Peptide-directed encapsulation of inorganic nanoparticles into protein containers

Encapsulation of Transketolase into In Vitro-Assembled Protein Nanocompartments Improves Thermal Stability

Protein compartments offer definitive structures with a large potential design space that are of particular interest for green chemistry and therapeutic applications. One family of protein compartments, encapsulins, are simple prokaryotic nanocompartments that self-assemble from a single monomer into selectively permeable cages of between 18 and 42 nm. Over the past decade, encapsulins have been developed for a diverse application portfolio utilizing their defined cargo loading mechanisms and repetitive surface display. Although it has been demonstrated that encapsulation of non-native cargo proteins provides protection from protease activity, the thermal effects arising from enclosing cargo within encapsulins remain poorly understood. This study aimed to establish a methodology for loading a reporter protein into thermostable encapsulins to determine the resulting stability change of the cargo. Building on previous in vitro reassembly studies, we first investigated the effectiveness of in vitro reassembly and cargo-loading of two size classes of encapsulins Thermotoga maritima T = 1 and Myxococcus xanthus T = 3, using superfolder Green Fluorescent Protein. We show that the empty T. maritima capsid reassembles with higher yield than the M. xanthus capsid and that in vitro loading promotes the formation of the M. xanthus T = 3 capsid form over the T = 1 form, while overloading with cargo results in malformed T. maritima T = 1 encapsulins. For the stability study, a Förster resonance energy transfer (FRET)-probed industrially relevant enzyme cargo, transketolase, was then loaded into the T. maritima encapsulin. Our results show that site-specific orthogonal FRET labels can reveal changes in thermal unfolding of encapsulated cargo, suggesting that in vitro loading of transketolase into the T. maritima T = 1 encapsulin shell increases the thermal stability of the enzyme. This work supports the move toward fully harnessing structural, spatial, and functional control of in vitro assembled encapsulins with applications in cargo stabilization.

Sortase A-Based Post-translational Modifications on Encapsulin Nanocompartments

Protein-based encapsulin nanocompartments, known for their well-defined structures and versatile functionalities, present promising opportunities in the fields of biotechnology and nanomedicine. In this investigation, we effectively developed a sortase A-mediated protein ligation system in Escherichia coli to site-specifically attach target proteins to encapsulin, both internally and on its surfaces without any further in vitro steps. We explored the potential applications of fusing sortase enzyme and a protease for post-translational ligation of encapsulin to a green fluorescent protein and anti-CD3 scFv. Our results demonstrated that this system could attach other proteins to the nanoparticles' exterior surfaces without adversely affecting their folding and assembly processes. Additionally, this system enabled the attachment of proteins inside encapsulins which varied shapes and sizes of the nanoparticles due to cargo overload. This research developed an alternative enzymatic ligation method for engineering encapsulin nanoparticles to facilitate the conjugation process.

Advances in nanomedicines: a promising therapeutic strategy for ischemic cerebral stroke treatment

Ischemic stroke, prevalent among the elderly, necessitates attention to reperfusion injury post treatment. Limited drug access to the brain, owing to the blood-brain barrier, restricts clinical applications. Identifying efficient drug carriers capable of penetrating this barrier is crucial. Blood-brain barrier transporters play a vital role in nutrient transport to the brain. Recently, nanoparticles emerged as drug carriers, enhancing drug permeability via surface-modified ligands. This article introduces the blood-brain barrier structure, elucidates reperfusion injury pathogenesis, compiles ischemic stroke treatment drugs, explores nanomaterials for drug encapsulation and emphasizes their advantages over conventional drugs. Utilizing nanoparticles as drug-delivery systems offers targeting and efficiency benefits absent in traditional drugs. The prospects for nanomedicine in stroke treatment are promising.

Potential utility of bacterial protein nanoreactor for sustainable in-situ biocatalysis in wide range of bioprocess conditions

