Development of SpyTag/SpyCatcher-Bacmid Expression Vector System (SpyBEVS) for Protein Bioconjugations Inside of Silkworms

An Overview of Recent Developments in the Application of Antigen Displaying Vaccine Platforms: Hints for Future SARS-CoV-2 VLP Vaccines

Recently, a great effort has been devoted to studying attenuated and subunit vaccine development against SARS-CoV-2 since its outbreak in December 2019. It is known that diverse virus-like particles (VLPs) are extensively employed as carriers to display various antigenic and immunostimulatory cargo modules for vaccine development. Single or multiple antigens or antigenic domains such as the spike or nucleocapsid protein or their variants from SARS-CoV-2 could also be incorporated into VLPs via either a genetic or chemical display approach. Such antigen display platforms would help screen safer and more effective vaccine candidates capable of generating a strong immune response with or without adjuvant. This review aims to provide valuable insights for the future development of SARS-CoV-2 VLP vaccines by summarizing the latest updates and perspectives on the vaccine development of VLP platforms for genetic and chemical displaying antigens from SARS-CoV-2.

Expression and Purification of Porcine Rotavirus Structural Proteins in Silkworm Larvae as a Vaccine Candidate

In this study, silkworm larvae were used for expression of porcine rotavirus A (KS14 strain) inner capsid protein, VP6, and outer capsid protein, VP7. Initially, VP6 was fused with Strep-tag II and FLAG-tag (T-VP6), and T-VP6 was fused further with the signal peptide of Bombyx mori 30k6G protein (30k-T-VP6). T-VP6 and 30 k-T-VP6 were then expressed in the fat body and hemolymph of silkworm larvae, respectively, with respective amounts of 330 μg and 50 μg per larva of purified protein. Unlike T-VP6, 30k-T-VP6 was N-glycosylated due to attached signal peptide. Also, VP7 was fused with PA-tag (VP7-PA). Additionally, VP7 was fused with Strep-tag II, FLAG-tag, and the signal peptide of Bombyx mori 30k6G protein (30k-T-ΔVP7). Both VP7-PA and 30k-T-ΔVP7 were expressed in the hemolymph of silkworm larvae, with respective amounts of 26 μg and 49 μg per larva of purified protein, respectively. The results from our study demonstrated that T-VP6 formed nanoparticles of greater diameter compared with the ones formed by 30k-T-VP6. Also, higher amount of VP6 expressed in silkworm larvae reveal that VP6 holds the potential for its use in vaccine development against porcine rotavirus with silkworm larvae as a promising host for the production of such multi-subunit vaccines.

Display of multiple proteins on engineered canine parvovirus-like particles expressed in cultured silkworm cells and silkworm larvae

Recent progress has been made dramatically in decorating virus-like particles (VLPs) on the surface or inside with functional molecules, such as antigens or nucleic acids. However, it is still challenging to display multiple antigens on the surface of VLP to meet the requirement as a practical vaccine candidate. Herein this study, we focus on the expression and engineering of the capsid protein VP2 of canine parvovirus for VLP display in the silkworm-expression system. The chemistry of the SpyTag/SpyCatcher (SpT/SpC) and SnoopTag/SnoopCatcher (SnT/SnC) are efficient protein covalent ligation systems to modify VP2 genetically, where SpyTag/SnoopTag are inserted into the N-terminus or two distinct loop regions (Lx and L2) of VP2. The SpC-EGFP and SnC-mCherry are employed as model proteins to evaluate their binding and display on six SnT/SnC-modified VP2 variants. From a series of protein binding assays between indicated protein partners, we showed that the VP2 variant with SpT inserted at the L2 region significantly enhanced VLP display to 80% compared to 5.4% from N-terminal SpT-fused VP2-derived VLPs. In contrast, the VP2 variant with SpT at the Lx region failed to form VLPs. Moreover, the SpT (Lx)/SnT (L2) double-engineered chimeric VP2 variants showed covalent conjugation capacity to both SpC/SnC protein partners. The orthogonal ligations between those binding partners were confirmed by both mixing purified proteins and co-infecting cultured silkworm cells or larvae with desired recombinant viruses. Our results indicate that a convenient VLP display platform was successfully developed for multiple antigen displays on demand. Further verifications can be performed to assess its capacity for displaying desirable antigens and inducing a robust immune response to targeted pathogens.

Chemical and Enzymatic Methods for Post-Translational Protein-Protein Conjugation

Fusion proteins play an essential role in the biosciences but suffer from several key limitations, including the requirement for N-to-C terminal ligation, incompatibility of constituent domains, incorrect folding, and loss of biological activity. This perspective focuses on chemical and enzymatic approaches for the post-translational generation of well-defined protein-protein conjugates, which overcome some of the limitations faced by traditional fusion techniques. Methods discussed range from chemical modification of nucleophilic canonical amino acid residues to incorporation of unnatural amino acid residues and a range of enzymatic methods, including sortase-mediated ligation. Through summarizing the progress in this rapidly growing field, the key successes and challenges associated with using chemical and enzymatic approaches are highlighted and areas requiring further development are discussed.

