related gene family, merging the von Willebrand factor type A domain and the Hedgehog/INTein (HINT) domains. We present the first three-dimensional structure of the Vint domain from Tetrahymena thermophila corresponding to the auto-processing domain of hedgehog proteins, shedding light on the unique features, including an adduct recognition region (ARR). Our results suggest a potential binding between the ARR and sulfated glycosaminoglycans like heparin sulfate. Moreover, we uncover a possible regulatory role of the ARR in the auto-processing by Vint domains, expanding our understanding of the HINT domain evolution and their use in biotechnological applications. Vint domains might have played a crucial role in the transition from unicellular to multicellular organisms.

Preorganized cyclic modules facilitate the self-assembly of protein nanostructures

The rational design of supramolecular assemblies aims to generate complex systems based on the simple information encoded in the chemical structure. Programmable molecules such as nucleic acids and polypeptides are particularly suitable for designing diverse assemblies and shapes not found in nature. Here, we describe a strategy for assembling modular architectures based on structurally and covalently preorganized subunits. Cyclization through spontaneous self-splicing of split intein and coiled-coil dimer-based interactions of polypeptide chains provide structural constraints, facilitating the desired assembly. We demonstrate the implementation of a strategy based on the preorganization of the subunits by designing a two-chain coiled-coil protein origami (CCPO) assembly that adopts a tetrahedral topology only when one or both subunit chains are covalently cyclized. Employing this strategy, we further design a 109 kDa trimeric CCPO assembly comprising 24 CC-forming segments. In this case, intein cyclization was crucial for the assembly of a concave octahedral scaffold, a newly designed protein fold. The study highlights the importance of preorganization of building modules to facilitate the self-assembly of higher-order supramolecular structures.

Approaches to the Full and Partial Chemical Synthesis of Proteins

Histones are proteins which help to organize DNA. The way in which they function is complex and is partially controlled by post-translational modifications (PTMs). Histone proteins from numerous organisms can be recombinantly produced in bacteria, but many bacterial strains are incapable of installing the variety of PTMs that histones possess. An alternative method of producing histones, which can be used to introduce PTMs, is native chemical ligation (NCL). This chapter provides a general NCL protocol which can be used to produce synthetic, post-translationally modified, histone proteins. The focus is on the NCL procedure itself and not on producing the modified histone protein fragments as there are many different ways in which these can be synthesized, depending on the modification(s) required. The same NCL protocol is also applicable for expressed protein ligation (EPL) with only small modifications to the purification procedure potentially required.

Streamlining the Detection of Human Thyroid Receptor Ligand Interactions with XL1-Blue Cell-Free Protein Synthesis and Beta-Galactosidase Fusion Protein Biosensors

Thyroid receptor signaling controls major physiological processes and disrupted signaling can cause severe disorders that negatively impact human life. Consequently, methods to detect thyroid receptor ligands are of great toxicologic and pharmacologic importance. Previously, we reported thyroid receptor ligand detection with cell-free protein synthesis of a chimeric fusion protein composed of the human thyroid receptor beta (hTRβ) receptor activator and a β-lactamase reporter. Here, we report a 60% reduction in sensing cost by reengineering the chimeric fusion protein biosensor to include a reporter system composed of either the full-length beta galactosidase (β-gal), the alpha fragment of β-gal (β-gal-α), or a split alpha fragment of the β-gal (split β-gal-α). These biosensor constructs are deployed using E. coli XL1-Blue cell extract to (1) avoid the β-gal background activity abundant in BL21 cell extract and (2) facilitate β-gal complementation reporter activity to detect human thyroid receptor ligands. These results constitute a promising platform for high throughput screening and potentially the portable detection of human thyroid receptor ligands.

An artificially split class 3 intein

Inteins excise themselves from precursor polypeptides through protein splicing, joining N- and C-exteins with a peptide bond. Split inteins are expressed as separate polypeptides that undergo protein trans splicing (PTS). Here, we demonstrate PTS can be achieved using an artificially split class 3 intein. Because class 3 inteins use an internal initiating nucleophile near the C-extein junction, rather than the first residue of the intein, both catalytic nucleophiles are present on a single polypeptide. This results in a compact arrangement of catalytic nucleophiles for PTS compared to the standard arrangement for split class 1 inteins.

Efficient splicing of the CPE intein derived from directed evolution of the Cryptococcus neoformans PRP8 intein

Intein-mediated protein splicing has been widely used in protein engineering; however, the splicing efficiency and extein specificity usually limit its further application. Thus, there is a demand for more general inteins that can overcome these limitations. Here, we study the trans-splicing of CPE intein obtained from the directed evolution of Cne PRP8, which shows that its splicing rate is ~29- fold higher than that of the wild-type. When the +1 residue of C-extein is changed to cysteine, CPE also shows high splicing activity. Faster association and higher affinity may contribute to the high splicing rate compared with wild-type intein. These findings have important implications for the future engineering of inteins and provide clues for fundamental studies of protein structure and folding.

Structural and biochemical analysis of a novel atypically split intein reveals a conserved histidine specific to cysteine-less inteins

Protein trans-splicing mediated by a split intein reconstitutes a protein backbone from two parts. This virtually traceless autoprocessive reaction provides the basis for numerous protein engineering applications. Protein splicing typically proceeds through two thioester or oxyester intermediates involving the side chains of cysteine or serine/threonine residues. A cysteine-less split intein has recently attracted particular interest as it can splice under oxidizing conditions and is orthogonal to disulfide or thiol bioconjugation chemistries. Here, we report the split PolB16 OarG intein, a second such cysteine-independent intein. As a unique trait, it is atypically split with a short intein-N precursor fragment of only 15 amino acids, the shortest characterized to date, which was chemically synthesized to enable protein semi-synthesis. By rational engineering we obtained a high-yielding, improved split intein mutant. Structural and mutational analysis revealed the dispensability of the usually crucial conserved motif N3 (block B) histidine as an obvious peculiar property. Unexpectedly, we identified a previously unnoticed histidine in hydrogen-bond forming distance to the catalytic serine 1 as critical for splicing. This histidine has been overlooked so far in multiple sequence alignments and is highly conserved only in cysteine-independent inteins as a part of a newly discovered motif NX. The motif NX histidine is thus likely of general importance to the specialized environment in the active site required in this intein subgroup. Together, our study advances the toolbox as well as the structural and mechanistic understanding of cysteine-less inteins.

