RANALEXIN - A NOVEL ANTIMICROBIAL PEPTIDE FROM BULLFROG (RANA-CATESBEIANA) SKIN, STRUCTURALLY RELATED TO THE BACTERIAL ANTIBIOTIC, POLYMYXIN

Antimicrobial peptides comprise a diverse class of molecules used in h ost defense by plants, insects, and animals. In this study we have iso lated a novel antimicrobial peptide from the skin of the bullfrog, Ran a catesbeiana. This 20 amino acid peptide, which we have termed Ranale xin, has the amino acid sequence: e-Leu-Gly-Gly-Leu-Ile-Lys-Ile-Val-Pr o-Ala-Met-Ile- Cys-Ala-Val-Thr-Lys-Lys-Cys-COOH, and it contains a sin gle intramolecular disulfide bond which forms a heptapeptide ring with in the molecule. Structurally, Ranalexin resembles the bacterial antib iotic, polymyxin, which contains a similar heptapeptide ring. We have also cloned the cDNA for Ranalexin from a metamorphic R. catesbeiana t adpole cDNA library. Based on the cDNA sequence, it appears that Ranal exin is initially synthesized as a propeptide with a putative signal s equence and an acidic amino acid-rich region at its amino-terminal end . Interestingly, the putative signal sequence of the Ranalexin cDNA is strikingly similar to the signal sequence of opioid peptide precursor s isolated from the skin of the South American frogs Phyllomedusa sauv agei and Phyllomedusa bicolor. Northern blot analysis and in situ hybr idization experiments demonstrated that Ranalexin mRNA is first expres sed in R. catesbeiana skin at metamorphosis and continues to be expres sed into adulthood.