Engineering protein nanocages as carriers for biomedical applications

Bhaskar, Sathyamoorthy; Lim, Sierin

Viruses and virus-based particles. Years of studies to deduce information about viral assembly, replication and infection pathways provide a clear idea on the stability and functionality of viruses. 3 Viral transduction has been observed to be facilitated by self-assembling protein structures called capsids, which interact with host cells and deliver their genome into the target cell compartment. 1 The outer surface of the viral capsid opens up several opportunities for diverse chemical functionalization and site-specific modifications to synthesize organic/inorganic materials and to link targeting moieties. 9 Viruses have an innate mechanism for protecting their genome, transferring it to the extracellular environment, escaping the immune system upon reaching target cells by interacting with their receptors, and delivering the packaged nucleic acids into the desired cell compartment. Therefore, viruses are used as functional carriers for targeted delivery with built-in advantages over synthetic delivery vehicles. 1 Viruses are the dominant class of the bionanoparticle family, with sizes ranging widely from 10 to 100 nm ( Figure 1 ). 3 In addition to cell targeting ability and gene delivery efficiency, these protein-based multifaceted systems have highly ordered spatial configurations, and the stability and functionality of these materials have already been established through intensive research with advances in understanding virus infection, replication and assembly pathways. 3 The cowpea mosaic virus (CPMV) (PDB ID 2BFU) was the first nanocage among viruses shown to be conjugated to a variety of functional molecules. This nanocage is a well-characterized plant virus that can be produced in high amounts. The composition of the amino-acid residues in the virus's structure is well understood for bioconjugation with functional molecules. The virus is stable over a wide range of pH levels, temperatures and organic solvents. It could therefore be used for assembling other inorganic nanoparticles. 10 Genetically engineered CPMV possessing thiol groups are produced and covalently attached to gold nanospheres. The nanospheres self-assemble into icosahedral plasmonic nanoclusters following the locations of the thiol groups on the CPMV and exhibit a 10-fold increase in local electromagnetic fields. These plasmonic nanoclusters have unique optical properties that can be used for spectroscopy and cancer treatment. 10 Multiple orthogonal reactive sites on viral nanocages can be established by genetic engineering for bioconjugation with signalling moieties and biological recognition motifs. 11 Two different amino-acid residues of turnip yellow mosaic virus (PDB ID 1AUY) are used for conjugation to luminescent terbium complexes and biotin molecules. The modified nanocages can be used as a scaffold for the development of sensor by time-resolved fluoroimmunoassay. 11 Recent advances in selective chemical modifications of viruses and viral-based particles have broadened the scope of modulating bionanoparticles beyond genetic mutations. 3 Ferritin. In 1937, ferritin was discovered as a novel protein structure for storing and transporting iron molecules. Ferritin isolated from horse spleen contained 20% iron in its native structure. 9 Subsequently, it has been found to exist almost ubiquitously in biological systems,