Standard Intein Gene Expression Ramps (SIGER) for Protein-Independent Expression Control

Coordination of multigene expression is one of the key challenges of metabolic engineering for the development of cell factories. Constraints on translation initiation and early ribosome kinetics of mRNA are imposed by features of the 5'UTR in combination with the start of the gene, referred to as the "gene ramp", such as rare codons and mRNA secondary structures. These features strongly influence the translation yield and protein quality by regulating the ribosome distribution on mRNA strands. The utilization of genetic expression sequences, such as promoters and 5'UTRs in combination with different target genes, leads to a wide variety of gene ramp compositions with irregular translation rates, leading to unpredictable levels of protein yield and quality. Here, we present the Standard Intein Gene Expression Ramp (SIGER) system for controlling protein expression. The SIGER system makes use of inteins to decouple the translation initiation features from the gene of a target protein. We generated sequence-specific gene expression sequences for two inteins (DnaB and DnaX) that display defined levels of protein expression. Additionally, we used inteins that possess the ability to release the C-terminal fusion protein in vivo to avoid the impairment of protein functionality by the fused intein. Overall, our results show that SIGER systems are unique tools to mitigate the undesirable effects of gene ramp variation and to control the relative ratios of enzymes involved in molecular pathways. As a proof of concept of the potential of the system, we also used a SIGER system to express two difficult-to-produce proteins, GumM and CBM73.

Enzymatic Spin-Labeling of Protein N- and C-Termini for Electron Paramagnetic Resonance Spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool for investigating the structure and dynamics of proteins. The introduction of paramagnetic moieties at specific positions in a protein enables precise measurement of local structure and dynamics. This technique, termed site-directed spin-labeling, has traditionally been performed using cysteine-reactive radical-containing probes. However, large proteins are more likely to contain multiple cysteine residues and cysteine labeling at specific sites may be infeasible or impede function. To address this concern, we applied three peptide-ligating enzymes (sortase, asparaginyl endopeptidase, and inteins) for nitroxide labeling of N- and C-termini of select monomeric and dimeric proteins. Continuous wave and pulsed EPR (double electron electron resonance) experiments reveal specific attachment of nitroxide probes to ether N-termini (OaAEP1) or C-termini (sortase and intein) across three test proteins (CheY, CheA, and iLOV), thereby enabling a straightforward, highly specific, and general method for protein labeling. Importantly, the linker length (3, 5, and 9 residues for OaAEP1, intein, and sortase reactions, respectively) between the probe and the target protein has a large impact on the utility of distance measurements by pulsed EPR, with longer linkers leading to broader distributions. As these methods are only dependent on accessible N- and C-termini, we anticipate application to a wide range of protein targets for biomolecular EPR spectroscopy.

Biomolecular Click Reactions Using a Minimal pH-Activated Catcher/Tag Pair for Producing Native-Sized Spider-Silk Proteins

A type of protein/peptide pair known as Catcher/Tag pair spontaneously forms an intermolecular isopeptide bond which can be applied for biomolecular click reactions. Covalent protein conjugation using Catcher/Tag pairs has turned out to be a valuable tool in biotechnology and biomedicines, but it is essential to increase the current toolbox of orthogonal Catcher/Tag pairs to expand the range of applications further, for example, for controlled multiple-fragment ligation. We report here the engineering of novel Catcher/Tag pairs for protein ligation, aided by a crystal structure of a minimal CnaB domain from Lactobacillus plantarum. We show that a newly engineered pair, called SilkCatcher/Tag enables efficient pH-inducible protein ligation in addition to being compatible with the widely used SpyCatcher/Tag pair. Finally, we demonstrate the use of the SilkCatcher/Tag pair in the production of native-sized highly repetitive spider-silk-like proteins with >90 % purity, which is not possible by traditional recombinant production methods.

Harnessing Split-Inteins as a Tool for the Selective Modification of Surface Receptors in Live Cells

Biochemical studies of integral membrane proteins are often hampered by low purification yields and technical limitations such as aggregation causing in vitro manipulations to be challenging. The ability of controlling proteins in live cells bypasses these limitations while broadening the scope of accessible questions owing to the proteins being in their native environment. Here we take advantage of the intein biorthogonality to mammalian systems, site specificity, fast kinetics, and auto-processing nature as an attractive option for modifying surface proteins. Using EGFR as a model, we demonstrate that the split-intein pair AvaN /NpuC can be used to efficiently and specifically modify target membrane proteins with a synthetic adduct for downstream live cell application.

Single-Step Non-Chromatographic Purification of Recombinant Mammalian Proteins Using a Split Intein ELP Tag System

Glycoprotein therapeutics are currently used by large patient populations and generate significant revenue for the biopharmaceutical industry. These therapeutic proteins are currently purified at industrial scale using individualized processes involving multiple chromatographic steps. In the absence of a viable affinity platform method, the required chromatographic steps are difficult to develop and inevitably lead to significant yield losses. Further, during preclinical development, there is a need for reliable platform technologies capable of performing high-throughput screening for biologic candidates. Although affinity tags can provide a solution to some of these challenges, they require specific affinity resins, and the tag itself can interfere with the target protein characteristics. Fusion protein systems consisting of elastin-like polypeptide (ELP) and self-cleaving split inteins such as Npu DnaE can serve as potential non-chromatographic platform technologies for the single-step purification of tagless glycoproteins expressed in mammalian cells. In this chapter, we demonstrate the use of this technology to obtain highly purified anti-ErbB2 ML39 single-chain variable fragment (scFv) expressed from Expi293F suspension cells.

