Improved non-chromatographic purification of a recombinant protein by cationic elastin-like polypeptides

Genetic Functionalization of Protein-Based Biomaterials via Protein Fusions

Proteins implement many useful functions, including binding ligands with unparalleled affinity and specificity, catalyzing stereospecific chemical reactions, and directing cell behavior. Incorporating proteins into materials has the potential to imbue devices with these desirable traits. This review highlights recent advances in creating active materials by genetically fusing a self-assembling protein to a functional protein. These fusion proteins form materials while retaining the function of interest. Key advantages of this approach include elimination of a separate functionalization step during materials synthesis, uniform and dense coverage of the material by the functional protein, and stabilization of the functional protein. This review focuses on macroscale materials and discusses (i) multiple strategies for successful protein fusion design, (ii) successes and limitations of the protein fusion approach, (iii) engineering solutions to bypass any limitations, (iv) applications of protein fusion materials, including tissue engineering, drug delivery, enzyme immobilization, electronics, and biosensing, and (v) opportunities to further develop this useful technique.

A Superlong-Acting Growth Hormone-Polypeptide Fusion for Growth Hormone Deficiency Treatment

Recombinant human growth hormone (rhGH) is clinically used to treat growth hormone deficiency (GHD). However, daily administration of rhGH is required due to its poor stability and short blood circulation, which causes pains and burdens as well as inconvenience to patients. In this study, a method for genetically fusing rhGH to a thermosensitive polymer of elastin-like polypeptide (ELP) is reported, using which the rhGH-ELP thermosensitive fusion protein can be purified by the thermosensitivity of ELP instead of chromatography. The ELP fusion not only drastically improves the stability of rhGH, but also enables the in situ formation of a sustained-release depot of rhGH-ELP upon subcutaneous (SC) injection, which exhibits gentle release with a platform-to-trough fluctuation in blood and a very long circulatory half-life of 594.6 h. In contrast, rhGH exhibits a peak-to-trough fluctuation in blood with a very short circulatory half-life of 0.7 h. As a result, a single subcutaneous injection of rhGH-ELP can consecutively promote the linear growth of rats and the development of major tissues and organs over 3 weeks without obvious side effects, whereas rhGH is required to be injected daily to achieve similar therapeutic results.

Multiresponsive Nanoscale Self-Assembly of Azurin-Elastin-like Polypeptide Fusion Protein for Enhanced Prostate Cancer Therapy

A fusion protein composed of a bacterial protein, azurin, having antineoplastic properties and a thermally responsive structural cationic elastin-like protein (ELP), is designed, cloned, expressed, and purified. A simple method of inverse transition cycle (ITC) is employed to purify the fusion protein azurin-ELP diblock copolymer (d-bc). The molecular weight of the azurin-ELP fusion protein is ∼32 kDa. Further, its self-assembly properties are investigated. Interestingly, the engineered azurin-ELP d-bc in response to increasing temperature shows a dual-step phase separation into biofunctional nanostructures. Around the physiological temperature, azurin-ELP d-bc forms stable coacervates, which is dependent on the concentration and time of incubation. These coacervates are formed below the lower critical solubility temperature (LCST) of the ELP block at physiological temperature. Above LCST, i.e., 50-55°C, micelles of size ranging from 25 to 30 nm are formed. The cytotoxicity of azurin-ELP d-bc depends on the size of the coacervates formed and their cellular uptake at physiological temperature. Further, MTT assay of azurin-ELP d-bc in the cross-linked micelles prepared ex situ shows > six times higher killing of LNCaP cells than the unimeric form of azurin-ELP at 5 μM concentration. The flow cytometric results of these micelles at 20 μM concentration show ∼97% LNCaP cells in the apoptotic phase. Thus, azurin-ELP cross-linked micelles have enhanced potential for anticancer therapy due to their higher avidity.

Vascular Endothelial Growth Factor Receptor 1 Targeting Fusion Polypeptides with Stimuli-Responsiveness for Anti-angiogenesis

Genetically engineered fusion polypeptides have been investigated to introduce unique bio-functionality and improve some therapeutic activity for anti-angiogenesis. We report herein that stimuli-responsive, vascular endothelial growth factor receptor 1 (VEGFR1) targeting fusion polypeptides composed of a VEGFR1 (fms-like tyrosine kinase-1 (Flt1)) antagonist, an anti-Flt1 peptide, and a thermally responsive elastin-based polypeptide (EBP) were rationally designed at the genetic level, biosynthesized, and purified by inverse transition cycling to develop potential anti-angiogenic fusion polypeptides to treat neovascular diseases. A series of hydrophilic EBPs with different block lengths were fused with an anti-Flt1 peptide, forming anti-Flt1-EBPs, and the effect of EBP block length on their physicochemical properties was examined. While the anti-Flt1 peptide decreased phase-transition temperatures of anti-Flt1-EBPs, compared with EBP blocks, anti-Flt1-EBPs were soluble under physiological conditions. The anti-Flt1-EBPs dose dependently inhibited the binding of VEGFR1 against vascular endothelial growth factor (VEGF) as well as tube-like network formation of human umbilical vein endothelial cells under VEGF-triggered angiogenesis in vitro because of the specific binding between anti-Flt1-EBPs and VEGFR1. Furthermore, the anti-Flt1-EBPs suppressed laser-induced choroidal neovascularization in a wet age-related macular degeneration mouse model in vivo. Our results indicate that anti-Flt1-EBPs as VEGFR1-targeting fusion polypeptides have great potential for efficacious anti-angiogenesis to treat retinal-, corneal-, and choroidal neovascularization.