Bacterial microcompartments (MCPs) are proteinaceous organelles that natively encapsulates the enzymes, substrates, and cofactors within a protein shell. They optimize the reaction rates by enriching the substrate in the vicinity of enzymes to increase the yields of the product and mitigate the outward diffusion of the toxic or volatile intermediates. The shell protein subunits of MCP shell are selectively permeable and have specialized pores for the selective inward diffusion of substrates and products release. Given their attributes, MCPs have been recently explored as potential candidates as subcellular nano-bioreactor for the enhanced production of industrially important molecules by exercising pathway encapsulation. In the current study, MCPs have been shown to sustain enzyme activity for extended periods, emphasizing their durability against a range of physical challenges such as temperature, pH and organic solvents. The significance of an intact shell in conferring maximum protection is highlighted by analyzing the differences in enzyme activities inside the intact and broken shell. Moreover, a minimal synthetic shell was designed with recruitment of a heterologous enzyme cargo to demonstrate the improved durability of the enzyme. The encapsulated enzyme was shown to be more stable than its free counterpart under the aforementioned conditions. Bacterial MCP-mediated encapsulation can serve as a potential strategy to shield the enzymes used under extreme conditions by maintaining the internal microenvironment and enhancing their cycle life, thereby opening new means for stabilizing, and reutilizing the enzymes in several bioprocess industries.

Encapsulins: Nanotechnology's future in a shell

Encapsulins, virus capsid-like bacterial nanocompartments have emerged as promising tools in medicine, imaging, and material sciences. Recent work has shown that these protein-bound icosahedral 'organelles' possess distinct properties that make them exceptionally usable for nanotechnology applications. A key factor contributing to their appeal is their ability to self-assemble, coupled with their capacity to encapsulate a wide range of cargos. Their genetic manipulability, stability, biocompatibility, and nano-size further enhance their utility, offering outstanding possibilities for practical biotechnology applications. In particular, their amenability to engineering has led to their extensive modification, including the packaging of non-native cargos and the utilization of the shell surface for displaying immunogenic or targeting proteins and peptides. This inherent versatility, combined with the ease of expressing encapsulins in heterologous hosts, promises to provide broad usability. Although mostly not yet commercialized, encapsulins have started to demonstrate their vast potential for biotechnology, from drug delivery to biofuel production and the synthesis of valuable inorganic materials. In this review, we will initially discuss the structure, function and diversity of encapsulins, which form the basis for these emerging applications, before reviewing ongoing practical uses and highlighting promising applications in medicine, engineering and environmental sciences.

Encapsulin cargo loading: progress and potential

Encapsulins are a recently discovered class of prokaryotic self-assembling icosahedral protein nanocompartments measuring between 24 and 42 nm in diameter, capable of selectively encapsulating dedicated cargo proteins in vivo. They have been classified into four families based on sequence identity and operon structure, and thousands of encapsulin systems have recently been computationally identified across a wide range of bacterial and archaeal phyla. Cargo encapsulation is mediated by the presence of specific targeting motifs found in all native cargo proteins that interact with the interior surface of the encapsulin shell during self-assembly. Short C-terminal targeting peptides (TPs) are well documented in Family 1 encapsulins, while more recently, larger N-terminal targeting domains (TDs) have been discovered in Family 2. The modular nature of TPs and their facile genetic fusion to non-native cargo proteins of interest has made cargo encapsulation, both in vivo and in vitro, readily exploitable and has therefore resulted in a range of rationally engineered nano-compartmentalization systems. This review summarizes current knowledge on cargo protein encapsulation within encapsulins and highlights select studies that utilize TP fusions to non-native cargo in creative and useful ways.

Artificial Protein Cages Assembled via Gold Coordination

Artificial protein cages made from multiple copies of a single protein can be produced such that they only assemble upon addition of a metal ion. Consequently, the ability to remove the metal ion triggers protein-cage disassembly. Controlling assembly and disassembly has many potential uses including cargo loading/unloading and hence drug delivery. TRAP-cage is an example of such a protein cage which assembles due to linear coordination bond formation with Au(I) which acts to bridge constituent proteins. Here we describe the method for production and purification of TRAP-cage.

Crystalline Biohybrid Materials Based on Protein Cages

Highly ordered superstructures of nanomaterials can be synthesized using protein cages as templates for the assembly of inorganic nanoparticles. Here, we describe in detail the creation of these biohybrid materials. The approach involves computational redesign of ferritin cages, followed by recombinant protein production and purification of the new variants. Metal oxide nanoparticles are synthesized inside the surface-charged variants. The composites are assembled using protein crystallization to yield highly ordered superlattices, which are characterized, for example, with small angle X-ray scattering. This protocol provides a detailed and comprehensive account on our newly established strategy for the synthesis of crystalline biohybrid materials.

