Protein assemblies can be designed for development of nano–bio materials. This has been achieved by modulating protein–protein interactions. However, fabrication of highly ordered protein assemblies remains challenging. Protein crystals, which have highly ordered arrangements of protein molecules, provide useful source matrices for synthesizing artificial protein assemblies. Here, we describe construction of a supramolecular filament structure by engineering covalent and non‐covalent interactions in a protein crystal. Performing in‐cell crystallization of Trypanosoma brucei cysteine protease cathepsin B (TbCatB), we achieved a precise arrangement of protein molecules while suppressing random aggregation due to disulfide bonds. We succeeded in synthesizing bundled filament from the crystals by autoxidation of cysteinyl thiols after the isolation of the crystals from living cells.

Protein assemblies can be designed for development of nano–bio materials. This has been achieved by modulating protein–protein interactions. However, fabrication of highly ordered protein assemblies remains challenging. Protein crystals, which have highly ordered arrangements of protein molecules, provide useful source matrices for synthesizing artificial protein assemblies. Here, we describe construction of a supramolecular filament structure by engineering covalent and non‐covalent interactions in a protein crystal. Performing in‐cell crystallization of Trypanosoma brucei cysteine protease cathepsin B (TbCatB), we achieved a precise arrangement of protein molecules while suppressing random aggregation due to disulfide bonds. We succeeded in synthesizing bundled filament from the crystals by autoxidation of cysteinyl thiols after the isolation of the crystals from living cells.

We have established a new method for encapsulating superfold green fluorescent protein (sfGFP) into polyhedra crystal (PhC) with high efficiency using a protein needle (PN). H1-sfGFP-PN, in which PN is fused to the C-terminus of H1-sfGFP, exhibits a 55-fold increase in encapsulation of sfGFP relative to H1-sfGFP with the improvement provided by the synergistic effects of the H1-helix, PN, and the His-tag at the C-terminus of PN. This method is expected to provide useful materials based on encapsulation of various functional proteins.