Cryopreservation of Cell Sheets for Regenerative Therapy: Application of Vitrified Hydrogel Membranes

Organ transplantation is the first and most effective treatment for missing or damaged tissues or organs. However, there is a need to establish an alternative treatment method for organ transplantation due to the shortage of donors and viral infections. Rheinwald and Green et al. established epidermal cell culture technology and successfully transplanted human-cultured skin into severely diseased patients. Eventually, artificial cell sheets of cultured skin were created, targeting various tissues and organs, including epithelial sheets, chondrocyte sheets, and myoblast cell sheets. These sheets have been successfully used for clinical applications. Extracellular matrix hydrogels (collagen, elastin, fibronectin, and laminin), thermoresponsive polymers, and vitrified hydrogel membranes have been used as scaffold materials to prepare cell sheets. Collagen is a major structural component of basement membranes and tissue scaffold proteins. Collagen hydrogel membranes (collagen vitrigel), created from collagen hydrogels through a vitrification process, are composed of high-density collagen fibers and are expected to be used as carriers for transplantation. In this review, the essential technologies for cell sheet implantation are described, including cell sheets, vitrified hydrogel membranes, and their cryopreservation applications in regenerative medicine.

Tagging Recombinant Proteins to Enhance Solubility and Aid Purification

Protein fusion technology has had a major impact on the efficient production and purification of individual recombinant proteins. The use of genetically engineered affinity and solubility-enhancing polypeptide "tags" has a long history, and there is a considerable repertoire of these that can be used to address issues related to the expression, stability, solubility, folding, and purification of their fusion partner. In the case of large-scale proteomic studies, the development of purification procedures tailored to individual proteins is not practicable, and affinity tags have become indispensable tools for structural and functional proteomic initiatives that involve the expression of many proteins in parallel. In this chapter, the rationale and applications of a range of established and more recently developed solubility-enhancing and affinity tags is described.

Purification of a Hydrophobic Elastin-Like Protein Toward Scale-Suitable Production of Biomaterials

Elastin-like proteins (ELPs) are polypeptides with potential applications as renewable bio-based high-performance polymers, which undergo a stimulus-responsive reversible phase transition. The ELP investigated in this manuscript—ELP[V2Y-45]—promises fascinating mechanical properties in biomaterial applications. Purification process scalability and purification performance are important factors for the evaluation of potential industrial-scale production of ELPs. Salt-induced precipitation, inverse transition cycling (ITC), and immobilized metal ion affinity chromatography (IMAC) were assessed as purification protocols for a polyhistidine-tagged hydrophobic ELP showing low-temperature transition behavior. IMAC achieved a purity of 86% and the lowest nucleic acid contamination of all processes. Metal ion leakage did not propagate chemical modifications and could be successfully removed through size-exclusion chromatography. The simplest approach using a high-salt precipitation resulted in a 60% higher target molecule yield compared to both other approaches, with the drawback of a lower purity of 60% and higher nucleic acid contamination. An additional ITC purification led to the highest purity of 88% and high nucleic acid removal. However, expensive temperature-dependent centrifugation steps are required and aggregation effects even at low temperatures have to be considered for the investigated ELP. Therefore, ITC and IMAC are promising downstream processes for biomedical applications with scale-dependent economical costs to be considered, while salt-induced precipitation may be a fast and simple alternative for large-scale bio-based polymer production.

Injectable Hydrogels of Stimuli-Responsive Elastin and Calmodulin-Based Triblock Copolypeptides for Controlled Drug Release

A variety of block copolypeptides with stimuli responsiveness have been of growing interest for dynamic self-assembly. Here, multistimuli-responsive triblock copolypeptides composed of thermosensitive elastin-based polypeptides (EBP) and ligand-responsive calmodulin (CalM) were genetically engineered, over-expressed, and nonchromatographically purified by inverse transition cycling. Diluted EBP-CalM-EBP (ECE) triblock copolypeptides under physiological conditions self-assembled into vesicles at the nanoscale by temperature-triggered aggregation of the EBP block with lower critical solution temperature behaviors. Furthermore, concentrated ECE triblock copolypeptides under identical conditions exhibited thermally induced gelation, resulting in physically crosslinked hydrogels. They showed controlled rheological and mechanical properties depending on the conformational change of the CalM middle block induced by binding either Ca²⁺ or Ca²⁺ and trifluoperazines (TFPs) as ligands. In addition, both Ca²⁺-free and Ca²⁺-bound ECE triblock copolypeptide hydrogels exhibited biocompatibility, while those bound to both Ca²⁺ and TFPs showed severe cytotoxicity because of controlled TFP release of the CalM blocks. The ECE triblock hydrogels with stimuli responsiveness would be useful as injectable drug delivery depots for biomedical applications.

