Published papers

Encapsulation of hydrophobic molecules in protein-based nanocages is a promising approach for dispersing these molecules in water. Here, we report a chemical modification approach to produce a protein nanocage with a hydrophobic interior surface based on our previously developed nanocage, TIP60. The large pores of TIP60 act as tunnels for small molecules, allowing modification of the interior surface by hydrophobic compounds without nanocage disassembly. We used four different hydrophobic compounds for modification. The largest modification group tested, pyrene, resulted in a modified TIP60 that could encapsulate aromatic photosensitizer zinc phthalocyanine (ZnPC) more efficiently than the other modification compounds. The encapsulated ZnPC generated singlet oxygen upon light activation in the aqueous phase, whereas ZnPC alone formed inert aggregates under the same experimental conditions. Given that chemical modification allows a wider diversity of modifications than mutagenesis, this approach could be used to develop more suitable nanocages for encapsulating hydrophobic molecules of interest.

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.

Many protein nanocages are polyhedral shapes with icosahedral symmetry owing to the geometric constraints defined by Euler's polyhedral theorem, which suggests that pentagonal faces play a key role in creating uniform structures. This principle has inspired a range of design approaches to create artificial protein nanocages. For example, this review mainly shows design, structures, and applications of TIP60, an artificially designed icosahedral protein nanocage produced through a fusion protein approach incorporating pentameric structures. Protein engineering techniques, such as chemical modifications and mutation strategies for TIP60, have successfully added properties useful for various applications, including drug delivery, nanoreactors, and hydrogels. Recent advances in computational protein design can enable precise structural customization. Therefore, the integration of rational design with targeted modifications is likely to become a powerful approach for developing functionalized proteins.

A split-protein system is a simple approach to introduce new termini which are useful as modification sites in protein engineering, but has been adapted mainly for monomeric proteins. Here we demonstrate the design of split subunits of the 60-mer artificial fusion-protein nanocage TIP60. The subunit fragments successfully reformed the cage structure in the same manner as prior to splitting. One of the newly introduced terminals at the interior surface can be modified using a tag peptide and green fluorescent protein. Therefore, the termini could serve as a versatile modification site for incorporating a wide variety of functional peptides and proteins.

Dyes exhibiting polarity-dependent color changes, known as solvatochromism, have great potential for creating sensors, smart materials, and responsive coatings. However, full-range color shifts require a technique to disperse dyes across a wide range of solvent polarities, which remains a persistent challenge. For example, hydrophobic dyes often aggregate in water, preventing effective color shifts. Although surfactants can assist in dye dispersion, they can also prevent solvent molecules from accessing the dye. To address this, we used a 60-mer protein nanocage, TIP60, with a densely pyrene-modified interior surface. The modification did not induce protein denaturation, as monitored by small-angle X-ray scattering, and greatly increased the aqueous solubility of a hydrophobic solvatochromic dye, Nile Red (NR), while preserving its fluorescence. The NR-loaded solution appeared blue, reflecting the polar environment surrounding NR. Cryogenic electron microscopy suggested that the pyrenes interacted with each other to form a binding site for NR. This interaction also contributed to thermostability of TIP60 (65 °C to 86 °C) and stability against sodium dodecyl sulfate, as observed by electrophoresis experiments. When brushed onto plain copy paper, the NR-loaded nanocage appeared bluish-purple and shifted reversibly to purplish red upon heating, returning on cooling—presumably via nanocage dissociation and reassembly. The color change was also sensitive to humidity. We term this material “misteINK”, a protein-based ink with reversible temperature- and humidity-dependent color changes. These findings demonstrate that a single-step interior modification enables the rational design of protein materials for tuning dye photophysics, providing a powerful strategy for designing protein-based functional materials.

Oligomeric protein nanocages often disassemble into their subunits and reassemble by external stimuli. Thus, using these nanocages as cross-linkers for hydrogel network structures is a promising approach to allow hydrogels to undergo stimuli-responsive gel-sol transitions or self-healing. Here, we report hydrogels that show a reversible gel-sol transition resulting from the heat-induced dissociation and reassociation of protein nanocages. The hydrogel contained the 60-mer artificial protein nanocage, TIP60, as a supramolecular cross-linker for polyethylene glycol network structures. The hydrogel showed a gel-to-sol transition upon heating at a temperature above the melting point of TIP60 and immediately returned to a gel state upon cooling to room temperature. During the heating and cooling treatment of the hydrogel, small-angle X-ray scattering analysis suggested the dissociation and reassociation of TIP60. Furthermore, we demonstrated redox-responsive cargo release from TIP60 in the hydrogel. These results showed the potential of TIP60 as a component of multi-stimuli-responsive hydrogels.

