Chemical modification of the interior and exterior surfaces of protein nanocages holds promise for various applications such as cosmetics, pharmaceuticals, and catalysts. However, dual-surface modification of these surfaces using different chemicals remains challenging, particularly when the same substituents, such as cysteine thiols, are modified. We recently produced an artificial protein nanocage called TIP60 that has 20 large surface pores. Chemical modification of cysteine residues introduced in the interior surface of TIP60 showed that these pores allow the passage of small molecules from the outside environment to the inside of the nanocage. In this study, we found that the surface pores on TIP60 function as size-dependent molecular filters. Modification experiments using different-sized polymers containing maleimide groups, which specifically react with thiols, showed that macromolecules with diameters larger than that of the pores could not penetrate into the inner cavity. This molecular size discrimination by the pores prompted us to perform stepwise dual-surface functionalization of a double mutant of TIP60 presenting cysteine residues on the interior and exterior surfaces. This was achieved by modifying the exterior cysteine residues with a polymer containing a maleimide group that cannot penetrate to the inside of the nanocage, followed by modification of the interior cysteine residues using thiol-containing small molecules. Dual-functionalized TIP60 released internal small molecules in a redox-responsive manner. This simple approach for dual-surface modification would make TIP60 a useful nanocarrier for a broad range of applications including drug-delivery and molecular filtration systems.
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