Labeling Ebola Virus with a Self-Splicing Fluorescent Reporter

Inteins (intervening proteins) are polypeptides that interrupt the sequence of other proteins and remove themselves through protein splicing. In this intein-catalyzed reaction, the two peptide bonds surrounding the intein are rearranged to release the intein from the flanking protein sequences, termed N- and C-exteins, which are concurrently joined by a peptide bond. Because of this unique functionality, inteins have proven exceptionally useful in protein engineering. Previous work has demonstrated that heterologous proteins can be inserted within an intein, with both the intein and inserted protein retaining function, allowing for intein-containing genes to coexpress additional coding sequences. Here, we show that a fluorescent protein (ZsGreen) can be inserted within the Pyrococcus horikoshii RadA intein, with the hybrid protein (ZsG-Int) maintaining fluorescence and splicing capability. We used this system to create a recombinant Ebola virus expressing a fluorescent protein. We first tested multiple potential insertion sites for ZsG-Int within individual Ebola virus proteins, identifying a site within the VP30 gene that facilitated efficient intein splicing in mammalian cells while also preserving VP30 function. Next, we successfully rescued a virus containing the ZsG-Int-VP30 fusion protein, which displayed fluorescence in the infected cells. We thus report a new intein-based application for adding reporters to systems without the need to add additional genes. Further, this work highlights a novel reporter design, whereby the reporter is only made if the protein of interest is translated and does not remain fused to the protein of interest.

SHTXTHHly, an extracellular secretion platform for the preparation of bioactive peptides and proteins in Escherichia coli

BACKGROUND

In previous work, we developed an E. coli extracellular secretion platform XTHHly based on the hemolysin A secretion system. It can produce bioactive peptides with simple purification procedures. However, the wider application of this platform is limited by poor secretion efficiency.

RESULTS

In this study, we first discovered a positive correlation between the isoelectric point (pI) value of the target protein and the secretion level of the XTHHly system. Given the extremely high secretion level of S tag, we fused it at the N-terminus and created a novel SHTXTHHly system. The SHTXTHHly system significantly increased the secretion levels of antimicrobial peptides (PEW300, LL37, and Aurein 1.2) with full bioactivities, suggesting its excellent capacity for secretory production of bioactive peptides. Furthermore, RGDS, IL-15, and alcohol dehydrogenase were successfully secreted, and their bioactivities were largely maintained in the fusion proteins, indicating the potential applications of the novel system for the rapid determination of protein bioactivities. Finally, using the SHTXTHHly system, we produced the monomeric Fc, which showed a high affinity for Fcγ Receptor I and mediated the antibody-dependent immunological effects of immune cells, demonstrating its potential applications in immunotherapies.

CONCLUSIONS

The SHTXTHHly system described here facilitates the secretory production of various types of proteins in E. coli. In comparison to previously reported expression systems, our work enlightens an efficient and cost-effective way to evaluate the bioactivities of target proteins or produce them.

High Level Expression and Purification of Cecropin-like Antimicrobial Peptides in Escherichia coli

Cecropins are a family of antimicrobial peptides (AMPs) that are widely found in the innate immune system of Cecropia moths. Cecropins exhibit a broad spectrum of antimicrobial and anticancer activities. The structures of Cecropins are composed of 34–39 amino acids with an N-terminal amphipathic α-helix, an AGP hinge and a hydrophobic C-terminal α-helix. KR12AGPWR6 was designed based on the Cecropin-like structural feature. In addition to its antimicrobial activities, KR12AGPWR6 also possesses enhanced salt resistance, antiendotoxin and anticancer properties. Herein, we have developed a strategy to produce recombinant KR12AGPWR6 through a salt-sensitive, pH and temperature dependent intein self-cleavage system. The His6-Intein-KR12AGPWR6 was expressed by E. coli and KR12AGPWR6 was released by the self-cleavage of intein under optimized ionic strength, pH and temperature conditions. The molecular weight and structural feature of the recombinant KR12AGPWR6 was determined by MALDI-TOF mass, CD, and NMR spectroscopy. The recombinant KR12AGPWR6 exhibited similar antimicrobial activities compared to the chemically synthesized KR12AGPWR6. Our results provide a potential strategy to obtain large quantities of AMPs and this method is feasible and easy to scale up for commercial production.

Intein-Mediated Protein Engineering for Cell-Based Biosensors

Cell-based sensors provide a flexible platform for screening biologically active targets and for monitoring their interactions in live cells. Their applicability extends across a vast array of biological research and clinical applications. Particularly, cell-based sensors are becoming a potent tool in drug discovery and cell-signaling studies by allowing function-based screening of targets in biologically relevant environments and enabling the in vivo visualization of cellular signals in real-time with an outstanding spatiotemporal resolution. In this review, we aim to provide a clear view of current cell-based sensor technologies, their limitations, and how the recent improvements were using intein-mediated protein engineering. We first discuss the characteristics of cell-based sensors and present several representative examples with a focus on their design strategies, which differentiate cell-based sensors from in vitro analytical biosensors. We then describe the application of intein-mediated protein engineering technology for cell-based sensor fabrication. Finally, we explain the characteristics of intein-mediated reactions and present examples of how the intein-mediated reactions are used to improve existing methods and develop new approaches in sensor cell fabrication to address the limitations of current technologies.