Switchable Self-Assembly of Elastin- and Resilin-Based Block Copolypeptides with Converse Phase Transition Behaviors

Self-assembly of thermally responsive polypeptides into unique nanostructures offers intriguing attributes including dynamic physical dimensions, biocompatibility, and biodegradability for the smart bio-nanomaterials. As elastin-based polypeptide (EBP) fusion proteins with lower critical solution temperature (LCST) are studied as drug delivery systems, EBP block copolypeptides with the resilin-based polypeptide (RBP) displaying an upper critical solution temperature (UCST) have been of great interest. In this study, we report thermally triggered, dynamic self-assembly of EBP- and RBP-based diblock copolypeptides into switched nanostructures with reversibility under physiological conditions. Molecular DNA clones encoding for the EBP-RBP diblocks at different block length ratios were biosynthesized via recursive directional ligation and overexpressed, followed by nonchromatographic purification by inverse transition cycling. Genetically engineered diblock copolypeptides composed of the EBP with an LCST and the RBP with a UCST showed converse phase transition behaviors with both a distinct LCST and a distinct UCST (LCST < UCST). As temperature increased, three phases of these EBP-RBP diblocks were observed: (1) self-assembled micelles or vesicles below both LCST and UCST, (2) whole aggregates above LCST and below UCST, and (3) reversed micelles above both LCST and UCST. In conclusion, these stimuli-triggered, dynamic protein-based nanostructures are promising for advanced drug delivery systems, regenerative medicine, and biomedical nanotechnology.

A novel all-in-one strategy for purification and immobilization of β-1,3-xylanase directly from cell lysate as active and recyclable nanobiocatalyst

BACKGROUND

Exploring a simple and versatile technique for direct immobilization of target enzymes from cell lysate without prior purification is urgently needed. Thus, a novel all-in-one strategy for purification and immobilization of β-1,3-xylanase was proposed, the target enzymes were covalently immobilized on silica nanoparticles via elastin-like polypeptides (ELPs)-based biomimetic silicification and SpyTag/SpyCatcher spontaneous reaction. Thus, the functional carriers that did not require the time-consuming surface modification step were quickly and efficiently prepared. These carriers could specifically immobilize the SpyTag-fused target enzymes from the cell lysate without pre-purification.

RESULTS

The ELPs-SpyCatcher hardly leaked from the carriers (0.5%), and the immobilization yield of enzyme was up to 96%. Immobilized enzyme retained 85.6% of the initial activity and showed 88.6% of the activity recovery. Compared with free ones, the immobilized β-1,3-xylanase showed improved thermal stability, elevated storage stability and good pH tolerance. It also retained more than 70.6% of initial activity after 12 reaction cycles, demonstrating its excellent reusability.

CONCLUSIONS

The results clearly highlighted the effectiveness of the novel enzyme immobilization method proposed here due to the improvement of overall performance of immobilized enzyme in respect to free form for the hydrolysis of macromolecular substrates. Thus, it may have great potential in the conversion of algae biomass as well as other related fields.

Human Granulocyte-Macrophage Colony-Stimulating Factor Fused to Elastin-Like Polypeptides Assembles Biologically-Active Nanoparticles

Human granulocyte-macrophage colony-stimulating factor (hGMCSF) is crucial in the immune system as it stimulates survival, proliferation, differentiation, and functional activation of myeloid hematopoietic cells. hGMCSF is integral to approved therapies, including monoclonal antibodies against checkpoint inhibitors, chimeric antigen receptors, and prevention of chemotherapy-induced neutropenia. Recombinant hGMCSF can be purified from Escherichia. coli; however, it forms inclusion bodies that require solubilization and refolding. Alternatively, this manuscript describes its fusion with an elastin-like polypeptide (ELP). Previously reported as purification tags and solubility enhancers, ELPs are recombinant polypeptides that undergo reversible temperature-dependent phase separation. This report is the first to show that fusion to an ELP enables direct purification of hGMCSF fusions from the soluble fraction of bacterial lysate. Surprisingly, these ELP-fusions assemble stable, small, spherical nanoparticles that maintain pro-mitotic activity of hGMCSF. These nanoparticles exhibit ELP-mediated phase separation; however, nanoparticle assembly significantly increases the entropic and enthalpic cost of phase separation compared to ELP alone. The attachment of a high molecular weight ELP to a difficult-to-express protein, like hGMCSF, appears to be a useful strategy to stabilize bioactive, protein-based nanoparticles, which may have broad applications in medicine and biology.