Protein nanocages, which have inner cavities and surface pores, are attractive materials for various applications, such as in catalysts and medicine. Recently, we produced an artificial protein nanocage, TIP60, and demonstrated its potential as a stimuli-responsive nanocarrier. In the present study, we report a simple purification method for TIP60 that can replace time-consuming and costly affinity chromatography purification. TIP60, which has an anionic surface charge, aggregated at mildly acidic pH and redissolved at neutral pH, maintaining its cage structure. This pH-responsive reversible precipitation allowed us to purify TIP60 from soluble fractions of the E. coli cell lysate by controlling the pH. Compared with conventional Ni-NTA column purification, the pH-responsive precipitation method provided purified TIP60 with similar purity (∼80%) and higher yield. This precipitation purification method should facilitate the large-scale investigation and practical use of TIP60 nanocages.

Protein-based nanoparticles hold promise for a broad range of applications. Here, we report the production of a uniform anionic hollow protein nanoparticle, designated TIP60, which spontaneously assembles from a designed fusion protein subunit based on the geometric features of polyhedra. We show that TIP60 tolerates mutation and both its interior and exterior surfaces can be chemically modified. Moreover, TIP60 forms larger structures upon the addition of a cationic protein. Therefore, TIP60 can be used as a modifiable nano-building block for further molecular assembly.

Enzymatic degradation of poly(ethylene terephthalate) (PET) is promising because this process is safer than conventional industrial approaches. Recently, a cationic PET hydrolase (PETase) was identified from Ideonella sakaiensis. Pre-incubation of a low-crystallinity PET film with anionic surfactants prior to initiating the reaction was found to improve PETase activity 120-fold. After 36 h at 30 °C, the film thickness decreased by 22 %. The binding of surfactants to the film makes the surface anionic, thereby attracting the cationic PETase. Mutagenesis of PETase showed that the surface cationic region formed by R53, R90, and K95, which are located on the same side as the substrate binding pocket, was crucial for efficient acceleration of activity by the anionic surfactant. Thus, surfactant bound on PET aligns the orientation of the active site to the surface, resulting in efficient hydrolysis. We believe that this approach using PETase could be further improved by designing surfactant molecules for the more efficient enzymatic PET degradation.

Cutinases are promising agents for poly(ethylene terephthalate) (PET) bio-recycling because of their ability to produce the PET monomer terephthalic acid with high efficiency under mild reaction conditions. In this study, we found that the low-crystallinity PET (lcPET) hydrolysis activity of thermostable cutinase from Thermobifida fusca (TfCut2), was increased by the addition of cationic surfactant that attracts enzymes near the lcPET film surface via electrostatic interactions. This approach was applicable to the mutant TfCut2 G62A/F209A, which was designed based on a sequence comparison with PETase from Ideonella sakaiensis. As a result, the degradation rate of the mutant in the presence of cationic surfactant increased to 31 ± 0.1 nmol min−1 cm−2, 12.7 times higher than that of wild-type TfCut2 in the absence of surfactant. The long-duration reaction showed that lcPET film (200 μm) was 97 ± 1.8% within 30 h, the fastest biodegradation rate of lcPET film thus far. We therefore believe that our approach would expand the possibility of enzyme utilization in industrial PET biodegradation.

Supramolecular protein nanoparticles and nanocages have potential in a broad range of applications. Recently, we developed a uniform supramolecular protein nanoparticle, TIP60, symmmetrically self-assembled from fusion proteins of a pentameric Sm-like protein and a dimeric MyoX-coil domain. Herein, we report the icosahedral 60-meric structure of TIP60 solved using single-particle cryo-electron microscopy. Interestingly, the structure revealed 20 regular-triangle-like pores on the surface. TIP60 and its mutants have many modifiable sites on their exterior and interior surfaces. The TIP60 architecture will be useful in the development of biomedical and biochemical nanoparticles/nanocages for future applications.

Chemical modification of the interior and exterior surfaces of protein nanocages holds promise for various applications such as cosmetics, pharmaceuticals, and catalysts. However, dual-surface modification of these surfaces using different chemicals remains challenging, particularly when the same substituents, such as cysteine thiols, are modified. We recently produced an artificial protein nanocage called TIP60 that has 20 large surface pores. Chemical modification of cysteine residues introduced in the interior surface of TIP60 showed that these pores allow the passage of small molecules from the outside environment to the inside of the nanocage. In this study, we found that the surface pores on TIP60 function as size-dependent molecular filters. Modification experiments using different-sized polymers containing maleimide groups, which specifically react with thiols, showed that macromolecules with diameters larger than that of the pores could not penetrate into the inner cavity. This molecular size discrimination by the pores prompted us to perform stepwise dual-surface functionalization of a double mutant of TIP60 presenting cysteine residues on the interior and exterior surfaces. This was achieved by modifying the exterior cysteine residues with a polymer containing a maleimide group that cannot penetrate to the inside of the nanocage, followed by modification of the interior cysteine residues using thiol-containing small molecules. Dual-functionalized TIP60 released internal small molecules in a redox-responsive manner. This simple approach for dual-surface modification would make TIP60 a useful nanocarrier for a broad range of applications including drug-delivery and molecular filtration systems.