The Evolution of Intein-Based Affinity Methods as Reflected in 30 years of Patent History

Self-cleaving affinity tags, based on engineered intein protein domains, have been touted as a universal single step purification platform for tagless non-mAb proteins. These approaches provide all of the power and flexibility of tag-based affinity methods, but deliver a tagless target protein suitable for clinical applications without complex process development. This combination of features might accelerate and de-risk biopharmaceutical development by bridging early discovery to full-scale manufacturing under a single platform. Despite this profound promise, intein-based technologies have yet to reach their full potential. This review examines the evolution of intein-based purification methods in the light of several significant intein patents filed over the last 3 decades. Illustrated with actual key figures from each of the relevant patents, key advances are described with a focus on applications in basic research and biopharmaceutical production. Suggestions for extending intein-based purification systems to emerging therapies and non-protein applications are presented as concluding remarks.

Structural Basis for the Propagation of Homing Endonuclease-Associated Inteins

Inteins catalyze their removal from a host protein through protein splicing. Inteins that contain an additional site-specific endonuclease domain display genetic mobility via a process termed “homing” and thereby act as selfish DNA elements. We elucidated the crystal structures of two archaeal inteins associated with an active or inactive homing endonuclease domain. This analysis illustrated structural diversity in the accessory domains (ACDs) associated with the homing endonuclease domain. To augment homing endonucleases with highly specific DNA cleaving activity using the intein scaffold, we engineered the ACDs and characterized their homing site recognition. Protein engineering of the ACDs in the inteins illuminated a possible strategy for how inteins could avoid their extinction but spread via the acquisition of a diverse accessory domain.

Reactive Chlorine Species Reversibly Inhibit DnaB Protein Splicing in Mycobacteria

Intervening proteins, or inteins, are mobile genetic elements that are translated within host polypeptides and removed at the protein level by splicing. In protein splicing, a self-mediated reaction removes the intein, leaving a peptide bond in place. While protein splicing can proceed in the absence of external cofactors, several examples of conditional protein splicing (CPS) have emerged. In CPS, the rate and accuracy of splicing are highly dependent on environmental conditions. Because the activity of the intein-containing host protein is compromised prior to splicing and inteins are highly abundant in the microbial world, CPS represents an emerging form of posttranslational regulation that is potentially widespread in microbes. Reactive chlorine species (RCS) are highly potent oxidants encountered by bacteria in a variety of natural environments, including within cells of the mammalian innate immune system. Here, we demonstrate that two naturally occurring RCS, namely, hypochlorous acid (the active compound in bleach) and N-chlorotaurine, can reversibly block splicing of DnaB inteins from Mycobacterium leprae and Mycobacterium smegmatis in vitro. Further, using a reporter that monitors DnaB intein activity within M. smegmatis, we show that DnaB protein splicing is inhibited by RCS in the native host. DnaB, an essential replicative helicase, is the most common intein-housing protein in bacteria. These results add to the growing list of environmental conditions that are relevant to the survival of the intein-containing host and influence protein splicing, as well as suggesting a novel mycobacterial response to RCS. We propose a model in which DnaB splicing, and therefore replication, is paused when these mycobacteria encounter RCS. IMPORTANCE Inteins are both widespread and abundant in microbes, including within several bacterial and fungal pathogens. Inteins are domains translated within host proteins and removed at the protein level by splicing. Traditionally considered molecular parasites, some inteins have emerged in recent years as adaptive posttranslational regulatory elements. Several studies have demonstrated CPS, in which the rate and accuracy of protein splicing, and thus host protein functions, are responsive to environmental conditions relevant to the intein-containing organism. In this work, we demonstrate that two naturally occurring RCS, including the active compound in household bleach, reversibly inhibit protein splicing of Mycobacterium leprae and Mycobacterium smegmatis DnaB inteins. In addition to describing a new physiologically relevant condition that can temporarily inhibit protein splicing, this study suggests a novel stress response in Mycobacterium, a bacterial genus of tremendous importance to humans.

Intein Inhibitors as Novel Antimicrobials: Protein Splicing in Human Pathogens, Screening Methods, and Off-Target Considerations

Protein splicing is a post-translational process by which an intervening polypeptide, or intein, catalyzes its own removal from the flanking polypeptides, or exteins, concomitant with extein ligation. Although inteins are highly abundant in the microbial world, including within several human pathogens, they are absent in the genomes of metazoans. As protein splicing is required to permit function of essential proteins within pathogens, inteins represent attractive antimicrobial targets. Here we review key proteins interrupted by inteins in pathogenic mycobacteria and fungi, exciting discoveries that provide proof of concept that intein activity can be inhibited and that this inhibition has an effect on the host organism's fitness, and bioanalytical methods that have been used to screen for intein activity. We also consider potential off-target inhibition of hedgehog signaling, given the similarity in structure and function of inteins and hedgehog autoprocessing domains.

Novel human cell expression method reveals the role and prevalence of posttranslational modification in nonmuscle tropomyosins

Biochemical studies require large quantities of proteins, which are typically obtained using bacterial overexpression. However, the folding machinery in bacteria is inadequate for expressing many mammalian proteins, which additionally undergo posttranslational modifications (PTMs) that bacteria, yeast, or insect cells cannot perform. Many proteins also require native N- and C-termini and cannot tolerate extra tag amino acids for proper function. Tropomyosin (Tpm), a coiled coil protein that decorates most actin filaments in cells, requires both native N- and C-termini and PTMs, specifically N-terminal acetylation (Nt-acetylation), to polymerize along actin filaments. Here, we describe a new method that combines native protein expression in human cells with an intein-based purification tag that can be precisely removed after purification. Using this method, we expressed several nonmuscle Tpm isoforms (Tpm1.6, Tpm1.7, Tpm2.1, Tpm3.1, Tpm3.2, and Tpm4.2) and the muscle isoform Tpm1.1. Proteomics analysis revealed that human-cell-expressed Tpms present various PTMs, including Nt-acetylation, Ser/Thr phosphorylation, Tyr phosphorylation, and Lys acetylation. Depending on the Tpm isoform (humans express up to 40 Tpm isoforms), Nt-acetylation occurs on either the initiator methionine or on the second residue after removal of the initiator methionine. Human-cell-expressed Tpms bind F-actin differently than their Escherichia coli-expressed counterparts, with or without N-terminal extensions intended to mimic Nt-acetylation, and they can form heterodimers in cells and in vitro. The expression method described here reveals previously unknown features of nonmuscle Tpms and can be used in future structural and biochemical studies with Tpms and other proteins, as shown here for α-synuclein.

