The ascaphins: A family of antimicrobial peptides from the skin secretions of the most primitive extant frog, Ascaphus truei

The tailed frog Ascaphus truei occupies a unique position in phylogeny as the most primitive extant anuran and is regarded as the sister taxon to the clade of all other living frogs. Eight structurally related peptides, termed ascaphins 1-8, were isolated from norepinephrine-stimulated skin secretions of A. truei and were shown to possess differential growth inhibitory activity against Escherichia coli and Staphylococcus aureus. Ascaphins 2-7 may be represented by the consensus amino acid sequence GX₂DX ₂KGAAKX₃KTVAX₂IANX·COOH whereas ascaphin-1 (GFRDVLKGAAKAFVKTVAGHIAN·NH₂) and ascaphin-8 (GFKDLLKGAAKALVKTVLF·NH₂) contain a C-terminally α-amidated residue. The ascaphins show no appreciable structural similarity with other families of antimicrobial peptides from frog skin but display limited sequence identity with the cationic, amphipathic α-helical peptides pandinin 1 and opistoporin 1, isolated from the venoms of African scorpions. Ascaphin-8 shows the highest potency against a range of pathogenic microorganisms but has the greatest haemolytic activity. The data indicate that the host defence strategy of using antimicrobial peptides in skin secretions arose early in the evolution of anurans.