Characterization of novel antimicrobial peptides from the skins of frogs of the Rana esculenta complex

Rana esculenta is a hybridogenetic hybrid between Rana ridibunda and Rana lessonae and so is best considered as a complex of interbreeding species rather than a discrete single species. In this study, antimicrobial peptides were isolated from a pooled extract of the skins of specimens of the R. esculenta complex collected in the wild. In addition to several peptides belonging to the brevinin and esculentin families that have been previously isolated from skin secretions of a single specimen of R. esculenta, three newly described members of the brevinin-2 family (brevinin-2Ei, brevinin-2Ej, and brevinin-2Ek) and one member of the temporin family (temporin-1Ec) were purified and characterized. In addition, three structurally related peptides with no sequence similarity with antimicrobial peptides isolated from other species of ranid frogs, that potently and selectively inhibit the growth of the Gram-positive bacterium Escherichia coli (minimal inhibitory concentration (MIC<5 μM)), were identified. These peptides show limited amino acid sequence similarity to the homologous exon gene products that encode the N-terminal flanking peptides of preprocaerulein, preproxenopsin, and preprolevitide and so have been termed caerulein precursor-related fragments (CPRF-Ea, CPRF-Eb, and CPRF-Ec). The data suggest that there may be considerable polymorphism among specimens from different populations of the R. esculenta complex. It is proposed that the distribution and amino acid sequences of skin antimicrobial peptides may be useful markers for taxonomic classification of particular sub-populations and for an understanding of phylogenetic interrelationships.