Production of IgG1-based bispecific antibody without extra cysteine residue via intein-mediated protein trans-splicing

A major class of bispecific antibodies (BsAbs) utilizes heterodimeric Fc to produce the native immunoglobulin G (IgG) structure. Because appropriate pairing of heavy and light chains is required, the design of BsAbs produced through recombination or reassembly of two separately-expressed antigen-binding fragments is advantageous. One such method uses intein-mediated protein trans-splicing (IMPTS) to produce an IgG1-based structure. An extra Cys residue is incorporated as a consensus sequence for IMPTS in successful examples, but this may lead to potential destabilization or disturbance of the assay system. In this study, we designed a BsAb linked by IMPTS, without the extra Cys residue. A BsAb binding to both TNFR2 and CD30 was successfully produced. Cleaved side product formation was inevitable, but it was minimized under the optimized conditions. The fine-tuned design is suitable for the production of IgG-like BsAb with high symmetry between the two antigen-binding fragments that is advantageous for screening BsAbs.

Expression, solubility monitoring, and purification of the co-folded LUBAC LTM domain by structure-guided tandem folding in autoinducing cultures

The linear ubiquitin chain assembly complex tethering motif (LUBAC-LTM) domain is composed of two different accessory LUBAC components (HOIL-1L and SHARPIN) but folds as a single globular domain. Targeted disruption of the intricate LTM-LTM interaction destabilizes LUBAC in lymphoma cells, thereby attenuating LUBAC stability, which highlights that targeting the interaction between the two LTM motifs is a promising strategy for the development of new agents against cancers that depend on LUBAC activity for their survival. To further screen for small-molecule inhibitors that can selectively disrupt the LTM-LTM interaction, it is necessary to obtain high-purity samples of the LTM domain. Ideally, such a sample would not contain any components other than the LTM itself, so that false positives (molecules binding to other parts of LUBAC) could be eliminated from the screening process. Here we report a simple strategy that enabled successful bacterial production of the isolated LUBAC LTM domain in high yield and at high purity. The strategy combines (1) structural analysis highlighting the possibility of tandem expression in the SHARPIN^L™ to HOIL-1L^L™ direction; (2) bacterial expression downstream of EGFP to efficiently monitor expression and solubility; (3) gentle low-temperature folding using autoinduction. Formation of stably folded LTM was verified by size-exclusion chromatography and heteronuclear NMR spectroscopy. From 200-ml cultures sufficient quantities (∼7 mg) of high-purity protein for structural studies could be obtained. The presented strategy will be beneficial for LUBAC LTM-based drug-screening efforts and likely serve as a useful primer for similar cases, i.e., whenever a smaller folded fragment is to be isolated from a larger protein complex for site-specific downstream applications.

Exploiting Protein N-Terminus for Site-Specific Bioconjugation

Although a plethora of chemistries have been developed to selectively decorate protein molecules, novel strategies continue to be reported with the final aim of improving selectivity and mildness of the reaction conditions, preserve protein integrity, and fulfill all the increasing requirements of the modern applications of protein conjugates. The targeting of the protein N-terminal alpha-amine group appears a convenient solution to the issue, emerging as a useful and unique reactive site universally present in each protein molecule. Herein, we provide an updated overview of the methodologies developed until today to afford the selective modification of proteins through the targeting of the N-terminal alpha-amine. Chemical and enzymatic strategies enabling the selective labeling of the protein N-terminal alpha-amine group are described.

Synthetic Biology-Empowered Hydrogels for Medical Diagnostics

Synthetic biology is strongly inspired by concepts of engineering science and aims at the design and generation of artificial biological systems in different fields of research such as diagnostics, analytics, biomedicine, or chemistry. To this aim, synthetic biology uses an engineering approach relying on a toolbox of molecular sensors and switches that endows cellular hosts with non-natural computing functions and circuits. Importantly, this concept is not only limited to cellular approaches. Synthetic biological building blocks have also conferred sensing and switching capability to otherwise inactive materials. This principle has attracted high interest for the development of biohybrid materials capable of sensing and responding to specific molecular stimuli, such as disease biomarkers, antibiotics, or heavy metals. Moreover, the interconnection of individual sense-and-respond materials to complex materials systems has enabled the processing of, for example, multiple inputs or the amplification of signals using feedback topologies. Such systems holding high potential for applications in the analytical and diagnostic sectors will be described in this chapter.

Strategies for Site-Specific Labeling of Receptor Proteins on the Surfaces of Living Cells by Using Genetically Encoded Peptide Tags

Fluorescence microscopy imaging enables receptor proteins to be investigated within their biological context. A key challenge is to site-specifically incorporate reporter moieties into proteins without interfering with biological functions or cellular networks. Small peptide tags offer the opportunity to combine inducible labeling with small tag sizes that avoid receptor perturbation. Herein, we review the current state of live-cell labeling of peptide-tagged cell-surface proteins. Considering their importance as targets in medicinal chemistry, we focus on membrane receptors such as G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). We discuss peptide tags that i) are subject to enzyme-mediated modification reactions, ii) guide the complementation of reporter proteins, iii) form coiled-coil complexes, and iv) interact with metal complexes. Given our own contributions in the field, we place emphasis on peptide-templated labeling chemistry.

