Dressing up artificial viral capsids self-assembled from C-terminal-modified β-annulus peptides

Enveloped Viral Replica Equipped with Spike Protein Derived from SARS-CoV-2

Synthetic viral nanostructures are useful as materials for analyzing the biological behavior of natural viruses and as vaccine materials. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an enveloped virus embedding a spike (S) protein involved in host cell infection. Although nanomaterials modified with an S protein without an envelope membrane have been developed, they are considered unsuitable for stability and functionality. We previously constructed an enveloped viral replica complexed with a cationic lipid bilayer and an anionic artificial viral capsid self-assembled from β-annulus peptides. In this study, we report the first example of an enveloped viral replica equipped with an S protein derived from SARS-CoV-2. Interestingly, even the S protein equipped on the enveloped viral replica bound strongly to the free angiotensin-converting enzyme 2 (ACE2) receptor as well as ACE2 localized on the cell membrane.

A supramolecular system mimicking the infection process of an enveloped virus through membrane fusion

Membrane fusion is an essential step for the entry of enveloped viruses, such as human immunodeficiency virus and influenza virus, into the host cell, often triggered by the binding of membrane proteins on the viral envelope to host cell membrane. Recently, external stimuli was shown to trigger membrane fusion in an artificial system. Direct observation of artificial membrane fusion using a giant unilamellar vesicle (GUV), which is similar in size to a cell, is useful as a biological model system. However, there are no model systems for studying membrane fusion of enveloped viruses with host cells. Here, we report a supramolecular model system for viral entry into a GUV or cell through membrane fusion. The system was constructed by complexing a cationic lipid bilayer on an anionic artificial viral capsid, self-assembled from viral β-annulus peptides. We demonstrate that the cationic enveloped artificial viral capsid electrostatically interacts with the anionic GUV or cell, and the capsid enters the GUV or cell through membrane fusion. The model system established in this study will be important for analyzing membrane fusion during infection of a natural virus.

Reversible Assembly of an Artificial Protein Nanocage Using Alkaline Earth Metal Ions

Protein nanocages are of increasing interest for use as drug capsules, but the encapsulation and release of drug molecules at appropriate times require the reversible association and dissociation of the nanocages. One promising approach to addressing this challenge is the design of metal-dependent associating proteins. Such designed proteins typically have Cys or His residues at the protein surface for connecting the associating proteins through metal-ion coordination. However, Cys and His residues favor interactions with soft and borderline metal ions, such as Au⁺ and Zn²⁺, classified by the hard and soft acids and bases concept, restricting the types of metal ions available to drive association. Here, we show the alkaline earth (AE) metal-dependent association of the recently designed artificial protein nanocage TIP60, which is composed of 60-mer fusion proteins. The introduction of a Glu (hard base) mutation to the fusion protein (K67E mutant) prevented the formation of the 60-mer but formed the expected cage structure in the presence of Ca, Sr, or Ba ions (hard acids). Cryogenic electron microscopy (cryo-EM) analysis indicated a Ba ion at the interface of the subunits. Furthermore, we demonstrated the encapsulation and release of single-stranded DNA molecules using this system. Our results provide insights into the design of AE metal-dependent association and dissociation mechanisms for proteins.

Protein Nanoparticles: Uniting the Power of Proteins with Engineering Design Approaches

Protein nanoparticles, PNPs, have played a long-standing role in food and industrial applications. More recently, their potential in nanomedicine has been more widely pursued. This review summarizes recent trends related to the preparation, application, and chemical construction of nanoparticles that use proteins as major building blocks. A particular focus has been given to emerging trends related to applications in nanomedicine, an area of research where PNPs are poised for major breakthroughs as drug delivery carriers, particle-based therapeutics or for non-viral gene therapy.

Peptide Self-assembly into stable Capsid-Like nanospheres and Co-assembly with DNA to produce smart artificial viruses

Smart artificial viruses have been successfully developed by co-assembly of de novo designed peptides with DNA, which achieved stimuli-responsibility and efficient gene transfection in cancer cells. The peptides were designed to incorporate several functional segments, including a hydrophobic aromatic segment to drive self-assembly, two or more cysteines to regulate the assemblage shape and stabilize the assembled nanostructures via forming disulfide bonds, several lysines to facilitate co-assembly with DNA and binding to cell membranes, and an enzyme-cleavable segment to introduce cancer sensitivity. The rationally designed peptides self-assembled into stable nanospheres with a uniform diameter of < 10 nm, which worked as capsid-like subunits to further interact with DNA to produce hierarchical virus-mimicking structures by encapsulating DNA in the interior. Such artificial viruses can effectively protect DNA from nuclease digestion and achieve efficient genome release by enzyme-triggered structure disassembly, which ensured a high level of gene transfection in tumor cells. The system emulates very well the structural and functional properties of natural viruses from the aspects of capsid formation, genome package and gene transfection, which is highly promising for application as efficient gene vectors.

Artificial Nanocage Formed via Self-Assembly of β-Annulus Peptide for Delivering Biofunctional Proteins into Cell Interiors

Nanocarriers that deliver functional proteins to cell interiors are an attractive platform for the intracellular delivery of intact proteins without further modification, with in vivo compatibility. Development of efficient methods for cargo protein encapsulation and release in recipient cell cytosol is needed. Herein, we assess the feasibility of the abovementioned requirements using a protein nanocage (artificial nanocage) without compromising the structure and functions of the original protein and allowing for design flexibility of the surfaces and interiors. The protein nanocage formed via the self-assembly of the β-annulus peptide (24-amino acid peptide) in water was used as a model framework. The nitrilotriacetic acid moiety was displayed on the nanocage lumen for effective encapsulation of hexahistidine-tagged proteins in the presence of Ni²⁺, and the amphiphilic cationic lytic peptide HAad was displayed on a nanocage surface to attain cell permeability. Successful intracellular delivery of cargo proteins and targeting of cytosolic proteins by a nanobody were achieved, indicating the validity of the approach employed in this study.