Enzymatic degradation of algal 1,3-xylan: from synergism of lytic polysaccharide monooxygenases with β-1,3-xylanases to their intelligent immobilization on biomimetic silica nanoparticles

Lytic polysaccharide monooxygenases (LPMOs) with synergistic effect on polysaccharide hydrolase represent a revolution in biotechnology, which may accelerate the conversion of biomass to the second-generation biofuels. Discovering more hydrolases that have synergism with LPMOs will considerably expand the knowledge and application of biomass degradation. The LPMOs named CgAA9 were verified to exhibit 1.52-fold synergism when incubated with β-1,3-xylanase at a molar ratio of 3:1. The ion chromatography results proved that CgAA9 did not alter the endogenous hydrolysis mode of β-1,3-xylanase. Meanwhile, to decrease the operational cost of enzymes, a novel strategy for immobilizing LPMOs and β-1,3-xylanases based on the biomimetic silica nanoparticles was developed. It enabled preparation of immobilized enzymes directly from the cell lysate. The immobilization efficiency and activity recovery reached 84.6 and 81.4%. They showed excellent reusability for 12 cycles by retaining 68% of initial activity. The optimum temperature for both free and immobilized biocatalyst were 40 and 37 °C, indicating they were ideal candidates for typical simultaneous saccharification and fermentation (SSF) in ethanol production from algea biomass. This was the first report on the synergy between LPMOs and β-1,3-xylanase, and the strategy for enzyme self-immobilization was simple, timesaving, and efficient, which might have great potentials in algae biomass hydrolysis. KEY POINTS: • The lytic polysaccharide monooxygenases (LPMOs) from Chaetomium globosum were firstly verified to boost the hydrolysis of β-1,3-xylanases for β-1,3-xylan. • A novel strategy for simple preparation of SpyCather-modifed silica nanopartilcles and intelligent immobilization of target enzymes from the cell lysate was proposed. • The immobilized LPMOs and β-1,3-xylanases could be reasonable alternatives for typical simultaneous saccharification and fermentation (SSF) in manipulation of algae biomass.

The Effects of Self-cleavage Intein-ELK16 Tag in the Transcript Steric Hindrance of IFN

Intervening proteins (Inteins) are identified as protein domains in a precursor protein structure. Inteins can excise itself from precursor protein and join the remaining portions which result in forming an active protein. In this study, the transcript expression level of recombinant human Interferon beta (rhIFNβ) connected to the self-cleavage Intein-ELK16 (LELELKLKLELELKLK) tag was measured by real-time PCR in HEK293T cell line. First, the sequence of Mycobacterium tuberculosis RecA (Mtu recA) was obtained from the InBase database to do appropriate changes including adding the restriction sites, kozak sequence, signal peptide and ELK16 sequence by SnapGene software. The RNA secondary structure were also examined using the online RNA Fold 2.2 web server. Next, the construct was inserted into pUC19 plasmid. The sequence of rhIFNβ was also cloned into pBudCE4.1 vector. In the next step, the rhIFNβ was ligated into the construct (self-cleavage tag of ELK16) using T4 DNA ligase and the recombinant construct was transfected into HEK293T cell line. Finally, expression of the cassette was evaluated by real-time PCR. The analysis of secondary RNA structure indicates a minimum free energy of MEF - 261.10 kcal/mol. Our results indicate that IFNβ was upregulated (37.8-fold, p < 0.0001) in cells which transfected by rhIFNβ-ELK16 compared to the mock and un-transfected conditions. Altogether, our results show that the presence of mini self-cleavage Intein-ELK16 tag along with the rhIFNβ had no interference in transcription of rhIFNβ in the HEK293T cell line.

Unique Phase Transition of Exogenous Fusion Elastin-like Polypeptides in the Solution Containing Polyethylene Glycol

Elastin-Like polypeptides (ELPs), as well-known temperature-controlled bio-macromolecules, are widely used. However, little is known about the interactions between ELPs and macromolecules, which is an important yet neglected problem. Here, the phase transition characteristics of an ELPs-SpyCatcher fusion protein (E-C) in the presence of polyethylene glycol (PEG) in single salts (Na2CO3, Na2SO4, NaCl) solutions were investigated using a UV spectrophotometer, DLC, and fluorescence spectroscopy, and we got some interesting results. The phases transition of E-C occurred at a concentration lower than 0.5 mol/L Na2CO3/PEG2000, while in single Na2CO3 (<0.5 mol/L), the phase transition of E-C did not occur. In the Na2CO3/PEG solution, we observed a unique two-step phase transition of E-C when the Na2CO3 concentration was 0.5 mol/L and PEG2000 concentration was less than 0.15 g/mL, respectively. In the Na2CO3/PEG2000 solution, the phase-transition temperature of E-C decreased with the increase of PEG concentration, but increased in the Na2SO4/PEG2000 solution, while it remained unchanged in the NaCl/PEG2000 solution. However, the phase-transition temperature of the linear ELPs40 decreased under the same salts/PEG2000 solutions. We also addressed the possible molecular mechanism of the interesting results. In contrast to the current well-understood salts-ELPs interactions, this work provides some new insights into the interaction between the PEG-salts-ELPs in solution.