Construction and Quantitation of a Selectable Protein Splicing Sensor Using Gibson Assembly and Spot Titers

Inteins (intervening proteins) are translated within host proteins and removed through protein splicing. Conditional protein splicing (CPS), where the rate and accuracy of splicing are highly dependent on environmental cues, has emerged as a novel form of post-translational regulation. While CPS has been demonstrated for several inteins in vitro, a comprehensive understanding of inteins requires tools to quantitatively monitor their activity within the cellular context. Here, we describe a method for construction of a splicing-dependent system that can be used to quantitatively assay for conditions that modulate protein splicing. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Construction of an intein-containing KanR2 library using Gibson assembly Basic Protocol 2: Phenotype determination using quantitative spot titers Support Protocol 1: Preparation of LB agar plates for spot titers Support Protocol 2: Preparation and transformation of competent M. smegmatis cells.

Production of recombinant venom peptides as tools for ion channel research

Animal venom is a rich source for peptide toxins that bind and modulate the function of ion channels. Owing to their ability to bind receptor sites on the channel protein with high affinity and specificity, peptide neurotoxins have become an indispensable tool for ion channel research. Recent breakthroughs in structural biology and advances in computer simulations of biomolecules have sparked a new interest in animal toxins as probes of channel protein structure and function. Here, we focus on methods used to produce animal toxins for research purposes using recombinant expression. The specific challenges associated with heterologous production of venom peptides are discussed, and several methods targeting these issues are presented with an emphasis on E. coli based systems. An efficient protocol for the bacterial expression, folding, and purification of recombinant venom peptides is described.

Intein mediated high throughput screening for bispecific antibodies

Bispecific antibodies comprise extremely diverse architectures enabling complex modes of action, such as effector cell recruitment or conditional target modulation via dual targeting, not conveyed by monospecific antibodies. In recent years, research on bispecific therapeutics has substantially grown. However, evaluation of binding moiety combinations often leads to undesired prolonged development times. While high throughput screening for small molecules and classical antibodies has evolved into a mature discipline in the pharmaceutical industry, dual-targeting antibody screening methodologies lack the ability to fully evaluate the tremendous number of possible combinations and cover only a limited portion of the combinatorial screening space. Here, we propose a novel combinatorial screening approach for bispecific IgG-like antibodies to extenuate screening limitations in industrial scale, expanding the limiting screening space. Harnessing the ability of a protein trans-splicing reaction by the split intein Npu DnaE, antibody fragments were reconstituted within the hinge region in vitro. This method allows for fully automated, rapid one-pot antibody reconstitution, providing biological activity in several biochemical and functional assays. The technology presented here is suitable for automated functional and combinatorial high throughput screening of bispecific antibodies.

Structural, Dynamic, and Functional Characterization of a DnaX Mini-intein Derived from Spirulina platensis Provides Important Insights into Intein-Mediated Catalysis of Protein Splicing

Protein splicing is a self-catalyzed post-translational modification in which the intein enzyme excises itself from a precursor protein and ligates the flanking sequences to produce a mature protein. We report the solution structure of a 136-residue DnaX mini-intein enzyme derived from the cyanobacterium Spirulina platensis. This sequence adopts a well-defined globular structure and forms a horseshoe-shaped fold commonly found in the HINT (hedgehog intein) topology. Backbone dynamics and hydrogen exchange experiments revealed conserved motions on various time scales, which is proposed to be a characteristic of the intein fold. Interestingly, several dynamic motions were found in symmetrically equivalent positions within the protein structure, which might be a consequence of the symmetrical intein fold. In cell splicing activity showed that Spl DnaX mini-intein is a highly active enzyme. The precursor protein was not detected at any timepoint of the assay. Apart from the splicing reaction, catalytic cleavage at the N- and C-termini of the precursor protein was also observed. To determine the roles of the catalytic residues in splicing and cleavage reactions, all combinations of alanine mutations of these residues were generated and functionally characterized. This in-depth analysis revealed cooperativity between these catalytic residues, which suppresses the N- and C-terminal cleavage reactions and enhances the yield of the spliced product. Overall, this study provides a thorough structural, dynamic, and functional characterization of a new intein sequence and adds to the collection of these unique enzymes that have found tremendous applications in biochemistry and biotechnology.

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.

Therapeutic applications of trans-splicing

BACKGROUND

RNA trans-splicing joins exons from different pre-mRNA transcripts to generate a chimeric product. Trans-splicing can also occur at the protein level, with split inteins mediating the ligation of separate gene products to generate a mature protein.

SOURCES OF DATA

Comprehensive literature search of published research papers and reviews using Pubmed.

AREAS OF AGREEMENT

Trans-splicing techniques have been used to target a wide range of diseases in both in vitro and in vivo models, resulting in RNA, protein and functional correction.

AREAS OF CONTROVERSY

Off-target effects can lead to therapeutically undesirable consequences. In vivo efficacy is typically low, and delivery issues remain a challenge.

GROWING POINTS

Trans-splicing provides a promising avenue for developing novel therapeutic approaches. However, much more research needs to be done before developing towards preclinical studies.

AREAS TIMELY FOR DEVELOPING RESEARCH

Increasing trans-splicing efficacy and specificity by rational design, screening and competitive inhibition of endogenous cis-splicing.

Intein-mediated intracellular production of active microbial transglutaminase in Corynebacterium glutamicum

The microbial transglutaminase (mTGase) from Streptomyces mobaraense is widely used in the food industry. However, recombinant production of mTGase is challenging because the mTGase is synthesized as an inactive zymogen, and needs to be activated by proteolytic processing. In this study, self-cleaving intein Ssp DnaB was applied to activate the mTGase in Corynebacterium glutamicum. Premature cleavage of intein Ssp DnaB also occurred, but instead of suppressing premature cleavage, this phenomenon was used to produce active mTGase in C. glutamicum. Both SDS-PAGE analysis and mTGase activity assays indicated that the premature cleavage of intein Ssp DnaB activated the mTGase intracellularly in C. glutamicum. The subsequent N-terminal amino acid sequencing and site-directed mutagenesis studies further showed that the premature cleavage activated the mTGase intracellularly, in a highly specific manner. Moreover, the growth performance of C. glutamicum was not noticeably affected by the intracellular expression of active mTGase. Finally, the mTGase was produced in a 2 L bioreactor, with activity up to 49 U/mL, the highest intracellular mTGase activity ever reported. Using premature cleavage of intein Ssp DnaB to activate mTGase in C. glutamicum, we produced high levels of intracellular active mTGase. Moreover, this approach did not require any further processing steps, such as protease treatment or lengthy incubation, greatly simplifying the production of active mTGase. This efficient and simple approach has great potential for the large-scale industrial production of active mTGase.

