The Mechanism of Fluorescence Quenching of Protein Photosensitizers Based on miniSOG During Internalization of the HER2 Receptor

The protein photosensitizer miniSOG is a promising agent for photodynamic therapy. The genetically encoded phototoxins 4D5scFv-miniSOG and DARPin-miniSOG specifically bind to the HER2 receptor overexpressed on the surface of cancer cells and promote receptor-mediated internalization of HER2. We show that ingestion of proteins in a complex with the receptor reduces the fluorescent signal of the phototoxic module in endosomes. In order to clarify the mechanism of decrease in the fluorescence intensity of miniSOG-based proteins as they enter a cancer cell during internalization, we analyzed the influence of different factors, including low pH, proteolysis, cofactor reduction, and shielding, on changes in the fluorescence of photosensitizers. Shielding and absorption of miniSOG fluorescence by cell fluorophores, including cytochrome c, were found to contribute significantly to the changes in the fluorescent properties of miniSOG.