Single Amino Acid Variation Underlies Species-Specific Sensitivity to Amphibian Skin-Derived Opioid-like Peptides

It has been suggested that the evolution of vertebrate opioid receptors (ORs) follow a vector of increased functionality. Here, we test this idea by comparing human and frog ORs. Interestingly, some of the most potent opioid peptides known have been isolated from amphibian skin secretions. Here we show that such peptides (dermorphin and deltorphin) are highly potent in the human receptors and inactive in frog ORs. The molecular basis for the insensitivity of the frog ORs to these peptides was studied using chimeras and molecular modeling. The insensitivity of the delta OR (DOR) to deltorphin was due to variation of a single amino acid, Trp7.35, which is a leucine in mammalian DORs. Notably, Trp7.35 is completely conserved in all known DOR sequences from lamprey, fish, and amphibians. The deltorphin-insensitive phenotype was verified in fish. Our results provide a molecular explanation for the species selectivity of skin-derived opioid peptides. [Display omitted] •Comparison of frog and human opioid receptors show distinct pharmacology•Frog-derived opioid peptides secreted are potent in human but inactive in frogs•The molecular basis for delta-receptor species selectivity due to a single amino acid•The structural basis for this selectivity filter is elucidated Vardy et al. compare the activation of opioid receptors from human and frogs. Deltorphin, the opioid peptide secreted by frogs, was shown to highly potent in the human DOR but inactive in the frog receptor. Bioinformatics, structural analysis, and extensive mutagenesis revealed that a single amino acid is responsible for the species selectivity exhibited by deltorphin.