Encapsulins: Structure, Properties, and Biotechnological Applications

In 1994 a new class of prokaryotic compartments was discovered, collectively called "encapsulins" or "nanocompartments". Encapsulin shell protomer proteins self-assemble to form icosahedral structures of various diameters (24-42 nm). Inside of nanocompartments shells, one or several cargo proteins, diverse in their functions, can be encapsulated. In addition, non-native cargo proteins can be loaded into nanocompartments, and shell surfaces can be modified via various compounds, which makes it possible to create targeted drug delivery systems, labels for optical and MRI imaging, and to use encapsulins as bioreactors. This review describes a number of strategies of encapsulins application in various fields of science, including biomedicine and nanobiotechnologies.

Microcompartment assembly around multicomponent fluid cargoes

This article describes dynamical simulations of the assembly of an icosahedral protein shell around a bicomponent fluid cargo. Our simulations are motivated by bacterial microcompartments, which are protein shells found in bacteria that assemble around a complex of enzymes and other components involved in certain metabolic processes. The simulations demonstrate that the relative interaction strengths among the different cargo species play a key role in determining the amount of each species that is encapsulated, their spatial organization, and the nature of the shell assembly pathways. However, the shell protein–shell protein and shell protein–cargo component interactions that help drive assembly and encapsulation also influence cargo composition within certain parameter regimes. These behaviors are governed by a combination of thermodynamic and kinetic effects. In addition to elucidating how natural microcompartments encapsulate multiple components involved within reaction cascades, these results have implications for efforts in synthetic biology to colocalize alternative sets of molecules within microcompartments to accelerate specific reactions. More broadly, the results suggest that coupling between self-assembly and multicomponent liquid–liquid phase separation may play a role in the organization of the cellular cytoplasm.

Protein Cages: From Fundamentals to Advanced Applications

Proteins that self-assemble into polyhedral shell-like structures are useful molecular containers both in nature and in the laboratory. Here we review efforts to repurpose diverse protein cages, including viral capsids, ferritins, bacterial microcompartments, and designed capsules, as vaccines, drug delivery vehicles, targeted imaging agents, nanoreactors, templates for controlled materials synthesis, building blocks for higher-order architectures, and more. A deep understanding of the principles underlying the construction, function, and evolution of natural systems has been key to tailoring selective cargo encapsulation and interactions with both biological systems and synthetic materials through protein engineering and directed evolution. The ability to adapt and design increasingly sophisticated capsid structures and functions stands to benefit the fields of catalysis, materials science, and medicine.

Encapsulation of Gold Nanoparticles into Redesigned Ferritin Nanocages for the Assembly of Binary Superlattices Composed of Fluorophores and Gold Nanoparticles

Nanomaterials with a defined composition and structure can be synthesized by exploiting natural templates or biomolecular matrices. In the present work, we use protein nanocages derived from human ferritin as a nanoscale building block for the assembly of gold nanoparticles and fluorescent molecules in the solid state. As a generalizable strategy, we show that prior to material synthesis, the cargo can be encapsulated into the protein nanocages using a dis- and reassembly approach. Toward this end, a new ligand system for gold nanoparticles enables efficient encapsulation of these particles into the nanocages. The gold nanoparticle-loaded protein nanocages are co-assembled with fluorophore-loaded protein nanocages. Binary superlattices are formed because two oppositely charged ferritin nanocages are used as templates for the assembly. The binary crystals show strong exciton-plasmon coupling between the encapsulated fluorophores and gold nanoparticles, which was spatially resolved with fluorescence lifetime imaging. The strategy outlined here offers a modular approach toward binary nanomaterials with highly ordered building blocks.

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.

Characterizing the Dynamic Disassembly/Reassembly Mechanisms of Encapsulin Protein Nanocages

Encapsulins, self-assembling icosahedral protein nanocages derived from prokaryotes, represent a versatile set of tools for nanobiotechnology. However, a comprehensive understanding of the mechanisms underlying encapsulin self-assembly, disassembly, and reassembly is lacking. Here, we characterize the disassembly/reassembly properties of three encapsulin nanocages that possess different structural architectures: T = 1 (24 nm), T = 3 (32 nm), and T = 4 (42 nm). Using spectroscopic techniques and electron microscopy, encapsulin architectures were found to exhibit varying sensitivities to the denaturant guanidine hydrochloride (GuHCl), extreme pH, and elevated temperature. While all three encapsulins showed the capacity to reassemble following GuHCl-induced disassembly (within 75 min), only the smallest T = 1 nanocage reassembled after disassembly in basic pH (within 15 min). Furthermore, atomic force microscopy revealed that all encapsulins showed a significant loss of structural integrity after undergoing sequential disassembly/reassembly steps. These findings provide insights into encapsulins' disassembly/reassembly dynamics, thus informing their future design, modification, and application.

