Synthetic peptides derived from the beta(2)-beta(3) loop of Raphanus sativus antifungal protein 2 that mimic the active site

Rs-AFPs are antifungal proteins, isolated from radish (Raphanus sativus) se ed or leaves, which consist of 50 or 51 amino acids and belong to the plant defensin family of proteins. Four highly homologous Rs-AFPs have been isol ated (Rs-AFP1-4). The structure of Rs-AFP1 consists of three beta -strands and an alpha -helix, and is stabilized by four cystine bridges. Small pepti des deduced from the native sequence, still having biological activity, are not only important tools to study structure-function relationships, but ma y also constitute a commercially interesting target. In an earlier study, w e showed that the antifungal activity of Rs-AFP2 is concentrated mainly in the beta2-beta3 loop. In this study, we synthesized linear 19-mer peptides, spanning the entire beta2-beta3 loop, that were found to be almost as pote nt as Rs-AFP2. Cysteines, highly conserved in the native protein, are essen tial for maintaining the secondary structure of the protein. Surprisingly, in the 19-mer loop peptides, cysteines can be replaced by alpha -aminobutyr ic acid, which even improves the antifungal potency of the peptides. Analog ous cyclic 19-mer peptides, forced to adopt a hairpin structure by the intr oduction of one or two non-native disulfide bridges, were also found to pos sess high antifungal activity. The synthetic 19-mer peptides, like Rs-AFP2 itself, cause increased Ca2+ influx in pregerminated fungal hyphae.