SELECT-GLYCOCIN: a recombinant microbial system for expression and high-throughput screening of glycocins

Glycosylated bacteriocins (glycocins) are potential clean label food preservatives and new alternatives to antibiotics. Further development requires the availability of a method for laboratory evolution of glycocins, wherein the challenges to overcome include ensuring glycosylation in a heterologous host, avoiding potential toxicity of active glycocins to the host, and provisioning of a one-pot screening assay for active mutants. Employing EntS, a sequential O/S- di-glycosyltransferase from Enterococcus faecalis TX0104, a proof of the concept microbial system and high throughput screening assay (SELECT-GLYCOCIN) is developed for generation of O/S- linked glycopeptide libraries and screening of glycocins for desired activity/property. The method enabled enzyme-dependent in vivo glycosylation in the heterologous host and rapid screening of mutants of enterocin 96 (Ent96)- a glycocin active against food-borne pathogen L. monocytogenes. Using SELECT-GLYCOCIN, a library of random (1.5 X 10^3) and rational (17) mutants of Ent96 was generated. The mutants were screened for bioactivity to identify a total of 376 random and 14 rational mutants as bioactive. Downstream detailed analysis of 16 random and 14 rational mutants led to the identification of sequence- and or glyco-variants namely, G16E-H24Q, C13T, and Ent96-K4_K5insYYGNGV (PedioEnt96) as improved antimicrobials. To summaries, SELECT-GLYCOCIN provides a system and a generic method for discovery and screening of glycocins that can further be adapted to any known/unknown glycocins and can be employed in food preservatives' and drug discovery programs.

Opportunities and challenges of the tag-assisted protein purification techniques: Applications in the pharmaceutical industry

Tag-assisted protein purification is a method of choice for both academic researches and large-scale industrial demands. Application of the purification tags in the protein production process can help to save time and cost, but the design and application of tagged fusion proteins are challenging. An appropriate tagging strategy must provide sufficient expression yield and high purity for the final protein products while preserving their native structure and function. Thanks to the recent advances in the bioinformatics and emergence of high-throughput techniques (e.g. SEREX), many new tags are introduced to the market. A variety of interfering and non-interfering tags have currently broadened their application scope beyond the traditional use as a simple purification tool. They can take part in many biochemical and analytical features and act as solubility and protein expression enhancers, probe tracker for online visualization, detectors of post-translational modifications, and carrier-driven tags. Given the variability and growing number of the purification tags, here we reviewed the protein- and peptide-structured purification tags used in the affinity, ion-exchange, reverse phase, and immobilized metal ion affinity chromatographies. We highlighted the demand for purification tags in the pharmaceutical industry and discussed the impact of self-cleavable tags, aggregating tags, and nanotechnology on both the column-based and column-free purification techniques.

Improved protein purification system based on C-terminal cleavage of Npu DnaE split intein

A purification system was constructed with the N-segment of the Npu DnaE split intein as an affinity ligand immobilized onto an epoxy-activated medium and the C-segment used as the cleavable tag fusing target protein. The affinity properties of C-tagged proteins adsorbed on I_N affinity chromatography medium were studied with GFP as a model target protein. The saturated adsorption capacity and dynamic adsorption capacity reached 51.9-21.0 mg mL^-1, respectively. With this system, two model proteins, GFP and alcohol dehydrogenase (ADH), has been successfully taglessly purified with regulation of Zn²⁺ and DTT. The yield, purification factor and purity of purified tagless GFP reached 39, 11.7 and 97%, respectively; while these values for purified tagless ADH were 38.2, 6.8 and 91%, respectively. These results showed that the system for Npu DnaE split intein-mediated affinity adsorption and in situ cleavage is a potential platform for recombinant protein production.

Deciphering protein post-translational modifications using chemical biology tools

Proteins carry out a wide variety of catalytic, regulatory, signalling and structural functions in living systems. Following their assembly on ribosomes and throughout their lifetimes, most eukaryotic proteins are modified by post-translational modifications; small functional groups and complex biomolecules are conjugated to amino acid side chains or termini, and the protein backbone is cleaved, spliced or cyclized, to name just a few examples. These modifications modulate protein activity, structure, location and interactions, and, thereby, control many core biological processes. Aberrant post-translational modifications are markers of cellular stress or malfunction and are implicated in several diseases. Therefore, gaining an understanding of which proteins are modified, at which sites and the resulting biological consequences is an important but complex challenge requiring interdisciplinary approaches. One of the key challenges is accessing precisely modified proteins to assign functional consequences to specific modifications. Chemical biologists have developed a versatile set of tools for accessing specifically modified proteins by applying robust chemistries to biological molecules and developing strategies for synthesizing and ligating proteins. This Review provides an overview of these tools, with selected recent examples of how they have been applied to decipher the roles of a variety of protein post-translational modifications. Relative advantages and disadvantages of each of the techniques are discussed, highlighting examples where they are used in combination and have the potential to address new frontiers in understanding complex biological processes.

Substrate specificities of inteins investigated by QuickDrop-cassette mutagenesis

Inteins catalyze self-excision from host precursor proteins while concomitantly ligating the flanking substrates (exteins) with a peptide bond. Noncatalytic extein residues near the splice junctions, such as the residues at the -1 and +2 positions, often strongly influence the protein-splicing efficiency. The substrate specificities of inteins have not been studied for many inteins. We developed a convenient mutagenesis platform termed "QuickDrop"-cassette mutagenesis for investigating the influences of 20 amino acid types at the -1 and +2 positions of different inteins. We elucidated 17 different profiles of the 20 amino acid dependencies across different inteins. The substrate specificities will accelerate our understanding of the structure-function relationship at the splicing junctions for broader applications of inteins in biotechnology and molecular biosciences.