Programmable stimuli-responsive polypeptides for biomimetic synthesis of silica nanocomposites and enzyme self-immobilization

Bioinspired silicification is an attractive route for achieving unique silica nanocomposites. Herein, a novel, facile and inexpensive route for biosilica synthesis is developed using the stimuli-responsive elastin-like polypeptide (ELP). The ELP is precisely tailored to a silica-mineralizing peptide by programming it with lysine residues. The resulting cationic ELP[KV₈F-40] is purified in ultrahigh yield using a chromatography-free ITC purification technique based on thermal-responsive property. Excitingly, the specific activity of ELP is 40-fold higher than that of silaffin. Besides, efficient and strong entrapment of ELP is achieved with over 98% of immobilization yield and less than 2% of leakage. These imply that cationic ELP may be used as a bifunctional tag (purification and immobilization) for fusion protein. An enzyme (xylanase) is therefore chosen to genetically fuse to ELP. The ELP-fused xylanase is purified by ELP with high purity (~98%) and enables the rapid (within minutes) self-immobilization. The immobilization yield was greater than 95%, and the immobilized xylanases hardly leaked from the silica matrix, demonstrating high efficiency of the self-immobilization process. The strategy developed here may provide a new opportunity for fabricating functional silica nanocomposites in a feasible and inexpensive pathway, which will have great potentials in the field of biotechnology.

Rapid and simple purification of elastin-like polypeptides directly from whole cells and cell lysates by organic solvent extraction

Elastin-like polypeptides (ELP) are a well-known class of proteins that are being increasingly utilized in a variety of biomedical applications, due to their beneficial physicochemical properties. A unifying feature of ELP is their demonstration of a sequence tunable inverse transition temperature (Tt) that enables purification using a simple, straightforward process called inverse transition cycling (ITC). Despite the utility of ITC, the process is inherently limited to ELP with an experimentally accessible Tt. Since the underlying basis for the ELP Tt is related to its high overall hydrophobicity, we anticipated that ELP would be excellent candidates for purification by organic extraction. We report the first method for rapidly purifying ELP directly from whole E. coli cells or clarified lysates using pure organic solvents and solvent mixtures, followed by aqueous back extraction. Our results show that small ELP and a large ELP-fusion protein can be isolated in high yield from whole cells or cell lysates with greater than 95% purity in less than 30 min and with very low levels of LPS and DNA contamination.

Engineering Globular Protein Vesicles through Tunable Self-Assembly of Recombinant Fusion Proteins

Vesicles assembled from folded, globular proteins have potential for functions different from traditional lipid or polymeric vesicles. However, they also present challenges in understanding the assembly process and controlling vesicle properties. From detailed investigation of the assembly behavior of recombinant fusion proteins, this work reports a simple strategy to engineer protein vesicles containing functional, globular domains. This is achieved through tunable self-assembly of recombinant globular fusion proteins containing leucine zippers and elastin-like polypeptides. The fusion proteins form complexes in solution via high affinity binding of the zippers, and transition through dynamic coacervates to stable hollow vesicles upon warming. The thermal driving force, which can be tuned by protein concentration or temperature, controls both vesicle size and whether vesicles are single or bi-layered. These results provide critical information to engineer globular protein vesicles via self-assembly with desired size and membrane structure.

Oligomerization triggered by foldon: a simple method to enhance the catalytic efficiency of lichenase and xylanase

BACKGROUND

Effective and simple methods that lead to higher enzymatic efficiencies are highly sough. Here we proposed a foldon-triggered trimerization of the target enzymes with significantly improved catalytic performances by fusing a foldon domain at the C-terminus of the enzymes via elastin-like polypeptides (ELPs). The foldon domain comprises 27 residues and can forms trimers with high stability.

RESULTS

Lichenase and xylanase can hydrolyze lichenan and xylan to produce value added products and biofuels, and they have great potentials as biotechnological tools in various industrial applications. We took them as the examples and compared the kinetic parameters of the engineered trimeric enzymes to those of the monomeric and wild type ones. When compared with the monomeric ones, the catalytic efficiency (k _cat /K _m ) of the trimeric lichenase and xylanase increased 4.2- and 3.9- fold. The catalytic constant (k _cat ) of the trimeric lichenase and xylanase increased 1.8- fold and 5.0- fold than their corresponding wild-type counterparts. Also, the specific activities of trimeric lichenase and xylanase increased by 149% and 94% than those of the monomeric ones. Besides, the recovery of the lichenase and xylanase activities increased by 12.4% and 6.1% during the purification process using ELPs as the non-chromatographic tag. The possible reason is the foldon domain can reduce the transition temperature of the ELPs.