Triggered Reversible Disassembly of an Engineered Protein Nanocage*

Protein nanocages play crucial roles in sub-cellular compartmentalization and spatial control in all domains of life and have been used as biomolecular tools for applications in biocatalysis, drug delivery, and bionanotechnology. The ability to control their assembly state under physiological conditions would further expand their practical utility. To gain such control, we introduced a peptide capable of triggering conformational change at a key structural position in the largest known encapsulin nanocompartment. We report the structure of the resulting engineered nanocage and demonstrate its ability to disassemble and reassemble on demand under physiological conditions. We demonstrate its capacity for in vivo encapsulation of proteins of choice while also demonstrating in vitro cargo loading capabilities. Our results represent a functionally robust addition to the nanocage toolbox and a novel approach for controlling protein nanocage disassembly and reassembly under mild conditions.

Bacterial Nanocompartments: Structures, Functions and Applications

Increasing efficiency is an important driving force behind cellular organization and often achieved through compartmentalization. Long recognized as a core principle of eukaryotic cell organization, its widespread occurrence in prokaryotes has only recently come to light. Despite the early discovery of a few microcompartments such as gas vesicles and carboxysomes, the vast majority of these structures in prokaryotes are less than 100 nm in diameter - too small for conventional light microscopy and electron microscopic thin sectioning. Consequently, these smaller-sized nanocompartments have therefore been discovered serendipitously and then through bioinformatics shown to be broadly distributed. Their small uniform size, robust self-assembly, high stability, excellent biocompatibility, and large cargo capacity make them excellent candidates for biotechnology applications. This review will highlight our current knowledge of nanocompartments, the prospects for applications as well as open question and challenges that need to be addressed to fully understand these important structures.

Load and Display: Engineering Encapsulin as a Modular Nanoplatform for Protein-Cargo Encapsulation and Protein-Ligand Decoration Using Split Intein and SpyTag/SpyCatcher

Protein cage nanoparticles have a unique spherical hollow structure that provides a modifiable interior space and an exterior surface. For full application, it is desirable to utilize both the interior space and the exterior surface simultaneously with two different functionalities in a well-combined way. Here, we genetically engineered encapsulin protein cage nanoparticles (Encap) as modular nanoplatforms by introducing a split-C-intein (Int^C) fragment and SpyTag into the interior and exterior surfaces, respectively. A complementary split-N-intein (Int^N) was fused to various protein cargoes, such as NanoLuc luciferase (Nluc), enhanced green fluorescent protein (eGFP), and Nluc-miniSOG, individually, which led to their successful encapsulation into Encaps to form Cargo@Encap through split intein-mediated protein ligation during protein coexpression and cage assembly in bacteria. Conversely, the SpyCatcher protein was fused to various protein ligands, such as a glutathione binder (GST-SC), dimerizing ligands (FKBP12-SC and FRB-SC), and a cancer-targeting affibody (SC-EGFRAfb); subsequently, they were displayed on Cargo@Encaps through SpyTag/SpyCatcher ligation to form Cargo@Encap/Ligands in a mix-and-match manner. Nluc@Encap/glutathione-S-transferase (GST) was effectively immobilized on glutathione (GSH)-coated solid supports exhibiting repetitive and long-term usage of the encapsulated luciferases. We also established luciferase-embedded layer-by-layer (LbL) nanostructures by alternately depositing Nluc@Encap/FKBP12 and Nluc@Encap/FRB in the presence of rapamycin and applied enhanced green fluorescent protein (eGFP)@Encap/EGFRAfb as a target-specific fluorescent imaging probe to visualize specific cancer cells selectively. Modular functionalization of the interior space and the exterior surface of a protein cage nanoparticle may offer the opportunity to develop new protein-based nanostructured devices and nanomedical tools.

Nanotechnological Applications Based on Bacterial Encapsulins

Encapsulins are proteinaceous nanocontainers, constructed by a single species of shell protein that self-assemble into 20-40 nm icosahedral particles. Encapsulins are structurally similar to the capsids of viruses of the HK97-like lineage, to which they are evolutionarily related. Nearly all these nanocontainers encase a single oligomeric protein that defines the physiological role of the complex, although a few encapsulate several activities within a single particle. Encapsulins are abundant in bacteria and archaea, in which they participate in regulation of oxidative stress, detoxification, and homeostasis of key chemical elements. These nanocontainers are physically robust, contain numerous pores that permit metabolite flux through the shell, and are very tolerant of genetic manipulation. There are natural mechanisms for efficient functionalization of the outer and inner shell surfaces, and for the in vivo and in vitro internalization of heterologous proteins. These characteristics render encapsulin an excellent platform for the development of biotechnological applications. Here we provide an overview of current knowledge of encapsulin systems, summarize the remarkable toolbox developed by researchers in this field, and discuss recent advances in the biomedical and bioengineering applications of encapsulins.