Intein-mediated cytoplasmic reconstitution of a split toxin enables selective cell ablation in mixed populations and tumor xenografts

The application of proteinaceous toxins for cell ablation is limited by their high on- and off-target toxicity, severe side effects, and a narrow therapeutic window. The selectivity of targeting can be improved by intein-based toxin reconstitution from two dysfunctional fragments provided their cytoplasmic delivery via independent, selective pathways. While the reconstitution of proteins from genetically encoded elements has been explored, exploiting cell-surface receptors for boosting selectivity has not been attained. We designed a robust splitting algorithm and achieved reliable cytoplasmic reconstitution of functional diphtheria toxin from engineered intein-flanked fragments upon receptor-mediated delivery of one of them to the cells expressing the counterpart. Retargeting the delivery machinery toward different receptors overexpressed in cancer cells enables selective ablation of specific subpopulations in mixed cell cultures. In a mouse model, the transmembrane delivery of a split-toxin construct potently inhibits the growth of xenograft tumors expressing the split counterpart. Receptor-mediated delivery of engineered split proteins provides a platform for precise therapeutic and experimental ablation of tumors or desired cell populations while also greatly expanding the applicability of the intein-based protein transsplicing.

An off-the-Shelf Approach for the Production of Fc Fusion Proteins by Protein Trans-Splicing towards Generating a Lectibody In Vitro

Monoclonal antibodies, engineered antibodies, and antibody fragments have become important biological therapeutic platforms. The IgG format with bivalent binding sites has a modular structure with different biological roles, i.e., effector and binding functions, in different domains. We demonstrated the reconstruction of an IgG-like domain structure in vitro by protein ligation using protein trans-splicing. We produced various binding domains to replace the binding domain of IgG from Escherichia coli and the Fc domain of human IgG from Brevibacillus choshinensis as split-intein fusions. We showed that in vitro protein ligation could produce various Fc-fusions at the N-terminus in vitro from the independently produced domains from different organisms. We thus propose an off-the-shelf approach for the combinatorial production of Fc fusions in vitro with several distinct binding domains, particularly from naturally occurring binding domains. Antiviral lectins from algae are known to inhibit virus entry of HIV and SARS coronavirus. We demonstrated that a lectin could be fused with the Fc-domain in vitro by protein ligation, producing an IgG-like molecule as a “lectibody”. Such an Fc-fusion could be produced in vitro by this approach, which could be an attractive method for developing potential therapeutic agents against rapidly emerging infectious diseases like SARS coronavirus without any genetic fusion and expression optimization.

Split Intein-Mediated Protein Ligation for detecting protein-protein interactions and their inhibition

Here, to overcome many limitations accompanying current available methods to detect protein-protein interactions (PPIs), we develop a live cell method called Split Intein-Mediated Protein Ligation (SIMPL). In this approach, bait and prey proteins are respectively fused to an intein N-terminal fragment (IN) and C-terminal fragment (IC) derived from a re-engineered split intein GP41-1. The bait/prey binding reconstitutes the intein, which splices the bait and prey peptides into a single intact protein that can be detected by regular protein detection methods such as Western blot analysis and ELISA, serving as readouts of PPIs. The method is robust and can be applied not only in mammalian cell lines but in animal models such as C. elegans. SIMPL demonstrates high sensitivity and specificity, and enables exploration of PPIs in different cellular compartments and tracking of kinetic interactions. Additionally, we establish a SIMPL ELISA platform that enables high-throughput screening of PPIs and their inhibitors.

Protein-protein interactions are fundamental to the regulation of protein activity and cellular phyisology. Here the authors present Split Intein-Mediated Protein Ligation, which uses bait and prey proteins fused to intein fragments to generate single intact proteins upon interaction.

Expression of Highly Active Bacterial Phospholipase A2 in Yeast Using Intein-Mediated Delayed Protein Autoactivation

Phospholipase A₂ (PLA₂) has found extensive use in industry. However, recombinant PLA₂ production in different expression systems is a difficult task because of its toxicity to cell membranes. We report here the development of an effective method for production of highly active PLA₂ from Streptomyces violaceoruber strain A-2688 in the yeast Saccharomyces cerevisiae. The method is based on the use of the PRP8 mini-intein (from Penicillium chrysogenum) inserted into the phospholipase sequence with the purpose of temporal inactivation of the enzyme and its subsequent delayed autoactivation. We demonstrate that the most effective site for intein insertion is Ser76 of the mature phospholipase. As a result of intein-containing precursor secretion from yeast cells and its subsequent autocatalytic splicing, highly active enzyme accumulated in the yeast culture fluid. The properties of the obtained recombinant phospholipase A₂ protein were similar to those of the native Streptomyces violaceoruber PLA₂ protein. A possible evolutionary role of delayed autoactivation of intein-containing proteins is also discussed.

<p>Recent Advances of Magnetic Nanomaterials in the Field of Oncology</p>

Nanomagnetic devices, such as nano-field effect transistor biosensors and radio frequency magnetic induction therapies, came into being with the development of medical nanomaterials. The application of nanomagnetic materials in the treatment of cancers is rapidly becoming increasingly important because of its ability to target therapy and diagnose early. In this review, an untechnical overview of the fundamental of magnetism in nanomaterials and an illustration of how these materials are applied are presented. The applications of nano-field effect transistor biosensors for the detection of tumor biomarker nanomaterials in the therapy and diagnosis of cancers and nanomagnetic materials are summarized in this paper. A systemic summary of the use of nanomagnetic materials and nano-filed effect transistor biosensors for the treatment and diagnosis of tumors is also provided in the review.