A pH-Responsive Protein\nAssembly through Clustering\nof a Charge-Tunable Single Amino Acid Repeat

Specific targeting of tumor cells is a key to achieving\nhigh therapeutic\nefficacy while minimizing off-target side effects. As a general approach\nto targeting diverse tumor cells, considerable attention has been\npaid to the tumor microenvironment, particularly its slightly acidic\npH (6.5–6.8). However, existing pH-sensitive nanomaterials,\nbased on organic polymers and proteins, often lack sufficient pH sensitivity\nand specificity. Here, we demonstrate a strategy to construct a pH-responsive\nprotein assembly through clustering of a single amino acid repeat\nas a charge-tunable moiety. As a proof of concept, a histidine peptide\nwith varying lengths was displayed on the surface of a ferritin assembly\ncomposed of 24 subunits by genetic fusion to a subunit. The resulting\nself-assembled ferritin particles, termed “pHerricle (pH-responsive\nferritin particle)”, were shown to exhibit a specific binding\nto tumor cells in response to pH changes through cooperative effects\nof histidine peptides. Increasing the histidine peptide length from\n0 to 12 residues increased the pHerricle’s cell-binding capacity\nby 21-fold and allowed modulation of the targetable pH range. General\napplicability as a tumor cell-targeting platform was shown by specific\ndelivery of a cytotoxic cargo by the pHerricle into tumor cells of\nvarious origins in a pH-dependent manner.