Role of unique basic residues in cytotoxic, antibacterial and antiparasitic activities of human eosinophil cationic protein

Eosinophil granule proteins, eosinophil cationic protein (ECP) and eosinophil-derived neurotoxin are members of the RNase A superfamily, which play a crucial role in host defense against various pathogens as they are endowed with several biological activities. Some of the biological activities possessed by ECP have been attributed to its strong basic character. In the current study, we have investigated the role of five unique basic residues, Arg22, Arg34, Arg61, Arg77 and His64 of ECP in its catalytic, cytotoxic, antibacterial and antiparasitic activities. These residues were changed to alanine to generate single and double mutants. None of the selected residues was found to be involved in the RNase activity of ECP. The substitution of all five residues individually was detrimental for the cytotoxic, antibacterial and antiparasitic activities of ECP; however, mutation of Arg22 and Arg34 resulted in the most significant effects. The double mutants also had reduced biological activities. All ECP mutants that had significantly reduced toxicity also had reduced membrane destabilization activity. Our study demonstrates that Arg22, Arg34, Arg61, Arg77 and His64 of ECP are crucial for its membrane destabilization activity, which appears to be the underlying mechanism of its cytotoxic, antibacterial and antiparasitic activities.