Encapsulin Nanocontainers as Versatile Scaffolds for the Development of Artificial Metabolons

The construction of non-native biosynthetic pathways represents a powerful, modular strategy for the production of valuable synthons and fine chemicals. Accordingly, artificially affixing enzymes that catalyze sequential reactions onto DNAs, proteins, or synthetic scaffolds has proven to be an effective route for generating de novo metabolons with novel functionalities and superior efficiency. In recent years, nanoscale microbial compartments known as encapsulins have emerged as a class of robust and highly engineerable proteinaceous containers with myriad applications in biotechnology and synthetic biology. Herein we report the concurrent surface functionalization and internal packaging of encapsulins from Thermotoga maritima to generate a catalytically competent two-enzyme metabolon. Encapsulins were engineered to covalently sequester up to 60 copies of a dihydrofolate reductase (DHFR) enzyme variant on their exterior surfaces using the SpyCatcher bioconjugation system, while their lumens were packaged with a tetrahydrofolate-dependent demethylase enzyme using short peptide affinity tags abstracted from the encapsulin's native protein cargo. Successful cross-talk between the two colocalized enzymes was confirmed as tetrahydrofolate produced by externally tethered DHFR was capable of driving the demethylation of a lignin-derived aryl substrate by packaged demethylases, albeit slowly. The subsequent introduction of a previously reported pore-enlarging deletion in the encapsulin shell was shown to enhance metabolite exchange such that the encapsulin-based metabolon functioned at speeds equivalent to those of the two enzymes freely dispersed in solution. Our work thus further emphasizes the engineerability of encapsulins and their potential use as flexile scaffolds for biocatalytic applications.

Mechanisms of Scaffold-Mediated Microcompartment Assembly and Size Control

This article describes a theoretical and computational study of the dynamical assembly of a protein shell around a complex consisting of many cargo molecules and long, flexible scaffold molecules. Our study is motivated by bacterial microcompartments, which are proteinaceous organelles that assemble around a condensed droplet of enzymes and reactants. As in many examples of cytoplasmic liquid-liquid phase separation, condensation of the microcompartment interior cargo is driven by flexible scaffold proteins that have weak multivalent interactions with the cargo. Our results predict that the shell size, amount of encapsulated cargo, and assembly pathways depend sensitively on properties of the scaffold, including its length and valency of scaffold-cargo interactions. Moreover, the ability of self-assembling protein shells to change their size to accommodate scaffold molecules of different lengths depends crucially on whether the spontaneous curvature radius of the protein shell is smaller or larger than a characteristic elastic length scale of the shell. Beyond natural microcompartments, these results have important implications for synthetic biology efforts to target alternative molecules for encapsulation by microcompartments or viral shells. More broadly, the results elucidate how cells exploit coupling between self-assembly and liquid-liquid phase separation to organize their interiors.

Bioengineering a Light-Responsive Encapsulin Nanoreactor: A Potential Tool for In Vitro Photodynamic Therapy

Encapsulins, a prokaryotic class of self-assembling protein nanocompartments, are being re-engineered to serve as "nanoreactors" for the augmentation or creation of key biochemical reactions. However, approaches that allow encapsulin nanoreactors to be functionally activated with spatial and temporal precision are lacking. We report the construction of a light-responsive encapsulin nanoreactor for "on demand" production of reactive oxygen species (ROS). Herein, encapsulins were loaded with the fluorescent flavoprotein mini-singlet oxygen generator (miniSOG), a biological photosensitizer that is activated by blue light to generate ROS, primarily singlet oxygen (¹O₂). We established that the nanocompartments stably encased miniSOG and in response to blue light were able to mediate the photoconversion of molecular oxygen into ROS. Using an in vitro model of lung cancer, we showed that ROS generated by the nanoreactor triggered photosensitized oxidation reactions which exerted a toxic effect on tumor cells, suggesting utility in photodynamic therapy. This encapsulin nanoreactor thus represents a platform for the light-controlled initiation and/or modulation of ROS-driven processes in biomedicine and biotechnology.