CONCLUSION

The trimeric lichenase and xylanase induced by foldon have advantages in the catalytic performances. Besides, they were easier to purify with increased purification fold and decreased the loss of activities compared to their corresponding monomeric ones. Trimerizing of the target enzymes triggered by the foldon domain could improve their activities and facilitate the purification, which represents a simple and effective enzyme-engineering tool. It should have exciting potentials both in industrial and laboratory scales.

Selective protein complexation and coacervation by polyelectrolytes

This review discusses the possible relationship between protein charge anisotropy, protein binding affinity, polymer structure, and selective phase separation. We hope that a fundamental understanding of primarily electrostatically driven protein-polyelectrolyte (PE) interactions can enable the prediction of selective protein binding, and hence selective coacervation through non-specific electrostatics. Such research will partially challenge the assumption that specific binding has to be realized through specific binding sites with a variety of short-range interactions and some geometric match. More specifically, the recent studies on selective binding of proteins by polyelectrolytes were examined from different assemblies in addition to the electrostatic features of proteins and PEs. At the end, the optimization of phase separation based on binding affinity for selective coacervation and some considerations relevant to using PEs for protein purification were also overviewed.

Tagging Recombinant Proteins to Enhance Solubility and Aid Purification

Protein fusion technology has had a major impact on the efficient production and purification of individual recombinant proteins. The use of genetically engineered affinity and solubility-enhancing polypeptide "tags" has increased greatly in recent years and there now exists a considerable repertoire of these that can be used to solve issues related to the expression, stability, solubility, folding, and purification of their fusion partner. In the case of large-scale proteomic studies, the development of purification procedures tailored to individual proteins is not practicable, and affinity tags have therefore become indispensable tools for structural and functional proteomic initiatives that involve the expression of many proteins in parallel. Here, the rationale and applications of a range of established and more recently developed solubility-enhancing and affinity tags is described.

Phenylboronic acid-incorporated elastin-like polypeptide nanoparticle drug delivery systems

Packaging hydrophobic drugs into nanoparticles can improve their aqueous solubility, tumor-specific accumulation and therapeutic effect.

Antibacterial activity and modes of action of phosvitin-derived peptide Pt5e against clinical multi-drug resistance bacteria

Pt5e, a mutant peptide derived from the C-terminal 55 residues of zebrafish phosvitin, has been suggested to be a novel antibacterial peptide. However, if it is applicable to clinical MDR bacteria remains to be tested. In this study, high-purity Pt5e was first expressed and purified by fusion with cationic elastin-like polypeptide. Pt5e was then shown to be capable of effectively killing all the five clinical MDR bacteria tested. Pt5e kill the MDR bacteria at several levels, including inserting into the bacterial membranes, causing the membrane depolarization and permeabilization, and inducing the intracellular apoptosis/necrosis. All these data suggest that Pt5e is a promising therapeutic potential as an antibiotics against clinical MDR bacteria.

Production of a polar fish antimicrobial peptide in Escherichia coli using an ELP-intein tag

An important aspect related to infectious pathogens is their exceptional adaptability in developing resistance, which leads to a perpetual challenge in the discovery of antimicrobial drugs with novel mechanisms of action. Among them, antimicrobial peptides (AMPs) stand out as promising anti-infective molecules. In order to overcome the high costs associated with isolation from natural sources or chemical synthesis of AMPs we propose the expression of Pa-MAP 2, a polyalanine AMP. Pa-MAP 2 was fused to an ELP-intein tag where the ELP (Elastin-like polypeptide) was used to promote aggregation and fast and cost-effective isolation after expression, and the intein was used to stimulate a controlled AMP release. For these, the vector pET21a was used to produce Pa-MAP 2 fused to the N-termini region of a modified Mxe GyrA intein followed by 60 repetitions of ELP. Purified Pa-MAP 2 showed a MIC of 25μM against E. coli ATCC 8739. Batch fermentation demonstrated that Pa-MAP-2 can be produced in both rich and defined media at yields 50-fold higher than reported for other AMPs produced by the ELP-intein system, and in comparable yields to expression systems with protease or chemical cleavage.

Chemical synthesis and characterization of elastin-like polypeptides (ELPs) with variable guest residues

The properties of elastin-like polypeptides (ELPs), specifically the fact that they are soluble in aqueous buffers below and aggregate reversibly above a well-defined transition temperature, are extensively used for protein purification, enzyme recycling, and more recently, for in vivo applications such as drug delivery and tissue engineering. ELPs are artificial but biocompatible polypeptides composed of pentameric repeats (Val-Pro-Gly-Xaa-Gly) containing different guest residues Xaa, derived from mammalian elastin. The temperature-dependent aggregation and desaggregation of ELPs is controlled by composition of the pentameric repeats as well as the number of repetitive units within the ELP. External parameters such as ELP concentration, pH, and most importantly, salt effects heavily influence the transition temperature. Here, we explore the chemical synthesis of a series of 51mer peptides consisting of 10 pentameric ELP repeats with hydrophobic as well as charged guest residues such as isoleucine, leucine, alanine, lysine, and/or glutamate all prepared by Boc-based solid phase peptide synthesis. These guest residues expand the available toolbox of synthetic ELPs and provide ELPs that can be chemically modified and tuned to specific environments. An N-terminal cysteine is added allowing disulfide-based crosslinking of ELPs and to link synthetic ELPs to a recombinantly produced protein using native chemical ligation. Transition temperatures of all synthetic ELPs and the fusion construct were determined by measuring turbidity in solution and spanned a large temperature range between 25 and 70 °C, providing synthetically accessible ELPs with transition temperatures suitable for in vitro and in vivo applications. Cycling between their soluble and aggregate state has been observed at least 6 times without significant loss of material for all synthetic ELPs. Copyright © 2016 European Peptide Society and John Wiley & Sons, Ltd.