Embedding a membrane protein into an enveloped artificial viral replica

Natural enveloped viruses, in which nucleocapsids are covered with lipid bilayers, contain membrane proteins on the outer surface that are involved in diverse functions, such as adhesion and infection of host cells. Previously, we constructed an enveloped artificial viral capsid through the complexation of cationic lipid bilayers onto an anionic artificial viral capsid self-assembled from β-annulus peptides. Here we demonstrate the embedding of the membrane protein Connexin-43 (Cx43), on the enveloped artificial viral capsid using a cell-free expression system. The expression of Cx43 in the presence of the enveloped artificial viral capsid was confirmed by western blot analysis. The embedding of Cx43 on the envelope was evaluated by detection via the anti-Cx43 antibody, using fluorescence correlation spectroscopy (FCS) and transmission electron microscopy (TEM). Interestingly, many spherical structures connected to each other were observed in TEM images of the Cx43-embedded enveloped viral replica. In addition, it was shown that fluorescent dyes could be selectively transported from Cx43-embedded enveloped viral replicas into Cx43-expressing HepG2 cells. This study provides a proof of concept for the creation of multimolecular crowding complexes, that is, an enveloped artificial viral replica embedded with membrane proteins.

We demonstrate the embedding membrane protein, Cx43, on the enveloped artificial viral capsid using a cell-free expression system. The embedding of Cx43 on the envelope was evaluated by detection with anti-Cx43 antibody using FCS and TEM.

A Photoresponsive Artificial Viral Capsid Self-Assembled from an Azobenzene-Containing β-Annulus Peptide

Photoinduced structural changes in peptides can dynamically control the formation and dissociation of supramolecular peptide materials. However, the existence of photoresponsive viral capsids in nature remains unknown. In this study, we constructed an artificial viral capsid possessing a photochromic azobenzene moiety on the peptide backbone. An azobenzene-containing β-annulus peptide derived from the tomato bushy stunt virus was prepared through solid-phase synthesis using Fmoc-3-[(3-aminomethyl)-phenylazo]phenylacetic acid. The azobenzene-containing β-annulus (β-Annulus-Azo) peptide showed a reversible trans/cis isomerization property. The β-annulus-azo peptide self-assembled at 25 μM into capsids with the diameters of 30–50 nm before UV irradiation (trans-form rich), whereas micrometer-sized aggregates were formed after UV irradiation (cis-form rich). The artificial viral capsid possessing azobenzene facilitated the encapsulation of fluorescent-labeled dextrans and their photoinduced release from the capsid.

Supramolecular Architectures of Nucleic Acid/Peptide Hybrids

Supramolecular architectures that are built artificially from biomolecules, such as nucleic acids or peptides, with structural hierarchical orders ranging from the molecular to nano-scales have attracted increased attention in molecular science research fields. The engineering of nanostructures with such biomolecule-based supramolecular architectures could offer an opportunity for the development of biocompatible supramolecular (nano)materials. In this review, we highlighted a variety of supramolecular architectures that were assembled from both nucleic acids and peptides through the non-covalent interactions between them or the covalently conjugated molecular hybrids between them.

Horseradish Peroxidase-Decorated Artificial Viral Capsid Constructed from β-Annulus Peptide via Interaction between His-Tag and Ni-NTA

Artificial construction of spherical protein assemblies has attracted considerable attention due to its potential use in nanocontainers, nanocarriers, and nanoreactors. In this work, we demonstrate a novel strategy to construct peptide nanocapsules (artificial viral capsids) decorated with enzymes via interactions between His-tag and Ni-NTA. A β-annulus peptide derived from the tomato bushy stunt virus was modified with Ni-NTA at the C-terminus, which is directed toward the exterior surface of the artificial viral capsid. The β-annulus peptide bearing Ni-NTA at the C-terminus self-assembled into capsids of about 50 nm in diameter. The Ni-NTA-displayed capsids were complexed with recombinant horseradish peroxidase (HRP) with a C-terminal His-tag which was expressed in Escherichia coli. The β-annulus peptide-HRP complex formed spherical assemblies whose sizes were 30–90 nm, with the ζ-potential revealing that the HRP was decorated on the outer surface of the capsid.

Encapsulation of mRNA into Artificial Viral Capsids via Hybridization of a β-Annulus-dT20 Conjugate and the Poly(A) Tail of mRNA

Messenger RNA (mRNA) drugs have attracted considerable attention as promising tools with many therapeutic applications. The efficient delivery of mRNA drugs using non-viral materials is currently being explored. We demonstrate a novel concept where mCherry mRNA bearing a poly(A) tail is encapsulated into capsids co-assembled from viral β-annulus peptides bearing a 20-mer oligothymine (dT20) at the N-terminus and unmodified peptides via hybridization of dT20 and poly(A). Dynamic light scattering measurements and transmission electron microscopy images of the mRNA-encapsulated capsids show the formation of spherical assemblies of approximately 50 nm. The encapsulated mRNA shows remarkable ribonuclease resistance. Further, modification by a cell-penetrating peptide (His16) on the capsid enables the intracellular expression of mCherry of encapsulated mRNA.