Structural diversity and species distribution of host-defense peptides in frog skin secretions

Abstract

Cationic peptides that adopt an amphipathic α-helical conformation in a membrane-mimetic environment are synthesized in the skins of many frog species. These peptides often display cytolytic activities against bacteria and fungi consistent with the idea that they play a role in the host's system of defense against pathogenic microorganisms, but their importance in the survival strategy of the animal is not clearly understood. Despite the common misconception that antimicrobial peptides are synthesized in the skins of all anurans, the species distribution is sporadic, suggesting that their production may confer some evolutionary advantage to the organism but is not necessary for survival. The low potency of many frog skin antimicrobial peptides is consistent with the hypothesis that cutaneous symbiotic bacteria may provide the major system of defense against pathogenic microorganisms in the environment with antimicrobial peptides assuming a supplementary role in some species.

Fig. 1

Antimicrobial peptides isolated from skin secretions of the tailed frog Ascaphus truei in the family Leiopelmatidae. The shaded amino acid residues are conserved in all peptides

Fig. 2

Antimicrobial peptides isolated from skin secretions of the midwife toad Alytes obstetricans in the family Alytidae

Fig. 3

Antimicrobial peptides isolated from skin secretions of species in the genus Bombina in the family Bombinatoridae. The residues shown in bold are d-amino acids

Fig. 4

Antimicrobial peptides isolated from skin secretions of species in the genera Xenopus and Silurana in the family Pipidae. In order to maximize structural similarity, amino acid deletions, denoted by asterisks, have been introduced into some sequences

Fig. 5

Antimicrobial peptides isolated from extracts of the dorsal skin glands of species in the genera Uperoleia and Crinia in the family Myobatrachidae

Fig. 6

Antimicrobial peptides isolated from skin secretions of species in the genus Leptodactylus in the family Leptodactylidae

Fig. 7

Antimicrobial peptides isolated from skin secretions of species in the genera Hoplobatrachus and Fejervarya in the family Dicroglossidae