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.

HaloTag mediated artificial cellulosome assembly on a rolling circle amplification DNA template for efficient cellulose hydrolysis

We report here the generation of artificial cellulosomes onto a DNA scaffold using the self-labeling HaloTag for DNA conjugation. Rolling circle amplification multiplexing templates were used to increase the complexity of this system with higher efficiency observed.

Soluble expression and purification of the recombinant bioactive peptide precursor BPP-1 in Escherichia coli using a cELP-SUMO dual fusion system

A bioactive peptide precursor (BPP-1, 14.3 kDa/115AA), a newly designed polypeptide that may exert a potential antihypertensive effect in vivo, is composed of many different ACE inhibitory peptides and antioxidant peptides tandemly linked according to the restriction sites of gastrointestinal proteases. In this report, we present a novel method to obtain soluble BPP-1 in Escherichia coli using cationic elastin-like polypeptide and SUMO (cELP-SUMO) tags. The cELP-SUMO-tagged fusion protein was expressed in soluble form at 20 °C for 20 h. After purification based on the inverse transition cycling (ITC) method, the purified cELP-SUMO-CFPP fusion protein was subsequently cleaved by a SUMO protease to release the mature BPP-1. After a subsequent simple salt precipitation process, approximately 167.2 mg of recombinant BPP-1 was obtained from 1 l of bacterial culture with at least 92% purity. The molecular mass (Mr) of the recombinant BPP-1 was confirmed by MALDI-TOF MS to equal 14,347. The purified BPP-1 was subjected to simulated gastrointestinal digestion, and the resulting hydrolysates exhibited notable ACE inhibitory and antioxidant activities in vitro. This report provides the first description of the soluble production of a bioactive peptide multimer with potential ACE inhibitory and antioxidant activities in E. coli using a cELP-SUMO tag.

Post-Translational Modification of Bionanoparticles as a Modular Platform for Biosensor Assembly

Context driven biosensor assembly with modular targeting and detection moieties is gaining significant attentions. Although protein-based nanoparticles have emerged as an excellent platform for biosensor assembly, current strategies of decorating bionanoparticles with targeting and detection moieties often suffer from unfavorable spacing and orientation as well as bionanoparticle aggregation. Herein, we report a highly modular post-translational modification approach for biosensor assembly based on sortase A-mediated ligation. This approach enables the simultaneous modifications of the Bacillus stearothermophilus E2 nanoparticles with different functional moieties for antibody, enzyme, DNA aptamer, and dye decoration. The resulting easy-purification platform offers a high degree of targeting and detection modularity with signal amplification. This flexibility is demonstrated for the detection of both immobilized antigens and cancer cells.

Sortase A-mediated multi-functionalization of protein nanoparticles

A new strategy was developed to create multi-functionalizaton of protein nanoparticles using Sortase A-mediated ligation, resulting in modified protein nanoparticles that are both thermally responsive and catalytic active.

Applications of elastin-like polypeptides in drug delivery

Elastin-like polypeptides (ELPs) are biopolymers inspired by human elastin. Their lower critical solution temperature phase transition behavior and biocompatibility make them useful materials for stimulus-responsive applications in biological environments. Due to their genetically encoded design and recombinant synthesis, the sequence and size of ELPs can be exactly defined. These design parameters control the structure and function of the ELP with a precision that is unmatched by synthetic polymers. Due to these attributes, ELPs have been used extensively for drug delivery in a variety of different embodiments-as soluble macromolecular carriers, self-assembled nanoparticles, cross-linked microparticles, or thermally coacervated depots. These ELP systems have been used to deliver biologic therapeutics, radionuclides, and small molecule drugs to a variety of anatomical sites for the treatment of diseases including cancer, type 2 diabetes, osteoarthritis, and neuroinflammation.

