Protein nanocages: Programmable biomaterial platforms for therapeutics, bioimaging, and diagnostics

Protein nanocages are monodisperse, hollow structures formed through the symmetric self-assembly of protein subunits. Their genetically encoded architectures combine the geometric precision of nanoscale assemblies with the intrinsic biocompatibility and functional versatility of naturally evolved proteins. Well-defined internal cavities and engineerable interior and exterior surfaces enable the spatial organization of therapeutic cargoes, molecular recognition elements, and bioactive components within a single nanoscale platform. In this Review, we examine the structural organization, self-assembly mechanisms, physicochemical properties, and functional diversity of representative protein nanocages, including ferritin, the pyruvate dehydrogenase E2 core, DNA-binding proteins from starved cells (Dps), small heat shock proteins (sHSPs), encapsulins, and lumazine synthase. We discuss how their structural features and assembly behavior govern biological performance and support biomedical applications in targeted drug delivery, cancer immunotherapy, vaccine development, and in vivo and in vitro diagnostics. Beyond describing individual platforms, we establish a unifying framework that positions protein nanocages as programmable molecular organizers and highlights five transferable design principles: multivalent ligand display, spatial compartmentalization, receptor-guided targeting, reporter organization and signal enhancement, and stimulus-responsive cargo release and functional activation. We also examine persistent challenges in structural stability, immunogenicity, scalable manufacturing, and clinical translation, alongside emerging opportunities in protein engineering, synthetic biology, and computational protein design. Collectively, these principles establish protein nanocages as versatile biomaterial platforms for integrating sensing, targeting, imaging, and therapy within a single molecular architecture, providing a foundation for next-generation precision nanomedicine. STATEMENT OF SIGNIFICANCE: Reviews typically treat protein nanocages individually, obscuring shared structural features and the extent to which design strategies transfer across systems. This Review addresses this gap by establishing a comparative framework spanning structurally diverse nanocages and distilling their similarities into a concise set of generalizable design principles. It reframes protein nanocages not merely as passive delivery vehicles but as programmable molecular scaffolds that organize cargo with controlled spatial arrangement and stoichiometry. This framework provides biomaterials researchers with a transferable basis for engineering protein assemblies for precision nanomedicine while identifying key challenges in structural stability, immunogenicity, and scalable manufacturing that must be addressed to facilitate clinical translation.