Engineering spatiotemporal organization and dynamics in synthetic cells

Constructing synthetic cells has recently become an appealing area of research. Decades of research in biochemistry and cell biology have amassed detailed part lists of components involved in various cellular processes. Nevertheless, recreating any cellular process in vitro in cell-sized compartments remains ambitious and challenging. Two broad features or principles are key to the development of synthetic cells-compartmentalization and self-organization/spatiotemporal dynamics. In this review article, we discuss the current state of the art and research trends in the engineering of synthetic cell membranes, development of internal compartmentalization, reconstitution of self-organizing dynamics, and integration of activities across scales of space and time. We also identify some research areas that could play a major role in advancing the impact and utility of engineered synthetic cells. This article is categorized under: Biology-Inspired Nanomaterials > Lipid-Based Structures Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Advances in encapsulin nanocompartment biology and engineering

Compartmentalization is an essential feature of all cells. It allows cells to segregate and coordinate physiological functions in a controlled and ordered manner. Different mechanisms of compartmentalization exist, with the most relevant to prokaryotes being encapsulation via self-assembling protein-based compartments. One widespread example of such is that of encapsulins-cage-like protein nanocompartments able to compartmentalize specific reactions, pathways, and processes in bacteria and archaea. While still relatively nascent bioengineering tools, encapsulins exhibit many promising characteristics, including a number of defined compartment sizes ranging from 24 to 42 nm, straightforward expression, the ability to self-assemble via the Hong Kong 97-like fold, marked physical robustness, and internal and external handles primed for rational genetic and molecular manipulation. Moreover, encapsulins allow for facile and specific encapsulation of native or heterologous cargo proteins via naturally or rationally fused targeting peptide sequences. Taken together, the attributes of encapsulins promise substantial customizability and broad usability. This review discusses recent advances in employing engineered encapsulins across various fields, from their use as bionanoreactors to targeted delivery systems and beyond. A special focus will be provided on the rational engineering of encapsulin systems and their potential promise as biomolecular research tools.

Encapsulins-Bacterial Protein Nanocompartments: Structure, Properties, and Application

Recently, a new class of prokaryotic compartments, collectively called encapsulins or protein nanocompartments, has been discovered. The shell proteins of these structures self-organize to form icosahedral compartments with a diameter of 25-42 nm, while one or more cargo proteins with various functions can be encapsulated in the nanocompartment. Non-native cargo proteins can be loaded into nanocompartments and the surface of the shells can be further functionalized, which allows for developing targeted drug delivery systems or using encapsulins as contrast agents for magnetic resonance imaging. Since the genes encoding encapsulins can be integrated into the cell genome, encapsulins are attractive for investigation in various scientific fields, including biomedicine and nanotechnology.

The biomedical and bioengineering potential of protein nanocompartments

Protein nanocompartments (PNCs) are self-assembling biological nanocages that can be harnessed as platforms for a wide range of nanobiotechnology applications. The most widely studied examples of PNCs include virus-like particles, bacterial microcompartments, encapsulin nanocompartments, enzyme-derived nanocages (such as lumazine synthase and the E2 component of the pyruvate dehydrogenase complex), ferritins and ferritin homologues, small heat shock proteins, and vault ribonucleoproteins. Structural PNC shell proteins are stable, biocompatible, and tolerant of both interior and exterior chemical or genetic functionalization for use as vaccines, therapeutic delivery vehicles, medical imaging aids, bioreactors, biological control agents, emulsion stabilizers, or scaffolds for biomimetic materials synthesis. This review provides an overview of the recent biomedical and bioengineering advances achieved with PNCs with a particular focus on recombinant PNC derivatives.

Highly ordered protein cage assemblies: A toolkit for new materials

Protein capsids are specialized and versatile natural macromolecules with exceptional properties. Their homogenous, spherical, rod-like or toroidal geometry, and spatially directed functionalities make them intriguing building blocks for self-assembled nanostructures. High degrees of functionality and modifiability allow for their assembly via non-covalent interactions, such as electrostatic and coordination bonding, enabling controlled self-assembly into higher-order structures. These assembly processes are sensitive to the molecules used and the surrounding conditions, making it possible to tune the chemical and physical properties of the resultant material and generate multifunctional and environmentally sensitive systems. These materials have numerous potential applications, including catalysis and drug delivery. This article is categorized under: Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.