The fusions of elastin-like polypeptides and xylanase self-assembled into insoluble active xylanase particles

We fused the genes of elastin-like polypeptides (ELPs) and xylanase and then expressed them in Escherichia coli. Unexpectedly, the fusion proteins self-assembled into insoluble active particles as the ELPs underwent a hardly reversible phase transition. The specific activity of the particles was 92% of the native counterparts, which means it can act as a pull-down handler for converting soluble proteins into active aggregates. We evaluated the characterizations of the insoluble active xylanase particles in detail and the results were encouraging. The pH optimum (6.0) of the particles was the same as the free one, but the optimum pH range was 5-7, while the free xylanase was 6-7. The free xylanase had an optimum temperature of 50°C, whereas the insoluble active xylanase particles shifted to 70°C. The pH stability, thermostability and storage stability of the xylanase particles increased significantly when compared with the free xylanase. We also observed an increase of the Km values of the free xylanase from 0.374gL(-1) to 0.980gL(-1) at the insoluble state. The considerable higher activity and stability of the xylanase particles were much like immobilized xylanases and could be valuable for its industrial application.

Challenges and opportunities in the purification of recombinant tagged proteins

The purification of recombinant proteins by affinity chromatography is one of the most efficient strategies due to the high recovery yields and purity achieved. However, this is dependent on the availability of specific affinity adsorbents for each particular target protein. The diversity of proteins to be purified augments the complexity and number of specific affinity adsorbents needed, and therefore generic platforms for the purification of recombinant proteins are appealing strategies. This justifies why genetically encoded affinity tags became so popular for recombinant protein purification, as these systems only require specific ligands for the capture of the fusion protein through a pre-defined affinity tag tail. There is a wide range of available affinity pairs "tag-ligand" combining biological or structural affinity ligands with the respective binding tags. This review gives a general overview of the well-established "tag-ligand" systems available for fusion protein purification and also explores current unconventional strategies under development.

Efficient soluble expression of secreted matrix metalloproteinase 26 in Brevibacillus choshinensis

Matrix metalloproteinase 26 (MMP-26) is a novel member of the matrix metalloproteinase family with minimal domain constitution and unknown physiological function. The three-dimensional (3D) structure of the enzyme also remains to be deciphered. Previous studies show that MMP-26 may be expressed in Escherichia coli (E. coli) as inclusion bodies and re-natured with catalytic activity. However, the low re-naturation rate of this method limits its usage in structural studies. In this paper, we tried to clone, express and purify the pro form and catalytic form of MMP-26 (ProMMP-26 and CatMMP-26) in several widely used expression vectors and express the recombinant MMP-26 proteins in E. coli cells. These constructs resulted in insoluble expressions or soluble expressions of MMP-26 with little catalytic activity. We then used Brevibacillus choshinensis (B. choshinensis) as the host system for the soluble and active expression of MMP-26. The enzyme was secreted in soluble form in the supernatant of cell culture medium and purified via a two-step purification process that included Ni(2+) affinity chromatography followed by gel filtration. The yields of purified ProMMP-26 and CatMMP-26 were 12 and 18mg/L, respectively, with high purity and homogeneity. Both ProMMP-26 and CatMMP-26 showed gelatin zymography activity and the purified CatMMP-26 had high enzymatic activity against DQ-gelatin substrate. The large-scale soluble and active protein production for future structural studies of MMP-26 is thus feasible using the B. choshinensis host system.

Predicting transition temperatures of elastin-like polypeptide fusion proteins

Elastin-like polypeptides (ELPs) are thermally sensitive peptide polymers that undergo thermally triggered phase separation and this behavior is imparted to soluble proteins when they are fused to an ELP. The transition temperature of the ELP fusion protein is observed to be different than that of a free ELP, indicating that the surface properties of the fused protein modulate the thermal behavior of ELPs. Understanding this effect is important for the rational design of applications that exploit the phase transition behavior of ELP fusion proteins. We had previously developed a biophysical model that explained the effect of hydrophobic proteins on depressing the transition temperature of ELP fusion proteins relative to free ELP. Here, we extend the model to elucidate the effect of hydrophilic proteins on the thermal behavior of ELP fusion proteins. A linear correlation was found between overall residue composition of accessible protein surface weighted by a characteristic transition temperature for each residue and the difference in transition temperatures between the ELP protein fusion and the corresponding free ELP. In breaking down the contribution of residues to polar, nonpolar, and charged, the model revealed that charged residues are the most important parameter in altering the transition temperature of an ELP fusion relative to the free ELP.

Expression and purification of the antimicrobial peptide cecropin AD by fusion with cationic elastin-like polypeptides

Cationic elastin-like polypeptides (CELP) are thermally responsive polypeptides that undergo an inverse temperature phase transition, and the recombinant CELP fusion proteins may be purified by inverse transition cycling (ITC). To obtain high-purity antimicrobial peptide cecropin AD (CAD), CELP was placed at the N-terminus of CAD and the expression vector pET28a-CELP-CAD was constructed. The expression vector was then transformed into Escherichia coli BL21 (DE3) to express the recombinant protein. After three rounds of ITC, enterokinase digestion and another hot spin, 1.2mg recombinant CAD was purified from 100ml culture medium. The antimicrobial test indicated that the high-purity CAD had strong antimicrobial activity against E. coli and Staphylococcus aureus.