Maximin 9, a novel free thiol containing antimicrobial peptide with antimycoplasma activity from frog Bombina maxima

Amphibian skin is a rich resource of antimicrobial peptides, like maximins and maximin Hs from frog Bombina maxima. Novel cDNA clones encoding a precursor protein, which comprises a novel maximin peptide (maximin 9) and reported maximin H3, were isolated from two constructed skin cDNA libraries of B. maxima. The predicted primary structure of maximin 9 is GIGRKFLGGVKTTFRCGVKDFASKHLY-NH2. A surprising substitution is at position 16, with a free cysteine in maximin 9 rather than usual conserved glycine in other reported maximins. Maximin 9, the homodimer form and its Cys16 to Gly16 mutant were synthesized and their antimicrobial activities were evaluated. Unlike previously reported maximin 3, the tested bacterial and fungal strains were resistant to maximin 9, its homodimer and the Cys16 to Gly16 mutant (with MICs>100 microM). On the other hand, interestingly, while eight clinical Mollicutes strains were generally resistant to maximin 9 homodimer and its Cys16 to Gly16 mutant, most of them are sensitive to maximin 9 at a peptide concentration of 30 microM, especially in the presence of dithiothreitol. These results indicate that the presence of a reactive Cys residue in maximin 9 is important for its antimycoplasma activity. The diversity of antimicrobial peptide cDNA structures encountered in B. maxima skin cDNA libraries and the antimicrobial specificity differences of the peptides may reflect well the species' adaptation to the unique microbial environments.