In vivo enzymatic digestion of HRV 3C protease cleavage sites-containing proteins produced in a silkworm-baculovirus expression system

Baculovirus expression vector system (BEVS) has been recognized as a potent protein expression system in engineering valuable enzymes and vaccines. Various fusion tags facilitate protein purification, leaving the potential risk to influence the target protein's biological activity negatively. It is of great interest to consider removing the additional tags using site-specific proteases, such as human rhinoviruses (HRV) 3C protease. The current study validated the cleavage activity of 3C protease in Escherichia coli and silkworm-BEVS systems by mixing the cell or fat body lysates of 3C protein and 3C site containing target protein in vitro. Further verification has been performed in the fat body lysate from co-expression of both constructs, showing remarkable cleavage efficiency in vivo silkworm larvae. We also achieved the glutathione-S-transferase (GST) tag-cleaved product of the VP15 protein from the White spot syndrome virus after purification, suggesting that we successfully established a coinfection-based recognition-and-reaction BEVS platform for the tag-free protein engineering.

Genetically engineered materials: Proteins and beyond

Information-rich molecules provide opportunities for evolution. Genetically engineered materials are superior in that their properties are coded within genetic sequences and could be fine-tuned. In this review, we elaborate the concept of genetically engineered materials (GEMs) using examples ranging from engineered protein materials to engineered living materials. Protein-based materials are the materials of choice by nature. Recent progress in protein engineering has led to opportunities to tune their sequences for optimal material performance. Proteins also play a central role in living materials where they act in concert with other biological components as well as nonbiological cofactors, giving rise to living features. While the existing GEMs are often limited to those constructed by building blocks of biological origin, being genetically engineerable does not preclude nonbiologic or synthetic materials, the latter of which have yet to be fully explored.

Ni-modified magnetic nanoparticles for affinity purification of His-tagged proteins from the complex matrix of the silkworm fat body

Purification of recombinant proteins is often a challenging matter because high purity and high recovery are desired. If the expressed recombinant protein is also in a complex matrix, such as from the silkworm expression system, purification becomes more challenging. Even if purification from the silkworm expression system is troublesome, it benefits from a high capacity for the production of recombinant proteins. In this study, magnetic nanoparticles (MNPs) were investigated as a suitable tool for the purification of proteins from the complex matrix of the silkworm fat body. The MNPs were modified with nickel so that they have an affinity for His-tagged proteins, as the MNP purification protocol itself does not need special equipment except for a magnet. Among the three different kinds of investigated MNPs, MNPs with sizes of 100 nm to 200 nm and approximately 20 nm-thick nickel shells were the most suitable for our purpose. With them, the total protein amount was reduced by up to at least approximately 77.7%, with a protein recovery of around 50.8% from the silkworm fat body. The minimum binding capacity was estimated to be 83.3 µg protein/mg MNP. Therefore, these MNPs are a promising tool as a purification pretreatment of complex sample matrices.

Power to the protein: enhancing and combining activities using the Spy toolbox

Proteins span an extraordinary range of shapes, sizes and functionalities. Therefore generic approaches are needed to overcome this diversity and stream-line protein analysis or application. Here we review SpyTag technology, now used in hundreds of publications or patents, and its potential for detecting and controlling protein behaviour. SpyTag forms a spontaneous and irreversible isopeptide bond upon binding its protein partner SpyCatcher, where both parts are genetically-encoded. New variants of this pair allow reaction at a rate approaching the diffusion limit, while reversible versions allow purification of SpyTagged proteins or tuned dynamic interaction inside cells. Anchoring of SpyTag-linked proteins has been established to diverse nanoparticles or surfaces, including gold, graphene and the air/water interface. SpyTag/SpyCatcher is mechanically stable, so is widely used for investigating protein folding and force sensitivity. A toolbox of scaffolds allows SpyTag-fusions to be assembled into defined multimers, from dimers to 180-mers, or unlimited 1D, 2D or 3D networks. Icosahedral multimers are being evaluated for vaccination against malaria, HIV and cancer. For enzymes, Spy technology has increased resilience, promoted substrate channelling, and assembled hydrogels for continuous flow biocatalysis. Combinatorial increase in functionality has been achieved through modular derivatisation of antibodies, light-emitting diodes or viral vectors. In living cells, SpyTag allowed imaging of protein trafficking, retargeting of CAR-T cell killing, investigation of heart contraction, and control of nucleosome position. The simple genetic encoding and rapid irreversible reaction provide diverse opportunities to enhance protein function. We describe limitations as well as future directions.