Protein cage architectures such as virus capsids and ferritins are versatile nanoscale platforms amenable to both genetic and chemical modification. Incorporation of multiple functionalities within these nanometer-sized protein architectures demonstrate their potential to serve as functional nanomaterials with applications in medical imaging and therapy. In the present study, we synthesized an iron oxide (magnetite) nanoparticle within the interior cavity of a genetically engineered human H-chain ferritin (HFn). A cell-specific targeting peptide, RGD-4C which binds alphavbeta3 integrins upregulated on tumor vasculature, was genetically incorporated on the exterior surface of HFn. Both magnetite-containing and fluorescently labeled RGD4C-Fn cages bound C32 melanoma cells in vitro. Together these results demonstrate the capability of a genetically modified protein cage architecture to serve as a multifunctional nanoscale container for simultaneous iron oxide loading and cell-specific targeting.

Cage architectures based on the cowpea chlorotic mottle virus (see Figure) have been employed to achieve a synthetic mimic of the iron storage protein ferritin. The electrostatic nature of the inner protein surface could be changed by up to 3240 units of charge, while still maintaining a stable cage structure. The spatial isolation within the protein cage prevents bulk aggregation of the mineral particles and results in a stable, mono‐disperse colloid.

In recent years, plant virus capsids, the protein shells that form the surface of a typical plant virus particle, have emerged as useful biotemplates for material synthesis. All virus capsids are assembled from virus-coded protein subunits. Many plant viruses assemble capsids with precise 3D structures providing nanoscale architectures that are highly homogeneous and can be produced in large quantities. Capsids are amenable to both genetic and chemical modifications allowing new functions to be incorporated into their structure by design. The three capsid surfaces, the interior surface, the exterior surface, or the interface between coat protein subunits, can be independently functionalized to produce multifunctional biotemplates. In this review, we examine the recent advances in using plant virus capsids as biotemplates for nanomaterials and their potential for applications in nanotechnology, especially medicine.

Nature has provided us with a range of reactive nanoscale platforms, in the form of protein cage architectures such as viral capsids and the cages of ferritin-like proteins. Protein cage architectures have clearly demarcated exterior, interior, and interface surfaces consisting of precisely located chemical functionalities. In the present work, we demonstrate that the small heat shock protein (MjHsp) cage from Methanococcus jannaschii is a new and versatile nanoscale platform whose exterior and interior surfaces are amenable to both genetic and chemical modification. Wild type and genetic mutants of the Hsp cage are shown to react with activated fluorescein molecules in a site specific manner. In addition, the 12 nm Hsp cage serves as a size constrained reaction vessel for the oxidative mineralization of iron, resulting in the formation of monodispersed 9 nm iron oxide nanoparticles. These results demonstrate the utility of the Hsp cage to serve as a nanoscale platform for the synthesis of both soft (organic) and hard (inorganic) materials.

In multistep reactions, stability of intermediates is critical to the rate of product formation and a significant factor in generating kinetic traps. The capsid protein of cowpea chlorotic mottle virus (CCMV) can be induced to assemble into spherical particles of 30, 60, and 90 dimers. Based on examining assembly kinetics and reaction end points, we find that formation of uniform, ordered structures is not always a result of reactions that reach equilibrium. Equilibration or, alternatively, kinetic trapping can be identified by a straightforward analysis. Altering the assembly path of "spherical" particles is a means of controlling the distribution of products, which has broad applicability to self-assembly reactions.