Nanocrystals are unique crystalline particles with a highly ordered atomic structure and distinctive physicochemical properties which arise from their high surface-area-to-volume ratios. These characteristics have made nanocrystalline systems increasingly attractive for biomedical applications, particularly in biomanufacturing, drug delivery and therapeutic protein stabilisation. A biologically derived example of such nanocrystals is found in the parasporal inclusions of Bacillus thuringiensis (Bt), composed of Cry proteins. These crystalline protein structures have traditionally been employed as bioinsecticides due to their specificity and toxicity toward target insect species. However, studies have demonstrated their non-toxicity in humans and non-target insects, thereby highlighting their potential for safe biotechnological applications. The inherent crystallinity and high stability of Cry proteins have prompted exploration into their use as platforms for recombinant protein encapsulation.

This study investigates the feasibility of utilising Cry protein nanocrystals as stable scaffolds for the display of functional peptide tags. Specifically, the SpyTag peptide sequence was genetically fused to Cry proteins, and the resulting recombinant nanocrystals were produced in E. coli, purified, and characterised. The structural analysis via electron microscopy revealed irregularly shaped particles ranging from 700 to 800 nm in size. Functional validation was performed by incubating the SpyTag-functionalised crystals with SpyCatcher, confirming covalent complex formation through SDS-PAGE. These findings demonstrate that Cry-based nanocrystals can preserve the functionality of fused peptide tags, supporting their use as versatile platforms for recombinant protein stabilisation. This approach offers promising implications for the easy production and stable formulation of therapeutically relevant proteins with poor intrinsic stability.